简单的末端烯烃异构化为具有 Z 立体化学的内部异构体的情况很少见,因为通常会形成更稳定的 E 异构体。我们在这里表明,由庞大的 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来催化一些简单的 1-烯烃的异构化,特别是作为稳定性较差的 Z-异构体的 2-烯烃。催化通过“烷基”机制进行,三配位钴 (II) 烷基配合物作为静止状态。如同位素标记实验所示,β-氢化物消除和 [1,2]-插入步骤都很快。立体模型通过在 β-氢化物消除的过渡态钴 (II) 处的方形平面几何结构来解释选择性。该催化剂不仅适用于简单的烯烃,还适用于高烯丙基硅烷、缩酮、和甲硅烷基醚。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之外,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,并证明了空间体
An Alternative Mechanism for the Cobalt-Catalyzed Isomerization of Terminal Alkenes to (<i>Z</i>)-2-Alkenes
作者:Anastasia Schmidt、Alexander R. Nödling、Gerhard Hilt
DOI:10.1002/anie.201409902
日期:2015.1.12
The cobalt‐catalyzed selective isomerization of terminal alkenes to the thermodynamically less‐stable (Z)‐2‐alkenes at ambient temperatures takes place by a new mechanism involving the transfer of a hydrogen atom from a Ph2PH ligand to the starting material and the formation of a phosphenium complex, which recycles the Ph2PH complex through a 1,2‐H shift.
Double-Bond Isomerization: Highly Reactive Nickel Catalyst Applied in the Synthesis of the Pheromone (9<i>Z</i>,12<i>Z</i>)-Tetradeca-9,12-dienyl Acetate
A highly reactive nickel catalyst comprising NiCl2(dppp) or NiCl2(dppe) with zinc powder, ZnI2 and Ph2PH, was applied in the isomerization of terminal alkenes to Z-2-alkenes. The double-bond geometry of the 2-alkene can be controlled via the reaction temperature to yield the 2-Z-alkenes in excellent yields and high Z-selectivities. The formation of other constitutional isomers, such as 3-alkenes, is suppressed on the basis of the proposed mechanism via a 1,2-hydride shift from the metal to the Ph2P ligand. The nickel-catalyzed isomerization reaction was then applied in the synthesis of (9Z,12Z)-tetradeca-9,12-dienyl acetate, a pheromone with a 2Z,5Z-diene subunit.