Rehydrogenation of Aminoboranes to Amine-Boranes Using H2O: Reaction Scope and Mechanism

Water has been successfully employed as a reagent with which to rehydrogenate aminoboranes (e.g., iPr(2)N=BH2, 2,2,6,6-Me4C5H6N=BH2, and also transient Me2N=BH2 derived from (1)/(2)[Me2N-BH2](2)) to amine-boranes (e.g., iPr(2)NHBH(3), 2,2,6,6-Me4C5H6NHBH3, Me2NHBH3) in approximately 30% yield. The conversion to amine-boranes from the corresponding aminoboranes using this method represents an example of a metal-free, single-step route for the hydrogenation of the B=N bond. Deuterium labeling studies indicated that the protic hydrogen (N-H) on the rehydrogenated amine-borane was derived from H2O, whereas the third hydridic hydrogen (B-H) on the amine-borane was generated from the formation of a postulated hydride-bridged intermediate H2B(-H)(-NR2)B(OH)H (R-2 = Me-2, iPr(2), 2,2,6,6-Me4C5H6), which requires a second equivalent of the starting aminoborane, thus explaining the low yield. Formation of insoluble borates (BxOyHz) provides a driving force for the reaction. Significantly, the yield can be increased by adding a sacrificial source of BH3 (e.g., to ca. 53% for BH3THF) or by adding a separate source of H- (e.g., to ca. 95% for LiBH4) to complement the H+ (from H2O) in a more atom-efficient reaction.