Mechanism of Ni-Catalyzed Oxidations of Unactivated C(sp3)–H Bonds

The Ni-catalyzed oxidation of unactivated alkanes, including the oxidation of polyethylenes, by meta-chloroperbenzoic acid (mCPBA) occur with high turnover numbers under mild conditions, but the mechanism of such transformations has been a subject of debate. Putative, high-valent nickel-oxo or nickel-oxyl intermediates have been proposed to cleave the C–H bond, but several studies on such complexes have not provided strong evidence to support such reactivity toward unactivated C­(sp3)–H bonds. We report mechanistic investigations of Ni-catalyzed oxidations of unactivated C–H bonds by mCPBA. The lack of an effect of ligands, the formation of carbon-centered radicals with long lifetimes, and the decomposition of mCPBA in the presence of Ni complexes suggest that the reaction occurs through free alkyl radicals. Selectivity on model substrates and deuterium-labeling experiments imply that the m-chlorobenzoyloxy radical derived from mCPBA cleaves C–H bonds in the alkane to form an alkyl radical, which subsequently reacts with mCPBA to afford the alcohol product and regenerate the aroyloxy radical. This free-radical chain mechanism shows that Ni does not cleave the C­(sp3)–H bonds as previously proposed; rather, it catalyzes the decomposition of mCPBA to form the aroyloxy radical.

以间氯过氧苯甲酸（meta-chloroperbenzoic acid, mCPBA）为氧化剂的镍催化未活化烷烃（包括聚乙烯）氧化反应，可在温和条件下实现高周转数转化，但此类转化的反应机理一直是学界争议的焦点。此前有研究提出，反应通过高价镍氧或镍氧自由基中间体断裂C(sp³)-H键，但针对这类配合物的多项研究并未提供有力证据以支撑其与未活化C(sp³)-H键发生该类反应的合理性。本工作针对mCPBA介导的镍催化未活化C(sp³)-H键氧化反应开展了系统的机理研究。配体对反应无明显影响、反应体系中生成长寿命碳中心自由基、镍配合物存在下mCPBA发生分解，这些实验结果均表明反应通过自由烷基自由基路径进行。模型底物选择性实验与氘标记实验结果显示，由mCPBA衍生的间氯苯甲酰氧基自由基可断裂烷烃中的C-H键生成烷基自由基，随后烷基自由基与mCPBA反应生成醇产物并再生芳酰氧基自由基。该自由基链式机理表明，镍并未如此前所提出的那样直接断裂C(sp³)-H键，而是通过催化mCPBA分解生成芳酰氧基自由基来发挥催化作用。