Carbene radical

Carbene radicals are a recently proposed class of organometallic carbene. The carbene radical state is formed from reaction between an open-shell transition metal porphyrin and organic ligand. In 2003, experimental work on Cobalt(II) cyclopropanation catalysts suggested the hypothesis that a carbene radical intermediate state was being generated in situ.[1] Theoretical calculations validated this hypothesis and explained the bonding mechanism underlying the stability of the carbene radical.[2] [3] To date, carbene radicals have been synthesized only as long-lived reactive intermediates.[4][5] The chemical bond present in carbene radicals is surprising in that it possesses aspects of both Fischer and Schrock carbenes.[2]

Bonding mechanism of the carbene radical

Example of carbene radical reaction intermediate generated by reaction of cobalt porphyrin and CHCO2Et ligand. σ donation from the ligand to the "dz2" orbital on the cobalt metal center occurs concurrent with π* back-donation from the t2g symmetry orbitals, in a manner similar to a Fischer carbene. Unlike a Fischer carbene, however, the excess electron density on the α-carbon must be relieved by overlap with an adjacent acceptor group such as a carbonyl or sulfonyl.

The mechanism of formation of a carbene radical, like the Fischer mechanism, involves electron donation to a σ molecular orbital of which a dz2 orbital on the metal is a participant.[6] But unlike the Fischer bond, the transition metal dz2 orbital is initially singly occupied with some resulting spin pairing with a donated electron, giving the bond partial Schrock-like character.[2]

In order for the σ bond to be stabilized (typically with a bond order slightly less than 1), a back-bonding action from the π molecular orbital to the anti-bonding π* molecular orbital is necessary and the porphyrin ring serves as an electron π-symmetry "buffer" to ensure this interaction is obtained.[2]

The back-donation to the π* orbital would result in unfavorable excess electron density on the carbene carbon but the presence of adjacent functional groups (carbonyl or sulfonyl groups have the desired electronegativity) relieve this electron build-up and yield the final radical electron, which occupies a single p atomic orbital state on the carbon.[2]

See also

References

  1. ↑ Huang, Lingyu; Chen, Ying; Gao, Guang-Yao; Zhang, X. Peter (2003). "Diastereoselective and enantioselective cyclopropanation of alkenes catalyzed by cobalt porphyrins". Journal of Organic Chemistry. 68 (21): 8179–8184. doi:10.1021/jo035088o.
  2. 1 2 3 4 5 Belof, Jonathan; Cioce, Christian; Xu, Xue; Zhang, X. Peter; Space, Brian; Woodcock, H. Lee (2011). "Characterization of tunable radical metal–carbenes: Key intermediates in catalytic cyclopropanation". Organometallics. 30 (10): 2739–2746. doi:10.1021/om2001348.
  3. ↑ ‘Carbene Radicals’ in CoII(por)-Catalyzed Olefin Cyclopropanation Wojciech I. Dzik, Xue Xu, X. Peter Zhang, Joost N. H. Reek, and Bas de Bruin Journal of the American Chemical Society 2010 132 (31), 10891-10902 doi:10.1021/ja103768r
  4. ↑ Lu, Hongjian; Dzik, Wojciech; Xu, Xue; Wojtas, Lukasz; de Bruin, Bas; Zhang, X. Peter (2011). "Experimental evidence for cobalt(III)-carbene radicals: Key intermediates in cobalt(II)-based metalloradical cyclopropanation". Journal of the American Chemical Society. 133 (22): 8518–8521. doi:10.1021/ja203434c.
  5. ↑ Russell, Sarah; Hoyt, Jordan; Bart, Suzanne; Milsmann, Carsten; Stieber, S. Chantal; Semproni, Scott; DeBeer, Serena; Chirik, Paul (2014). "Synthesis, electronic structure and reactivity of bis(imino)pyridine iron carbene complexes: evidence for a carbene radical". Chemical Science. 5: 1168–1174. doi:10.1039/C3SC52450G.
  6. ↑ Frenking, Gernot; Sola, Miquel; Vyboishchikov, Sergei (2005). "Chemical bonding in transition metal carbene complexes". Journal of Organometallic Chemistry. 690 (24-25): 6178–6204. doi:10.1016/j.jorganchem.2005.08.054.
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