Alkaline-earth metal complexes of thiol-pendant crown ethers as turnover catalysts of ester cleavage

Abstract

Methanolysis of pNPOAc under moderately alkaline conditions is catalysed by the alkaline-earth metal ion (Ca, Sr, Ba) complexes of 2-(mercaptomethyl)-18-crown-6 according to biphasic kinetics in which an initial burst of pNPOH release is followed by a zeroth-order phase. Two competing catalytic mechanisms contribute to the overall process, (i) a double displacement mechanism in which the complexed metal ion enables the SH group of the catalyst to undergo an acylation–deacylation cycle, and (ii) a bypass mechanism in which a catalytically active complex-bound methoxide ion is involved and no covalently bound intermediate is formed. Calcium ion is much more efficient than its larger analogues both in the acylation–deacylation cycle of the double displacement mechanism, and in the direct delivery of methoxide ion to the ester substrate.