Dedicated to Professor Ernst Otto Fischer on the occasion of his 65th birthdayFollowing a survey of the asymmetric hydrogenation of prochiral olefins with transition metal/phosphane catalysts, the problem of chirality transfer from the optically active ligands of the catalyst to the substrate is discussed. A new concept for this chirality transfer is introduced ; the conformational analysis of model compounds as well as the development of catalysts for enantioselective hydrosilylation demonstrate the usefulness of this concept.For every reaction in which a new asymmetric center is created there exists a transition metal catalyst which-when added in small amounts-leads stereospecijkally to one enantiomer. One onZy has to find it.
Introductory RemarksEnzymes catalyze the synthesis of optically active compounds from prochiral precursors. The reactions are usually stereospecific. Since an enzyme molecule converts its substrate 10000 to 1000000 times per minute"], the optical activity contained in it is multiplied by this factor each minute. Enzyme reactions are thus prime examples for "asymmetric catalysis" in which large amounts of optically active products are formed from a small amount of "optically active information". However, enzyme reactions are often restricted to natural substrates, and reactivity as well as stereoselectivity rapidly decrease when the substrate is varied. This poses the following questions: How good are synthetic catalysts in asymmetric catalysis? Can they make a contribution to the increasing demand for enantiomerically pure optically active pharmaceuticals, foods, animal feeds, pesticides, organic chemicals, etc.?
Enantioselective Catalysis with Transition MetaVPhosphane ComplexesIn 1968 Horner et a1.[" and Knowles et ~1 . '~' independently discovered that prochiral olefins can be hydrogenated enantioselectively using rhodium complexes similar to Wilkinson's catalystE4] but having optically active phosphane ligands. Scheme 1 shows this for one of the first asymmetric hydrogenations-the conversion of a-methoxystyrene 1 into a-methoxyethylbenzene 2a, b. sure) and the possibility of converting dehydroamino acids into amino acids['' soon resulted in world-wide attention for this area of research. Although the use of monodentate optically active phosphanes led to high optical inductions for some special substrates, it was only the switch to chelating phosphanes that brought decidedly better optical yields.
3Scheme 2
*a
CbThe synthesis of N-acetylphenylalanine 4a, b from (aa-(N-acetamido)cinnamic acid 3 according to Scheme 2 may be quoted as an example. The catalyst is formed in situ from the precatalyst [Rh(cod)Cl], (cod = 1,s-cyclooctadiene) and the cocatalyst diop 5 (Scheme 3), and gives an optical induction of 81% ee. Thus (-)-diop, which is derived from naturally occurring tartaric acid, leads to N-acetylphenylalanine with an ( R ) : ( S ) ratio of 90.5 :9.5[91. These studies culminated several years ago in the development of an industrial process for the synthesis of L-Dopa, a drug used to...
