Department of Chemistry, University of Rochester , 120 Trustee Road, Rochester, New York 14627, United States.
J Am Chem Soc. 2014 Jan 22;136(3):945-55. doi: 10.1021/ja408238n. Epub 2014 Jan 3.
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 β-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. β-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 β-hydride elimination. The catalyst works not only with simple alkenes, but also with homoallyl silanes, 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.
简单末端烯烃异构化为 Z 立体化学的内型异构体很少见，因为通常形成更稳定的 E-异构体。我们在这里表明，由大位阻β-二酮亚胺配体支撑的钴(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-烯烃异构化为 2-烯烃，即不太稳定的 Z-异构体。该催化通过“烷基”机理进行，其中三配位钴(II)烷基配合物是休眠状态。通过同位素标记实验表明，β-氢化物消除和[1,2]-插入步骤都很快。一个空间模型通过β-氢化物消除过渡态中的钴(II)的平面正方形几何结构解释了选择性。该催化剂不仅适用于简单的烯烃，还适用于偕丙基硅烷、缩醛和硅醚。从与不良底物的反应中分离出钴(I)或钴(II)产物表明，关键催化剂分解途径是双分子的，降低催化剂浓度通常可以提高选择性。除了具有潜在有用性和选择性的转化外，这些研究还为高自旋钴配合物催化烯烃异构化提供了机理理解，并证明了空间位阻在控制烯烃形成的立体选择性方面的有效性。
