Rehydrogenation of Aminoboranes to Amine–Boranes Using H 2 O: Reaction Scope and Mechanism

Water has been successfully employed as a reagent with which to rehydrogenate aminoboranes (e.g., i Pr 2 N=BH 2 , 2,2,6,6‐Me 4 C 5 H 6 N=BH 2 , and also transient Me 2 N=BH 2 derived from 1 / 2 [Me 2 N‐BH 2 ] 2 ) to amine–boranes (e.g., i Pr 2 NH · BH 3 , 2,2,6,6‐Me 4 C 5 H 6 NH · BH 3 , Me 2 NH · BH 3 ) 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 H 2 O, whereas the third hydridic hydrogen (B–H) on the amine–borane was generated from the formation of a postulated hydride‐bridged intermediate H 2 B(μ‐H)(μ‐NR 2 )B(OH)H (R 2 = Me 2 , i Pr 2 , 2,2,6,6‐Me 4 C 5 H 6 ), which requires a second equivalent of the starting aminoborane, thus explaining the low yield. Formation of insoluble borates (B x O y H z ) provides a driving force for the reaction. Significantly, the yield can be increased by adding a sacrificial source of BH 3 (e.g., to ca. 53 % for BH 3 · THF) or by adding a separate source of H – (e.g., to ca. 95 % for LiBH 4 ) to complement the H + (from H 2 O) in a more atom‐efficient reaction.