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 (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.