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

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 degrees C and shown to follow the following empirical rate law for degrees of ionisation (alpha) ranging from 0.1 to 1 : k(pol) = a .alpha + b . e(-c/alpha). The slopes of the plots of k(pol) against alpha measures the reactivity of the polymer (k(react)) at given values of alpha and these increase as a is increased. The reactivity against substituted phenyl acetates at given alpha values increases with increasing cross-linking content of the microgel support. The reactivities at alpha = 0.1 and 0.9 obey the Bronsted equations: logk(react) (alpha = 0.1) = -0.55 +/- 0.065pK(a)(ArOH) + 3.26 +/- 0.44 (n = 5; r = 0.9799) logk(react) (alpha = 0.9) = -0.37 +/- 0.06pK(a)(ArOH) + 3.13 + 0.43 (n = 5; r = 0.9594) The similarity between the Bronsted beta values for the microgel-supported hydroxamate nucleophile at low degree of ionisation (alpha) and monomer hydroxamate ion is consistent with the microscopic medium of the transition-state's being water-like. At a high alpha value the less negative beta value indicates a smaller charge change in the transition state; this result and the effect of crosslinking 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 microgoel-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.