Mechanism of Metal-Free Hydrogen Transfer between Amine–Boranes and Aminoboranes

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 (ΔG 1°(295) = 10 ± 7 kJ·mol–1; ΔG 1 ⧧ (295) = 91 ± 5 kJ·mol–1), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me2NBH2 (ΔG 2°(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 (k H/k D = 0.9 ± 0.2) and a large normal KIE at nitrogen (k H/k D = 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.