A Theoretical Investigation of the Ni(II)-Catalyzed Hydrovinylation of Styrene.
A Theoretical Investigation of the Ni(II)-Catalyzed Hydrovinylation of Styrene.
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DOI：
10.1021/om900045p
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发表时间：
2009-06-22
期刊：
影响因子：
2.8
通讯作者：
Jemmis ED
中科院分区：
文献类型：
--
作者：
Joseph J;Rajanbabu TV;Jemmis ED
我们报告了对苯乙烯与Ni(II)-膦烷催化体系的加氢乙烯化反应的详细和充分的计算研究，该反应最初被认为是通过涉及氢化镍中间体的阳离子机制进行的。从本研究中发现的具有定量产率和中等选择性的特定催化剂络合物的一般特征：(a)由于源自Ni(II)-烯丙基催化剂前体的低方形平面戊基螯合中间体，引发反应的激活势垒（18.8 kcal/mol）高于反应的激活势垒（18.8 kcal/mol）。因此，存在一个催化诱导期；(b)产物通过β-H转移步骤从催化剂中退出，而不是通常的β-H消除途径，由于磷烷配体的反式作用，β-H消除途径具有很高的活化能；(c)限制周转和决定对映体的过渡态也是β- h转移；(d)由于不含氢化物中间体，防止了产物的不需要的异构化；(e)由于对映体选择性是在h转移阶段本身决定的，因此催化循环中产物的构型会影响随后循环中的对映体选择性；(f) d平方平面Ni（II）中唯一强配体的反式效应，η - 3-苄基中间体的稳定性和三个配位的可用性使得烯烃底物的低聚/聚合和线性加氢乙烯化的区域选择性加氢乙烯化成为可能。这项工作也证实了以前认识到的半可变性群体在该机制的各个阶段的作用。
We report a detailed and full computational investigation on the hydrovinylation reaction of styrene with the Ni(II)-phospholane catalytic system, which was originally presumed to proceed through a cationic mechanism involving a nickel hydride intermediate. The following general features emerge from this study on a specific catalyst complex that was found to give quantitative yield and moderate selectivity: (a) the activation barrier for the initiation (18.8 kcal/mol) is higher than that for the reaction due to a low-lying square-planar pentenyl chelate intermediate originating from a Ni(II)-allyl catalyst precursor. Consequently there is an induction period for the catalysis; (b) the exit of product from the catalyst is via a β-H-transfer step instead of the usual β-H elimination pathway, which has a very high activation energy due to a trans effect of the phospholane ligand; (c) the turnover-limiting and enantio- determining transition state is also the β-H-transfer; (d) because of the absence of a hydride intermediate, the unwanted isomerization of the product is prevented; (e) since the enantio-discrimination is decided at the H-transfer stage itself, the configuration of the product in a catalytic cycle influences the enantioselectivity in the subsequent cycle; (f) the trans effect of the sole strong ligand in the d square-planar Ni(II), the stability of the η3-benzyl intermediate, and the availability of three coordination sites enable regioselective hydrovinylation over the possible oligomerization/polymerization of the olefin substrates and linear hydrovinylation. This work has also confirmed the previously recognized role of the hemilabile group at various stages in the mechanism.
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