Chiral rhodium carboxylate complexes

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

Hashimoto and co-workers (55) reported that generation of ylide 152 from aryl ester 151 in the presence of a chiral rhodium complex Rh2(S-PTTL)4, a chiral phthalimide substimted carboxylate, followed by cycloaddition with DMAD, led to the formation of adduct 153 in good yield and in 74% enantiomeric excess (ee). 

Although the first catalysts were copper-based, the insertion of metal-associated carbenes into carbon-hydrogen bonds has undergone a renaissance with the advent of rhodium(II) carboxylate catalysts [56]. Metal-catalyzed enan-tioselective C-H insertions of carbenes have not been studied in great detail. Most of the efficient enantioselective versions of this reaction involve chiral rhodium complexes and until recently, the use of chiral catalysts derived from metals other than copper and rhodium for the asymmetric C-H insertion of metal-associated carbenes are still unexplored. 

On this basis, in a joint effort with Martin and Muller (1991a), Doyle developed a series of dinuclear rhodium 2-pyrrolidone-5-carboxylate complexes that might give better enantiomeric ratios in cyclopropanations (see also Muller and Polleux, 1994). This was indeed the case for a series of intramolecular cyclopropanations of allyl diazoacetates with the complex Rh2((55)-MEPY)4 obtained with chiral methyl 2-pyrrolidone-5-carboxylate (MEPY = 8.178) an ee between 65 and 94 o was found. Doyle et al. (1993 a) continued that work with additional inter- and intramolecular cyclopropanations as well as with intramolecular CH insertions. Doyle and his coworkers again obtained good-to-excellent enantioselectivity with the same catalyst. Examples are given in Schemes 8-75 to 8-77. 

Asymmetric cyclopropanation. The ability to effect ligand exchange between rhodium(II) acetate and various amides has lead to a search for novel, chiral rhodium(II) catalysts for enantioselective cyclopropanation with diazo carbonyl compounds. The most promising to date are prepared from methyl (S)- or (R)-pyroglutamate (1), [dirhodium(ll) tetrakis(methyl 2-pyrrolidone-5-carboxylate)]. Thus these complexes, Rh2[(S)- or (R)-l]4, effect intramolecular cyclopropanation of allylic diazoacetates (2) to give the cyclo-propanated y-lactones 3 in 65 S 94% ee (equation 1). In general, the enantioselectivity is higher in cyclopropanation of (Z)-alkenes. 

The hydrogenation of various a/3-unsaturated acids in the presence of rhodium-phosphine complex catalysts whose ligands are chiral at both phosphorus and carbon gives saturated carboxylic acids with enantiomeric excesses up to 70%. The addition of carbon tetrachloride, catalysed by copper(ll) chloride, to (—)-men thy 1 acrylic and methacrylic esters, followed by hydrolysis, results in /3-tri-chloromethyl derivatives (6) having ca. 50% enantiomeric enrichment at the a-position. ... 

Rhodium(II)-MEPY and rhodium(II)-MACIM (methyl 1-acetylimidazolidin-2-one-4-carboxylate) complexes are efficient chiral catalysts for intramolecular carbon-hydrogen insertion reactions of diazoacetates (224) and metal carbene transformations (225). Dirhodium(II) carboxylates of similar structure (eg, piperidinonate complexes of the Rh2(ligand)4 type) have been found efficient catalysts for asymmetric cyclopropanation of olefins (226). 

Unsaturated Carboxylic Add Amides, Esters, and Nitriles Enantioselective hydroformylation of dialkylacrylamides was investigated by Clarke and coworkers in detail (Scheme 4.81) [56]. It was found that these substrates undergo hydroformylation more slowly than styrene. Up to 82% ee was realized in the best trials. A serious problem was caused by the epimerization of chiral aldehydes by the intermediarily formed umnodified rhodium hydride complexes. Therefore, the reaction times should be kept short, and low temperatures are recommended. 

The enantioselective synthesis of axially chiral hydroxy carboxylic acid derivatives 42 was accomplished by the cationic rhodium(I)/BINAP complex-catalyzed [2 + 2 + 2] cycloaddition of a,w-diynes 40 with 2-alkoxynaphthalene-derived alkynyl esters 41 with high yields and ee values (Scheme 9.15) [17],... 

Glaser, R., M. Twaik, S. Geresh, and J. Blumenfeld Structural Requirements in Chiral Diphosphine-Rhodium Complexes. VIII. Asymmetric Hydrogenation of N-Acetyldehydroamino Acids with Rhodium(I) Complexes Containing Chiral Carboxylic Analogues of DIOP. Tetrahedron Letters 1977, 4635. 

Rhodium compounds and complexes are also commercially important catalysts. The hydroformylation of propene to butanal (a precursor of hfr(2-ethyUiexyl) phthalate, the PVC plasticizer) is catalyzed by hydridocarbonylrhodium(I) complexes. Iodo(carbonyl)rhodium(I) species catalyze the production of acetic acid from methanol. In the flne chemical industry, rhodium complexes with chiral ligands catalyze the production of L-DOPA, used in the treatment of Parkinson s disease. Rhodium(II) carboxylates are increasingly important as catalysts in the synthesis of cyclopropyl compounds from diazo compounds. Many of the products are used as synthetic, pyrethroid insecticides. Hexacyanorhodate(III) salts are used to dope silver halides in photographic emulsions to reduce grain size and improve gradation. 

Rhodium(II) carboxylates and carbonyls offer a number of advantages over copper. Palladiummolybdenum complexes and chiral cobalt complexesare also often... 

Enantiomeric excesses up to 43% were obtained in the catalytic transfer hydrogenation of some a, j9-unsaturated carboxylic acids in water using sodium formate in the presence of rhodium complexes associated with chiral sulfonated ligands such as Cyclobutanediop 3 [48]. 

Only a few chiral catalysts based on metals other than rhodium and ruthenium have been reported. The titanocene complexes used by Buchwald et al. [109] for the highly enantioselective hydrogenation of enamines have aheady been mentioned in Section 3.4 (cf. Fig. 32). Cobalt semicorrin complexes have proven to be efficient catalysts for the enantioselective reduction of a,P-unsaturated carboxylic esters and amides using sodium borohydride as the reducing agent [ 156, 157]. Other chiral cobalt complexes have also been studied but with less success... 

As already mentioned (Section 2.5.1.2.1.3), rhodium(III) borohydride complexes with chiral carboxamide ligands have also been used as enantioselective catalysts for the hydrogenation of a,/ -unsaturated carboxylic esters89. In one case, with methyl 3-phenyl-2-butenoate as substrate, optical yields in the range of 60% have been achieved. 

The dimeric rhodium(II) compounds become enantioselective catalysts if they contain optically active ligands. For this the anions of optically active carboxylic acids seem to be most appropriate. The complex in which the Rhz unit is clamped by four mande-late anions was synthesized and structurally characterized some time ago. [7] As a catalyst, however, this complex results in only small enantiomeric excesses. [8] The reason for this is probably that the asymmetric centers lie in a plane between the two Rh atoms and are thus too far away from the coordination sites directed to the outside, at which the catalysis occurs. Chiral substituents at the nitrogen atoms of carboxamide anions would be considerably closer to these reaction centers, and... 

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