Comparative study of the reactivities of substituted 3-(benzoyl)benzyl carbanions in water and in DMSO.

Benzyl-substituted carbanions produced by photodecarboxylation of ketoprofen derivatives have been examined in basic aqueous and DMSO solutions. Product studies, combined with kinetic measurements from laser flash photolysis, have allowed the determination of absolute rate constants for protonation and intra-SN2 reactions leading to five- and six-membered ring cyclizations; the former are significantly faster. Many of the well-known trends in carbanion reactivity are placed on an absolute rate basis; thus, intra-SN2 are favored in polar nonprotic solvents, and the effect is larger for the more hindered carbanion centers. Protonation by water is slightly dependent on the nature of the carbanion center and is ∼400 times faster in nonhydroxylic solvents, compared with bulk water. As expected, the reactivity for halide leaving groups follows the usual order of decreasing bond strengths, i.e., I- > Br- > Cl-.