Physical organic chemistry of transition metal carbene complexes. 13. Kinetics of proton transfer from (5-methyl-2-oxacyclopentylidene)pentacarbonylchromium(O) and hydrolysis of its conjugate anion in aqueous acetonitrile

The pK(a) = 13.41 of the title compound (5), determined by a kinetic method, is about 1.3 units lower than the pK(a) of (2-oxacyclopentylidene)pentacarbonylchromium(0) (1) in 50% MeCN-50% water at 25 degrees C:, The acidifying effect of the methyl group is attributed to its stabilizing effect on the C=C double-bond resonance structure of the anion (5(-)). The rate constant for deprotonation of 5 by OH- is about the same as for deprotonation of 1, despite the higher acidity of 5. This means that the intrinsic rate constant for proton transfer from 5 is significantly lower than that from 1. This reduction in the intrinsic rate constant is mainly the result of the imbalanced nature of the transition state which deprives the transition state from the stabilizing effect of the methyl group. Based on precedents, the most likely mechanism for the hydrolysis of 5 involves rate-limiting protonation of 5(-) concerted with metal-carbon band cleavage. Even though the relatively small kinetic solvent isotope effect cannot rigorously exclude an alternative nucleophilic substitution mechanism, a comparison of the reactivity of 5 and 1 allows one to rule out this latter mechanism.