Reactions of some copper(II) complexes of the Schiff bases of amino-acid esters

Abstract

Bis[ethyl N-(o-hydroxybenzylidene)valinato]copper(II)[Cu(salvalEt)₂] and bis[ethyl N-(o-hydroxybenzylidene)-leucinato]copper(II)[Cu(salleuEt)₂] have been prepared for the first time. Bis(o-hydroxybenzylideneamine)-copper(II)[Cu(salNH)₂] is isolated in small yield from the oxidation by molecular oxygen at room temperature of these two complexes in absolute ethanol solution. The general formation of Cu(salα),xH₂O by hydrolysis of Cu(salαR)₂ in aqueous ethanol is observed. Cu(salval) and Cu(salleu), are assigned dimeric structures on the basis of their analyses and subnormal magnetic moments. The complex Cu(salphe),xH₂O is formulated as a carbinolamine species rather than a Schiff base. The copper(II) complex of the Schiff base of methyl anthranilate has been prepared for the first time and is converted into the corresponding ethyl ester complex upon being heated in ethanol. Evidence was also found for the hydrolysis of this complex, and of the copper(II) complex of the tetradentate double Schiff-base formed between lysine ethyl ester and salicylaldehyde (1 : 2). The rapid racemisation of the complexes Cu(sal[graphic omitted]R)₂ is shown to be a general reaction. The complexes Cu(sal[graphic omitted]),xH₂O are optically stable in neutral solution, but racemise quickly under alkaline conditions. A general mechanism of catalysis in the complexes Cu(salαR)₂ is proposed involving the addition of water or alcohols to the azomethine links. The reverse reaction, the reformation of the Schiff base, provides a nucleophile in a favourable position to effect the various reactions found in this system. It is suggested that a similar carbinolamine mechanism may be involved in the catalysis of amino-acid reactions by pyridoxal in model systems and enzymes.