Intramolecular general base catalysis and the rate-determining step in the nucleophilic cleavage of ionized phenyl salicylate with primary and secondary amines

Abstract

The nucleophilic second-order rate constants for the reactions of ionized phenyl salicylate (PS–) with primary and secondary amines have revealed Bronsted plots of slopes βnuc1= 0.52 ± 0.06 and βnuc2= 0.27 ± 0.05, respectively. The suggested stepwise reaction mechanism involves the intramolecular proton transfer from cationic nitrogen to the anionic phenolic oxygen to form the monoanionic tetrahedral addition intermediate as the rate-determining step. The low value of βnuc2 is attributed to the extensive of proton transfer in the late transition state while the large value of βnuc1 is ascribed to the proton transfer in the early transition state of the rate-determining step. However, these low and high values of βnuc2 and βnuc1 respectively, are also compatible with the occurrence of respective early and late transition states in the rate-determining step involving concerted intramolecular general base-catalysed nucleophilic attack at the carbonyl carbon of PS–. Significantly large positive deviations from Bronsted plots have been observed for the reactivities of α-nucleophiles toward PS–. Monoprotonated ethane-1,2-diamine is ca. 20-fold more reactive than would be expected from its basicity, and this is attributed to the occurrence of the intramolecular general acid catalysis. The reactions of PS– with hydroxylamine and N-methylhydroxylamine involve ca.70% aminolysis and ⩽30% transesterification while those with tris-(2-hydroxyethyl)amine involve 100% transesterification. The extremely low reactivity of ammonia compared with that of primary amines of similar basicity indicates that it does not belong to a series of homologous primary amines in its nucleophilic reactivity toward PS–.