title: B-Methylated Amine-Boranes: Substituent Redistribution, Catalytic Dehydrogenation, and Facile Metal-Free Hydrogen Transfer Reactions.
abstract: 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·BH3-xMex (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 (11)B 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.
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Naomi E
authors.initials: NE
authors.lastname: Stubbs
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: André
authors.initials: A
authors.lastname: Schäfer
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Alasdair P M
authors.initials: AP
authors.lastname: Robertson
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Erin M
authors.initials: EM
authors.lastname: Leitao
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Titel
authors.initials: T
authors.lastname: Jurca
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Hazel A
authors.initials: HA
authors.lastname: Sparkes
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Christopher H
authors.initials: CH
authors.lastname: Woodall
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Mairi F
authors.initials: MF
authors.lastname: Haddow
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Ian
authors.initials: I
authors.lastname: Manners