New, Axially Chiral, Bimetallic Catalysts for Asymmetric Alkylation of Aldehydes with Diethylzinc

Abstract

Axially chiral bis(salicylidene)ethylenediamine (H2salen)-type ligands 3 (cf. Schemes 1 and 3) are efficient ligands for the enantioselective addition of diethylzinc to aldehydes. There is ample evidence that an active bimetallic catalyst forms an effective chiral pocket (see Fig. 2); of a series of first-row transition-metal complexes with these ligands, the most stereoselective were the CoII complexes (see Fig. 1). Best ee values as well as the fastest rates (see Tables 2 and 3) were obtained with these CoII complexes when an EtO substituent was present at C(3) of the salicylaldehyde residues of ligand 3 (R1=EtO), i.e., complex [CoII(3′h)] produced up to 93% ee with aromatic aldehydes and 78% ee for aliphatic aldehydes (see Table 4).