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 Brønsted plots of slopes β_nuc1= 0.52 ± 0.06 and β_nuc2= 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 β_nuc2 is attributed to the extensive of proton transfer in the late transition state while the large value of β_nuc1 is ascribed to the proton transfer in the early transition state of the rate-determining step. However, these low and high values of β_nuc2 and β_nuc1 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 Brønsted 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^–.