Dihydrogen Bond Intermediated Alcoholysis of Dimethylamine–Borane in Nonaqueous Media

Dimethylamine–borane (DMAB)
acid/base properties, its dihydrogen-bonded
(DHB) complexes and proton transfer reaction in nonaqueous media were
investigated both experimentally (IR, UV/vis, NMR, and X-ray) and
theoretically (DFT, NBO, QTAIM, and NCI). The effects of DMAB concentration,
solvents polarity and temperature on the degree of DMAB self-association
are shown and the enthalpy of association is determined experimentally
for the first time (−ΔH°assoc = 1.5–2.3 kcal/mol). The first case of “improper”
(blue-shifting) NH···F hydrogen bonds was observed
in fluorobenzene and perfluorobenzene solutions. It was shown that
hydrogen-bonded complexes are the intermediates of proton transfer
from alcohols and phenols to DMAB. The reaction mechanism was examined
computationally taking into account the coordinating properties of
the reaction media. The values of the rate constants of proton transfer
from HFIP to DMAB in acetone were determined experimentally [(7.9 ±
0.1) × 10–4 to (1.6 ± 0.1) × 10–3 mol–1·s–1] at 270–310 K. Computed activation barrier of this reaction
ΔG‡theor298 K(acetone) = 23.8 kcal/mol is in good agreement with the experimental
value of the activation free energy ΔG‡exp270 K = 21.1 kcal/mol.