An Altered Mechanism of Hydrolysisfor a Metal-Complexed Phosphate Diester
Journal of the American Chemical Society
Isotope effects in the nucleophile and in the leaving group were measured to gain information about the mechanism and transition state of the hydrolysis of methyl p-nitrophenyl phosphate complexed to a dinuclear cobalt complex. The complexed diester undergoes hydrolysis about 1011 times faster than the corresponding uncomplexed diester. The kinetic isotope effects indicate that this rate acceleration is accompanied by a change in mechanism. A large inverse 18O isotope effect in the bridging hydroxide nucleophile (0.937 ± 0.002) suggests that nucleophilic attack occurs before the rate-determining step. Large isotope effects in the nitrophenyl leaving group (18Olg = 1.029 ± 0.002, 15N = 1.0026 ± 0.0002) indicate significant fission of the P−O ester bond in the transition state of the rate-determining step. The data indicate that in contrast to uncomplexed diesters, which undergo hydrolysis by a concerted mechanism, the reaction of the complexed diester likely proceeds via an addition−elimination mechanism. The rate-limiting step is expulsion of the p-nitrophenyl leaving group from the intermediate, which proceeds by a late transition state with extensive bond fission to the leaving group. This represents a substantial change in mechanism from the hydrolysis of uncomplexed aryl phosphate diesters.
T. Humphry, M. Forconi, N. H. Williams and A. C. Hengge. “An Altered Mechanism of Hydrolysis for a Metal-Complexed Phosphate Diester.” J. Am. Chem. Soc., 2002, 124, 14860-14861.