Aminolyses of 4-nitrophenyl phenyl carbonate and thionocarbonate: Effect of modification of electrophilic center from C=O to C=S on reactivity and mechanism

Abstract

A kinetic study is reported for nucleophilic substitution reactions of 4-nitrophenyl phenyl carbonate (5) and 4-nitrophenyl phenyl thionocarbonate (6) with a series of primary amines. The thiono compound 6 is less reactive than its oxygen analogue 5 toward strongly basic amines but is more reactive toward weakly basic CF 3CH 2NH 2. The Brønsted-type plots obtained from the aminolyses of 5 and 6 are curved downwardly. The reactions are proposed to proceed through a stepwise mechanism with a change in the RDS on the basis of the curved Brønsted-type plots. The microscopic rate constants (k 1 and k 2/k -1 ratio) associated with the current aminolyses are consistent with the proposed reaction mechanism. The replacement of the C=O bond in 5 by a polarizable C=S group results in a decrease in the k 1 value but an increase in the k 2/k -1 ratio. Besides, such a modification of the electrophilic center causes a decrease in pK a c, defined as the pK a at the curvature center of curved Brønsted-type plots, but does not alter the reaction mechanism. The larger k 2/k -1 ratio for the reactions of 6 compared to those of 5 is proposed to be responsible for the decreased pK a c value. © 2006 American Chemical Society.