PHYSICAL ORGANIC-CHEMISTRY OF TRANSITION-METAL CARBENE COMPLEXES - 13- KINETICS OF PROTON-TRANSFER FROM HYL-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 m ethod, is about 1.3 units lower than the pK(a) of (2-oxacyclopentylide ne)pentacarbonylchromium(0) (1) in 50% MeCN-50% water at 25 degrees C: , The acidifying effect of the methyl group is attributed to its stabi lizing 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 s ame as for deprotonation of 1, despite the higher acidity of 5. This m eans that the intrinsic rate constant for proton transfer from 5 is si gnificantly lower than that from 1. This reduction in the intrinsic ra te constant is mainly the result of the imbalanced nature of the trans ition state which deprives the transition state from the stabilizing e ffect of the methyl group. Based on precedents, the most likely mechan ism 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 alt ernative nucleophilic substitution mechanism, a comparison of the reac tivity of 5 and 1 allows one to rule out this latter mechanism.