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

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.