Methanolysis esters complex mechanisms

Early Examples with Esters Having Complex Mechanisms for Methanolysis Promoted by Lanthanides and a Triazacyclododecane Zn" Complex... 

Analysis ofthe kinetic data shows that the barium salt of 7, as well as the analogous salts of its higher homologues, perform much less efficiently than 4-Ba. The Ba complex of 7 turns over with a very low efficiency, caused by the extreme slowness of the deacylation step. Only a minor fraction ofthe liberated pNPOH in the steady-state phase is due to the expected double displacement mechanism. A larger fraction is most likely ascribable to the metal ion not sequestered by 7, and thereby available in solution for electrophilic assistance to direct methanolysis of the ester reactant. 

A fairly efficient nucleophile catalyst with transacylase activity (10—Ba cat) was developed by us [9]. Previous investigations indicated that alkaline-earth metal ions complexed by crown ethers are efficient promoters of acyl transfer reactions from esters to anionic nucleophiles [10]. The barium complex 10—Ba acts as a turnover catalyst in the methanolysis of aryl acetates, (diisopropylethylamine buffer in MeCN-MeOH 9 1, (v/v), 25 °C). Experimental evidence points to the operation of a ping-pong mechanism as outlined in Scheme 26.1. 

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