Dehydrocoupling of Phosphine‐Borane Adducts Under Ambient Conditions Using Aminoboranes as Hydrogen Acceptors

We report on the reactivity of aminoboranes (R₂N=BH₂; R = iPr, Et, Me) with phosphine-borane adducts (PhR'PH ⋅ BH₃; R' = H, Ph): Sufficiently sterically unencumbered aminoboranes can accept hydrogen from phosphine-borane adducts. The hydrogen transfer results in the formation of amine-borane adducts (R₂NH ⋅ BH₃) and transient phosphinoboranes (PhR'P-BH₂) in situ. These phosphinoboranes undergo subsequent reactivity to yield either polyphosphinoborane, [PhPH-BH₂]_n, or the linear dimer, Ph₂PH ⋅ BH₂-Ph₂P ⋅ BH₃. Unlike metal catalyzed phosphine-borane dehydrocoupling, which occurs at elevated temperatures (2 M, toluene, ≥100 °C, ≥24 h), these dehydrocoupling reactions occur under ambient conditions (2 M, Et₂O or C₆D₆, 20 °C, ≤24 h). We performed a computational mechanistic study in which we identified that this transformation potentially occurs via a P-to-N and B-to-B hydrogen transfer through a 6-membered transition state.