Structural evidence of a ketimine as the product of an amino acid with an aldehyde. An intermediate in the racemization and transamination of amino acids

Abstract

The aldimine and ketimine forms of two Schiff base complexes formed by the condensation of two isomeric imidazole carboxaldehydes with an amino acid are reported. Reaction of L1, the Schiff base condensate of 5-methyl-4-imidazolecarboxaldehyde (5Me4Im) and valine, with copper(II) perchlorate results in the isolation of [Cu(L1)(5Me4Im)(ClO4)] while the analogous reaction of L2, the Schiff base condensate of 1-methyl-2-imidazolecarboxaldehyde (1Me2Im) with alanine, and nickel(II) results in the isolation of [Ni(L2)2]. L1 exhibits the expected aldimine form of the amino acid derived Schiff base, 5Me4Im-CH=N-CH(R)CO2−, while L2 exhibits the tautomeric ketimine form, 1Me2Im-CH2–N=C(R’)CO2−. Structural data clearly support the two tautomeric forms. The ketimine form, observed in [Ni(L2)2], has been proposed as an intermediate in the racemization and transamination of amino acids.