Facile heterolytic H2 activation by amines and B(C6F5)3.
Facile heterolytic H2 activation by amines and B(C6F5)3.
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DOI：
10.1002/anie.200800935
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发表时间：
2008-07
影响因子：
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通讯作者：
V. Sumerin;Felix Schulz;M. Nieger;M. Leskelä;T. Repo;B. Rieger
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文献类型：
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作者：
V. Sumerin;Felix Schulz;M. Nieger;M. Leskelä;T. Repo;B. Rieger
在工业上重要的反应中，例如加氢裂化和氢化，H2气体用作还原剂和/或氢原子源。[1]即使这些反应效率的微小改进也会转化为大量的货币节省。这些转变的关键步骤是H2在过渡金属上的活化。能量可及d轨道的节点特性允许过渡金属中心直接与H2在具有低活化势垒的协同反应中反应。[2]然而，不仅过渡金属络合物昂贵，而且由于毒性问题，在药物中间体的生产中通常需要从反应产物中完全除去金属杂质。[3]虽然无数的合成配合物和酶与过渡金属在其反应性的核心是众所周知的，有显着较少的例子，H3 OH键活化促进单独的非金属。[4]H2与含有主族元素的化合物在低温基质中的几种反应已经被报道;[5]然而，在温和条件下，H2在非金属上的活化仅被Power和同事在二锗烷、二锗烷和初级锗烷的产物混合物中观察到，[6]直到最近，当Stephan和同事报道了H2从硼酸盐中的热释放。所得产物（C6 H2 Me 3）2 P（C6 F4）B（C6 F5）2在258 ℃下经历H2加成以改革原始盐。[7]以类似的方式，空间要求高的膦和硼烷的混合物（“受抑的刘易斯对”）[8]也可以异相裂解H2以形成硼酸鳞[R3 PH][HBr '3]。[9]最近，Bertrand及其同事报道了所选择的有机卡宾的亲核性足以裂解H2和NH3。[10]本文扩展了“阻挫刘易斯对”的家族，证明了不仅大体积的膦和硼烷或有机卡宾可以裂解H2，而且廉价、稳定的胺与B（C6 F5）3的组合也可以裂解H2。和19 FNMR光谱。二异丙基乙胺和二异丙基胺与B（C6 F5）3的反应得到了盐1a或1B和两性环戊二烯基2a或2B的混合物（反应路线1）;[11]然而，没有观察到2，2，6，6-四甲基哌啶（一种没有α氢原子的大体积仲胺）的反应。尽管二异丙胺和B（C6 F5）3之间的反应在升高的温度下是可逆的，但来自与二异丙基乙胺的反应的1a和2a的混合物是热稳定的（方案1）。
In industrially important reactions, such as hydroformylation and hydrogenation, H2 gas serves as a reducing agent and/or a hydrogen-atom source.[1] Even small improvements in the efficiency of these reactions translate into large monetary savings. The key step in these transformations is the activation of H2 at a transition metal. The nodal character of the energetically accessible d orbitals allows a transition-metal center to react directly with H2 in a concerted reaction with a low activation barrier.[2] However, not only are transitionmetal complexes expensive, but the complete removal of metal impurities from the reaction product is generally required in the production of pharmaceutical intermediates owing to toxicity concerns.[3] Although countless synthetic complexes and enzymes with transition metals at their reactive core are well known, there are significantly fewer examples of HÀH bond activation facilitated solely by a nonmetal.[4] Several reactions of H2 with compounds containing main-group elements in lowtemperature matrices have been reported;[5] however, H2 activation at nonmetals under mild conditions had only been observed by Power and co-workers in product mixtures of digermenes, digermanes, and primary germanes,[6] until recently, when Stephan and co-workers reported the thermal liberation of H2 from a phosphonium borate salt. The resulting product,(C6H2Me3) 2P (C6F4) B (C6F5) 2, undergoes the addition of H2 at 258C to reform the original salt.[7] In an analogous fashion, mixtures of sterically demanding phosphanes and boranes (“frustrated Lewis pairs”)[8] can also cleave H2 heterolytically to form phosphonium borates [R3PH][HBR’3].[9] More recently, Bertrand and co-workers reported that selected organic carbenes are just nucleophilic enough to cleave H2 and NH3.[10] Herein we extend the family of “frustrated Lewis pairs” and demonstrate that not only bulky phosphanes and boranes or organic carbenes can cleave H2, but also inexpensive, stable amines in combination with B (C6F5) 3.Solutions of stoichiometric mixtures of diisopropylethylamine, diisopropylamine, or 2, 2, 6, 6-tetramethylpiperidine and B (C6F5) 3 in toluene were investigated by 1H, 11B, and 19FNMR spectroscopy. The reactions of diisopropylethylamine and diisopropylamine with B (C6F5) 3 gave mixtures of the salt 1a or 1b and the zwitterion 2a or 2b as expected (Scheme 1);[11] however, no reaction was observed for 2, 2, 6, 6-tetramethylpiperidine, a bulky secondary amine with no α hydrogen atoms. Whereas the reaction between diisopropylamine and B (C6F5) 3 is reversible at elevated temperature, the mixture of 1a and 2a from the reaction with diisopropylethylamine is thermally stable (Scheme 1).