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

The kinetics of the metal-free hydrogen transfer from amine-borane Me2NH center dot BH3 to aminoborane iPr(2)N=BH2, yielding iPr(2)NH center dot BH3 and cyclodiborazane [Me2N center dot 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 B-11{H-1} 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 (Delta G(1)((295))(degrees) = 10 +/- 7 kJ.mol(-1); Delta G(1)((295))(double dagger) = 91 +/- 5 kJ.mol(-1)), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me2N=BH2 (Delta G(2)((295))(degrees) = -28 +/- 14 kJ.mol(-1)). Systematic deuterium labeling of the NH and BH moieties in Me2NH center dot BH3 and iPr(2)N=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.