Z‑Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes

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.

简单端烯烃异构为具有Z型立体化学的内烯烃异构体的反应较为罕见，因为通常更稳定的E型异构体更易生成。本文证实，由位阻庞大的β-二亚胺配体（β-diketiminate ligands）负载的钴(II)催化剂具备合适的动力学选择性，可催化部分简单1-烯烃发生异构化，专一性地生成稳定性更差的Z型2-烯烃。该催化过程经由“烷基”机制进行，以三配位钴(II)烷基络合物作为反应静息态。同位素标记实验表明，β-氢消除与[1,2]-插入步骤均为快反应步骤。位阻模型可通过β-氢消除过渡态中钴(II)的平面正方形几何构型，对该立体选择性作出解释。该催化剂不仅适用于简单烯烃，还可应用于高烯丙基硅烷、缩酮及硅醚类底物。从与低活性底物的反应体系中分离得到的钴(I)或钴(II)产物表明，关键的催化剂分解路径为双分子过程，降低催化剂浓度通常可提升反应选择性。除了获得具有潜在应用价值的选择性转化方法外，本研究还为高自旋钴络合物催化烯烃异构化反应提供了机理层面的认知，并验证了空间位阻在调控烯烃生成立体选择性方面的有效性。