A Simple Chiral Cu(II) Complex as an Effective Phase-Transfer Catalyst for the Enantioselective Alkylation of Dissymmetric Glycinate Ketimines.

Catalytic asymmetric benzylation of a dissymmetric tert-butylglycinate ketimine, incorporating 1-naphthyl and phenyl groups as the Schiff base substituents, under phase-transfer conditions was investigated. It was interesting to note that the sense of asymmetric induction of the alkylation of Z-imine stereoisomer is opposite to that of the corresponding E stereoisomer with a similar degree of enantioselectivity. More interestingly, the chiral Cu(II) complex of the Schiff base derived from (R)-2-phenylglycinol and 2-hydroxy-1-naphthaldehyde was found to catalyze the same reaction under solid-liquid conditions with comparable enantioselectivity (up to 60% ee) with respect to known cinchona alkaloid catalysts. The solvent/base-system parameter was shown to control the optimal catalytic activity.