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

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• 作者：Naomi E. Stubbs ; Andr茅 Sch盲fer ; Alasdair P. M. Robertson ; Erin M. Leitao ; Titel Jurca ; Hazel A. Sparkes ; Christopher H. Woodall ; Mairi F. Haddow ; Ian Manners
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：November 16, 2015
• 年：2015
• 卷：54
• 期：22
• 页码：10878-10889
• 全文大小：627K
• ISSN：1520-510X

文摘

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鈥睳H路BH₂Me (1a: R = R鈥?= H; 1b: R = Me, R鈥?= H; 1c: R = R鈥?= Me; 1d: R = R鈥?= iPr), were heated to 70 掳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鈥睳H路BH_3鈥?i>xMe_x (x = 0鈥?). These reactions were postulated to arise via amine-borane dissociation followed by the reversible formation of diborane intermediates and adduct reformation. Dehydrocoupling of 1a鈥?b>1d with Rh(I), Ir(III), and Ni(0) precatalysts in THF at 20 掳C resulted in an array of products, including aminoborane RR鈥睳鈺怋HMe, cyclic diborazane [RR鈥睳鈥揃HMe]₂, and borazine [RN鈥揃Me]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鈺怋H₂, to yield iPr₂NH路BH₃ (together with dehydrogenation products derived from 1a) was complete within only 10 min at 20 掳C in THF, substantially faster than for the N-substituted analogue MeNH₂路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路BH₃ and iPr₂N鈺怋H₂. 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.