The Ba(II) complex of a crown ether bearing a sulphydryl side-arm as turnover catalyst of ester cleavage

When activated by barium(II), 18-mercaptometyl-1,4,7,10,13,16-hexaoxacyclononadecane acts as turnover catalyst in the methanolysis of p-nitrophenyl acetate (pNPOAc) under slightly basic conditions in CH3CN-CH3OH 9:1 (v/v). The function of the complexed barium ion is to enhance the nucleophilicity of the pendant SH group towards the acetyl group of the ester. and to assist as a built-in electrophile the subsequent transfer of the acetyl group from the acetylated intermediate to the solvent. Besides the above double displacement mechanism of ester cleavage, a direct metal-ion assisted methanolysis involving reaction of pNPOAc with the ternary complex (5 BaOMe)(+) has been detected. (C) 1996 Elsevier Science Ltd