Kinetic investigation of the reduction of pinacolone by borane catalyzed by oxazaborolidines in THF. Hydride shift as rate determining step

The kinetics of the reduction of the ketone pinacolone (P) by borane (B) catalyzed by the oxazaborolidines (OAB) (S)-1-methyl-3,3-diphenylperhydropyrrolo[1,2-c][1,3,2]oxazaborolidine and (S)-2,4-dimethyl-3,3-diphenyl-1,3,2-oxazaborolidine have been studied in THF. The findings are discussed in comparison with the kinetic results obtained previously for the same reaction using two different OABs as catalysts (C). The reaction order in the ketone P changes in the series of OABs from first-order to zero-order. This surprising behaviour excludes the addition of P to the binary complex C-B forming the ternary complex P-C-B as the rate determining step. However, the kinetic data are consistently explained if the intramolecular hydride shift in P-C-B with subsequent rearrangement leading to the complex of monoalkoxyborane and catalyst is assumed to be rate determining. The rate equations of the catalytic reactions of the four OABs, the corresponding ee values, and apparent activation energies are given.