Details
| Original language | English |
|---|---|
| Pages (from-to) | 16805-16816 |
| Number of pages | 12 |
| Journal | Journal of the American Chemical Society |
| Volume | 134 |
| Issue number | 40 |
| Publication status | Published - 10 Oct 2012 |
| Externally published | Yes |
Abstract
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.
ASJC Scopus subject areas
- Chemical Engineering(all)
- Catalysis
- Chemistry(all)
- General Chemistry
- Biochemistry, Genetics and Molecular Biology(all)
- Biochemistry
- Chemical Engineering(all)
- Colloid and Surface Chemistry
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In: Journal of the American Chemical Society, Vol. 134, No. 40, 10.10.2012, p. 16805-16816.
Research output: Contribution to journal › Article › Research › peer review
}
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 -