Potassium and silver chiral cobaltate(III) complexes as precatalysts for asymmetric C-C bond formation

Chiral, coordinatively-saturated cobaltate(Ill) complexes of Schiff bases obtained from salicylaldehyde and the optically active amino acids, (S)-Val, (S)-Thr, and (S)-Trp, are formed as C-2-symmetrical, octahedral, anionic Lambda(S,S)-, and Delta(S,S)-diastereoisomeric complexes, which are easily separable by chromatography. The complexes are stereochemically inert and, thus, are not transformed into each other under normal conditions. The counter-cations of the complexes can be easily interchanged and the sodium, potassium, and silver salts of some of the complexes were prepared. The structures of diastereoisomeric Lambda-[(Sal-(S)-Val)(2)Co(III)]Ag--(+) and Delta-[(Sal-(S)-Val)(2)Co(III)]Ag--(+) were established by single crystal X-ray analysis. All of the sodium and potassium complexes effectively catalyzed the reaction between benzaldehyde and Me3SiCN, but only the Lambda-[(Sal-(S)-Trp)(2)Co(III)]K--(+) complex gave rise to enantiomerically-enriched mandelonitrile with up to 77% ee. The silver salts catalyzed a Mukaiyama reaction with low enantioselectivity (ee in the range 6-27%). (C) 2008 Elsevier Ltd. All rights reserved.