Department of Chemistry, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
J Am Chem Soc. 2017 Jul 26;139(29):9909-9920. doi: 10.1021/jacs.7b03548. Epub 2017 Jul 12.
The mechanisms of Ni-catalyzed C-H arylation, alkylation, and sulfenylation with N,N-bidentate directing groups are investigated using density functional theory (DFT) calculations. While the C-H cleavage occurs via the concerted metalation-deprotonation (CMD) mechanism in all types of reactions, the subsequent C-C and C-X bond formation steps may occur via either oxidative addition to form a Ni(IV) intermediate or radical pathways involving Ni(III) complexes generated from homolytic dissociation of disulfides/peroxides or halide-atom transfer from alkyl halides. DFT calculations revealed that radical mechanisms are preferred in reactions with sterically hindered coupling partners with relatively low bond dissociation energies (BDE) such as dicumyl peroxide, heptafluoroisopropyl iodide and diphenyl disulfide. In contrast, these radical processes are highly disfavored when generating unstable phenyl and primary alkyl radicals. In such cases, the reaction proceeds via an oxidative addition/reductive elimination mechanism involving a Ni(IV) intermediate. These theoretical insights into the substrate-controlled mechanisms in the C-H functionalizations were employed to investigate a number of experimental phenomena including substituent effects on reactivity, chemo- and regioselectivity and the effects of oxidant in the intermolecular oxidative C-H/C-H coupling reactions.
使用密度泛函理论（DFT）计算研究了具有 N，N-双齿导向基团的 Ni 催化 C-H 芳基化、烷基化和亚磺酰化反应的机理。虽然所有类型的反应中的 C-H 断裂都通过协同金属化-去质子化（CMD）机制发生，但随后的 C-C 和 C-X 键形成步骤可能通过氧化加成形成 Ni(IV) 中间体或涉及由二硫代物/过氧化物或卤化物原子从卤代烷转移的均裂产生的 Ni(III) 配合物的自由基途径发生。DFT 计算表明，在与空间位阻较大的偶联伙伴反应中，自由基机制是优选的，这些偶联伙伴的键离解能（BDE）相对较低，例如二枯基过氧化物、七氟异丙基碘化物和二苯基二硫醚。相比之下，当生成不稳定的苯基和伯烷基自由基时，这些自由基过程极不受青睐。在这种情况下，反应通过涉及 Ni(IV) 中间体的氧化加成/还原消除机制进行。这些对 C-H 官能化中底物控制机制的理论见解被用于研究许多实验现象，包括反应性、化学和区域选择性的取代基效应以及在分子间氧化 C-H/C-H 偶联反应中的氧化剂的影响。
