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

Abstract

Chiral, coordinatively-saturated cobaltate(III) 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 Λ( S, S)-, and Δ( 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 Λ-[(Sal-( S)-Val) 2Co(III)] −Ag + and Δ-[(Sal-( S)-Val) 2Co(III)] −Ag + were established by single crystal X-ray analysis. All of the sodium and potassium complexes effectively catalyzed the reaction between benzaldehyde and Me 3SiCN, but only the Λ-[(Sal-( S)-Trp) 2Co(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%).