Redox Noninnocence of Carbene Ligands: Carbene Radicals in (Catalytic) C-C Bond Formation

被引:170
作者
Dzik, Wojciech I. [1 ]
Zhang, X. Peter [2 ]
de Bruin, Bas [1 ]
机构
[1] Univ Amsterdam, Vant Hoff Inst Mol Sci, NL-1098 XH Amsterdam, Netherlands
[2] Univ S Florida, Dept Chem, Tampa, FL 33620 USA
基金
欧洲研究理事会; 美国国家科学基金会;
关键词
HYDROGEN-ATOM TRANSFER; ASYMMETRIC CYCLOPROPANATION; ELECTRON-TRANSFER; ORGANOMETALLIC CHEMISTRY; CENTERED REACTIVITY; OXIDATIVE ADDITION; ORGANIC RADICALS; COMPLEXES; METAL; COBALT;
D O I
10.1021/ic200043a
中图分类号
O61 [无机化学];
学科分类号
070301 ; 081704 ;
摘要
In this Forum contribution, we highlight the radical-type reactivities of one-electron-reduced Fischer-type carbenes. Carbene complexes of group 6 transition metals (Cr, Mo, and W) can be relatively easily reduced by an external reducing agent, leading to one-electron reduction of the carbene ligand moiety. This leads to the formation of "carbene-radical" ligands, showing typical radical-type reactivities. Fischer-type. carbene ligands are thus clearly redox-active and can behave as so-called "redox noninnocent ligands". The "redox noninnocence" of Fischer-type carbene ligands is most clearly illustrated at group 9 transition metals in the oxidation state II+ (Co-II, Rh-II, and Ir-II). In such carbene complexes, the metal effectively reduces the carbene ligand by one electron in an intramolecular redox process. As a result, the thus formed "carbene radicals" undergo a variety of radical-type C-C and C-H bond formations. The redox noninnocence of Fischer-type carbene ligands is not just a chemical curiosity but, in fact, plays an essential role in catalytic cyclopropanation reactions by cobalt(II) porphyrins. This has led to the successful development of new chiral cobalt(II) porphyrins as highly effective catalysts for asymmetric cyclopropanation with unprecedented reactivity and stereocontrol. The redox noninnocence of the carbene intermediates results in the formation of carbene-radical ligands with nucleophilic character, which explains their effectiveness in the cyclopropanation of electron-deficient olefins and their reduced tendency to mediate carbene dimerization. To the best of our knowledge, this represents the first example in which the redox noninnocence of a reacting ligand plays a key role in a catalytic organometallic reaction. This Forum contribution ends with an outlook on further potential applications of one-electron-activated Fischer-type carbenes in new catalytic reactions.
引用
收藏
页码:9896 / 9903
页数:8
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