Kinetics of the borane reduction of pinacolone in THF catalyzed by two different oxazaboroles

Abstract

For the first time the kinetics of the borane reduction of a ketone catalyzed by oxazaboroles have been studied experimentally. Pinacolone (P) and BH₃–THF are the reagents used, 1-phenyl-3,3-diethylperhydropyrrolo[1,2-c][1,3,2]oxazaborole 1a and 1,3,3-triphenylperhydropyrrolo[1,2c][1,3,2]oxazaborole 1b the catalysts. The reaction of the ketone P with the oxazaborole–borane complex is the rate determining step of the catalytic cycle. The corresponding rate constant is about 40 times larger for 1b than for 1a. The presence of NaBH₄, which is the stabilizer in commercial BH₃–THF solutions, leads to a drastic acceleration of both the catalyzed and the direct borane reduction.