Enantioselectivity of nickel(II) and copper(II) complexes of Schiff bases derived from amino acids and (S)-o-[(N-benzylprolyl)amino]-acetophenone or (S)-o-[(N-benzylprolyl)amino]benzaldehyde. Crystal and molecular structures of [Ni{(S)-bap-(S)-Val}] and [Cu{(S)-bap-(S)-Val}]

Abstract

Copper(II) and nickel(II) complexes of Schiff bases derived from (S)-o-[(N-benzylprolyl)amino]-acetophenone [(S)-bap] and the amino acids (aa) : glycine, (R)- and (S)- and (S)-valine (val), (R)- and (S)-adamant-1-ylalanine, and (R)- and (S)-adamant-1-ylglycine have been synthesized. The structure of the complexes has been determined by physical and chemical methods; in addition the structures of [Cu{(S)-bap-(S)-Val}] and [Ni{(S)-bap-(S)-Val}] have been determined by X-ray crystallographic analysis. The kinetic CH-acidity and deuterium exchange of the hydrogen of an amino acid moiety under the action of bases has been studied. The deuterium exchange is accompained with epimerization which results in 80% excess of the (S)-2-[²H]amino acid. The stereoselectivity in the nickel complexes has been found to be higher than in the copper ones. The epimerization rate constant lies within the range 5 × 10^–4–1.2 × 10^–1 dm³ mol^–1 s^–1. The observed stereoselective effects are interpreted on the basis of X-ray analysis, circular dichroism, and ¹H n.m.r. spectroscopic data. It is suggested that the [Ni{(S)-bap-aa}] complexes can be used for determining the absolute configuration of the amino acids.