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 <i>i</i>Pr₂NBH₂, yielding <i>i</i>Pr₂NH·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 (Δ<i>G</i>₁°₍₂₉₅₎ = 10 ± 7 kJ·mol^–1; Δ<i>G</i>₁^⧧₍₂₉₅₎ = 91 ± 5 kJ·mol^–1), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me₂NBH₂ (Δ<i>G</i>₂°₍₂₉₅₎ = −28 ± 14 kJ·mol^–1). Systematic deuterium labeling of the NH and BH moieties in Me₂NH·BH₃ and <i>i</i>Pr₂NBH₂ allowed the kinetic isotope effects (KIEs) attending the hydrogen transfer to be determined. A small inverse KIE at boron (<i>k</i>_H/<i>k</i>_D = 0.9 ± 0.2) and a large normal KIE at nitrogen (<i>k</i>_H/<i>k</i>_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