Two-State Reactivity in Low-Valent Iron-Mediated C–H Activation and the Implications for Other First-Row Transition Metals

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• 作者：Yihua Sun ; Hao Tang ; Kejuan Chen ; Lianrui Hu ; Jiannian Yao ; Sason Shaik ; Hui Chen
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：March 23, 2016
• 年：2016
• 卷：138
• 期：11
• 页码：3715-3730
• 全文大小：1332K
• ISSN：1520-5126

文摘

C–H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. Our key mechanistic discovery for C–H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C–H bond cleavage. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe(II) to Fe(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C–C coupling and finalizing the C–H functionalization. Regeneration of the Fe(II) catalyst for the next round of C–H activation involves SET oxidation of the Fe(I) species generated after the C–C bond coupling. The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C–H activation. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C–H activation. It is predicted that stabilization of low-spin Mn(II) using an appropriate ligand sphere should produce another promising candidate for efficient C–H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C–H activation reactions.