Mechanism of metal-free hydrogen transfer between amine-boranes and aminoboranes

The kinetics of the metal-free hydrogen transfer from amine-borane Me(2)NH·BH(3) to aminoborane iPr(2)N═BH(2), yielding iPr(2)NH·BH(3) and cyclodiborazane [Me(2)N-BH(2)](2) via transient Me(2)N═BH(2), 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 (11)B{(1)H} 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 Me(2)N═BH(2) (ΔG(2)°((295)) = -28 ± 14 kJ·mol(-1)). Systematic deuterium labeling of the NH and BH moieties in Me(2)NH·BH(3) and iPr(2)N═BH(2) 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.