Alkoxide-Catalyzed Reduction of Ketones with Pinacolborane

The reduction of ketones with pinacolborane (4,4,5,5-tetramethyl-1,3,2-dioxaborolane) is catalyzed by 5 mol % NaOt-Bu at ambient temperature. The reaction is high yielding and general, providing complete conversion of aryl and dialkyl ketones. Although spectroscopic studies of the active hydride source in benzene-d(6) were complicated due to poor solubility, the data are consistent with the active hydride source being the trialkoxyborohydride, which is believed to be present in low concentration under the reaction conditions. Performing analogous studies in tetrahydrofuran resulted in a complex equilibrium between several different boron-containing species in which the trialkoxyborohydride compound was the major species.