ChemInform Abstract: MECHANISM AND STEREOCHEMISTRY OF ASYMMETRIC CATALYSIS BY METAL COMPLEXES

Abstract

The use of chiral catalysts to effect the asymmetnc hydrogenation of prochiral olefinic substrates with high optical yields represents one of the most impressive achievements to date in catalytic selectivity. Notably high optical yields, approaching 1OO7 enantiomeric excess, have been achieved in the hydrogenation of prochiral enamides to the corresponding amino acid derivatives, using homogeneous cationic rhodium catalysts containing chiral phosphine, especially bis(tertiaryphosphine) , ligands . The following aspects of such systems are discussed: (a) the coordination chemistry of the cationic rhodium phosphine catalysts, (b) the mechanism of catalytic hydrogenation, and (c) the origin of the enantioselection. A remarkable conclusion of the studies described is that, contrary to the hitherto prevailing lock and key view concerning these and related stereoselective catalysts, the enantioselection in these systems is determined not by the preferred initial binding of the prochiral substrate to the chiral catalyst but, rather, by the much higher reactivity of the minor diastereomer of the catalyst-substrate adduct corresponding to the less favored binding mode. INTRODUCTION The use of chiral catalysts to effect the asymmetric hydrogenation of prochiral olefinic substrates with high optical yields represents one of the most remarkable achievements to date in catalytic selectivity, rivaling the corresponding stereoselectivity of enzymic catalysts (1). Notably high optical yields, approaching lOO enantiomeric excess (e.e.), have been obtained in the hydrogenation of ct-acylaminoacrylic acid derivatives such as 1 to the corresponding amino acid derivatives (eq. 1) using cationic rhodium 'omplexes containing chiral phosphine (especially chelating diphosphine) ligands as catalysts (1-4). The commercial synthesis of L-dopa (3,4-dihydroxyphenylalanine) by such a route exemplifies an important practical application of this extraordinarily stereoselective catalysis. * + (-'Or' H COOR {Rh(P )] R2 2 + H2 2 R1CH2C—H (1) R1 1 NHCOR3 NHCOR Examples of some of the chiral ligands that have been found to be effective in such asymmetric catalytic hydrogenation reactions are depicted by 2 to 2 (1-6). These catalyst systems ar impressive not only for their remakable stereoselectivities but also for their very high activities. Extrapolation from low temperature measurements(7) yields turnover frequencies under saturation conditions approaching 10 sec1 at room temperature for reaction 1 catalyzed by cationic Rh rhodium complexes of DIPHOS (8) and its chiral derivatives. Even higher catalytic activities are exhibted by rhodium complexes of chelating diphosphine ligands that form larger chelate rings, e.g., DIOP (3) (8). Such activities are unusually high for homogeneous hydrogenation c'talysts (9) and, indeed, lie well up on the scale of activities characteristic of enzymes. Thus, in respect of both selectivity and rate, the behavior of these synthetic catalysts rivals, to an unprecedented degree, that of enzymic catalysts. 99