Chiral coordinatively saturated cobalt(III) complexes with Schiff bases of enantio pure amino acids are formed as Λ and ∆ isomers, which are not transformed into each other under normal conditions. These complexes catalyze the formation of enantiomerically enriched cyanohydrins from aldehydes and Me 3 SiCN under homo and heterogeneous ca talysis.Catalysts containing chiral elements in the cationic part are widely used in asymmetric catalysis. 1 However, the chemistry of polynuclear catalysts, which are salts of chiral anions with achiral cations, has started to develop actively in the recent time. 1-3 For instance, a family of bimetallic catalysts based on chiral binaphthol with an element of asymmetry in the anionic part has recently been developed. 1 Such catalysts are used, in particular, in the aldol condensation and Michael addition. Chiral octa hedral phosphates are used to shift the equilibrium in solutions of mutually transforming enantiomers toward one of them with enantiomeric excess up to 98%. 2 A new class of chiral borate anions, viz., derivatives of enan tiomerically pure amino acids, was created. 3 In the pres ence of copper salts, these compounds catalyze the enantioselective cyclopropanation of styrene. We have previously 4 synthesized potassium bis(N salicylidene aminoacidato)cobaltates, which are coordinatively satu rated cobalt complexes with two perpendicular tridentate ligands, the latter being the Schiff bases of salicylaldehyde and (S) amino acids. These complexes were used as chiral substrates for asymmetric alkylation in syntheses of enantiomerically enriched amino acids. 4
Results and DiscussionIn the present work, we report the use of the chiral anionic* complexes, viz., potassium Λ bis[N salicylidene (S) aminoacidato]cobaltates, 4 as catalysts in the synthe ses of enantiomerically enriched cyanohydrins. These complexes were synthesized according to the somewhat modified literature procedure 5 (Scheme 1).The isomers were separated and purified by column chromatography. The complexes exist as meridian Λ and ∆ stereoisomers, which are not transformed into each other under normal conditions (i.e., they are stereo chemically inert) and can be separated chromatographi cally. Thus, we prepared individual Λ and ∆ stereo isomeric cobalt complexes of the Schiff bases of salicyl aldehyde and the following amino acids: glycine**, (S) valine, (S) phenylalanine, (S) tryptophan, and * The electrophoresis data that confirm the ionic structure of the complexes are given in Experimental. ** In the case of glycine, two enantiomeric complexes are formed, which were separated by the crystallization of the diastereomeric salts with brucine. 6
