Bonding scheme of carbene radical complexes as compared to Schrock and Fischer-type carbene complexes
Carbene radicals are a special class of organometallic carbenes. The carbene radical can be formed by one-electron reduction of Fischer-type carbenes using an external reducing agent, or directly upon carbene formation at an open-shell transition metal complex (in particular low-spin cobalt(II) complexes) using diazo compounds and related carbene precursors.[1]
Cobalt(III)-carbene radicals have found catalytic applications in cyclopropanation reactions,[2][3][4] as well as in a variety of other catalytic radical-type ring-closing reactions.[5][6][7][8]
Theoretical calculations and EPR studies confirmed their radical-type behaviour and explained the bonding interactions underlying the stability of the carbene radical.[9][10]
Stable carbene radicals of other metals are known,[1] but the catalytically relevant cobalt(III)-carbene radicals have thus far only been synthesized as long-lived reactive intermediates.[11][12]
Examples of radical-type reactions involving carbene radical complexes
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Ikeno, T.; Iwakura, I.; Yamada, T. (2002). "Cobalt−Carbene Complex with Single-Bond Character: Intermediate for the Cobalt Complex-Catalyzed Cyclopropanation". Journal of the American Chemical Society. 124 (51): 15152–15153. doi:10.1021/ja027713x. PMID 12487572.
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Huang, L.; Chen, Y.; Gao, G.-Y.; Zhang, X.P. (2003). "Diastereoselective and enantioselective cyclopropanation of alkenes catalyzed by cobalt porphyrins". Journal of Organic Chemistry. 68 (21): 8179–8184. doi:10.1021/jo035088o. PMID 14535801.
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Chirila, A.; Das, B.G.; Paul, N.D.; de Bruin, B. (2017). "Diastereoselective Radical-Type Cyclopropanation of Electron Deficient Alkenes mediated by the Highly Active [Co(MeTAA)] Catalyst". ChemCatChem. 9 (8): 1413–1421. doi:10.1002/cctc.201601568. PMC 5413858. PMID 28529668.
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Das, B.G.; Chirila, A.; Tromp, M.; Reek, J.N.H.; de Bruin, B. (2016). "Co(III)-Carbene Radical Approach to Substituted 1H-Indenes". Journal of the American Chemical Society. 138 (28): 8968–8975. doi:10.1021/jacs.6b05434. PMID 27340837. S2CID 197200873.
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Paul, N.D.; Chirila, A.; Lu, H.; Zhang, X.P.; de Bruin, B. (2013). "Carbene Radicals in Cobalt(II) Porphyrin-Catalysed Carbene Carbonylation Reactions; A Catalytic Approach to Ketenes". Chemistry: A European Journal. 19 (39): 12953–12958. doi:10.1002/chem.201301731. PMC 4351769. PMID 24038393.
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Lu H, Zhang XP, de Bruin B (September 2013). "Carbeneradicals in cobalt(II)-porphyrin-catalysed carbene carbonylation reactions; a catalytic approach...
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detected: the phenyl anion (C6H−5), the phenyl cation (C6H+5), and the phenyl radical (C 6H• 5). Although Ph and phenyl uniquely denote C6H5−, substituted derivatives...
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