Amine–Borane Dehydropolymerization Using Rh-Based Precatalysts: Resting State, Chain Control, and Efficient Polymer Synthesis

A detailed study of H3B·NMeH2 dehydropolymerization using the cationic precatalyst [Rh­(DPEphos)­(H2BNMe3(CH2)2tBu)]­[BArF4] identifies the resting state as dimeric [Rh­(DPEphos)­H2]2 and boronium [H2B­(NMeH2)2]+ as the chain-control agent. [Rh­(DPEphos)­H2]2 can be generated in situ from Rh­(DPEphos)­(benzyl) and catalyzes polyaminoborane formation (H2BNMeH)n [Mn = 15 000 g mol–1]. Closely related Rh­(Xantphos)­(benzyl) operates at 0.1 mol % to give a higher molecular weight polymer [Mn = 85 000 g mol–1] on the gram scale with low residual [Rh], 81 ppm. This insight offers a mechanistic template for dehydropolymerization.