Kinetic isotope effect study of reductions of benzophenone with complex metal hydrides

Abstract

Carbon-14 and deuterium kinetic isotope effects were determined for reductions of benzophenone with NaBH/sub 4/, LiBH/sub 4/, and LiAlH/sub 4/ at 25.0/sup 0/C. The observed /sup 12/k//sup 14/k at the carbonyl carbon and k/sub H4//k/sub D4/ of the reducing agents were 1.066 and 0.75, 1.043 and 1.089 and 1.024 and 1.10, respectively, for NaBH/sub 4/, LiBH/sub 4/ and LiAlH/sub 4/ reductions. Kinetic isotope effect calculations with various transition-state models suggested that the extent of hydride transfer from Al or B to the carbonyl carbon is increasing in the order LiAlH/sub 4/ (0.35) < LiBH/sub 4/ (0.55) < NaBH/sub 4/ (0.75). Nucleophilic solvent participation to the BH/sub 4/ moiety is weak if present for the SBH reduction.