Photocatalysis [1] and electrosynthesis [2] have been recognized as environmentally friendly methodologies for redox reactions, as they obviate the need for toxic or dangerous reagents. These approaches also allow for unstable reagents to be produced in situ, thereby permitting mild conditions. The reactive intermediates can be generated by the photoexcitation [1], or ionization of a mediator [2] at an electrode. The reagent can then perform redox chemistry with the substrate, and return to its original form to complete the catalytic cycle.

A class of oxidative electrochemical mediators based on the triarylimidazole framework has recently been investigated by Little, Zeng and Francke [2-5]. These mediators undergo one electron oxidation at the anode to form a radical cation that serves as an oxidative catalyst.

Recently, we have explored the potential of imidazole and phenanthroimidazole mediators to serve as photocatalysts. With an amine playing the role of a sacrificial reductant, the excited state serves as a reducing agent suitable for dehalogenation of aromatics, 5- and 6-exo,trig cyclizations of allyloxy and homoallyloxy iodo aromatics, as well as the photoredox equivalent of electrohydrocyclization [6].

Reference

[1] Tucker, J. W.; Stephenson, R. J. J. Org. Chem., 2012, 77, 1617-1622.

[2] Francke, R.; Little, R. D. Chem. Soc. Rev., 2014, 43, 2492-2521.

[3] Francke, R.; Little, R. D. J. Am. Chem. Soc., 2014, 136, 427-435.

[4] Zhang, N.-T.; Zeng, C.-C.; Lam, C. M.; Gbur, R. K.; Little, R. D. J. Org. Chem. 2013, 78, 2104–2110.

[5] Zeng, C.-C.; Zhang, N.-T.; Lam, C. M.; Little, R. D. Org. Lett., 2012, 14, 1314-1317.

[6a] Miranda, J. A.; Wade, C. J.; Little, R. D. J. Org. Chem. 2005, 70, 8017-8026.
[6b] Moëns, L.; Baizer, M. M.; Little, R. D. J. Org. Chem. 1986, 51, 4497-4498.