Nucleophilic Catalysts with Transacylase Activity

Host structures that, like many hydrolytic enzymes [16], contain an OH/SH group that acts as acyl-receiving/acyl-releasing unit have been reported by several authors [18]. Such host structures are acylated by suitably functionalized esters at enhanced rates thanks to the presence of tailored binding sites proximal to the nucleophilic group. However, since no provision was made to activate the cleavage ofthe acylated host, many of the investigated systems showed little or no turnover capability. 

3 Nucleophilic Catalysts with Transacylase Activity I 119 ArOAc ArO  

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A first example of nucleophilic catalyst with transacylase activity is given by an equimolar mixture of a Ba + salt (either bromide or perchlorate) and p-tert-butylcalix [4]arene-crown-5 (1), which catalyzes the methanolysis of aryl acetates in MeCN— MeOH (1 1, v/v) under slightly basic conditions (3 1 diisopropylethylamine-per-chlorate salt buffer) at 25°C [19,20]. 

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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