Influence of the metal and chiral diamine on metal(II)salen catalysed, asymmetric synthesis of α-methyl α-amino acids

The influence of the metal ion and chiral diamine used to form a metal(salen) complex on the catalytic activity of the complex in the asymmetric benzylation of an alanine enolate was investigated. Only metal ions which could form square-planar complexes gave catalytically active complexes, and best results were obtained with metal ions from the first row of transition metals, particularly copper(II) and cobalt(II). Salen ligands derived from acyclic, chiral 1,2-diamines were found to generate poor catalysts, an effect which seems to correlate with the ability of the substituents within the diamine to adopt a conformation in which they are anti to one another. Complexes derived from a variety of 5- and 6-membered cyclic 1,2-diamines did form active catalysts, but the enantioselectivity was always far lower than that of the parent cyclohexane-1,2-diamine derived complex.