B-Methylated Amine-Boranes: Substituent Redistribution, Catalytic Dehydrogenation, and Facile Metal-Free Hydrogen Transfer Reactions.
B-Methylated Amine-Boranes: Substituent Redistribution, Catalytic Dehydrogenation, and Facile Metal-Free Hydrogen Transfer Reactions.
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DOI：
10.1021/acs.inorgchem.5b01946
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发表时间：
2015-11
影响因子：
4.6
通讯作者：
Naomi E. Stubbs;A. Schäfer;A. Robertson;E. Leitao;T. Jurca;H. Sparkes;C. H. Woodall;M. Haddow
中科院分区：
文献类型：
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作者：
Naomi E. Stubbs;A. Schäfer;A. Robertson;E. Leitao;T. Jurca;H. Sparkes;C. H. Woodall;M. Haddow
虽然胺硼烷的脱氢化学在氮取代引起了相当大的关注，少得多的是已知的关于其B-取代类似物的反应性。当B-甲基化胺-硼烷加合物时，（1a：R = R' = H; 1b：R = Me，R' = H; 1c：R = R' = Me; 1d：R = R' = iPr）在溶液中加热至70 °C，（THF或甲苯），观察到再分配反应，涉及硼上的甲基和氢取代基的明显混乱，以提供物种RR'NH·BH 3-xMex（x = 0-3）。这些反应被假定为通过胺硼烷解离，然后可逆地形成乙硼烷中间体和加合物重整而产生。在20 °C下，在THF中，1a-1d与Rh（I）、Ir（III）和Ni（0）预催化剂的脱氢偶联产生一系列产物，包括氨基硼烷RR 'N-BHMe、环状二硼氮烷[RR' N-BHMe]2和环硼氮烷[RN-BMe]3，基于通过原位（11）B NMR光谱的分析，其中峰归属进一步得到密度泛函理论（DFT）计算的支持。值得注意的是，1a和单体氨基硼烷iPr 2N·BH 2之间非常快速的无金属氢转移，生成iPr 2NH·BH 3（以及1a衍生的脱氢产物），在20 °C的THF中仅在10分钟内完成，比N-取代的类似物MeNH 2·BH 3快得多。DFT计算表明，氢转移是通过一个六元环过渡态的协同机制进行的，类似于之前报道的N-二甲基物种Me 2NH·BH 3和iPr 2N·BH 2的反应.然而，由于在硼上而不是在氮上存在供电子甲基取代基，该过程在化学上更有利，并且活化能势垒降低。
Although the dehydrogenation chemistry of amine-boranes substituted at nitrogen has attracted considerable attention, much less is known about the reactivity of their B-substituted analogues. When the B-methylated amine-borane adducts, RR'NH·BH2Me (1a: R = R' = H; 1b: R = Me, R' = H; 1c: R = R' = Me; 1d: R = R' = iPr), were heated to 70 °C in solution (THF or toluene), redistribution reactions were observed involving the apparent scrambling of the methyl and hydrogen substituents on boron to afford a mixture of the species RR'NH·BH3-xMex (x = 0-3). These reactions were postulated to arise via amine-borane dissociation followed by the reversible formation of diborane intermediates and adduct reformation. Dehydrocoupling of 1a-1d with Rh(I), Ir(III), and Ni(0) precatalysts in THF at 20 °C resulted in an array of products, including aminoborane RR'N═BHMe, cyclic diborazane [RR'N-BHMe]2, and borazine [RN-BMe]3 based on analysis by in situ (11)B NMR spectroscopy, with peak assignments further supported by density functional theory (DFT) calculations. Significantly, very rapid, metal-free hydrogen transfer between 1a and the monomeric aminoborane, iPr2N═BH2, to yield iPr2NH·BH3 (together with dehydrogenation products derived from 1a) was complete within only 10 min at 20 °C in THF, substantially faster than for the N-substituted analogue MeNH2·BH3. DFT calculations revealed that the hydrogen transfer proceeded via a concerted mechanism through a cyclic six-membered transition state analogous to that previously reported for the reaction of the N-dimethyl species Me2NH·BH3 and iPr2N═BH2. However, as a result of the presence of an electron donating methyl substituent on boron rather than on nitrogen, the process was more thermodynamically favorable and the activation energy barrier was reduced.