Transition Metal-Catalyzed Formation of Boron−Nitrogen Bonds: Catalytic Dehydrocoupling of Amine-Borane Adducts to Form Aminoboranes and Borazines

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Abstract
A mild, catalytic dehydrocoupling route to aminoboranes and borazine derivatives from either primary or secondary amine-borane adducts has been developed using late transition metal complexes as precatalysts. The adduct Me2NH·BH3 thermally eliminates hydrogen at 130 °C in the condensed phase to afford [Me2N−BH2]2 (1). Evidence for an intermolecular process, rather than an intramolecular reaction to form Me2NBH2 as an intermediate, was forthcoming from “hot tube” experiments where no appreciable dehydrocoupling of gaseous Me2NH·BH3 was detected in the range 150−450 °C. The dehydrocoupling of Me2NH·BH3 was found to be catalyzed by 0.5 mol % [Rh(1,5-cod)(μ-Cl)]2 in solution at 25 °C to give 1 quantitatively after ca. 8 h. The rate of dehydrocoupling was significantly enhanced if the temperature was raised or if the catalyst loading was increased. The catalytic activity of various other transition metal complexes (Ir, Ru, Pd) for the dehydrocoupling of Me2NH·BH3 was also demonstrated. This new catalytic method was extended to other secondary adducts RR‘NH·BH3 which afforded the dimeric species [(1,4-C4H8)N−BH2]2 (2) and [PhCH2(Me)N−BH2]2 (3) or the monomeric aminoborane iPr2NBH2 (4) under mild conditions. A new synthetic approach to the linear compounds R2NH−BH2−NR2−BH3 (5: R = Me; 6: R = 1,4-C4H8) was developed and subsequent catalytic dehydrocoupling of these species yielded the cyclics 1 and 2. The species 5 and 6 are postulated to be intermediates in the formation of 1 and 2 directly from the catalytic dehydrocoupling of the adducts R2NH·BH3. The catalytic dehydrocoupling of NH3·BH3, MeNH2·BH3, and PhNH2·BH3 at 45 °C to give the borazine derivatives [RN−BH]3 (10: R = H; 11: R = Me; 12: R = Ph) was demonstrated. TEM analysis of the contents of the reaction solution for the [Rh(1,5-cod)(μ-Cl)]2 catalyzed dehydrocoupling of Me2NH·BH3 together with Hg poisoning experiments suggested a heterogeneous catalytic process involving Rh(0) colloids.