Transition metal-catalyzed formation of boron-nitrogen bonds: Catalytic dehydrocoupling of amine-borane adducts to form aminoboranes and borazines

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 (Me2NHBH3)-B-. thermally eliminates hydrogen at 130 degreesC in the condensed phase to afford [Me2N-BH2](2) (1). Evidence for an intermolecular process, rather than an intramolecular reaction to form Me2N=BH2 as an intermediate, was forthcoming from "hot tube" experiments where no appreciable dehydrocoupling of gaseous Me2NH-BH3 was detected in the range 150-450 degreesC. The dehydrocoupling of Me2NH-BH3 was found to be catalyzed by 0.5 mol % [Rh(1,5-cod)(mu-Cl)](2) in solution at 25 degreesC 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'(NHBH3)-B-. which afforded the dimeric species [(1,4-C4H8)N-BH2](2) (2) and [PhCH2(Me)N-BH2](2) (3) or the monomeric aminoborane (Pr2N)-Pr-1=BH2 (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 (R2NHBH3)-B-.. The catalytic dehydrocoupling of (NH3BH3)-B-., MeNH2. BH3, and (PhNH2BH3)-B-. at 45 degreesC 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)(mu-Cl)](2) catalyzed dehydrocoupling of Me2NH-BH3 together with Hg poisoning experiments suggested a heterogeneous catalytic process involving Rh(0) colloids.