Hydrolysis of phosphinic esters: general-base catalysis by imidazole

Abstract

Imidazole catalyses the hydrolysis of aryl diphenylphosphinates (I). At 25° the Hammett-σ-ρ value for the bimolecular rate constant (k_im) is 2·88 and at 55° is 2·64; hydroxide-ion catalysis (k_OH) at 25° has a smaller ρ value 1·55. Ratio of phosphate dianion catalysis (k_HPO4=) to k_im is ca. 5 for 4-nitrophenyl diphenylphosphinate; the deuterium oxide solvent isotope effect on k_im is 2·15 and no departure from 1:1 stoicheiometry was observed in the imidazole-catalysed hydrolysis of the 4-nitrophenyl ester. These results indicate that imidazole acts as a general-base catalyst. Nucleophilic catalysis is observed with imidazole-catalysed hydrolysis of 4-nitrophenyl dimethyl-phosphate, diphenylphosphate and di-4-nitrophenylphosphate (k_im^H/k_im^D= 1·03, 1·02, 1·53 respectively). Steric crowding of the transition state renders nucleophilic catalysis inefficient for the diphenylphosphinate esters.

A diagnostic tool not previously employed to distinguish general-base from nucleophilic catalysis by imidazole is discussed. The good Hammett-σ plot for imidazole catalysis in diphenylphosphinate ester hydrolysis argues for a general-base mechanism and for a stepwise rather than a concerted displacement.