US20100010235A1

US20100010235A1 - Method of using rhodium quinonoid catalysts

Info

Publication number: US20100010235A1
Application number: US12/456,633
Authority: US
Inventors: William C. Trenkle; Julia L. Barkin; Seung Uk Son; Dwight A. Sweigart; Marcus D. Faust, Jr.
Original assignee: Brown University
Current assignee: Brown University
Priority date: 2005-11-30
Filing date: 2009-06-19
Publication date: 2010-01-14
Also published as: US7569734B2; US20070123715A1

Abstract

In accordance with aspects of the invention methods of using rhodium hydroquinone catalysts for the conjugate addition of boronic acids are disclosed.

Description

PRIORITY CLAIM

This application claims priority from U.S. Provisional Patent Application No. 60/740,762 filed on Nov. 30, 2005, the contents of which are hereby incorporated by reference.

RELATED APPLICATION

The subject application shares certain attributes with co-pending application Ser. No. ______, entitled, Rhodium Quinonoid Catalysts, filed on even date herewith, the contents of which are hereby incorporated by reference in its entirety.

STATEMENT OF GOVERNMENT RIGHTS

The invention was made in part under U.S. Government contract number NSF CHE-0308640. Accordingly, the Government has certain rights in this invention.

TECHNICAL FIELD

The invention concerns the use of hydroquinone catalysts.

BACKGROUND

In fields such as the development of pharmaceuticals, agrochemicals and in medicinal discovery chemistry, there is an unmet need for conjugate additions with electron deficient boronic acids and esters. Some recent advances in the development of transition-metal-catalyzed carbon-carbon bond-forming chemistries are reviewed in Rouhi, “Fine Chemicals”, Chemical & Engineering News, Sep. 6, 2004, pages 49-67.
Additionally, few π-bonded hydroquinones are known. See Moussa, et al., “η5-Semiquinone Complexes and the η4-Benzoquinone of (Pentamethylcyclopentadienyl)rhodium and -irridium: Synthesis, Structures, Hydrogen Bonding, and Electrochemical Behavior”, Organometallics 23: 6231-38 (2004) and Amouri, Eur. J. Inorg. Chem., page 3808 (2005). For example, complexes containing 1,4-hydroquinone (H₂Q) π-bonded to a transition metal are of substantial interest because of the importance of quinonoid molecules in mediating proton and electron transfer reactions. See: Pierpont, C. G.; Langi, C. W. Prog. Inorg Chem. 1994, 41, 331. Ebadi, M.; Lever, A. B. P. Inorg Chem. 1999, 38, 467. Coenzyme Q: Biochemistry, Bioenergetics and Clinical Applications of Ubiquinone; Lenaz, G., Ed.; Wiley: New York, 1985.
Those skilled in the art desire new uses of hydroquinone complexes. Embodiments of the subject invention satisfy this need and others.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, disclosed is a method of making an acyclic or cyclic compound comprising combining in a reaction mixture an aryl or vinylic boronic compound, a conjugate acceptor and a rhodium hydroquinone catalyst under suitable reaction conditions, allowing the reaction to proceed to its end, and isolating the desired acylic or cyclic compound from the reaction mixture.
In accordance with another aspect, a method comprises using the rhodium hydroquinone catalyst for the conjugate addition of a boronic substrate of the boronic compound comprising transferring a carbon group from the boronic compound to the conjugate acceptor via the rhodium hydroquinone catalyst; and forming a product of conjugate addition.
In accordance with another aspect, a method comprises combining the boronic compound, rhodium hydroquinone catalyst, conjugate acceptor and a solvent, wherein the boronic compound is an aryl boronic acid, the conjugate acceoptor is enone and the solvent is dimethoxyethane; adding an aqueous solution of LiOH base followed by deoxygenated H₂O to obtain a mixture; stirring the mixture at about 50° C. for about 1 hour; then diluting the mixture with a solution of NH₄Cl, extracted with EtOAc/hexanes and dried Na₂S0₄; followed by filtering to obtain the desired compound.
In accordance with another aspect, the above catalyst comprises the formula formula (I)
wherein X⁻ is selected from the group consisting of BF₄ ⁻, SbF₆ ⁻, PO₂F₂ ⁻, PF₆ ⁻, OTf⁻, ⁻OTs, SO₄ ²⁻, B(C₆F₅)₄ ⁻, B(C₆H₅)₄ ⁻, ClO₄ ⁻, NO₃ ⁻, NO₂ ⁻, HOSO₃ ⁻, CO₃ ²⁻, O₃SCF₂CF₂CF₂CF₃ ⁻;
wherein OTf=O₃SCF₃ ⁻; OTs=O₃SC₆H₄CH₃ ⁻; R′CO₂ ⁻;
wherein R′ is selected from the group consisting of hydrogen or an alkyl, aryl or carbon atom bearing three identical or non-identical substituents;
wherein L₁and L₂are each a ligand that donates electron density to rhodium to stabilize it;
wherein
is either chiral or achiral and M₁and M₂comprise hydroxo (OH) groups in the ortho-, meta-, and para-positions and R is selected from the group consisting of H, C, O, N and S, with or without substituents, said substituents being identical or non-identical. L₁and L₂may also be identical or non-identical ligands that are either chiral or achiral and selected from the group consisting of alkenes, dialkenes, alkyene, phosphines, water, phosphites, sulfides, sulfoxides, sulfonates, sulfonamides, sulfones, ethers, amines, imines, amides, aldehydes, ketones, esters, nitrites, and combinations thereof.
In accordance with a further aspect, the above catalyst comprises the formula (II)
wherein X⁻ is selected from the group consisting of BF₄ ⁻, SbF₆ ⁻, PO₂F₂ ⁻, PF₆ ⁻, OTf⁻, ⁻OTs, SO₄ ²⁻, B(C₆F₅)₄ ⁻, B(C₆H₅)₄, ClO₄ ⁻, NO₃, NO₂, HOSO₃ ⁻, CO₃ ²⁻, O₃SCF₂CF₂CF₂CF₃ ⁻ wherein OTf=O₃SCF₃ ⁻; OTs=O₃SC₆H₄CH₃ ⁻; R′CO₂ ⁻;
wherein R′ is selected from the group consisting of hydrogen or an alkyl, aryl or carbon bearing three identical or non-identical substituents;
wherein L₁and L₂are each a ligand that donates electron density to rhodium to stabilize it;
wherein R is selected from the group consisting of H, C, O, N and S, with or without substituents, said substituents being identical or non-identical; and L₁and L₂may be identical or non-identical ligands that are either chiral or achiral and selected from the group consisting of alkenes, dialkenes, alkyene, phosphines, water, phosphites, sulfides, sulfoxides, sulfonates, sulfonamides, sulfones, ethers, amines, imines, amides, aldehydes, ketones, esters, nitrites, and combinations thereof.
In accordance with a further aspect the catalyst comprises the formula (III)
wherein Z is nitrogen, oxygen or sulfur;
wherein R is selected from the group consisting of H, C, O, N and S, with or without substituents, said substituents being identical or non-identical;
wherein X⁻ is selected from the group consisting of BF₄ ⁻, SbF₆ ⁻, PO₂F₂ ⁻, PF₆ ⁻, OTf⁻, ⁻OTs, SO₄ ²⁻, B(C₆F₅)₄, B(C₆H₅)₄, ClO₄, NO₃, NO₂, HOSO₃ ⁻, CO₃ ⁻, O₃SCF₂CF₂CF₂CF₃ ⁻ wherein OTf=O₃SCF₃ ⁻; OTs=O₃SC₆H₄CH₃ ⁻; R′CO₂ ⁻; and R′ is selected from the group consisting of hydrogen or an alkyl, aryl or carbon bearing three identical or non-identical substituents;
wherein L₁and L₂are each a ligand that donates electron density to rhodium to stabilize it;
or
formula (IV)
wherein L₁and L₂are each a ligand that donates electron density to rhodium to stabilize it; or
formula (V)
wherein L₁and L₂are each a ligand that donates electron density to rhodium to stabilize it; wherein M⁺ is a positively charged ion including any metal ion having an oxidation state at or higher than +1. M⁺ is selected from the group consisting of cationic Li, K, Cs, Be, Sr, Ba, Al, Ti, Zr, B, Si, Cd, Ag, Ph₃PNPPh₃, Rb, Mg²⁺, Ca²⁺, Na, R₄N⁺, Zn²⁺, ammonium salts including tetraalkylammonium cations, tetraalkylarsonium cations, guanidinium salts, amidinium salts, and combinations thereof; and L₁and L₂are identical or non-identical ligands that are either chiral or achiral and selected from the group consisting of alkenes, dialkenes, alkyene, phosphines, water, phosphites, sulfides, sulfoxides, sulfonates, sulfonamides, sulfones, ethers, amines, imines, amides, aldehydes, ketones, esters, nitriles, and combinations thereof.
In accordance with further aspects, the catalyst comprises 1,2-, 1,3- or 1,4-hydroquinone π-bonded to rhodium, and may further comprise [1,4-(hydroquine)Rh(COD)]⁺ cation or 1,3-(hydroquine)Rh(COD)]⁺ cation, wherein COD is cyclooctadiene.
In accordance with further aspects, the catalyst comprises 1,2-hydroquinone, 1,3-hydroquinone or 1,4-hydroquinone π-bonded to Rh(P(OPh)₃)₂ ⁺ cation.
In accordance with another aspect, the method comprises a reaction:
wherein the reaction comprises providing an active rhodium hydroquinone catalyst, Rh¹, and reacting the catalyst with the boronic compound comprising a sp2 hybridized carbon-center bearing a boron to transfer the sp2 hybridized carbon to rhodium and subsequently to the conjugate acceptor, which is an electron deficient olefin, an olefin bearing one or more electron withdrawing groups (EWG), through carbo-metallation followed by proto-demetallation in a presence of a base;
wherein EWG is selected from the group consisting of a ketone, aldehyde, imide, amide, ester, thioester, acid anhydride, nitro, sulfone, nitrile, sulfoxide, phosphinate, electron deficient aromatic ring or other suitable electron withdrawing substituent that withdraws electron density either through inductive or resonance effects from olefins, and combinations thereof; and R is selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium, and combinations thereof.
In accordance with another aspect, a method comprises the reaction:
wherein the reaction comprises providing the boronic compound and the catalyst and reacting the compound and catalyst under conditions sufficient to cause the reaction,
X is selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium, and combinations thereof.
M⁺ is a positively charged ion including any metal ion having an oxidation state at or higher than +1, and is selected from the group consisting of cationic Li, K, Cs, Be, Sr, Ba, Al, Ti, Zr, B, Si, Cd, Ag, Ph₃PNPPh₃, Rb, Mg²⁺, Ca²⁺, Na, R₄N⁺, Zn²⁺, ammonium salts including tetraalkylammonium cations, tetraalkylarsonium cations, guanidinium salts, amidinium salts, and combinations thereof;
Y is selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium, and combinations thereof;
L is a ligand that donates electron density to the rhodium to stabilize it and each L is an identical or non-identical ligand that is either chiral or achiral and selected from the group consisting of alkenes, dialkenes, alkyene, phosphines, water, phosphites, sulfides, sulfoxides, sulfonates, sulfonamides, sulfones, ethers, amines, imines, amides, aldehydes, ketones, esters, nitriles, and combinations thereof;
wherein EWG is selected from the group consisting of a ketone, aldehyde, imide, amide, ester, thioester, acid anhydride, nitro, sulfone, nitrile, sulfoxide, phosphinate, electron deficient aromatic ring or other suitable electron withdrawing substituent that withdraws electron density either through inductive or resonance effects from olefins, and combinations thereof, R is selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium, and combinations thereof; and
—BR₂is any boronic containing species neutral or anionically charged where boron is bound to a transfer group.
In accordance with a further aspect, disclosed is a reagent for the conjugate addition of aryl or vinylic boronic acids wherein the reagent comprises a rhodium hydroquinone catalyst.
Another aspect comprises using the rhodium hydroquinone catalyst for the conjugate addition of boronic acid comprising a reaction:
wherein the reaction comprises mixing reagents (a) and (b) and reacting the reagents and the rhodium hydroquinone catalyst under conditions sufficient to cause the reaction in the presence of a base and a solvent, wherein:
X is selected from the group consisting of p-Me, m-NO₂, H, o-Me, 4NH-Boc, p-OMe, p-Cl, p-F, 3Cl, 4F, m-NO₂, 3,4,5 triF, 2,3,4triF;
the solvent is selected from the group consisting of DME/H ₂0, H₂O and THF; and
the base is LiOH or KOH.
A further aspect comprises using a rhodium hydroquinone catalyst for the conjugate addition of aryl boronic acid to 2-cyclohexen-1-one comprising a reaction
wherein:
Rh+ is the catalyst; and
X is selected from the group consisting of p-Me and m-NO₂.
In accordance with another aspect, a method comprising a reaction:
wherein the reaction comprises mixing reagents (a) and (b) and reacting the reagents with a catalyst in the presence of a base and a solvent under conditions sufficient to cause the reaction,
wherein:
R is selected from the group consisting of H and Boc;
the catalyst is a rhodium quinone catalyst; the solvent is selected from the group
consisting of DME/H₂O and THF; and the base is LiOH.
Also disclosed according to a further aspect is a method for double deprotonation of a rhodium hydroquinone catalyst to active quinone complex 3•K comprising a reaction:
wherein the reaction comprises mixing reagents (a) and (b) and reacting the reagents and the catalyst in the presence of water and DME under conditions sufficient to cause the reaction.
Still further disclosed is a method comprising a reaction
wherein:
Rh+ is a rhodium hydroquinone catalyst; and
X is selected from the group consisting of: H, p-Me, o-Me, 4NH-Boc, p-OMe, p-Cl, p-F, 3Cl, 4F, m-NO₂, 3,4,5 triF, 2,3,4triF.
Also disclosed is a method of using a catalyst for the conjugate addition of aryl or vinyl boronic substrates comprising the steps of: a) providing a rhodium hydroquinone catalyst; b) transferring a sp2 hybridized carbon group from boron to a conjugate acceptor via the catalyst; and c) forming a product of conjugate addition.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein:

FIG. 1. shows deprotonation and concomitant oxidation of hydroquinonone to quinone with the π-bonded metal fragment acting as an internal electron acceptor;

FIG. 2. shows catalytic arylation of benzaldehydes;

FIG. 3 is a photographic reproduction of an X-ray crystallography showing the structure of an embodiment of the rhodium species;

FIG. 4 shows the ¹H NMR spectrum of diphenyl alcohol, the product of the rhodium-quinonoid catalyzed addition of phenyl boronic acid to benzaldahyde;

FIG. 5 shows ¹H NMR data of diaryl alcohols obtained using the rhodium quinonoid catalyst;

FIG. 6 shows cationic complexes and associated anions used to study the structural consequences charge assisted hydrogen bonding;

FIG. 7 shows hydrogen bonded structural patterns found in the solid state for [14⁺-16⁺]X⁻ can be (a) dimeric, (b) 1-D chain, (c) C₂-helical, (d) C₃-helical;

FIG. 8 shows hydrogen bonded structure of crystalline 14⁺SbF₆ ⁻ as a diethyl ether solvate (left) and slippage to a π-π stacked structure upon drying (right);

FIG. 9 shows dimeric structures of 14⁺OTf⁻ (left) and 14⁺OPf⁻, (right), both of which features charge assisted hydrogen bonding and π-π stacking interactions;

FIG. 10 shows the dimeric structure of 15⁺BF₄ ⁻;

FIG. 11 shows (a) 1-D hydrogen bonded chain structure in 16⁺BF₄ ⁻ with (b) disordered solvent (violet) in channels that are lined with phenyl rings;

FIG. 12 showing the C₂helical hydrogen bonded structure found in 14⁺OTs⁻. The helices all pack with the same twist direction, resulting in a chiral crystal;

FIG. 13 shows. the C₃helical hydrogen bonded structure found in 14⁺ClO₄ ⁻and 14⁺BF₄ ⁻;

FIG. 14 shows (a) 3-D packing of the C₃helices in 14⁺ClO₄ ⁻ and 14⁺BF₄ ⁻, and (b) a depiction of the resultant hydrophobic channels;

FIG. 15 shows IR spectra in the ν_OHregion (11 mM, CH₂Cl₂solvent) for (a) free 1,4-hydroquinone; (b) 14⁺X⁻ without hydrogen bonding between 14⁺ and X⁻; (c-e) 14⁺X⁻ with hydrogen bonding between 14⁺ and the X⁻; and

FIG. 16 shows views perpendicular and parallel to the channel axis in 14⁺BF₄ ⁻; two types of phenyl groups are shown.

DETAILED DESCRIPTION

Non-limiting embodiments of the invention are further described below. However, it should be appreciated that some of the features of embodiments of the invention could be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles of the invention, and not in limitation thereof. Further, one skilled in the art will appreciate that the invention can be practiced by other than the described embodiments; that these described embodiments are presented for the purposes of illustration and not of limitation. For example, while various substituents are noted herein, other substituents known to those of skill in the art may also be employed in the compounds and formulas referenced herein.
By way of introduction, there is a great need for efficient catalysts in carbon-carbon coupling reactions of boronic acid derivatives (in general synthesis, pharmaceuticals, etc.) and for coupling monomers to give polymers. Due to the unique mechanism provided by the synergistic action of the rhodium and quinone components described herein, the rhodium quinone catalysts promote reactions at a higher level of efficiency than previously available with non-quinone catalysts.
Advantageously, embodiments of the invention provide enhanced yields of conjugate addition products via the use of rhodium hydroquinone catalysts. Details of processing steps for the conjugate addition of aryl- and vinyl-boronic compounds to conjugate addition accepts are set forth in detail below. Examples of compounds and reactions are provided, along with reaction results.
Accordingly, embodiments of the invention are directed to uses and processes for the conjugate addition of aryl- and vinyl-boronic compounds (e.g. esters, boronic acids and boroxines) to cyclic and acyclic conjugate addition acceptors, including but not limited to, vinyl-nitro compounds; α,β-unsaturated amides, anhydrides, esters, thioesters, aldehydes and ketone; and vinyl-sulfones. It has been demonstrated that the use of the novel cationic hydroquinone-Rh complex set forth in the afore-reference application entitled “Rhodium Quinonoid Catalysts” and described herein provides a superior catalytic system than those believed to be currently reported in the literature.
Thus, an embodiment of the invention comprises the process:
wherein EWG may be selected from the group consisting of a ketone, aldehyde, imide, amide, ester, thioester, acid anhydride, nitro, sulfone, nitrile, sulfoxide, phosphinate, electron deficient aromatic ring or other suitable electron withdrawing substituent that withdraws electron density either through inductive or resonance effects from olefins, and combinations thereof; and R may be selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium or other suitable atom, and combinations thereof. These we term “conjugate acceptors” thus referring to an alkene or olefin with an electron withdrawing group (EWG).
A majority of experiments found in literature show that electron-deficient aryl boronic acids can provide poor yield of the desired addition product while requiring vast excess (3-10+ equiv) of the boronic acid for these results. Some other reports appear to give no results for electron poor systems and only report electron rich ones.
In contrast, the novel rhodium catalyst system has been demonstrated to be highly active and provide good amounts of the desired conjugate addition product with exceptionally low catalyst loadings (about 0.5-1.0 mol % Rh^Icatalyst) even with strongly electron withdrawing group present on the aromatic ring (3 fluorines meta, meta and para attempted and was successful).
Thus, in accordance with an embodiment of the invention, the process may comprise:

Accordingly,

X may be selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium or other suitable group, and combinations thereof.
M⁺ is a positively charged ion including any metal ion having an oxidation state at or higher than +1, and may also be selected from the group consisting of cationic Li, K, Cs, Be, Sr, Ba, Al, Ti, Zr, B, Si, Cd, Ag, Ph₃PNPPh₃, Rb, Mg²⁺, Ca²⁺, Na, R₄N⁺, Zn²⁺, ammonium salts including tetraalkylammonium cations, tetraalkylarsonium cations, guanidinium salts, amidinium salts, or other suitable counter-ions, and combinations thereof;
Y may be selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium or other suitable group, and combinations thereof;
L is a ligand that donates electron density to the rhodium to stabilize it and each L shown above may be identical or non-identical ligands that are either chiral or achiral and may be selected from the group consisting of alkenes, dialkenes, alkyene, phosphines, water, phosphites, sulfides, sulfoxides, sulfonates, sulfonamides, sulfones, ethers, amines, imines, amides, aldehydes, ketones, esters, nitrites or other ligands, and combinations thereof;
wherein EWG may be selected from the group consisting of a ketone, aldehyde, imide, amide, ester, thioester, acid anhydride, nitro, sulfone, nitrile, sulfoxide, phosphinate, electron deficient aromatic ring or other suitable electron withdrawing substituent that withdraws electron density either through inductive or resonance effects from olefins, and combinations thereof; and R may be selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium or other suitable atom, and combinations thereof; and
—BR₂may be any boronic containing species neutral or anionically charged where boron is bound to a transfer group, and includes for example boronic acid, boroxime, boronic ester, boron-ate complex or other suitable species, and combinations thereof.
As shown by the above reaction, the rhodium catalyst promotes the transfer of the substituent attached to the boron (−BR₂) to the electron deficient olefin to give the product of conjugate addition.
The above starting materials represent a class of boron containing substrates where the carbon that is transferred can be sp2. The carbon may be part of ring, e.g., a cyclic alkene, aromatic structure or heterocycle and may also be acyclic, e.g., an olefin or other suitable group.
Some representative examples of structures defining the conjugate acceptor include:
where X═H, O, N, C, S or other suitable group.
As a further specific example of an embodiment of the inventive process and as shown below, whereby an aryl boron species is added in a conjugate addition to an electron deficient olefin (in this depicted case: 2-cylcohexen-1-one).
The enone, boron compound and Rh⁺ complex may be combined in DME, then LiOH (aq. 1 M) may be added. The reaction mixture may be diluted with DME and water and heated until the reaction is complete.
Improvements of the embodiments of the inventive process in comparison to the use of alternative catalyst systems include in situ generation of the active catalyst from a stable precursor, alternate solvent system and reduced temperature profile. Additional advantages include the high isolated yields of product employing extremely electron deficient aromatic boronic acids (such as tri-halogenated examples) while maintaining low catalyst loadings (0.5 mol % Rh), short reaction times and mild temperatures (50° C.).
Additional advantages include reduced catalyst loading, reduced boronic acid loading and increased scope of useful boronic acids, differing metal salt and alternative substrate scope.
Further details regarding the preferred, novel rhodium quinonoid complex and processing thereof are set forth below. For ease of reference, the following sections are set forth as I-III.
I. An Anionic Rhodium η⁴-Quinonoid Complex as a Multifunctional Catalyst for the Arylation of Aldehydes with Arylboronic Acids
Hydroquinone has been coordinated in a π-bonded η⁶-manner to the metal fragments Cr(CO)₃, Mn(CO)₃ ⁺, and Cp*M²+(M=Rh, Ir). See: Huang, Y.-S.; Sabo-Etienne, S.; He, X.-D.; Chaudret, B. Organometallics 1992, 11, 303. Koelle, U.; Weisschadel, C.; Englert, U. J. Organomet. Chem. 1995, 490, 101. Schumann, H.; Arif, A. M.; Richmond, T. G. Polyhedron 1990, 9, 1677. Sun, S.; Carpenter, G. B.; Sweigart, D. A. J. Organomet. Chem. 1996, 512, 257. Le Bras, J.; Amouri, H.; Vaissermann, J. Organometallics 1998, 17, 1116. Oh, M.; Carpenter, G. B.; Sweigart, D. A. Organometallics 2002, 21, 1290. Moussa, J.; Guyard-Duhayon, C.; Herson, P.; Amouri, H.; Rager, M. N.; Jutand, A. Organometallics 2004, 23, 6231. Fairhurst, G.; White, C. J. Chem. Soc., Dalton Trans. 1979, 1531. An important chemical property displayed by some of these complexes is facile deprotonation of the —OH groups, which is accompanied by electron transfer to the metal and changes in the hapticity of the quinonoid ring. See, e.g., Sun et al., and Oh et al. above.
The foregoing is illustrated in FIG. 1 for the new η⁶-H₂Q complex [(1,4-hydroquinone)Rh(COD)]BF₄(1⁺BF₄ ⁻), synthesized in 74% isolated yield by the reaction of [Rh(COD)Cl]₂with AgBF₄and H₂Q. 1⁺BF₄ ⁻ cleanly undergoes deprotonation to afford stable neutral semiquinone (2) and anionic quinone (3⁻) complexes.
With a catalytically-active metal such as rhodium, it was thought that the ability to alter the charge on the metal center by simple reversible deprotonation at the quinonoid center may constitute a powerful way to tune catalytic activity. In addition, the anionic doubly deprotonated η⁴-quinone complex 3⁻ may be able to function as a ligand (“organometalloligand”), thereby offering the possibility of bifunctional activation of appropriate substrates by simultaneous interaction at the rhodium and quinonoid centers. The ability of a quinone complex to function as an organometalloligand has been demonstrated in the case of (η⁴-benzoquinone)Mn(CO)₃ ⁻. See: Oh, M.; Carpenter, G. B.; Sweigart, D. A., Acc. Chem. Res. 2004, 37, 1.
Catalysts able to operate in a bifunctional manner are of considerable current interest. See: Casey, C. P.; Johnson, J. B.; Singer, S. W.; Cui, Q. J. Am. Chem. Soc. 2001, 123, 11594. Mermerian, A. H.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 4050. Herein it is demonstrated that the hydroquinone complex 1⁺BF₄ ⁻ is a convenient precursor to M ⁺3⁻, where M⁺ can be any of a variety of cations such as an alkali metal or tetraalkylammonium cation or other suitable cation known to those knowledgeable in the art, which serves as a catalyst for the coupling of arylboronic acids and benzaldehydes to produce diaryl alcohols (see FIG. 2). It is shown that M ⁺3⁻acts in a multifunctional manner by simultaneously activating both the boronic acid and the aldehyde, the former by coordination of a quinonoid oxygen in 3⁻ to the boron and the latter through a Lewis acid interaction among the aldehyde, the counterion M⁺ and a quinonoid oxygen.
The X-ray structure of [(H₂Q)Rh(COD)]BF₄.Et₂O (1⁺BF₄ ⁻) established the anticipated η⁶-bonding mode. The structure obtained from X-ray analysis is shown in FIG. 3. The solid state structure of 1 BF₄displays several types of crystallographic disorder, but the connectivity and chemical structure indicated is certain; the structure was solved to a acceptable R1 factor of 8.9%.
Deprotonation of 1⁺BF₄ ⁻ with KO^tBu in THF (tetrahydrofuran) occurred readily to afford the semiquinone (2) and the quinone (K⁺3⁻) analogues (FIG. 1). X-ray quality crystals of K ⁺3⁻ could not be grown, but the butylammonium salt was readily obtained by metathesis and its X-ray structure determined as Bu₄N⁺ [(1,4-Q)Rh(COD)]⁻.3 Bu₄NBF₄. The Rh—C bond lengths clearly indicated an η⁴-bonding mode, with the quinone Rh—C distances being ca. 0.2 Å greater for the C(O) carbons in comparison to the other four quinone carbons. Deprotonation of 1⁺BF₄ ⁻ with KO^tBu in the presence of 18-crown-6 produced the salt K(18-C-6)⁺[(1,4-Q)Rh(COD)]⁻.K(18-C-6)BF₄, in which each quinone oxygen is linked to a crown ether encapsulated potassium ion (FIG. 1). It is noted that X-ray data for this salt were of moderate quality, but sufficient to establish the connectivity shown and establish that the indicated structure is correct.
The cross-coupling of organoborates and organic electrophiles has become an important synthetic tool in organic chemistry. See: Suzuki, A. Acc. Chem. Res. 1982, 15, 178. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. While palladium is often used as the transition metal in the catalyst for this reaction, rhodium can also be effective. Especially noteworthy are the rhodium-catalyzed addition of arylboronic acids to aldehydes and the 1,4-addition of arylboronic acids to enones. See, respectively, (1) Sakai, M.; Ueda, M.; Miyaura, N. Angew. Chem. Int. Ed. 1998, 37, 3279. Ueda, M.; Miyaura, N. J. Org. Chem. 2000, 65, 4450. Fürstner, A.; Krause, H. Adv. Synth. Catal. 2001, 343. Pucheault, M.; Darses, S.; Genet, J. P. J. Am. Chem. Soc. 2004, 126, 15356; (2) Takaya, Y.; Ogasawara, M.; Hayashi, T. J. Am. Chem. Soc. 1998, 120, 5579. Batey, R. A.; Thadani, A. N.; Smil, D. V. Org. Lett. 1999, 1, 1683. Ramnauth, J.; Poulin, O.; Bratovanov, S. S.; Rakhit, S.; Maddaford, S. P. Org. Lett. 2001, 3, 2571. Kuriyama, M.; Nagai, K.; Yamada, K.; Miwa, Y.; Taga, T.; Tomioka, K. J. Am. Chem. Soc. 2002, 124, 8932. (e) Hayashi, T.; Takahashi, M.; Takaya, Y.; Ogasawara, M. J. Am. Chem. Soc. 2002, 124, 5052. Yoshida, K.; Ogasawara, M.; Hayashi, T. J. Am. Chem. Soc. 2002, 124, 10984. Itooka, R.; Iguchi, Y.; Miyaura, N. J. Org. Chem. 2003, 68, 6000. Duursma, A.; Boiteau, J.-G.; Kefort, L.; Boogers, J. A. F.; de Vries, A. H. M.; de Vries, J. G.; Minnaard, A. J.; Fering a, B. L. J. Org. Chem. 2004, 69, 8045. The results obtained for arylboronic acid addition to benzaldehydes as catalyzed by rhodium quinone complexes are given in Table 1.
An inspection of the data in Table 1 shows some remarkable behavior. From entries 1-6 it is shown that the cationic rhodium hydroquinone complex 1⁺BF₄ ⁻ had catalytic activity when a base (KOH) is present. Addition of the neutral salt 1⁺BF₄ ⁻ had no effect (entry 5). It is concluded that the base likely functions to deprotonate the -quinonoid —OH groups. In agreement with this, the anionic quinone complex K ⁺3⁻ was found to be a very effective catalyst, giving high yields at 75° C. or higher temperatures. Interestingly, the yield drops dramatically when a crown ether is added to the reaction mixture or when K(18-C-6)⁺3⁻ is used as the catalyst in place of K⁺3⁻ (entries 13, 15). In a similar vein, the activity is reduced by the inclusion of n-Bu₄N⁺BF₄ ⁻ (entries 14, 16). Likely related to this is the observation that Li ⁺3⁻ is a more effective catalyst than K ⁺3⁻, as indicated by entries 10 and 12 compared to 17 and 18. This behavior clearly signals heterobimetallic or dual function catalysis in which the alkali metal Li⁺ or K⁺ enhances the electrophilic activation of the aldehyde carbon by interacting with the carbonyl oxygen, thus facilitating aryl transfer from the rhodium catalyst. See: Sammis, G. M.; Danjo, H.; Jacobsen, E. N. J. Am. Chem. Soc. 2004, 126, 9928. Shibasaki, M.; Yoshikawa, N. Chem. Rev. 2002, 102, 2187. Yamagiwa, N.; Matsunaga, S.; Shibasaki, M. J. Am. Chem. Soc. 2003, 125, 16178. Li, C.; Eidjaja, E.; Garland, M. J. Am. Chem. Soc. 2003, 125, 5540. Guo, N.; L₁, L.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542. Comte, V.; Le Gendre, P.; Richard, P.; Moïse, C. Organometallics 2005, 24, 1439. This hypothesis is in accord with the reduced reactivity that is found when the alkali metal is chelated with a crown ether or replaced with the much larger n-Bu₄N⁺ ion.
See Table 1 below setting forth results of the rhodium-catalyzed arylation of ArCHO with Ar′B(OH)₂in water solvent.

TABLE 1

Results for Rhodium-Catalyzed Arylation of Aldehydes

entry	aldehyde	catalyst	additives (eq)	T/° C.	time (h)	yield (%)^b

1	C₆H₅CHO	1⁺BF₄ ⁻	none	95	3	NR^c
2^d	C₆H₅CHO	1⁺BF₄ ⁻	none	95	3	NR
3	C₆H₅CHO	1⁺BF₄ ⁻	none	75	3	NR
4	C₆H₅CHO	1⁺BF₄ ⁻	KOH (1.2)	75	3	97
5	C₆H₅CHO	1⁺BF₄ ⁻	K⁺BF₄ ⁻(1.2)	75	3	NR
6	C₆H₅CHO	1⁺BF₄ ⁻	none	50	16	NR
7	C₆H₅CHO	K⁺3⁻	none	95	3	96
8	C₆H₅CHO	K⁺3⁻	none	75	3	93 (90)
9	C₆H₅CHO	K⁺3⁻	none	60	3	81
10	C₆H₅CHO	K⁺3⁻	none	50	3	48
11	C₆H₅CHO	K⁺3⁻	none	50	16	84
12	C₆H₅CHO	K⁺3⁻	none	25	16	19
13	C₆H₅CHO	K⁺3⁻	18-C-6 (0.075)	75	3	14
14	C₆H₅CHO	K⁺3⁻	n-Bu₄N⁺BF₄ ⁻(0.075)	75	3	24
15	C₆H₅CHO	K⁺(18-C-6)3⁻	none	75	3	13
16	C₆H₅CHO	n-Nu₄N⁺3⁻	none	75	3	2
17	C₆H₅CHO	Li⁺3⁻	none	50	3	96 (91)
18	C₆H₅CHO	Li⁺3⁻	none	25	16	40
19	C₆H₅CHO	[Rh(COD)Cl]₂	none	75	3	NR
20	C₆H₅CHO	[Rh(COD)₂]⁺BF₄ ⁻	none	75	3	NR
21	C₆H₅CHO	[Rh(COD)₂]⁺BF₄ ⁻	KOH (0.025)	75	3	1
22	C₆H₅CHO	[Rh(COD)₂]⁺BF₄ ⁻	KOH (1.2)	75	3	99
23	C₆H₅CHO	none	KOH (1.2)	75	3	NR
24	4-MeOC₆H₄CHO	K⁺3⁻	none	75	3	81 (78)
25	2,4,6-Me₃C₆H₂CHO	K⁺3⁻	none	75	3	69 (68)
26	4-MeC₆H₄CHO	K⁺3⁻	none	75	3	99 (97)
27	4-ClC₆H₄CHO	K⁺3⁻	none	75	3	99 (97)
28	4-PhC₆H₄CHO	K⁺3⁻	none	75	3	98 (93)
29	4-O₂NC₆H₄CHO	K⁺3⁻	none	75	3	99 (92)
30^e	C₆H₅CHO	K⁺3⁻	none	75	3	96 (91)
31^f	C₆H₅CHO	K⁺3⁻	none	75	3	94 (90)
32^g	C₆H₅CHO	K⁺3⁻	none	75	3	20
33^b	C₆H₅CHO	K⁺3⁻	none	75	3	2

Entries 7 and 30-33 show that electron-withdrawing para-substituents on the aryl group in Ar′B(OH)₂hinder the reaction, as has been found with other catalyst systems. See: Sakai, M.; Ueda, M.; Miyaura, N. Angew. Chem. Int. Ed. 1998, 37, 3279. Table 1 also indicates that the catalytic conditions are tolerant of a wide range of aryl substituents in the aldehyde reactant (entries 8 and 24-29). See also FIGS. 4 and 5, which present typical proton NMR data for the products of the arylation reactions.
Suzuki-Miyaura and Miyaura-Hiyashi type coupling reactions involving boronic acids are usually facilitated by the presence of stoichiometric external base (e.g., compare entries 20 and 22). It has been debated whether the base serves to increase the rate of transmetallation from boron to the transition metal catalyst by binding to the former or by binding to the latter. Recent theoretical studies suggest that the hard base OH⁻ functions by binding to the electrophilic boron, and that this increases the rate of subsequent transmetallation. See: Braga, A. A. C.; Morgon, N. H.; Ujaque, G.; Maseras, F. J. Am. Chem. Soc. 2005, 127, 9298.
The data in Table 1 show that K ⁺3⁻ and Li ⁺3⁻ are effective catalysts without the necessity of adding an external base. From this we conclude that the 3 complex itself can function as the base by binding to the boron via the quinonoid oxygens. In the present case, the binding of 3⁻ to the boronic acid assists the transmetallation step by decreasing the electrophilicity of the boron and by placing the transition metal in the vicinity of the transferring group (Ar′). ¹H NMR spectra of PhB(OH)₂in D₂O with and without K ⁺3⁻ present indicate that an interaction occurs.
The ability of the quinone ring system to undergo facile hapticity changes (η⁴→η⁵, etc.) may play a role in the ability of 3⁻ to function as an organometalloligand in this manner. It is concluded that catalyst 3⁻ is able to act in a bifunctional (and cooperative) manner, as has recently been suggested for other types of catalytic reactions. See: Casey, C. P.; Johnson, J. B.; Singer, S. W.; Cui, Q. J. Am. Chem. Soc. 2005, 127, 3100. (b) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97. (c) Josephsohn, N. S.; Kuntz, K. W.; Snapper, M. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2001, 123, 11594. (d) Mermerian, A. H.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 4050.
In summarizing the above, Applicants have characterized a π-bonded rhodium quinonoid complex that functions as a good catalyst for the coupling of arylboronic acids and aldehydes. The catalysis is heterobimetallic in that both the transition metal and concomitant alkali metal counterion play an integral part in the reaction. In addition, the anionic quinonoid catalyst itself plays a bifunctional role by acting as a ligand to the boronic acid and as a Lewis acid receptor site for the aryl group in the requisite transmetallation. In the reaction with aldehydes, the anionic rhodium catalyst appears to operate in an intriguing multifunctional manner with one quinone oxygen acting as a ligand by binding to the boron center, thus facilitating transmetallation, while the other quinone oxygen binds to the alkali metal counterion of the catalyst and indirectly activates the aldehyde electrophile (rate: M⁺=Li⁺>K⁺>>Bu₄N⁺). These interactions from the quinone oxygen atoms imply a supramolecular assembly of the boronic acid donor, the catalyst and the organic acceptor, as illustrated in Scheme 1. Such a preorganization is unprecedented and offers opportunities to significantly improve product yields and stereoselectivities in comparison to standard catalytic systems.
Thus, it can be seen that Applicants have successfully synthesized an anionic rhodium quinone complex that can function as a catalyst for Miyaura-Hiyashi coupling of arylboronic acids and aldehydes. Advantageously, the catalytic reactivity can be adjusted/tuned by protonation/deprotonation of the quinone complex. The catalyst as a potassium salt also functions in a heterobimetallic manner in that both the rhodium and the alkali metal play an integral role in the reaction. Moreover, the anionic rhodium complex is itself bifunctional in that it acts as a ligand in activating the boronic acid towards transmetallation of the rhodium center. The determination of a heterobimetallic catalyst that is also bifunctional (cooperative) and pH-tunable for an important class of reactions is believed to be unique.
Synthetic Procedure and Characterization of the New Materials
General: All reactions were carried out under N₂in flame-dried glassware. HPLC grade THF and Diethyl Ether solvents were used as received. [Rh(COD)Cl]₂was provided by Strem Chemicals. The ¹H NMR spectra were recorded by Bruker(300 MHz) spectrometers. Elementary analyses were performed by Quantitative Technologies Inc.
Synthesis of 1⁺BF₄ ⁻: After flame drying the glassware, [Rh(COD)Cl]₂(0.20 g, 0.41 mmol) and AgBF₄(0.19 g, 0.97 mmol) were mixed for 1 h at room temperature in a mixed solution of methylene chloride (4 mL) and acetone (1 mL). While stirring, a white precipitate formed on the bottom of the glassware. 1,4-hydroquinone (0.18 g, 1.63 mmol) was dissolved in acetone(2 mL) and added to the reaction mixture. After stirring for 2 h at r.t., the solvent was removed via rotary evaporation. The residue was taken up in methylene chloride (3 mL) and slowly added dropwise to ether through a glass filter. The yellow precipitate was filtered and washed three times with 10 mL aliquots of diethyl ether. The isolated yield was 72% (0.24 g, 0.59 mmol). To get the single crystals: 1+BF₄-(25 mg) was dissolved in a mixture of acetone (0.1 mL) and methylene chloride (1.5 mL) in a 5 mL-vial. Diethyl ether (3 mL) was carefully added to the upper layer. The solution was placed in a refrigerator for 3 days. Yellow crystals formed on the wall of vial.
¹H NMR (CD₂Cl₂): δ 7.64 (brs, OH, 2H), 6.53 (s, hydroquinone ring, 4H), 4.41 (br, COD, 4H), 2.40 (m, COD, 4H), 2.12 (m, COD, 4H) ppm. Elemental Anal. Calcd for C₁₄O₂H₁₈Rh₁B₁F₄: C, 41.21; H, 4.45. Found: C, 41.44; H, 4.31.
Synthesis of 2: 1⁺BF₄ ⁻ (0.1 g, 0.24 mmol) was dissolved in THF (5 mL) in a 20 mL-one neck Schlenk flask and the solution was mixed with 1 eq. K^tBuO (0.027 g, 0.25 mmol) and stirred at r.t. for 2 h. While, stirring the solution became turbid and a yellow precipitate formed. The precipitate was filtered under N₂and the collected solid was washed three times with THF and dried in vacuum. The isolated yield was 83% (0.065 g, 0.20 mmol).
¹H NMR (DMSO-d⁶): δ 5.94 (d, J=6.3 Hz, arene ring, 2H), 5.51 (br, OH, 1H) 5.20 (d, J=6.3 Hz, arene ring, 2H), 3.83 (br, COD, 4H), 2.20 (m, COD, 4H), 1.97 m, COD, 4H) ppm. Elemental Anal. Calcd for C₁₄O₂H₁₇Rh₁: C, 52.52; H, 5.35. Found: C, 50.82; H, 5.33.
Synthesis of K⁺3⁻: 1 (0.1 g, 0.24 mmol) was dissolved in THF (5 mL) in 20 mL-one neck Schlenk flask and the solution was mixed with 3 eq. K^tBuO (0.082 g, 0.74 mmol) and stirred at r.t. for 2 h. While stirring, the solution became turbid and a yellow precipitate formed. The precipitate was filtered under N₂and the collected solid was washed five times with THF and dried in vacuum. The isolated yield was 50% (0.074 g, 0.12 mmol).
¹H NMR (DMSO-d⁶): δ 4.89 (s, benzoquinone ring, 4H), 3.47 (br, COD, 4H), 2.18 (m, COD, 4H), 1.96 (m, COD, 4H) ppm. ¹H NMR (D₂O): δ 5.67 (s, benzoquinone ring, 4H), 4.01 (br, COD, 4H), 2.35 (m, COD, 4H), 2.13 (m, COD, 4H) ppm. Elemental Anal. Calcd for C₁₄O₂H₁₆Rh₁K₃B₂F₈: C, 27.56; H, 2.64. Found; C, 27.20; H, 2.59.
Synthesis of K(18-crown-6)⁺3⁻: 18-crown-6 (0.19 g, 0.74 mmol) was dissolved in THF (5 mL) in 20 ml-one neck Schlenk flask and K^tBuO solution (0.082 g, 0.74 mmol) in 5 mL was added to this solution. The solution was stirred for 30 minutes. To this solution, a THF (5 mL) solution of 1⁺BF₄ ⁻ (0.1 g, 0.24 mmol) was added. The solution was stirred for 5 hours at room temperature. Compared to the synthetic procedure of K ⁺3⁻, no precipitate formed. The solvent was evaporated and the resulting yellow solid was washed five times with diethyl ether (15 mL, five times). After drying in vacuum, the solid was dissolved in THF and diethyl ether was added carefully on the layer of THF. After a few days yellow crystals were collected and the isolated yield was 79% (0.20 g, 0.19 mmol).
¹H NMR (DMSO-d⁶): δ 4.88 (s, benzoquinone ring, 4H), 3.54 (s, crown ether, 48H), 3.47 (br, COD, 4H), 2.19 (m, COD, 4H), 1.96 (m, COD, 4H) ppm. ¹H NMR (CD₂Cl₂): δ 5.24 (s, benzoquinone ring, 4H), 3.69 (br, COD, 4H), 3.60 (s, crown ether, 48H), 2.26 (m, COD, 4H), 2.03 (m, COD, 4H) ppm. ¹H NMR (D₂O): δ 5.68 (s, benzoquinone ring, 4H), 4.02 (br, COD, 4H), 3.72 (s, crown ether, 48H), 2.35 (m, COD, 4H), 2.14 (m, COD, 4H) ppm. Elemental Anal. Calcd. for C₃₈O₁₅H₆₆Rh₁K₂B₁F₄: C, 44.28; H, 6.45. Found; C, 44.48; H, 6.61.
Synthesis of Li³⁰3⁻:1⁺BF₄ ⁻: (0.1 g, 0.24 mmol) was dissolved in THF (5 mL) in a 20 mL-one neck Schlenk flask and the solution was mixed with 3 eq. Li^tBuO (0.060 g, 0.75 mmol) and stirred at r.t. for 5 h. The solvent was evaporated and the resulting yellow solid was washed five times with mixture of THF and diethyl ether (v/v=1:5, 15 mL, five times). The precipitate was filtered under N₂and the collected solid was washed five times with THF and dried in vacuum. The isolated yield was 56% (0.057 g, 0.14 mmol)
¹H NMR (DMSO-d⁶): δ 4.96 (s, benzoquinone ring, 4H), 3.51 (br, COD, 4H), 2.20 (m, COD, 4H), 1.98 (m, COD, 4H) ppm. Elemental Anal. Calcd. for C₁₄O₂H₁₆Rh₁Li₂B₁F₄: C, 40.05; H, 3.84. Found; C, 40.04; H, 4.17.
Synthesis of n-Bu₄N⁺3⁻: 1⁺BF₄ ⁻ (0.1 g, 0.24 mmol) and Bu₄N⁺BF₄ ⁻ (0.21 g, 0.75 mmol) were dissolved in THF (5 mL) in a 20 mL-one neck Schlenk flask and the solution was mixed with 3 eq. K^tBuO (0.082 g, 0.74 mmol) and stirred at r.t. for 4 h. After reaction, in comparison to the synthetic procedure for K ⁺3⁻, there was no precipitate. The solvent was evaporated and the resulting yellow solid was washed five times with diethyl ether (15 mL, five times). After drying in vacuum, the solid was dissolved in THF and the diethyl ether was added carefully on the layer of THF. After a few days yellow crystals were collected and the isolated yield was 61% (0.23 g, 0.15 mmol).
¹H NMR (DMSO-d 6): δ 4.90 (s, benzoquinone ring, 4H), 3.48 (br, COD, 4H), 3.17 (t, J=7.8 Hz, Bu, 32H), 2.19 (m, COD, 4H), 1.97 (m, COD, 4H), 1.57 (brm, Bu, 32H), 1.32 (m, Bu, 32H), 0.94 (t, J=7.2 Hz, Bu, 48H) ppm. Elemental Anal. Calcd. for C₇₈H₁₆₀O₂N₄Rh₁B₃F₁₂: C, 60.46; H, 10.41; N, 3.62. Found; C, 59.23; H, 10.52; N, 3.52.
General Procedure of Catalytic Reaction
Distilled water (2 mL) was added to an elongated 15 mL-Schlenk flask. The water was bubbled for 5 minutes with nitrogen gas. After bubbling, the catalyst K ⁺3− (15 mg, 0.024 mmol), phenylboronic acid (0.15 g, 1.23 mmol) and benzaldehyde (0.1 ml, 0.98 mmol) were added. The mixture solution was heated at 75° C. for 3 hours. After reaction, the solution was cooled to room temperature and CDCl₃(3.5 mL) was added. The solution was shaken for 1 minute and the CDCl₃part was directly analyzed by ¹H NMR. The yield was calculated by the comparison of peak area of aldehyde reactant and the benzyl proton of the product alcohol.
Crystallography. X-ray data collection was carried out using a Bruker single-crystal diffractometer equipped with an APEX CCD area detector and controlled by SMART version 5.0. Collection was done either at room temperature or 100 K. Data reduction was performed by SAINT version 6.0 and absorption corrections were applied by SADABS version 2.0. The structures were typically determined by direct methods and refined on F squared by use of programs in SHELXTL version 5.0. Most hydrogen atoms appeared in a difference map, or they were generally inserted in ideal positions, riding on the atoms to which they are attached.
The X-ray structure of [(H₂Q)Rh(COD)]BF₄.Et₂O established the anticipated η⁶bonding mode. The solved structure contained two independent complexes of rhodium with hydroquinone (HQ) and (COD) ligands, a BF₄counterion and a diethyl ether molecule filling the void. Each HQ-Rh—COD complex is positioned on a two-fold axis, Rh(1) along an axis parallel to b, Rh(2) along one parallel to a (the asymmetric unit is charge-balanced, +1 for the two half complexes, and −1 for the BF₄). Notable features remained, particularly a “ghost” atom, and the R value remained high—around 13%. Rechecking by different methods showed that orthorhombic-symmetry (mmm) produced R(int) and R(symm) convincingly below 5%. Cell_now ranked the original cell first (C-centered). The space group was uniquely determined to be C222(1) by the systematic absences. Solving by use of the Patterson method yielded the same solution for the heavy atoms as found earlier. However, some original difficulties were highlighted: three independent heavy atoms found, with estimated atomic numbers 48, 39, and 24—although the only heavy atom is believed to be rhodium. The third heavy atom had been tentatively treated as carbon, but it may be something bigger, and its position mirrored the second rhodium atom on the other side of the HQ ligand. This suggested that the atom might be a fractional part of a disordered rhodium atom. When its identification as Rh(3) was tested, the R values dropped and the bonds to the expected ligands became apparent. Rh(3) occupied a position between the HQ coordinated to Rh(2) and the COD coordinated to the Rh(2) in the adjacent cell. Since neither ligand can bond to two rhodium atoms, disorder of the whole Rh(2) complex is believed to be present. The HQ ligand on Rh(2) is approximately overlapped by another on Rh(3), and the COD on the adjacent Rh(2) is approximately overlapped by another on Rh(3). This “whole molecule disorder” is difficult to model accurately, so an approximate model was constructed by restraining the ligands on Rh(2) and Rh(3) to be similar to the better-defined ligands on Rh(1), by treating all atoms except rhodium in the Rh(3) complex as isotropic, and by ignoring the likely disorder in the diethyl ether. With this model, the occupancy of the Rh(2) complex was about 68%, the Rh(3) complex about 32%; hence the electron counts on the three rhodium atoms are roughly consistent with the Patterson results. That is not the end of the disorder besetting this crystal; it is also a racemic twin, with enantiomers in a 55:45 ratio. Some complications in the analysis may preclude placing much weight on bond lengths and angles, but the connectivity is certainly determined (R1=0.089%).
X-ray quality crystals of K ⁺3− could not be grown, but the butylammonium salt was readily obtained by metathesis and its X-ray structure determined as Bu₄N⁺[(1,4-Q)Rh(COD)]⁻.3 Bu₄NBF₄(R1=0.076%). The Rh—C bond lengths clearly indicated an η⁴-bonding mode, with the quinone Rh—C distances being ca. 0.2 Å greater for the C(O) carbons in comparison to the other four quinone carbons. Deprotonation of 1⁺BF₄ ⁻ with KO^tBu in the presence of 18-crown-6 produced the salt K(18-C-6)⁺[(1,4-Q)Rh(COD)]⁻.K(18-C-6)BF₄, in which each quinone oxygen is linked to a crown ether encapsulated potassium ion. X-ray data for this salt were only moderate in quality (R1=12.8%), but sufficient to establish connectivity.
II. Highly Efficient 1,4-Additions of Electron Deficient Aryl Boronic Acids with a Novel Rhodium(I) Quinonoid Catalyst
As noted above, rhodium(I) quinonoid catalysts are a remarkably efficient new class of reagents for the conjugate addition of aryl boronic acids. In this section, the use of these reagents in catalyzing the highly efficient 1,4-additions of a broad range of boronic acids, including heteroaromatic and an example of trihalogenated aryl boronic acids is described. The rhodium(I) catalyzed conjugate addition of aryl boronic acids to electron deficient olefins is a mild approach to carbon-carbon bond formation. See Sakai, M.; Hayashi, H.; Miyaura, N. Organometallics, 1997, 16, 4229-4231; Fagnou, K.; Lautens, M. Chem. Rev. 2003, 103, 169-196; Hayashi, T.; Yamasaki, K. Chem. Rev. 2003, 103, 2829-2844.d) Hayashi, T. Pure Appl. Chem. 2004, 76, 465-475. This approach has been shown to be more chemoselective and widely applicable for molecules with reactive functionality than traditional cuprate or grignard chemistry. See Chapman, C. J.; Frost, C. G. Adv. Synth. Catal. 2002, 345, 353-355; Moss, R. J.; Wadsworth, K. J.; Chapman, C. J.; Frost, C. G. Chem. Commun. 2004, 1984-1985; Paquin, J.; Defieber, C.; Stephenson, C. R. J.; Carreira, E. M. J. Am. Chem. Soc. 2005, 127, 10850-10851. In addition, the enantioselective rhodium catalyzed conjugate addition reaction with chiral ligands, as shown by Hayashi and others, demonstrates the application of this methodology toward asymmetric synthesis. See Chapman, C. J.; Frost, C. G. Adv. Synth. Catal. 2002, 345, 353-355; Moss, R. J.; Wadsworth, K. J.; Chapman, C. J.; Frost, C. G. Chem. Commun. 2004, 1984-1985; Paquin, J.; Defieber, C.; Stephenson, C. R. J.; Carreira, E. M. J. Am. Chem. Soc. 2005, 127, 10850-10851. While this methodology is mild and highly effective for most substrates, aryl boronic acids with electron withdrawing substituents undergo competitive proto-deborylation. Attempts to favor 1,4-addition have included increasing aryl boronic acid equivalents, increasing catalyst loading, altering the aqueous/organic solvent ratio, decreasing temperature and in situ generation of aryl boronate reactants. The highest reported yields are afforded with 2-10 equivalents of aryl boronic acid, ≧3 mol % catalyst loading and prolonged reaction at 90-100° C. In this section, it is reported that the use of a new rhodium quinone catalyst provides a mild, highly effective and operationally facile procedure for conjugate addition of aryl boronic acids to 1-cyclohexen-2-one.
As noted in the previous sections, the development of the anionic rhodium 77-quinonoid complex K ⁺3− (or “3•K”) is described and it was found that it efficiently catalyzes the 1,2-addition of aryl boronates to aldehydes according to Eq. 1 below. See, Son, S. U.; Kim, S. B.; Reingold, J. A.; Carpenter, G. B.; Sweigart, D. A. J. Am. Chem. Soc. 2005, 127, 12238-12239. The oxygen sensitive 3•K was synthesized and isolated by double deprotonation of the hydroquinone precursor complex 1 in THF, via the neutral semiquinone 2.
In this section it is reported that preformed catalyst 3•K is also effective in conjugate 1,4-addition reactions, as described below. The air-sensitivity of anionic catalyst 3•K may be a drawback in this procedure, potentially necessitating greater catalyst loadings than may otherwise be required. In addressing this problem, it was found that the operational ease of the catalyzed conjugate additions of aryl boronic acids can be greatly facilitated by the in situ generation of catalyst 3•M (M=Li, Na, K, Cs) from the air stable rhodium hydroquinone salt 1. The conjugate addition reactions in DME/H₂O with catalyst 3-L₁, which is generated in situ from 1 and LiOH, are highly efficient and afford excellent yields with negligible side products in short reaction times at 50° C. (Table 2).
A significant aspect of the new procedure is the low catalyst loading (0.5 mol % reduced
from 2.5 mol % in Scheme 3) and low boronic acid equivalency (1.2 equiv) relative to the conjugate acceptor as compared to the conditions reported in the literature. An evaluation of counter ions indicates that 3•Li is more efficacious than the corresponding potassium salt 3•K (Table 2, entries 4-6). This counter ion effect was also evident in catalyzed 1,2-additions and may be ascribed to general acid activation of the organic electrophile (vide infra).

TABLE 2

Base study-electron rich and electron poor

entry	1 (mol %)	LiOH (mol %)	6b yield^[a] (%)	6e yield^[a] (%)

1	0.5	0	4	0
2	0.5	0.25	17	23
3	0.5	0.5	30	38
4	0.5	0.75	63 (31)^[b]	46 (33)^[b]
5	0.5	1.0	90 (31)^[b]	57 (59)^[b]
6	0.5	2.0	96 (81)^[b]	87 (56)^[b]
7	0.5	120	99	19

^[a] Isolated yield after silica gel chromatography.
^[b] Yield in parentheses arises from substitution of KOH in place of LiOH.

Preliminary results showed that the reaction exhibited a marked dependence on base equivalency, which was studied through systematic variation (Table 2). The role of base in the conjugate addition of aryl boronic acids to electron deficient olefins is not well understood. For the conjugate addition of p-tolyl boronic acid to 2-cyclohexen-1-one, stoichiometric LiOH (1.2 eq) affords excellent yield of desired ketone 6b (99%, entry 7, Table 2), although the reaction is equally productive with 2.0 mol % of base (96%, entry 6). The most dramatic change in yield was seen upon increasing base from 0.75 mol % (63%) to 1.0 mol % (90%) (entries 4-5). This behavior is consistent with complete activation of hydroquinone 1 into the active quinonoid catalyst 3•Li where 1 mol % of LiOH is required for complete double deprotonation of the precatalyst. In contrast, the m-nitrophenyl boronic acid analogue affords the highest yield of desired ketone 6e (87%) with 2.0 mol % of LiOH. Increasing the base quantity to 120 mol % results in reduced yield (19%, entry 7) and a prevalent side product, nitrobenzene, resulting from proto-deborylation. This study demonstrates that base is required for the reaction and 2.0 mol % provides optimal yield of the desired conjugate addition product. During further studies to optimize the reaction conditions, a series of additives and bases were examined. Catalytic amounts of carbonate bases, Na₂CO₃(2.0 mol %) or Cs₂CO₃(2.0 mol %), are effective at producing high yielding conjugate additions with boronic acid 4a (Table 3, entries 6 & 8), while stoichiometric amounts (120 mol %) of carbonate bases (entries 7 & 9) attenuated reactivity. Pyridine, either catalytic or quantitative, arrests all reactivity and consistent with this observation is the lack of product with pyridine boronic acids. Additional hydroquinone shows no detectable effect upon reaction outcome while lithium salts, such as LiCl or LiBF₄, either diminish the amount of product or completely arrest the reaction. The addition reaction can be run in the absence of organic solvent, however, stoichiometric base (120 mol %) is required for efficient reaction (entry 10 versus 11). This result is presumably due to the solubilization of the boronic acid into the aqueous phase by formation of the corresponding -ate complex. Preferred reaction conditions, outlined as a general procedure in the experimental section, are highly effective and facile for a range of boronic acid substrates. Using 2-cyclohexen-1-one as our conjugate acceptor, a number of different aryl boronic acids were studied with our optimized reaction conditions (Table 4). Ketone products 6a-g (entries 1-7) are afforded in high yields, with low catalyst loading (0.5 mol %) and low boronic acid equivalency (1.2 eq). Electron deficient boronic acids (entries 5-9) are afforded in excellent yields (94-99%) without any procedural modification from the earlier analogues. Improved yields (92-93%) of meta-nitro analogue 61 were achieved either by increasing the catalyst loading (2.0 mol %, entry 12) or increasing equivalencies of boronic acid (1.5 equiv, entry 13). Tri-fluoro analogues 6j and 6k (entry 14,15) were afforded in good to moderate yields (70% and 30% respectively). This is believed to be the first report of conjugate addition of a tri-halogenated aryl boronic acid. Efforts are underway to further optimize the additions of tri-fluorophenyl boronic acids 4j and 4k. Both 2,4-bis(trifluoromethyl)phenyl boronic acid and ortho-nitro phenyl boronic acid failed to produce the desired addition products under our standard conditions. The 4-, 5- or 6-indoloboronic acids (Table 5) undergo conjugate addition while N-Boc-2-indoloboronic acid (entry 1) does not afford any product. The additions of 4-indoloboronic acid (entry 2, Table 4) and o-tolyl boronic acid (entry 3, Table 4) show that ortho substitution can be tolerated, despite the attenuated reactivity observed for o-substituted boronic acids and documented difficulties of reactions with N-Boc protected pyrrole-2-boronic acids. See Lautens, M.; Mancuso, J.; Grover, H. Synthesis 2004, 12, 2006-2014.

TABLE 3

The role of additives and alternate bases upon conjugate
addition reaction.

entry	LiOH (mol %)	additive (mol %)	solvent	6a yield^[b] (%)

1	0.5	hydroquinone (2.0)	DME/H₂O	31^[c]
2	0.5	LiBF₄ (120)	DME/H₂O	—
3	0.5	LiCl (120)	DME/H₂O	20
4	—	pyridine (2.0)	DME/H₂O	—
5	—	pyridine (120)	DME/H₂O	—
6	—	Cs₂CO₃ (2.0)	DME/H₂O	97
7	—	Cs₂CO₃ (120)	DME/H₂O	68
8	—	Na₂CO₃ (2.0)	DME/H₂O	93
9	—	Na₂CO₃ (120)	DME/H₂O	74
10	2.0	—	H₂O	—
11	120	—	H₂O	99

^[a] 0.5 mol %
^[b] Isolated yield after silica gel chromatography.
^[c] Compare to Table 1 entry 3.

TABLE 4

Conjugate addition of a variety of boronic acids to 2-cyclohexen-1-one.

entry	boronic acid^b] (4)	1 (mol %)	product	yield^[c] (%)

1	a. X = H	0.5	6a	98
2	b. X = p-Me	0.5	6b	97
3	c. X = o-Me	0.5	6c	99
4	d. X = 4-NH-Boc	0.5	6d	99
5	e. X = p-OMe	0.5	6e	97
6	f. X = p-Cl	0.5	6f	99
7	g. X = p-F	0.5	6g	94
8	g. X = p-F^[d]	0.5	6g	99
9	h. X = 3-Cl,4-F	0.5	6h	96
10	i. X = m-NO₂	0.5	6i	85
12	i. X = m-NO₂	2.0	6i	93
13	i. X = m-NO₂ ^[e]	0.5	6i	92
14	j. X = 3,4,5-tri-F^[d]	2.0	6j	70
15	k. X = 2,3,4-tri-F	2.0	6k	30

^[a] 2.0 mol %
^[b] 1.2 equiv. relative to 1-cyclohexen-2-one
^[c] Isolated yield after silica gel chromatography.
^[d] fromboroxime
^[e] 1.5 eq of boronic acid

TABLE 5

Indole boronic acid conjugate addition to 1-cyclohexene-2-one.

entry	boronic acid (7)	1 (mol %)	product	yield^[b] (%)

1	a. 2-B(OH)₂ R = Boc	0.5	8a	—
2	b. 4-B(OH)₂ R = H	0.5	8b	80
3	c. 5-B(OH)₂ R = H	0.5	8c	63
4	d. 6-B(OH)₂ R = H	0.5	8d	86

^[a] 2.0 mol %
^[b] Isolated yield after silica gel chromatography.

The catalytic system presented is similarly efficient with a range of substrates (Table 6). The conjugate addition of p-tolyl boronic acid (4b) was studied with a selection of conjugate acceptors. The addition to cinnamaldehyde (9a) and ethyl cinnamate (9b) were highly efficient providing the products in 96% and 95% isolated yields, respectively (Table 6, entries 1 & 2). The β,β-disubstituted olefins are a problem for many catalytic systems providing no observed conjugate additions, but our preliminary studies have found that 4b can be added to ethyl 3,3-dimethyl-acrylate (9c) in modest yield (Table 6, entry 3). Reactions with α,β-unsaturated secondary amide 9d failed to provide any of the desired conjugate addition (Table 6, entry 4), however, cyclic imide 9e produced the corresponding product 10e in 97% isolated yield (Table 6, entry 5). Further studies are underway to determine the scope of substrates and to improve addition to highly hindered systems.

TABLE 6

Conjugate addition to a variety of conjugate acceptors.

entry	substrate	product	yield^[c] (%)

1			96^[d]

2			95^[d]

3			14^[e]

4			97

^[a] 0.5 mol %
^[b] 2.0 mol %
^[c] Isolated yield after silica gel chromatography based on 9.
^[d] 1 mol % 1 and 4.0 mol % LiOH.
^[e] 1.5 equiv of 4b and 150 mol % LiOH.

We hypothesize that the reactivity observed for quinone complex 3•Li is due to bifunctional activation in catalyzing the reaction of boronic acids and electron deficient olefins. In the activation of the boronic acid, the alkoxide of thehydroquinone ligand can act as a nucleophile to activate the boronic acid directly for transmetallation to the rhodium center (Scheme 4). After formation of the rhodium aryl species, the lithium counter-ion can act as a generalacid to pre-organize and activate the conjugate acceptor for carbo-metallation. This pre-organization acts to accelerate the rate of conjugate addition in relation to the rate of proto-deborylation, thereby allowing the use of extremely electron deficient aryl boronic acids. Mechanistic studies are underway to elucidate the details of this new class of Rh^Icatalysts.
Thus, Applicants have developed and disclose an efficient procedure for the conjugate addition of electron deficient aryl boronic acids to 2-cyclohexen-1-one and other activated conjugate acceptors (Table 6). Accordingly, examples of conjugate acceptors also include those of Table 6 above.
Scheme 5 provides a summary of conditions and reaction scope. This catalyst system is noteworthy due to the operational ease of use and high isolated yields with low levels of catalyst and boronic acid loading in an aqueous solution. The yields of addition products, using extremely electron deficient aryl boronic acids including the first report of trihalogenated aryl boronic acids, are excellent with minimal proto-deborylation and a complete absence of Heck type products. See Zou, G.; Wang, Z.; Zhu, J.; Tang, J. Chem. Commun. 2003, 2438-2439; Mori, A.; Danda, Y.; Fujii, T.; Hirabayashi, K.; Osakada, K. J. Am. Chem. Soc. 2001, 123, 10774-10775.
In summary, disclosed is a new and highly efficient catalytic system using a rhodium quinonoid complex to catalyze the conjugate addition of aryl boronic acids. The process is characterized by high isolated yields of product using extremely electron deficient aromatic boronic acids while maintaining low catalyst loadings in short reaction times at about 50° C. Additionally, examples of trifluoronated aryl additions are presented above which may be of interest to the biomedical and pharmaceutical communities.
Experimental Section—The rhodium(I) hydroquinone catalyst was synthesized as described above. Also see, for customary synthetic procedures, e.g. Y.-S. Huang, S. Sabo-Etienne, X.-D. He, B. Chaudret, Organometallic 1992, 11, 303; S. Sun, G. B. Carpenter, D. A. Sweigart, J. Organomet. Chem. 1996, 512, 257; J. Le Bras, H. Amouri, J. Vaissermann, Organometallics 1998, 17, 1116; M. Oh, G. B. Carpenter, D. A. Sweigart, Organometallics 2002, 21, 1290; J. Moussa, C. Guyard-Duhayon, P. Herson, H. Amouri, M. N. Ragwr, A. Jutand, Organometallics 2004, 23, 6231.
General Procedure: A 1-dram vial fitted with a Teflon cap was charged with aryl boronic acid (1.2 mmol) and enone (1.0 mmol) and dimethoxyethane (DME, 1.0 mL). A solution of 1 (0.02 M DME, 0.250 mL, 0.005 mmol, 0.5 mol %) followed by an aqueous LiOH solution (1.0 M, 0.020 mL, 0.020 mmol, 2.0 mol %). The headspace of the vial was flushed with N₂and deoxygenated H₂O (1.5 mL) was added. The vial was capped and the resulting mixture was stirred at 50° C. for 1 h. The reaction mixture was diluted with a saturated solution of NH₄Cl (5 mL), extracted with 25% EtOAc/hexanes (2×5 mL), dried (Na₂SO₄), filtered through a silica plug, and concentrated to afford pure product as characterized by ¹H and ¹³C NMR and high resolution mass spectrometry.
Note, it was also found that diethyl ether and THF can be used in place of DME, however no reaction is observed in toluene as solvent.
An alternate procedure was found to be efficacious for the conjugate addition to hindered poly-substituted electron deficient olefins such as tri-substituted olefin, acetyl-cyclohexene (Scheme 7). These conditions provide conjugate addition to hindered systems in the presence of minute amounts of organic solvent.
Alternate Reaction Conditions: General Procedure: In a one-dram vial, p-tolyl boronic acid (410 mg, 3.0 mmol, 3.0 equiv.) was combined with 1-acetylcyclohexene (124.1 mg, 1.0 mmol, 1.0 equiv.) before addition of a solution of Rh catalyst (500 μL, 0.02 M in DME, 1.0 mole %), LiOH (2.85 mL, 1.0 M in water, 2.85 equiv.), and deionized water (1.0 ml, deoxygenated by sparging with nitrogen gas for 30-60 minutes). The headspace of the vial was flushed with nitrogen before sealing the vessel with a teflon cap. The reaction mixture was vigorously stirred to induce phase mixing for 19 hours at 75° C. The reaction mixture was quenched with ammonium chloride (3.0 mL) and extracted with ethyl acetate/hexanes (1:3, 2×2 mL). The combined organic layers were washed sequentially with NaOH (2 ml, 1 N) and brine (2.0 ml), dried (sodium sulfate) and filtered through a short plug of silica gel, using 25% ethyl acetate/hexanes as the eluent. The desire product was obtained in 60% yield (128.7 mg) after concentration and removal of residual 1-acetylcyclohexene under reduced pressure (250 mTorr, rt, 12 h).
Rhodium quinonoid catalysts, arising from precatalyst 1, are believed to be a remarkably efficient new class of reagents for the conjugate addition of aryl boronic acids. Thus, as explained above, herein Applicants describe the use of these reagents in catalyzing the highly efficient addition of a broad range of boronic acids, including heteroaromatic and the first believed example of trihalogenated-aryl boronic acids.
III. Organometallic Crystal Engineering of [1,4- and 1,3-hydroquinone)Rh(P(OPh₃)₂] Salts by Charge Assisted Hydrogen Bonding
Organometallic crystal engineering has attracted significant recent attention due to potential catalytic and materials applications. See: D. Braga, F. Grepioni and G. R. Desiraju, Chem. Rev., 1998, 98, 1375; A. D. Burrows, C.-W. Chan, M. M. Chowdhry, J. E. McGrady and D. M. P. Mingos, Chem. Soc. Rev., 1995, 24, 329; S.-S. Sun and A. J. Lees, Inorg. Chem., 2001, 40, 3154; C. J. Kuehl, T. Yamamoto, S. R. Seidel and P. J. Stang, Org. Lett., 4, 913; D. M. Shin, Y. K. Chung and I. S. Lee, Cryst. Growth Des., 2002, 2, 493; Y. Kim and J. G. Verkade, Inorg. Chem., 2003, 42, 4262; R. D. Hartnell and D. P. Arnold, Organometallics, 2004, 23, 391; Y.-B. Dong, Y. Geng, J.-P. Ma and R.-Q. Huang, Inorg. Chem., 2005, 44, 1693. D. F. Eaton, A. G. Anderson, W. Tam and W. Wang, J. Am. Chem. Soc., 1987, 109, 1886; I. S. Lee, Y. K. Chung, J. Mun and C. S. Yoon, Organometallics, 1999, 18, 5080; I. R. Whittall, A. M. McDonagh, M. G. Humphrey and M. Samoc, Adv. Organomet. Chem., 1999, 43, 349; S. Barlow and S. R. Marder, Chem. Commun., 2000, 1555; M. Albrecht, M. Lutz, A. L. Spek and G. van Koten, Nature, 2000, 406, 970; M. Albrecht and G. van Koten, Angew. Chem., Int. Ed., 2001, 40, 3750; P. H. Dinolfo, J. T. Hupp, Chem. Mater., 2001, 13, 3113; S. J. Lee, A. Hu and W. Lin, J. Am. Chem. Soc., 2002, 124, 12948; M. J. E. Resendiz, J. C. Noveron, H. Disteldorf, S. Fischer and P. J. Stang, Org. Lett., 2004, 6, 651.
A variety of inorganic-organometallic coordination polymers using [η⁴-benzoquinone)Mn(CO)₃]⁻ as the fundamental building block connected to metallic nodes via the quinone oxygen atoms have been reported. See: M. Oh, G. B. Carpenter and D. A. Sweigart, Acc. Chem. Res., 2004, 37, 1; S. U. Son, S. B. Kim, J. A. Reingold, G. B. Carpenter and D. A. Sweigart, J. Am. Chem. Soc., 2005, 127, 12238. Braga and coworkers have reported the syntheses of hydrogen-bond directed organometallic and organic-organometallic supramolecules based on ferrocene, cobaltocene and bis-benzene chromium units. See: D. Braga, L. Maini and F. Grepioni, Organometallics, 2001, 20, 1875; D. Braga, G. Cojazzi, D. Emiliani, L. Maini and F. Grepioni, Organometallics, 2002, 21, 1315; D. Braga, M. Polito, D. D'Addario, E. Tagliavini, D. M. Proserpio, F. Grepioni and J. W. Steed, Organometallics, 2003, 22, 4532; D. Braga, M. Polito, M. Bracaccini, D. D'Addario, E. Tagliavini and L. Sturba, Organometallics, 2003, 22, 2142; D. Braga, M. Polito, D. D'Addario and F. Grepioni, Cryst. Growth Des., 2004, 4, 1109; D. Braga, M. Polito and F. Grepioni, Cryst. Growth Des., 2004, 4, 769. In the latter studies it was suggested that charge assisted hydrogen bonding, which can occur in ionic or zwitterionic systems and refers to hydrogen bonding accompanied by coulombic interactions resulting from the inherent electronic charges, can be an effective strategy for fully utilizing the directional properties of hydrogen-bonding mediated assembly. See: D. Braga and F. Grepioni, Acc. Chem. Res., 2000, 33, 601.
The self-assembly of molecules or molecular units into supramolecular arrays can be driven by covalent bond formation and/or can be driven by noncovalent interactions such as π-π stacking, hydrogen bonding and van der Waals forces. Hydrogen bonding has been recognized as a particularly powerful tool in this regard because of its unique directionality and specificity. Supramolecular assemblies predicated on hydrogen bonding can be reinforced by the cooperative action of multi-point H-bonds, or additional cooperative interactions between the modular components of the assembly. An important example of this is so-called charge-assisted hydrogen bonding. This can lead to an exceptionally strong interaction between the oppositely charged components.
Recently, it has been recognized that the structural and chemical versatility of organometallic building blocks can be utilized to prepare supramolecular assemblies with distinct physical and chemical properties that cannot be replicated in purely organic systems. For example, self-assembled coordination networks that feature transition metal nodes and the anionic complex [(η⁴-quinone)Mn(CO)₃]⁻ as organometalloligand spacers have been extensively reported by us. See Sweigart, et al., Accounts of Chemical Research, 2004, 37, 1. In addition to coordination mediated self-assembly, there has been a considerable interest in supramolecular organometallic assemblies formed via non-covalent interactions. Braga and coworkers, referenced above, for example, have described the self-assembly of a variety of organometallic sandwich compounds through charge-assisted hydrogen bonding.
In this section, Applicants present the structural consequences of hydrogen bonding within the ionic organometallic complexes of the type [(η⁶-hydroquinone)Rh(P(OPh)₃)₂]³⁰X⁻ (14⁺; X=BF₄, ClO₄, SbF₆, OTf, OTs, OPf), [(η⁶-resorcinol)Rh(P(OPh)₃)₂]⁺BF₄ ⁻ (15^+BF ₄ ⁻) and [(η⁶-4,4′-biphenol)Rh(P(OPh)₃)₂]BF₄(16⁺BF₄ ⁻). FIG. 5 illustrates the complexes. In these complexes, the —OH groups are activated by the electrophilic rhodium moiety to participate in charge-assisted hydrogen bonding to the anionic counterion. The crystal structures feature three kinds of non-covalent interactions: hydrogen bonding, coulombic attraction and π-π stacking, which result in an intriguing array of architectures: dimeric, 1-D chain, C₂-helical, and C₃-helical. The nature of the charge-assisted hydrogen bonding and the resulting 3-D structure in these systems is remarkably dependent on the identity of the anion. Robust porous networks are formed rapidly (minutes or less) with [(η⁶hydroquinone)Rh(P(OPh)₃)₂]⁺X⁻ (X=BF₄, ClO₄) and [(η⁶resorcinol)Rh(P(OPh)₃)₂]⁺BF₄ ⁻. The hydrophobic pores in [η⁶-hydroquinone)Rh(P(OPh)₃)₂]ClO₄bind toluene reversibly. This work demonstrates that self-assembly of well-designed organometallic building blocks via charge-assisted hydrogen bonding is an effective strategy for the construction of robust porous networks. With counterions containing both oxygen and fluorine, it was found that the former is invariably the hydrogen bond acceptor, a result in agreement with atomic charge calculations. It is anticipated that self-assembly via charge-assisted hydrogen bonding is an approach applicable to many organometallic systems.
Complexes 15⁺BF₄and 16⁺BF₄were synthesized in good yields by treatment of the precursor [Rh(P(OPh)₃)₂Cl]₂with AgBF₄in methylene chloride to generate [Rh(P(OPh)₃)₂]⁺ in situ, which was then reacted with resorcinol and 4,4′-biphenol, respectively. The 1,4-hydroquinone salts 14+X (X-=BF₄ ⁻, SbF₆ ⁻, PF₆ ⁻, ClO₄ ⁻, OTs⁻, OTf⁻) were synthesized in a similar manner, with the anion X⁻ deriving from the silver salt (AgX) utilized. The bulky phosphite ligands were introduced to minimize the probability of interpenetration in the solid state. Examples of the utilization of bulky groups to get non-interpenetrated porous structures include X. Xu, M. Nieuwenhuyzen and S. L. James, Angew. Chem. Int. Ed., 2002, 41, 764; N. G. Pschirer, D. M. Ciurtin, M. D. Smith, U. H. F, Bunz and H. C. Zur Loye, Angew. Chem. Int. Ed., 2002, 41, 583; B. Moulton and M. J. Zaworotko, Curr. Opin. Sol. State Mat. Sci., 2002, 6, 117.
Cartoon diagrams of the different types of solid state structural patterns found are shown in FIG. 6. The cationic hydroxybenzene complexes (14⁺-16⁺) and the anionic companion (X⁻) can assemble to generate dimeric, 1-D chain, C₂-helical or C₃-helical motifs, most of which feature charge-assisted hydrogen bonding. Relevant sample X-ray crystallographic data are summarized in Table 7.
Crystals of [(η⁶-1,4-hydroquinone)Rh(P(OPh)₃)₂]⁺SbF₆ ⁻(14⁺SbF₆ ⁻) suitable for the single crystal X-ray analysis were prepared by layering a methylene chloride solution at −20° C. with diethyl ether or hexane. Cubic-shaped orange crystals and plate-shaped yellow crystals were obtained with diethyl ether and hexane co-solvents, respectively. The X-ray structure of the orange crystals revealed that the hydroquinone —OH groups are hydrogen bonded to diethyl ether present in the crystal lattice (O . . . O=2.6 Å), as shown in FIG. 13 (left). The hydroquinone rings are arranged in pairs due to an edge-to-edge π-π stacking interaction involving two carbon atoms of each ring. The average C . . . C contact between the edges of adjacent rings is 3.3 Å. After drying under vacuum for one day, the XRPD (X-ray powder diffraction) pattern of 14⁺SbF₆ ⁻ changed significantly, from which it is inferred that the solid remains crystalline but undergoes a substantial structural change upon solvent loss. It proved possible to ascertain the nature of this change because the simulated XRPD obtained from single crystal data for 14⁺SbF₆ ⁻ grown with hexane co-solvent matched that obtained after drying 14⁺SbF₆ ⁻.2 Et₂O, suggesting that they have the same structure. The structure of the former, reveals a π-π stacked dimeric aggregate with nearly eclipsed hydroquinone rings that are separated by an average of 3.4 {acute over (Å)}. It is concluded that, upon drying, 14⁺SbF₆ ⁻.2 Et₂O undergoes a remarkable concerted hydroquinone ring slippage of ca. 3 {acute over (Å)} with concomitant loss of hydrogen bonding to the ether and gain of π-π stacking interactions, all without the loss of crystallinity.

TABLE 7

Crystallographic Data

	14 + SbF6−	14 + SbF6−(hex)	14 + OTf−

formula	C₅₀H₅₆F₆O₁₀P₂RhSb	C₄₂H₃₆F₆O₈P₂RhSb	C₄₄H₃₈Cl₂F₃O₁₁P₂RhS
fw	1217.55	1069.31	1067.55
T, K	100(2)	100(2)	100(2)
cryst syst	Triclinic	Triclinic	Triclinic
space group	P-1	P-1	P1
a, Å	12.834(1)	10.669(2)	10.597(1)
b, Å	13.259(1)	14.186(3)	13.952(1)
c, Å	17.158(1)	16.884(3)	16.587(1)
α, deg	94.600(1)	65.795(4)	74.722(1)
β, deg	99.863(1)	85.774(4)	84.604(2)
γ, deg	116.234(1)	70.863(4)	70.769(1)
V, Å³	2540(1)	2196(1)	2233(1)
Z	2	2	2
D_calcd,g/cm³	1.592	1.617	1.587
F(000)	1232	1064	1079
cryst size, mm	0.17 × 0.16 × 0.15	0.15 × 0.11 × 0.05	0.10 × 0.09 × 0.05
θ range, deg	1.74 to 28.42	1.65 to 26.47	1.60 to 26.45
no. of rflns collected
no. of data/restraints/params	12161/6/635	8995/0/541	17842/657/1123
Goodness-of-fit on F²	1.026	1.074	1.023
final R indices [I > 2σ(I)]	R1 = 0.0377	R1 = 0.1121	R1 = 0.0758
	wR2 = 0.0808	wR2 = 0.3021	wR2 = 0.1221

	14 + OPf−	16 + BF4−	14 + OTs−	14 + ClO4−

formula	C₄₂H₃₆F₂O₁₀P₃Rh	C₄₈H₄₀BF₄O₈P₂Rh	C₅₀H₄₅Cl₂O₁₁P₂RhS	C₄₂H₃₆ClO₁₂P₂Rh
fw	934.53	996.46	1089.67	933.01
T, K	100(2)	100(2)	293(2)	100(2)
cryst syst	Triclinic	Monoclinic	Orthorombic	Rhombohedral
space group	P1	P2/c	P2₁2₁2₁	R-3
a, Å	10.843(1)	17.960(5)	11.947(6)	38.625(1)
b, Å	11.245(1)	11.306(3)	17.660(8)	38.625(1)
c, Å	17.908(1)	23.267(7)	24.201(11)	15.096(1)
α, deg	105.394(1)	90	90	90
β, deg	90.398(1)	105.502(5)	90	90
γ, deg	107.279(1)	90	90	120
V, Å³	2001(1)	4553(2)	5106(4)	19505(2)
Z	2	4	4	18
D_calcd,g/cm³	1.551	1.454	1.417	1.43
F(000)	952	2032	2232	8568
cryst size, mm	0.07 × 0.05 × 0.05	0.14 × 0.14 × 0.10	0.135 × 0.11 × 0.038	0.12 × 0.114 × 0.096
θ range, deg	1.98 to 26.55	1.82 to 23.25	1.43 to 28.79	1.48 to 28.38
no. of rflns collected	21442	35882	57426	77869
no. of data/restraints/params	16047/603/1020	6529/730/618	12656/6/606	10732/0/523
Goodness-of-fit on F²	1.033	1.067	0.735	0.930
final R indices [I > 2σ(I)]	R1 = 0.0688	R1 = 0.1212	R1 = 0.0705	R1 = 0.0762
	wR2 = 0.1385	wR2 = 0.2917	wR2 = 0.1151	wR2 = 0.2386

Crystals of the triflate salt 14⁺OTf⁻ were grown by layering hexane on a methylene chloride solution at −20° C. The solid state structure consists of the dimeric unit illustrated in FIG. 9 and follows the general pattern depicted in FIG. 7 a. The two hydroquinone rings are π-π stacked (3.6 Å) and the —OH groups are hydrogen bonded to the sulfonate oxygens of the triflate anion (average O . . . O=2.65 Å). Since the sulfonate end of the triflate anion contains most of the net negative charge (vide infra), the hydrogen bonding would be expected to involve the oxygens rather than the fluorines, and may be classified as charge assisted.

The synthesis of 14⁺PF₆ ⁻, with AgPF₆as the anion source, proceeded smoothly and gave a product with a satisfactory elemental analysis. After slow recrystallization from methylene chloride, however, it became evident from subsequent single crystal X-ray analysis and altered bulk elemental analysis that hydrolysis of the anion to PF₂O₂ ⁻ (OPf⁻) had occurred during the recrystallization process. The hydrolysis reaction probably stems from trace water and may have been accelerated by the acidic nature of the coordinated hydroquinone. Hydrolysis of PF₆— in this manner has been observed previously. See Kannan, S.; James, A. J.; Sharp, P. R., Inorg. Chim. Acta, 2003, 345, 8. The X-ray structure of 14⁺OPf⁻ (FIG. 14) is very similar to that found for 14⁺OTf⁻. Charge-assisted hydrogen bonding and 7-7 stacking (3.4 Å) interactions dominate the observed dimeric units. Careful analysis of the X-ray data confirmed that the hydrogen bonding from the hydroquinone —OH groups is to oxygen and not fluorine acceptors on the OPf⁻ anion (average O . . . O=2.65 Å).
The dimeric structure found for 14⁺OPf⁻ and 14⁺OTf⁻ combines in a cooperative manner three types of non-covalent interactions: charge-pairing, hydrogen bonding and π-π stacking. A different type of dimeric assembly was found for [(η⁶-1,3-hydroquinone)Rh(P(OPh)₃)₂]+BF₄ ⁻ (15⁺BF₄ ⁻). In this case, the dimer is held together by charge-assisted hydrogen bonding but geometric restrictions prevent π-π stacking between the 1,3-hydroquinone rings (FIG. 10). The hydrogen bond distances in 15⁺BF₄ ⁻ average O . . . F=2.8 Å.
[(η⁶-4,4′-Biphenol)Rh(P(OPh)₃)₂]⁺BF₄ ⁻(16⁺BF₄ ⁻) forms the hydrogen bonding network depicted in FIG. 12. Only one F atom in the BF₄ ⁻ anion participates in hydrogen bond formation with the phenolic —OH groups. A 1-D polymeric chain structure results, shown in FIG. 11 a, with the hydrogen bond distances O . . . F=2.6 Å and O . . . O=2.8 Å. The 3-D crystal structure features small channels which are lined with phenyl groups from the triphenyl phosphite ligands that undergo 7-7E stacking. The channels were found to be filled with unidentified disordered solvent molecules, (FIG. 11 b).
The C₂-helical chain motif shown in FIG. 7 c was found for the tosylate salt of [(η⁶-1,4-hydroquinone)Rh(P(OPh)₃)₂]⁺ (14⁺OTs⁻). Long rod-shaped single crystals of 14⁺OTs⁻ were grown by layering a methylene chloride solution with hexane at 0° C. The helical hydrogen bonding network has C₂projection symmetry (FIG. 12). The space group (P2 ₁2₁2¹) implies the generation of chirality during the crystallization process, which means that the helices pack such that all possess the same direction of rotation (CW or CCW). The two independent hydrogen bonds in 14⁺OTs⁻ have O . . . O=2.43 and 2.67 Å.
Single crystals of 14⁺BF₄ ⁻and 14⁺ClO₄ ⁻ were grown by layering a methylene chloride solution with diethyl ether. These two salts have virtually identical structures, which feature the intriguing C₃-helical hydrogen bonded network shown in FIG. 7 d. Structural details for 14⁺ClO₄ ⁻ are shown in FIG. 13. The hydrogen bonding distances in 14⁺BF₄ ⁻ are F . . . O=2.47, 2.60 Å and those in 14⁺ClO₄ ⁻ are O . . . O=2.41, 2.91 Å. In each compound, six C₃helices assemble to generate the hexagonal channels or pores illustrated in FIG. 14. The structure belongs to the centrosymmetric space group R-3 and the direction of rotation of the helices alternates around the channels. The channels themselves located at the core of the six helices consist of hydrophobic phosphite phenyl groups (FIG. 14). Two of the three phenyl groups from each P(OPh)₃ligand contribute to the channels, which have a diameter of ca. 10.5 Å and are separated by ca. 23 Å.
The ease of formation of the pore structure shown in FIG. 14 b for 14⁺ClO₄ ⁻ and 14⁺BF₄was investigated by comparing the XRPD pattern of slowly grown macrocrystals with that found for microcrystals obtained by rapid precipitation. The addition of diethyl ether to a methylene chloride solution of 14⁺ClO₄ ⁻ led to rapid precipitation of a powder that appeared under a microscope to consist of good quality microcrystals. XRPD patterns showed that microcrystalline 1⁺ClO₄ ⁻ formed by simple rapid precipitation is (i) indeed crystalline and (ii) has the same porous structure possessed by slowly grown single crystals (FIG. 14). We come to the significant conclusion that the dynamic processes occurring in the assembly of organometallic building block 14⁺ClO₄ ⁻ into an intricate 3-D supramolecular architecture with hexagonal channels operate on a fast preparative time scale. Thus, the synthesis of crystalline porous materials such as 14⁺ClO₄ ⁻ can be accomplished within seconds (precipitation) rather than requiring days (slow single crystal growth). See also: Son UK Seung, Reingold Jeffrey A., Carpenter Gene B., Czech Paul T., Sweigart Dwight A., Organometallics 2006. Analogous conclusions obtain for the 14⁺BF₄ ⁻ analogue.
Experiments were done to probe the possible interaction of appropriate aromatic molecules with the hydrophobic channels present in 14⁺ClO₄(FIG. 14). The XRPD pattern of solid 14⁺ClO₄ ⁻ changes significantly after exposure to toluene for five days and then reverts to the original pattern after drying under vacuum. It may be concluded that toluene interacts reversibly with the host channels in 14⁺ClO₄ ⁻.
The hydrogen bonding interactions between the organometallic cations and the counter anions shown in FIG. 6 were studied in methylene chloride solution via FT-IR. The results are summarized in FIG. 15 and in Table 8. As shown in FIG. 15, ν_OHin 14⁺ is red-shifted by hydrogen bonding to the anion. With 14⁺ SbF₆ ⁻, two IR peaks are seen at 3517 and 3405 cm⁻¹(labeled b and c). The peak at 3517 cm⁻¹is assigned to “free” 14,i.e., the complex not hydrogen bonded to the counter anion. In support of this assertion, peak “b” also appears at the same frequency with counterions BF₄ ⁻ and ClO₄ ⁻. The much greater intensity of this peak in the case of SbF₆ ⁻ reflects the relatively poor ability of SbF₆ ⁻ to function as a hydrogen bond acceptor, a fact also indicated by the X-ray structures.
Peaks c-e in FIG. 15 are assigned to hydrogen bonded —OH groups. The shift of these ν_OHbands from the “free” position (peak b) can be used to estimate the strength of the H-bonding between the hydroquinone —OH groups and the counterion by application of logansen's equation. It is noted that the Iogansen equation related to hydrogen bonding enthalpy is ΔHo=−1.28(Δν)½. See also S. G. Kazarian, P. A. Hamley and M. Poliakoff, J. Am. Chem. Soc., 1993, 115, 9069; A. V. Iogansen, G. A. Kurkchi, V. M. Furman, V. P. Glazunov and S. E. Odinokov, Zh. Prikl. Spektrosk., 1980, 33, 460. The results, presented in Table 8, indicate that H-bonding between 14 or 15 and the counter anion is greater for O-based acceptors than for F-based acceptors. The hydrogen bonding strength spans the range 14-27 kJ/mol and follows the order SbF₆ ⁻<BF₄ ⁻<ClO₄ ⁻<OTf⁻<OPf⁻, OTs⁻.

TABLE 8

Summary of IR Study of Hydrogen Bonding^a

	Free	H-bonded	Shift in ν_OH	−ΔH^b
Compound	ν_OH(cm⁻¹)	ν_OH(cm⁻¹)	(cm⁻¹)	(kJ/mol)

hydroquinone	3585^c
Resorcinol	3580
4,4′-biphenol	3598
1⁺ClO₄ ⁻	3517^d	3231	286	21.6
1⁺OTf⁻	3517^d	3170	347	23.8
1⁺OPf⁻	3517^d	3078	439	26.8
1⁺OTs⁻	3517^d	3058	459	27.4
1⁺BF₄ ⁻	3517^d	3330	187	17.5
1⁺SbF₆ ⁻	3517^d	3405	112	13.5
2⁺BF₄ ⁻	3505^d	3321	184	17.4
3⁺BF₄ ⁻	3573^e	3296

The ν_OHbands in the IR spectra of free hydroquinone, resorcinol and 4,4′-biphenol were found to be invariant over the concentration range utilized (3-11 mM), indicating the absence of intermolecular hydrogen bonding at these concentrations. In contrast, FIG. 15 clearly shows that hydrogen bonding in 14⁺X⁻ can be extensive at 11 mM. The enhanced hydrogen bonding in 14⁺X⁻ can be attributed to (1) the positive charge on the cation brought about by the electrophilic rhodium fragment and (2) the obligatory anionic counterion that can act as a hydrogen bond acceptor. Charge pairing of the species in 14⁺X⁻ undoubtedly complements the hydrogen bonding. In order to probe the “charge assisted” nature of the hydrogen bonding, IR spectra of CH₂Cl₂solutions of 1,4-hydroquinone (11 mM) containing varying amounts of Bu₄ClO₄were recorded. One equivalent of Bu₄NClO₄has little effect on the IR spectrum and even with ten equivalents of Bu₄NClO₄present, a significant amount of free hydroquinone remains. It is concluded that the hydrogen bonding observed with (14⁺-16⁺), X⁻ has as important components both ionic charge pairing and electrophilic activation imparted by coordination to the transition metal.
Next, molecular orbital calculations were performed using Spartan to assign atomic charges to the key terminal atoms for the range of counterions. Atomic charges are notoriously difficult to define which led us to include the results from three differing approaches. See Spartan '04, Version 1.0.3; Wavefunction, Inc., Irvine, Calif. 2004 and Hehre, W. J. A Guide to Molecular Mechanics and Quantum Chemical Calcultions, Chapter 16, Wavefunction, Inc., Irvine Calif. 2003. Regardless of the charge partitioning scheme used, the oxygen atoms are calculated to be more electron-rich than the fluorine atoms. These results are in agreement with the observed preference for charge-assisted hydrogen-bonding to oxygen over fluorine in OTf⁻ and OPf⁻, as well as the trends observed in the IR spectra.
In crystal engineering, it is common for slight modification in ligand geometry and/or reaction conditions to result in supramolecular isomerization. This is typically the reason it is difficult to rationally design or predict supramolecular structures. In the case of 14⁺BF₄ ⁻ and 15⁺BF₄ ⁻ it is interesting that two iso-structures can be obtained from the self-assembly of geometrically different building blocks. This suggests that the bulky triphenyl phosphite groups, which are common to 14⁺BF₄ ⁻, 14 ClO₄and 15 BF₄ ⁻, play a major role in the supramolecular construction. This hypothesis is strengthened by an examination of the chemical composition of the channels.
The channels or pores located at the core of the six helices in 14+BF₄ ⁻ and in 14⁺ClO₄ ⁻ include hydrophobic phosphite phenyl groups (FIG. 14). Two of the three phenyl groups from each P(OPh)₃ligand contribute to the channels. FIG. 16 illustrates the view perpendicular and parallel to the channel axis. There are two types of phenyl groups present: half are situated parallel to the channel axis and define a pore diameter of ca. 10.5 Å; the other half are inclined by ca. 45° to the axis, reducing the effective pore diameter to ca. 6 Å. These two subgroups together form interesting sphere like units linked by linear units. In 15⁺BF₄ ⁻ completely analogous channels exist with the difference that the phenyl groups are more inclined (ca. 75°), with the result that the channels are much more blocked in comparison to the situation in 14⁺BF₄ ⁻ and 14⁺ClO₄ ⁻.
The micropores in 14⁺BF₄ ⁻ are likely to be robust because the hydrogen bonding is “charge-assisted” by coulombic interactions. Complementing this is the relatively small pore size and the large distance of ca. 23 Å between successive pore centers. See: B. Moulton and M. J. Zaworotko, Curr. Opin. Sol. State Mat. Sci., 2002, 6, 117. Metal-organic networks with pores in the size range reported herein have recently been found to be excellent hosts for suitably small guest molecules (e.g., acetylene). Aside from size the important factor influencing adsorption is the chemical environment of the pore interior. In the case of 14⁺BF₄ ⁻ and 14⁺ClO₄ ⁻, the channels or pores consist of aromatic rings, which may make the material a particularly useful model for hydrogen adsorption. See: B. Kesanli, Y. Cui, M. R. Smith, E. W. Bittner, B. C. Bockrath and W. Lin, Angew. Chem. Int. Ed., 2005, 44, 72.
In summary, the complexes [(1,4- and 1,3-hydroquinone)Rh(P(OPh)₃)₂]BF₄(14⁺BF₄ ⁻, 15⁺BF₄ ⁻) were found to exhibit charge assisted hydrogen bonding between the —OH groups and the BF₄ ⁻ or ClO₄ ⁻ counterion. In the solid state, this hydrogen bonding interaction gives rise to iso-structural supramolecular networks containing hydrophobic channels that consist of phenyl groups from the triphenyl phosphite ligands. Applications of these materials to guest-host chemistry are being examined.
Additionally, porous media for gas storage has potential applications in the development of hydrogen storage systems. Rhodium quinones are not believed to have been used before for this purpose. The rhodium hydroquinone cationic complex [(hydroquinone)RhL₂]⁺X⁻ has a solid state structure that is dominated by charge assisted hydrogen bonding and pi-pi stacking of the aromatic rings. With anions such as tetrafluoroborate and perchlorate, the solid material possesses hydrophobic channels that are lined with aromatic rings and that may provide an excellent environment for modeling the storage of hydrogen gas for application in storage and transport for energy applications.
It is further noted that additional crystallographic (CIF) data have been been deposited with the Cambridge Crystallographic Data Center as registry numbers CCDC 285472, 285473 and 299584-299590. See Table 8 for crystal data. Additional data are as follows: for 14⁺BF₄ ⁻: C₄₂H₃₆O₈P₂B₁F₄Rh₁, M=920.37, rhombohedral, space group R-3, a=38.46(1), b=38.46(1), c=14.93(1) Å, α=90, β=90, γ=120°, V=19125(2)A , Z=18, F(000)=8424, gof=0.855, final R1=0.0743; Crystal Data for 15+BF₄ ⁻: C₄₂H₃₆O₈P₂B₁F₄Rh₁, M=920.37, rhombohedral, space group R-3, a=38.72(1), b=38.72(1), c=14.66(1) A, α=90, β=90, γ=120°, V=19037(4)A³, Z=18, F(000)=8424, gof=1.098, final R1=0.0895
Synthetic Procedures and Characterization
General Considerations All reactions were carried out under N₂in flame-dried glassware. HPLC grade methylene chloride and diethyl ether solvents were used as received without further purification. [Rh(COD)Cl]₂was provided by Strem Chemicals. The ¹H NMR spectra were recorded on Bruker (300 MHz) spectrometers. Elementary analyses were performed by Quantitative Technologies Inc. (QTI, New Jersey). Thermogravimetric analyses (TGA, Q500 from Texas Instruments) and differential scanning calorimetry (DSC, DuPont DSC 2910) were performed at a scan rate of 5° C./min and 10° C./min using N₂, respectively. X-ray powder diffraction (XRPD) data were recorded on a Bruker D8 ADVANCE at 40 kV and 40 mA with Cu Kα radiation (λ=1.54050 Å) and a scan speed of 0.3°/sec and a step size of 0.1° in 20.
(η⁶-1,4-Hydroquinone)Rh[bis(triphenylphosphite)]BF₄(14⁺BF₄ ⁻). After flame drying the glassware, [Rh(P(OPh)₃)₂Cl]₂(0.36 g, 0.24 mmol) and AgBF₄(0.11 g, 0.56 mmol) were mixed for 1 h at room temperature in methylene chloride (5 mL). While stirring, a white precipitate was formed on the bottom of the glassware. 1,4-hydroquinone (0.10 g, 0.91 mmol) was added to the reaction mixture. After stirring for 2 h at r.t., the solvent was removed using rotary evaporator. The residue was dissolved in methylene chloride (3 mL) and slowly dropped to ethereal solution trough the Celite pad. The formed yellow solid in ether was collected by filter and washed with diethyl ether (10 mL, three times). The isolated yield was 71% (0.31 g, 0.34 mmol). To get the crystals: η⁶-1,4-hydroquinone Rh[bis(triphenylphosphite)]⁺BF₄ ⁻ (30 mg) was dissolved in methylene chloride (1.0 mL) in 5 mL-vial. Diethylether (3 mL) was carefully added to upper layer. After standing in a refrigerator for 3 days, reddish-yellow crystals formed on the wall of vial. ¹H NMR (CD₂Cl₂): δ 7.36(t, J=7.8 Hz, OPh, 12H) 7.26 (t, J=7.6 Hz, OPh, 6H), 7.02 (d, J=8.0 Hz, OPh, 12H), 6.56 (brs, OH, 2H), 5.63 (s, hydroquinone ring, 4H) ppm. Elemental Anal. Calcd for C₄₂O₈H₃₆P₂Rh₁B₁F₄: C, 54.81; H, 3.94. Found: C, 54.66; H, 3.86.
(η⁶-1,4-Hydroquinone)Rh[bis(triphenylphosphite)] ClO₄(14⁺ClO₄ ⁻). The same procedure was followed using AgClO₄instead of AgBF₄. The isolated yield was 79%. Crystals of 14⁺ClO₄ ⁻ were grown by layering a methylene chloride solution with hexane and cooling in a refrigerator for four days. ¹H NMR (CD₂Cl₂): δ 7.37(t, J=7.9 Hz, OPh, 12H), 7.25 (t, J=7.8 Hz, OPh, 6H), 7.01 (d, J=7.8 Hz, OPh, 12H), 6.96 (br s, OH, 2H), 5.67 (s, hydroquinone ring, 4H). Elemental anal. Calcd (%) for C₄₂O₁₂H₃₆P₂Rh₁C11: C, 54.07; H, 3.89. Found (%): C, 54.08; H, 4.01.
(η⁶-1,4-Hydroquinone)Rh[bis(triphenylphosphite)]SbF₆(14⁺SbF₆ ⁻). After flame drying the glassware, [Rh(P(OPh)₃)₂Cl]₂(0.36 g, 0.24 mmol) and AgSbF₆(0.19 g, 0.56 mmol) were mixed for 1 h at room temperature in methylene chloride (5 mL). While stirring, a white precipitate formed on the bottom of the glassware after which 1,4-hydroquinone (0.10 g, 0.91 mmol) was added to the reaction mixture. After stirring for 2 h at RT, the solvent was removed by rotary evaporation. The residue was dissolved in methylene chloride (3 mL) and slowly dropped into an ether solution through a Celite pad. A yellow solid formed in the ether solution and was collected by filtration (washed with diethyl ether, 10 mL, three times). The isolated yield was 83% (0.42 g, 0.39 mmol). Crystals were grown by dissolving 14⁺SbF₆ ⁻ (30 mg) in methylene chloride (1.0 mL) in a 5 mL-vial and layering with 3 mL of diethyl ether. The solution was placed in a refrigerator for 2 weeks, after which yellow crystals formed on the wall of the vial. ¹H NMR (CD₂Cl₂): δ 7.37(t, J=7.8 Hz, OPh, 12H), 7.27 (t, J=7.6 Hz, OPh, 6H), 7.03 (d, J=7.8 Hz, OPh, 12H), 6.11 (br s, OH, 2H), 5.68 (s, hydroquinone ring, 4H). Elemental anal. calcd (%) for C₄₂O₈H₃₆P₂Rh₁Sb₁F₆: C, 47.18; H, 3.39. Found: C, 47.85; H, 3.48.
(η⁶-1,4-Hydroquinone)Rh[bis(triphenylphosphite)]TfO(14⁺OTf). The same procedure as above was followed using AgOTf instead of AgSbF₆. The isolated yield was 91%. Crystals of 14⁺OTf⁻ were grown by layering a methylene chloride solution with hexane and cooling in a refrigerator for two days. Yellow crystals formed on the wall of vial. ¹H NMR (CD₂Cl₂): δ 8.26 (br s, OH, 2H), 7.31 (t, J=8.0 Hz, OPh, 12H), 7.21 (t, J=7.9 Hz, OPh, 6H), 6.97 (d, J=8.0 Hz, OPh, 12H), 5.47 (s, hydroquinone ring, 4H). Elemental anal. calcd (%) for C₄₃O₁₁H₃₆P₂Rh₁S₁F₃: C, 52.56; H, 3.69. Found (%): C, 53.08; H, 3.63.
(η⁶-1,4-Hydroquinone)Rh[bis(triphenylphosphite)]PF₂O₂(14⁺OPf⁻). The same procedure as above was followed using AgPF₆instead of AgSbF₆. Before recrystallization, the complex had PF₆ ⁻ as the counter anion. Elemental anal. calcd (%) for C₄₂0₈H₃₆P₃Rh₁F₆: C, 51.55; H, 3.71. Found (%): C, 52.04; H, 3.69. During recrystallization from methylene chloride, however, hydrolysis of the anion to PF₂O₂ ⁻ (OPf⁻) occurred to afford 14⁺OPf⁻ in a 66% isolated yield. ¹H NMR (CD₂Cl₂): δ 9.59 (brs, OH, 2H), 7.37 (t, J=8.0 Hz, OPh, 12H), 7.20 (t, J=7.9 Hz, OPh, 6H), 6.98 (d, J=8.0 Hz, OPh, 12H), 5.50 (s, hydroquinone ring, 4H). Elemental anal. Calcd (%) for C₄₂O₁₀H₃₆P₃Rh₁F₂: C, 53.98; H, 3.88. Found (%): C, 53.50; H, 3.73.
(η⁶-1,4-Hydroquinone)Rh[bis(triphenylphosphite)]OTs (14⁺OTs⁻). The same procedure was followed using silver tosylate instead of AgSbF₆. The isolated yield was 95%. Crystals of 14⁺OTs⁻ were grown by layering a methylene chloride solution with hexane and cooling in a refrigerator for three days. ¹H NMR (CD₂Cl₂): δ 7.38 (d, J=7.5 Hz, OTs, 2H), 7.27 (t, J=7.8 Hz, OPh, 12H), 7.25 (d, J=7.5 Hz, OTs, 2H), 7.15 (t, J=7.6 Hz, OPh, 6H), 6.95 (d, J=7.8 Hz, OPh, 12H), 6.69 (br s, OH, 2H), 5.55 (s, hydroquinone ring, 4H), 2.39 (s, OTs methyl, 3H). Elemental anal. Calcd (%) for C₅₀O₁₁H₄₃P₂Rh₁S₁: C, 54.81; H, 3.94. Found (%): C, 54.66; H, 3.86.
(η⁶-Resorcinol)Rh[bis(triphenylphosphite)]BF₄(14⁺BF₄ ⁻). The same procedure as above was followed but using resorcinol instead of hydroquinone. The isolated yield was 89%. To get the crystals: (η⁶-resorcinol)Rh[bis(triphenylphosphite)]⁺BF₄ ⁻ (25 mg) was dissolved in methylene chloride (0.7 mL) in a 5 mL vial. Toluene (3 mL) was carefully added to upper layer. The solution stands in refrigerator for 3 days. The orange crystals were formed on the wall of vial. ¹H NMR (CD₂Cl₂): δ 8.41 (brs, OH, 2H), 7.35 (t, J=7.8 Hz, protons in OPh, 12H), 7.25 (t, J=7.6 Hz, OPh, 6H), 7.02 (t, J=7.00, resorcinol, 1H), 7.00 (d, J=7.8 Hz, OPh, 12H), 6.31 (s, resorcinol, 1H), 4.88 (d, J=7.8 Hz, resorcinol, 2H) ppm. Elemental Anal. Calcd for C₄₂0₈H₃₆P₂RhlBlF₄: C, 54.81; H, 3.94. Found: C, 54.55; H, 4.10.
(η⁶-4,4-Biphenol)Rh[bis(triphenylphosphite)]BF₄(16⁺BF₄ ⁻). The same procedure was as above was followed using 4,4-biphenol instead of hydroquinone. The isolated yield was 87%. Crystals of 16⁺BF₄ ⁻ were grown by layering a methylene chloride solution with hexane and cooling in a refrigerator for three days. Orange crystals formed on the wall of vial. ¹H NMR (CD₂Cl₂): δ 8.39 (br s, OH, 1H), 7.26 (t, J=7.5 Hz, OPh, 12H), 7.22 (t, J=7.5 Hz, OPh, 6H), 6.90 (t, J=7.6 Hz, OPh, 6H), 6.83 (d, J=8.9 Hz, biphenol, 2H), 6.75 (d, J=6.75 Hz, biphenol, 2H), 6.00 (br s, OH, 1H), 5.92 (s, biphenol, 4H). Elemental anal. Calcd (%) for C₄₈O₈H₄OP₂Rh₁BIF₄: C, 57.86; H, 4.05. Found (%): C, 57.74; H, 3.91.
Single Crystal X-ray Structure. X-ray data collection was carried out using a Bruker single-crystal diffractometer equipped with an APEX CCD area detector and controlled by SMART version 5.0. Collection was done either at 100 K or 293K. Data reduction was performed by SAINT version 6.0. The structures were generally determined by direct methods and refined on F squared by use of programs in SHELXTL version 5.0. Most hydrogen atoms appeared in a difference map, or they were generally inserted in ideal positions, riding on the atoms to which they are attached.
In view of the foregoing, it can be seen that novel embodiments include, for example, the combination of rhodium and quinones for use in catalysis and use in generation of new organolithium reagents. Only a few rhodium quinones have been previously reported and none with the chemical formula or constitution set forth herein, and none that are believed to have been applied or are believed likely to be useful for any of the uses mentioned above. The rhodium and quinone components both play an integral role in the uses. In catalysis, the compounds function as multifunctional catalysts, which is also believed to be unique, in that, for example, the quinone part binds to a substrate while the rhodium center acts as a receptor site for a second substrate.
In conclusion, it should be noted that some of the features of the various non-limiting embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles, teachings and exemplary embodiments of this invention, and not in limitation thereof. Also, the numerical values, such as temperature, weight percent, etc., may also be understood in approximate (about) values.

Claims

1. A method of making an acyclic or cyclic compound comprising combining in a reaction mixture an aryl or vinylic boronic compound, a conjugate acceptor and a rhodium hydroquinone catalyst under suitable reaction conditions, allowing the reaction to proceed to its end, and isolating the desired acylic or cyclic compound from the reaction mixture.

2. The method of claim 1 comprising using the rhodium hydroquinone catalyst for conjugate addition of a boronic substrate of the boronic compound comprising transferring a carbon group from the boronic compound to the conjugate acceptor via the rhodium hydroquinone catalyst; and forming a product of conjugate addition.

3. The method of claim 2 comprising:

combining the boronic compound, rhodium hydroquinone catalyst, conjugate acceptor and a solvent, wherein the boronic compound is an aryl boronic acid, the conjugate acceoptor is enone and the solvent is dimethoxyethane;

adding an aqueous solution of LiOH base followed by deoxygenated H₂O to obtain a mixture;

stirring the mixture at about 50° C. for about 1 hour;

then diluting the mixture with a solution of NH₄Cl, extracted with EtOAc/hexanes and dried Na₂S0₄; followed by filtering to obtain the desired compound.

4. The method of claim 2 wherein the carbon group is a sp2 carbon group.

5. The method of claim 2 wherein the catalyst comprises the formula formula (I)

wherein X⁻ is selected from the group consisting of BF₄ ⁻, SbF₆ ⁻, PO₂F₂ ⁻, PF₆ ⁻, OTf⁻, ⁻OTs, SO₄ ²⁻, B(C₆F₅)₄ ⁻, B(C₆H₅)₄ ⁻, ClO₄ ⁻, NO₃ ⁻, NO₂ ⁻, HOSO₃ ⁻, CO₃ ²⁻, O₃SCF₂CF₂CF₂CF₃ ⁻; wherein OTf=O₃SCF₃ ⁻; OTs=O₃SC₆H₄CH₃ ⁻; R′CO₂ ⁻;

wherein R′ is selected from the group consisting of hydrogen or an alkyl, aryl or carbon atom bearing three identical or non-identical substituents;

wherein L₁and L₂are each a ligand that donates electron density to rhodium to stabilize it;

wherein

is either chiral or achiral and M₁and M₂comprise hydroxo (OH) groups in the ortho-, meta-, and para-positions and R is selected from the group consisting of H, C, O, N and S, with or without substituents, said substituents being identical or non-identical.

6. The method of claim 5 wherein L₁and L₂are identical or non-identical ligands that are either chiral or achiral and selected from the group consisting of alkenes, dialkenes, alkyene, phosphines, water, phosphites, sulfides, sulfoxides, sulfonates, sulfonamides, sulfones, ethers, amines, imines, amides, aldehydes, ketones, esters, nitriles, and combinations thereof.

7. The method of claim 2 wherein the catalyst comprises the formula (II)

wherein X⁻ is selected from the group consisting of BF₄ ⁻, SbF₆ ⁻, PO₂F₂ ⁻, PF₆ ⁻, OTf⁻, ⁻OTs, SO₄ ²⁻, B(C₆F₅)₄ ⁻, B(C₆H₅)₄ ⁻, ClO₄ ⁻, NO₃ ⁻, NO₂ ⁻, HOSO₃ ⁻, CO₃ ²⁻, O₃SCF₂CF₂CF₂CF₃ ⁻ wherein OTf=O₃SCF₃ ⁻; OTs=O₃SC₆H₄CH₃ ⁻; R′CO₂ ⁻;

wherein R′ is selected from the group consisting of hydrogen or an alkyl, aryl or carbon bearing three identical or non-identical substituents;

wherein R is selected from the group consisting of H, C, O, N and S, with or without substituents, said substituents being identical or non-identical.

8. The method of claim 7 wherein L₁and L₂are identical or non-identical ligands that are either chiral or achiral and selected from the group consisting of alkenes, dialkenes, alkyene, phosphines, water, phosphites, sulfides, sulfoxides, sulfonates, sulfonamides, sulfones, ethers, amines, imines, amides, aldehydes, ketones, esters, nitriles, and combinations thereof.

9. (canceled)

10. (canceled)

11. The method of claim 2 wherein the catalyst comprises 1,4-hydroquinone π-bonded to rhodium.

12. The method of claim 11 wherein the catalyst comprises [1,4-(hydroquine)Rh(COD)]⁺ cation, wherein COD is cyclooctadiene.

13. The method of claim 2 wherein the catalyst comprises 1,3-hydroquinone π-bonded to rhodium.

14. The method of claim 13 wherein the catalyst comprises [1,3-(hydroquine)Rh(COD)]⁺ cation, wherein COD is cyclooctadiene.

15. The method of claim 2 wherein the catalyst comprises 1,2-hydroquinone π-bonded to rhodium.

16. The method of claim 2 comprising 1,2-hydroquinone, 1,3-hydroquinone or 1,4-hydroquinone α-bonded to Rh(P(OPh)₃)₂ ⁺ cation.

17.-18. (canceled)

19. The method of claim 2 wherein the complex comprises η⁶-hydroquinone complex 1⁺BF₄ ⁻ synthesized by reaction of [Rh(COD)Cl)₂with AgBF₄and hydroquinone.

20. The method of claim 2 wherein the rhodium complex comprises two independent complexes of rhodium with hydroquinone and COD ligands, a BF₄counterion and a diethyl ether molecule.

21.-22. (canceled)

23. The method of claim 2 comprising about 0.1 to 1.0 mol % Rh¹catalyst.

24. The method of claim 1 wherein the conjugate acceptor is selected from the group consisting of:

wherein X═H, O, N, C or S.

25. The method of claim 2 comprising a reaction:

wherein the reaction comprises providing an active rhodium hydroquinone catalyst, Rh¹, and reacting the catalyst with the boronic compound comprising a sp2 hybridized carbon-center bearing a boron to transfer the sp2 hybridized carbon to rhodium and subsequently to the conjugate acceptor, which is an electron deficient olefin, an olefin bearing one or more electron withdrawing groups (EWG), through carbo-metallation followed by proto-demetallation in a presence of a base;

wherein EWG is selected from the group consisting of a ketone, aldehyde, imide, amide, ester, thioester, acid anhydride, nitro, sulfone, nitrile, sulfoxide, phosphinate, electron deficient aromatic ring or other suitable electron withdrawing substituent that withdraws electron density either through inductive or resonance effects from olefins, and combinations thereof; and R is selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium, and combinations thereof.

26. The method of claim 2 comprising a reaction:

wherein the reaction comprises providing the boronic compound and the catalyst and reacting the compound and catalyst under conditions sufficient to cause the reaction,

X is selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium, and combinations thereof.

M⁺ is a positively charged ion including any metal ion having an oxidation state at or higher than +1, and is selected from the group consisting of cationic L₁, K, Cs, Be, Sr, Ba, Al, Ti, Zr, B, Si, Cd, Ag, Ph₃PNPPh₃, Rb, Mg²⁺, Ca²⁺, Na, R4N⁺, Zn²⁺, ammonium salts including tetraalkylammonium cations, tetraalkylarsonium cations, guanidinium salts, amidinium salts, and combinations thereof;

Y is selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium, and combinations thereof;

L is a ligand that donates electron density to the rhodium to stabilize it and each L is an identical or non-identical ligand that is either chiral or achiral and selected from the group consisting of alkenes, dialkenes, alkyene, phosphines, water, phosphites, sulfides, sulfoxides, sulfonates, sulfonamides, sulfones, ethers, amines, imines, amides, aldehydes, ketones, esters, nitriles, and combinations thereof;

wherein EWG is selected from the group consisting of a ketone, aldehyde, imide, amide, ester, thioester, acid anhydride, nitro, sulfone, nitrile, sulfoxide, phosphinate, electron deficient aromatic ring or other suitable electron withdrawing substituent that withdraws electron density either through inductive or resonance effects from olefins, and combinations thereof; R is selected from the group consisting of hydrogen, carbon, halide, oxygen, sulfur, nitrogen, silicon, phosphorus, sulfur, selenium, and combinations thereof; and

—BR₂is any boronic containing species neutral or anionically charged where boron is bound to a transfer group.

27. A reagent for the conjugate addition of aryl or vinylic boronic acids wherein the reagent comprises a rhodium hydroquinone catalyst.

28. The method of claim 2 comprising using the rhodium hydroquinone catalyst for the conjugate addition of boronic acid comprising a reaction:

wherein the reaction comprises mixing reagents (a) and (b) and reacting the reagents and the rhodium hydroquinone catalyst under conditions sufficient to cause the reaction in the presence of a base and a solvent, wherein:

X is selected from the group consisting of p-Me, m-NO₂, H, o-Me, 4NH-Boc, p-OMe, p-Cl, p-F, 3Cl, 4F, m-NO₂, 3,4,5 triF, 2,3,4-triF;

the solvent is selected from the group consisting of DME/H₂0, H₂O and THF; and

the base is LiOH or KOH.

29. The method of claim 28 comprising using the catalyst for the conjugate addition of aryl boronic acid to 2-cyclohexen-1-one comprising a reaction

wherein:

Rh+ is the catalyst; and

X is selected from the group consisting of p-Me and m-NO₂.

30. (canceled)

31. A method for double deprotonation of a rhodium hydroquinone catalyst to active quinone complex 3•K comprising a reaction:

wherein the reaction comprises mixing reagents (a) and (b) and reacting the reagents and the catalyst in the presence of water and DME under conditions sufficient to cause the reaction.

32. The method of claim 2 comprising a reaction

wherein:

Rh+ is a rhodium hydroquinone catalyst; and

X is selected from the group consisting of: H, p-Me, o-Me, 4NH-Boc, p-OMe, p-Cl, p-F, 3Cl, 4F, m-NO₂, 3,4,5 triF, 2,3,4-triF.

33. The method of claim 32 further comprising an additive, wherein the additive is selected from the group consisting of hydroquinone, LiBF₄, LiCl, pyridine, Cs₂CO3, Cs₂CO₃and Na₂CO₃.

34. A method of using a catalyst for the conjugate addition of aryl or vinyl boronic substrates comprising the steps of:

a) providing a rhodium hydroquinone catalyst;

b) transferring a sp2 hybridized carbon group from boron to a conjugate acceptor via the catalyst; and

c) forming a product of conjugate addition.

35. The method of claim 2 comprising using the hydroquinone rhodium catalyst for the conjugate addition to tri-substituted olefins comprising at least one of the following reactions:

36. The method of claim 35 wherein the reaction is carried out at about 55° C.

37. The method of claim 35 wherein the reaction is carried out at about 75° C.