TY - JOUR

T1 - Mechanism of metal-free hydrogen transfer between amine-boranes and aminoboranes

AU - Leitao, Erin M.

AU - Stubbs, Naomi E.

AU - Robertson, Alasdair P.M.

AU - Helten, Holger

AU - Cox, Robert J.

AU - Lloyd-Jones, Guy C.

AU - Manners, Ian

PY - 2012/10/10

Y1 - 2012/10/10

N2 - The kinetics of the metal-free hydrogen transfer from amine-borane Me 2NH•BH3 to aminoborane iPr2N=BH 2, yielding iPr2NH•BH3 and cyclodiborazane [Me2N-BH2]2 via transient Me 2N=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; ΔG 1(295) = 91 ± 5 kJ•mol-1), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me2N=BH2 (ΔG 2°(295) = -28 ± 14 kJ•mol-1). Systematic deuterium labeling of the NH and BH moieties in Me 2NH•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.

AB - The kinetics of the metal-free hydrogen transfer from amine-borane Me 2NH•BH3 to aminoborane iPr2N=BH 2, yielding iPr2NH•BH3 and cyclodiborazane [Me2N-BH2]2 via transient Me 2N=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; ΔG 1(295) = 91 ± 5 kJ•mol-1), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me2N=BH2 (ΔG 2°(295) = -28 ± 14 kJ•mol-1). Systematic deuterium labeling of the NH and BH moieties in Me 2NH•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.

UR - http://www.scopus.com/inward/record.url?scp=84961982786&partnerID=8YFLogxK

U2 - 10.1021/ja307247g

DO - 10.1021/ja307247g

M3 - Article

AN - SCOPUS:84961982786

VL - 134

SP - 16805

EP - 16816

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 40

ER -