School of Chemistry, Cantock's Close, University of Bristol, Bristol BS8 1TS, United Kingdom.
J Am Chem Soc. 2012 Oct 10;134(40):16805-16. doi: 10.1021/ja307247g. Epub 2012 Sep 27.
The kinetics of the metal-free hydrogen transfer from amine-borane Me(2)NH·BH(3) to aminoborane iPr(2)N═BH(2), yielding iPr(2)NH·BH(3) and cyclodiborazane Me(2)N-BH(2) via transient Me(2)N═BH(2), 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 (11)B{(1)H} 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 (ΔG(1)°((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 Me(2)N═BH(2) (ΔG(2)°((295)) = -28 ± 14 kJ·mol(-1)). Systematic deuterium labeling of the NH and BH moieties in Me(2)NH·BH(3) and iPr(2)N═BH(2) allowed the kinetic isotope effects (KIEs) attending the hydrogen transfer to be determined. A small inverse KIE at boron (k(H)/k(D) = 0.9 ± 0.2) and a large normal KIE at nitrogen (k(H)/k(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.
无金属的氢从胺硼烷 Me(2)NH·BH(3)转移到氨基硼烷 iPr(2)N═BH(2)，生成 iPr(2)NH·BH(3)和环二硼氮烷 Me(2)N-BH(2)，通过瞬态 Me(2)N═BH(2)，已在详细研究中，进一步的信息来自同位素标记和 DFT 计算。在 THF 中，在不同溶剂和不同温度下，通过 (11)B{(1)H}NMR 光谱监测体系向平衡的接近程度。根据简单的两步氢转移/二聚化过程对所得时间-浓度数据进行模拟，得到了两个平衡的速率常数和热力学参数。在环境温度下，双分子氢转移在正向略微是吸热的（ΔG(1)°((295)) = 10 ± 7 kJ·mol(-1)；ΔG(1)(‡)((295)) = 91 ± 5 kJ·mol(-1)），随后氨基硼烷 Me(2)N═BH(2)的放能二聚化推动了整体平衡向正向进行（ΔG(2)°((295)) = -28 ± 14 kJ·mol(-1)）。Me(2)NH·BH(3)和 iPr(2)N═BH(2)中 NH 和 BH 部分的系统氘标记允许确定氢转移的动力学同位素效应 (KIE)。硼上的小逆 KIE（k(H)/k(D) = 0.9 ± 0.2）和氮上的大正常 KIE（k(H)/k(D) = 6.7 ± 0.9）与涉及 B 到 B 氢转移的预平衡或通过环状六元过渡态的协同但异步氢转移一致，其中 B 到 B 氢转移高度先进。DFT 计算完全符合协同但异步过程。
