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 via a 6-membered transition state.