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Abstract: Over the past decade it has been shown that amine–boranes RR′R″N·BH3 (R, R′, R″ = H, alkyl, aryl) undergo efficient catalytic dehydrocoupling/dehydrogenation using transition metal or main group precatalysts to produce various classes of products including aminoboranes, borazanes, borazines, and, most recently, the first examples of soluble high molecular weight polyaminoboranes. Furthermore, related studies have also demonstrated that amine–boranes can act as hydrogen sources in metal-catalyzed or metal-free transfer hydrogenation reactions of unsaturated organic species, including imines and polar olefins. Our group has shown that metal-free hydrogen exchange reactions are also possible between amine–boranes and aminoboranes under mild conditions. This review provides an overview of our recent work in each of these areas. [Copyright &y& Elsevier]

The kinetics of the metal-free hydrogen transfer from amine-borane Me2NH·BH3 to aminoborane iPr2N=BH2, yielding iPr2NH·BH3 and cyclodiborazane [Me2N-BH2]2 via transient Me2N=BH2, have been investigated in detail, with further information derived from isotopic labeling and DFT computations. The approach of the system toward equilibrium was monitored in both directions by 11B{1H} NMR spectroscopy in a range of solvents and at variable temperatures in THF. Simulation of the resulting temporal-concentration data according to a simple two-stage hydrogen transfer/dimerization process yielded the rate constants and thermodynamic parameters attending both equilibria. At ambient temperature, the bimolecular hydrogen transfer is slightly endergonic in the forward direction (ΔG1°(295) = 10 ± 7 kJ·mol-1; ΔG1≠(295) = 91 ± 5 kJ·mol-1), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me2N=BH2 (ΔG2°(295) = -28 ± 14 kJ·mol-1). Systematic deuterium labeling of the NH and BH moieties in Me2NH·BH3 and iPr2N=BH2 allowed the kinetic isotope effects (KIEs) attending the hydrogen transfer to be determined. A small inverse KIE at boron (kH/kD = 0.9 ± 0.2) and a large normal KIE at nitrogen (kH/kD = 6.7 ± 0.9) are consistent with either a pre-equilibrium involving a B-to-B hydrogen transfer or a concerted but asynchronous hydrogen transfer via a cyclic six-membered transition state in which the B-to-B hydrogen transfer is highly advanced. DFT calculations are fully consistent with a concerted but asynchronous process. [ABSTRACT FROM AUTHOR]