CH-Acidity of amino-acid esters co-ordinated to cobalt(III). Racemization during the formation of peptides

Abstract

Several dipeptide complexes of general formula [Co(tren)(NH₂CHRCONHCHR′COOCH₃)][CH₃C₆H₄SO₃-p]₃ have been obtained by coupling a chelated ester complex with a free amino-acid ester. The rate of the reaction appears to be quite sensitive to steric hindrance. With chiral amino-acids, the product is a mixture of diastereomers; this result is consistent with a fast racemization coupled with a slight asymmetric induction. The half-life for proton exchange in the complex [Co(en)₂(GlyOCH₃)][CH₃C₆H₄SO₃-p]₃ under the conditions of peptide synthesis is 20 s. The hydrolysis of this ester in methanol (1 mol dm^–3 in H₂O) is nevertheless faster than recemization.