Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes
Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes
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DOI：
10.1021/ja408238n
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发表时间：
2014-01-22
影响因子：
15
通讯作者：
Holland, Patrick L.
中科院分区：
文献类型：
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作者：
Chen, Chi;Dugan, Thomas R.;Holland, Patrick L.
简单的末端烯烃异构为具有z -立体化学的内异构体是罕见的，因为通常会形成更稳定的e -异构体。研究表明，由大体积β -二酮酸配体支撑的钴（II）催化剂具有适当的动力学选择性，可以催化一些简单的1-烯烃异构化，特别是催化2-烯烃作为不太稳定的z -异构体。催化作用通过“烷基”机制进行，三坐标的钴（II）烷基配合物为静息态。同位素标记实验表明，β -氢化物消除和[1,2]插入步骤都是快速的。立体模型解释了钴（II）在过渡态的选择性，通过方形平面几何来消除β -氢化物。该催化剂不仅对简单烯烃起作用，而且对纯烯丙基硅烷、酮和硅醚也起作用。从不良底物反应中分离钴(I)或钴（II）产物表明，关键的催化剂分解途径是双分子的，降低催化剂浓度通常会提高选择性。除了潜在有用的选择性转化外，这些研究还为高自旋钴配合物催化烯烃异构化提供了机制理解，并证明了立体体在控制烯烃形成的立体选择性方面的有效性。
The isomerization of simple terminal alkenes to internal isomers with Z-stereochemistry is rare, because the more stable E-isomers are typically formed. We show here that cobalt(II) catalysts supported by bulky beta-diketiminate ligands have the appropriate kinetic selectivity to catalyze the isomerization of some simple 1-alkenes specifically to the 2-alkene as the less stable Z-isomer. The catalysis proceeds via an "alkyl" mechanism, with a three-coordinate cobalt(II) alkyl complex as the resting state. beta-Hydride elimination and [1,2]-insertion steps are both rapid, as shown by isotopic labeling experiments. A steric model explains the selectivity through a square-planar geometry at cobalt(II) in the transition state for beta-hydride elimination. The catalyst works not only with simple alkenes, but also with homoallylsilanes, ketals, and silyl ethers. Isolation of cobalt(I) or cobalt(II) products from reactions with poor substrates suggests that the key catalyst decomposition pathways are bimolecular, and lowering the catalyst concentration often improves the selectivity. In addition to a potentially useful, selective transformation, these studies provide a mechanistic understanding for catalytic alkene isomerization by high-spin cobalt complexes, and demonstrate the effectiveness of steric bulk in controlling the stereoselectivity of alkene formation.