Ester fission catalysed by microgels possessing hydroxamic acid groups: structure-reactivity studies

Abstract

Second order kinetics of the reaction of polymer-attached hydroxamic acids with substituted phenyl acetates ( k pol) have been measured in aqueous solution at 25°C and shown to follow the following empirical rate law for degrees of ionisation (α) ranging from 0.1 to 1: k pol = a·α + b· e − c α . The slopes of the plots of k pol against α measures the reactivity of the polymer ( k react) at given values of α and these increase as α is increased. The reactivity against substituted phenyl acetates at given α values increases with increasing cross-linking content of the microgel support. The reactivities at α = 0.1 and 0.9 obey the Brønsted equations: logk react ( α = 0.1) = −0.55±0.065 pK ArOH a+3.26±0.44 (n = 5; r = 0.9799) logk react ( α = 0.9) = −0.37±0.06 pK ArOH a+3.13±0.43 (n = 5; r = 0.9594) The similarity between the Brønsted β values for the microgel-supported hydroxamate nucleophile at low degree of ionisation (α) and monomer hydroxamate ion is consistent with the microscopic medium of the transition-state's being water-like. At a high α value the less negative β value indicates a smaller charge change in the transition state; this result and the effect of cross-linking on reactivity supports the hypothesis that the substantial reactivity increases are due to hydroxamate ions buried in the microgel particle and electrophilic assistance of nucleophilic attack. De-cinnamoylation of O-cinnamoylated microgel-supported hydroxamic acid is only some ten-fold less reactive to hydroxide ion than is the model O-cinnamoyl- N-benzoylhydroxylamine. The polymer deacylation possesses a significant water term which means that at low pH it is faster than that of the model reaction; intramolecular catalysis by carboxylate ions is held responsible for this acceleration.