Mechanism of Ni-Catalyzed Reductive 1,2-Dicarbofunctionalization of Alkenes

Ni-catalyzed cross-electrophile
coupling reactions have emerged
as appealing methods to construct organic molecules without the use
of stoichiometric organometallic reagents. The mechanisms are complex:
plausible pathways, such as “radical chain” and “sequential
reduction” mechanisms, are dependent on the sequence of the
activation of electrophiles. A combination of kinetic, spectroscopic,
and organometallic studies reveals that a Ni-catalyzed, reductive
1,2-dicarbofunctionalization of alkenes proceeds through a “sequential
reduction” pathway. The reduction of Ni by Zn is the turnover-limiting
step, consistent with Ni­(II) intermediates as the catalyst resting-state.
Zn is only sufficient to reduce (phen)­Ni­(II) to a Ni­(I) species. As
a result, commonly proposed Ni(0) intermediates are absent under these
conditions. (Phen)­Ni­(I)–Br selectively activates aryl bromides
via two-electron oxidation addition, whereas alkyl bromides are activated
by (phen)­Ni­(I)–Ar through single-electron activation to afford
radicals. These findings could provide insight into achieving selectivity
between different electrophiles.

镍催化交叉亲电偶联反应（Ni-catalyzed cross-electrophile coupling reactions）已成为无需使用化学计量有机金属试剂即可构建有机分子的极具吸引力的方法。其反应机制较为复杂：诸如"自由基链式"与"分步还原"等合理反应路径，取决于亲电试剂的活化顺序。结合动力学、光谱学及有机金属化学研究结果可知，镍催化的烯烃还原性1,2-双碳官能团化反应遵循"分步还原"路径。锌对镍的还原为该反应的决速步，这与以Ni(II)中间体作为催化剂驻留态的结论一致。锌仅能将邻菲罗啉（phen）合Ni(II)还原为Ni(I)物种，因此在该反应条件下，此前普遍提出的Ni(0)中间体并不存在。邻菲罗啉（phen）合Ni(I)-溴可通过双电子氧化加成选择性活化芳基溴，而烷基溴则由邻菲罗啉（phen）合Ni(I)-芳基通过单电子活化生成自由基。上述研究结果可为实现不同亲电试剂间的选择性调控提供理论参考。