Experimental Evidence for Cobalt(III)-Carbene Radicals: Key Intermediates in Cobalt(II)-Based Metalloradical Cyclopropanation
Experimental Evidence for Cobalt(III)-Carbene Radicals: Key Intermediates in Cobalt(II)-Based Metalloradical Cyclopropanation
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DOI：
10.1021/ja203434c
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发表时间：
2011-06-08
影响因子：
15
通讯作者：
Zhang, X. Peter
中科院分区：
文献类型：
--
作者：
Lu, Hongjian;Dzik, Wojciech I.;Zhang, X. Peter
已经获得了新的和决定性的证据，钴（III）-卡宾自由基的存在，以前已提出的关键中间体的金属自由基环丙烷化的钴（II）配合物的卟啉的潜在机制。在不存在烯烃底物的情况下，发现[Co（TPP）]与苯乙烯基重氮乙酸乙酯的反应生成相应的钴（III）-乙烯基卡宾自由基，其随后通过其γ-自由基烯丙基共振形式二聚，得到双核钴（III）卟啉络合物。X-射线结构分析揭示了一个高度紧凑的二聚体结构，其中两个金属卟啉单元通过两个Co（III）中心之间的四取代的1，5-己二烯C-6-桥以面对面的方式排列。钴（III）-乙烯基卡宾自由基中间体的γ-自由基烯丙基共振形式可以通过形成C-O键被克里思有效地捕获，得到单核钴（III）络合物而不是二聚体产物。的烯丙基自由基的性质和相关的反应性曲线的钴（III）-卡宾自由基，包括其无法从甲苯溶剂中提取氢原子，建立了DFT计算。
New and conclusive evidence has been obtained for the existence of cobalt(III)-carbene radicals that have been previously proposed as the key intermediates in the underlying mechanism of metalloradical cyclopropanation by cobalt(II) complexes of porphyrins. In the absence of olefin substrates, reaction of [Co(TPP)] with ethyl styryldiazoacetate was found to generate the corresponding cobalt(III)-vinylcarbene radical that subsequently dimerizes via its gamma-radical allylic resonance form to afford a dinuclear cobalt(III) porphyrin complex. X-ray structural analysis reveals a highly compact dimeric structure wherein the two metalloporphyrin units are arranged in a face-to-face fashion through a tetrasubstituted 1,5-hexadiene C-6-bridge between the two Co(III) centers. The gamma-radical allylic resonance form of the cobalt(III)-vinylcarbene radical intermediate could be effectively trapped by TEMPO via C-O bond formation to give a mononuclear cobalt(III) complex instead of the dimeric product. The allylic radical nature and related reactivity profile of the cobalt(III)-carbene radical, including its inability to abstract hydrogen atoms from toluene solvent, were established by DFT calculations.