School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China.
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China.
J Am Chem Soc. 2023 Jan 11;145(1):359-376. doi: 10.1021/jacs.2c10126. Epub 2022 Dec 20.
The intermediacy of alkoxy radicals in cerium-catalyzed C-H functionalization via H-atom abstraction has been unambiguously confirmed. Catalytically relevant Ce(IV)-alkoxide complexes have been synthesized and characterized by X-ray diffraction. Operando electron paramagnetic resonance and transient absorption spectroscopy experiments on isolated pentachloro Ce(IV) alkoxides identified alkoxy radicals as the sole heteroatom-centered radical species generated via ligand-to-metal charge transfer (LMCT) excitation. Alkoxy-radical-mediated hydrogen atom transfer (HAT) has been verified via kinetic analysis, density functional theory (DFT) calculations, and reactions under strictly chloride-free conditions. These experimental findings unambiguously establish the critical role of alkoxy radicals in Ce-LMCT catalysis and definitively preclude the involvement of chlorine radical. This study has also reinforced the necessity of a high relative ratio of alcohol vs Ce for the selective alkoxy-radical-mediated HAT, as seemingly trivial changes in the relative ratio of alcohol vs Ce can lead to drastically different mechanistic pathways. Importantly, the previously proposed chlorine radical-alcohol complex, postulated to explain alkoxy-radical-enabled selectivities in this system, has been examined under scrutiny and ruled out by regioselectivity studies, transient absorption experiments, and high-level calculations. Moreover, the peculiar selectivity of alkoxy radical generation in the LMCT homolysis of Ce(IV) heteroleptic complexes has been analyzed and back-electron transfer (BET) may have regulated the efficiency and selectivity for the formation of ligand-centered radicals.
烷氧基自由基在铈催化 C-H 官能化中通过 H 原子转移的中间体作用已得到明确证实。通过 X 射线衍射合成并表征了与催化相关的 Ce(IV)-烷氧基配合物。对孤立的五氯代 Ce(IV)烷氧基进行了在位电子顺磁共振和瞬态吸收光谱实验，确定烷氧基自由基是通过配体到金属电荷转移（LMCT）激发产生的唯一杂原子中心自由基物种。通过动力学分析、密度泛函理论（DFT）计算和在严格无氯条件下的反应验证了烷氧基自由基介导的氢原子转移（HAT）。这些实验结果明确确立了烷氧基自由基在 Ce-LMCT 催化中的关键作用，并明确排除了氯自由基的参与。该研究还强调了醇与 Ce 的相对比例对选择性烷氧基自由基介导 HAT 的必要性，因为醇与 Ce 的相对比例的微小变化可能导致截然不同的反应机制。重要的是，以前提出的氯自由基-醇络合物，被假定为解释该体系中烷氧基自由基实现选择性的原因，已经通过区域选择性研究、瞬态吸收实验和高级计算进行了仔细检查和排除。此外，还分析了 Ce(IV)杂配体配合物 LMCT 均裂中烷氧基自由基生成的特殊选择性，并且可能通过反向电子转移（BET）调节形成配体中心自由基的效率和选择性。
