Hydrolysis of Mixed O,S-Ortho Esters of p-Methoxybenzoic Acid. Reactivity and Partitioning of the Tetrahedral Intermediates

Article abstract of DOI:10.1021/jo00389a005

Hydrolyses of four ortho esters of p-methoxybenzoic acid, ArC(OMe)3-n(SMe)n (n = 0-3), have been examined at 30 deg C in aqueous solution.All the hydrolyses are catalyzed by acid, rate decreasing with increasing number of methylthio groups (n).Uncatalyzed reaction was observed for the thio ortho esters (n = 1-3).Products from the mixed O,S-ortho esters (n = 1 and 2) are mixtures of the ester ArCOOMe and the thio ester ArCOSMe.Fraction of the latter increases with acidity of the medium, following a sigmoid curve (pK_3b = 0.8) from 0 to 0.68 in the case of n = 1 and following a biphasic sigmoid from 0 to 0.86 (pK_1c = 4.73) and then to 0.96 (pK_3b = 0.8) in the case of n = 2.These results are interpreted in terms of different partitioning in acid-catalyzed and spontaneous breakdowns of the tetrahedral intermediate as well as O,S-ortho esters.