'Carbene Radicals' in CoII(por)-Catalyzed Olefin Cyclopropanation
'Carbene Radicals' in CoII(por)-Catalyzed Olefin Cyclopropanation
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DOI：
10.1021/ja103768r
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发表时间：
2010-08-11
影响因子：
15
通讯作者：
de Bruin, Bas
中科院分区：
文献类型：
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作者：
Dzik, Wojciech I.;Xu, Xue;de Bruin, Bas
研究了钴卟啉介导的烯烃与重氮酯的环丙烷化反应机理。用EPR和ESI-MS技术研究了钴卟啉与重氮乙酸乙酯（EDA）的第一步反应。EDA与钴（II）卟啉反应形成1：1的Co（por）（CHCOOEt）加合物，其以两种异构体存在：“桥连卡宾”C“，其中”卡宾“与金属和在金属上具有d（7）构型的卟啉的吡咯氮结合，和“末端卡宾”C，其中“卡宾”表现为具有d（6）的氧化还原非无辜配体，钴中心和未成对电子驻留在“卡宾”碳原子上。使用DFT计算研究了由此形成的“钴卡宾自由基”与丙烯，苯乙烯和丙烯酸甲酯的后续反应性。计算表明，卡宾的形成是非官能化的Co-II（POR）的限速步骤，环丙烷环的形成通过逐步自由基过程进行：自由基加成的“卡宾自由基”C的烯烃的C=C双键的结果形成的γ-烷基自由基中间体D。然后，物质D在几乎无屏障的闭环反应（TS 3）中容易地崩溃以形成环丙烷。这种自由基机制很容易解释的高活性的Co-II（POR）物种在环丙烷化的缺电子烯烃，如丙烯酸甲酯。
The mechanism of cobalt(II)-porphyrin-mediated cyclopropanation of olefins with diazoesters was studied. The first step reaction of cobalt(II) porphyrin with ethyl diazoacetate (EDA) was examined using EPR and ESI-MS techniques. EDA reacts with cobalt(II) porphyrin to form a 1:1 Co(por)(CHCOOEt) adduct that exists as two isomers: the 'bridging carbene' C' in which the 'carbene' is bound to the metal and the pyrrolic nitrogen of the porphyrin that has a d(7) configuration on the metal, and the 'terminal carbene' C in which the 'carbene' behaves as a redox noninnocent ligand having a d(6) cobalt center and the unpaired electron residing on the 'carbene' carbon atom. The subsequent reactivities of the thus formed 'cobalt carbene radical' with propene, styrene, and methyl acrylate were studied using DFT calculations. The calculations suggest that the formation of the carbene is the rate-limiting step for the unfunctionalized Co-II(por) and that the cyclopropane ring formation proceeds via a stepwise radical process: Radical addition of the 'carbene radical' C to the C=C double bonds of the olefins results in formation of the gamma-alkyl radical intermediates D. Species D then easily collapse in almost barrierless ring-closure reactions (TS3) to form the cyclopropanes. This radical mechanism readily explains the high activity of Co-II(por) species in the cyclopropanation of electron-deficient olefins such as methyl acrylate.