B‑Methylated Amine-Boranes: Substituent Redistribution, Catalytic Dehydrogenation, and Facile Metal-Free Hydrogen Transfer Reactions

Although the dehydrogenation chemistry of amine-boranes substituted at nitrogen has attracted considerable attention, much less is known about the reactivity of their B-substituted analogues. When the B-methylated amine-borane adducts, RR′NH·BH₂Me (1a: R = R′ = H; 1b: R = Me, R′ = H; 1c: R = R′ = Me; 1d: R = R′ = iPr), were heated to 70 °C in solution (THF or toluene), redistribution reactions were observed involving the apparent scrambling of the methyl and hydrogen substituents on boron to afford a mixture of the species RR′NH·BH_3–xMe_x (x = 0–3). These reactions were postulated to arise via amine-borane dissociation followed by the reversible formation of diborane intermediates and adduct reformation. Dehydrocoupling of 1a–1d with Rh­(I), Ir­(III), and Ni(0) precatalysts in THF at 20 °C resulted in an array of products, including aminoborane RR′NBHMe, cyclic diborazane [RR′N–BHMe]₂, and borazine [RN–BMe]₃ based on analysis by in situ ¹¹B NMR spectroscopy, with peak assignments further supported by density functional theory (DFT) calculations. Significantly, very rapid, metal-free hydrogen transfer between 1a and the monomeric aminoborane, iPr₂NBH₂, to yield iPr₂NH·BH₃ (together with dehydrogenation products derived from 1a) was complete within only 10 min at 20 °C in THF, substantially faster than for the N-substituted analogue MeNH₂·BH₃. DFT calculations revealed that the hydrogen transfer proceeded via a concerted mechanism through a cyclic six-membered transition state analogous to that previously reported for the reaction of the N-dimethyl species Me₂NH·BH₃ and iPr₂NBH₂. However, as a result of the presence of an electron donating methyl substituent on boron rather than on nitrogen, the process was more thermodynamically favorable and the activation energy barrier was reduced.