Cyclizations. Part 1. Electrochemical and photochemical reactions of 1-(4-fluorophenyl)-5-(2-halogenophenyl)tetrazoles

Abstract

Electrochemical reduction of the title compounds, where the halogen substituent is Cl, Br or I, leads to cleavage of the carbon–halogen bond to leave a phenyl radical. Competition then follows between intramolecular radical substitution giving 7-fluorotetrazolo[1,5-f]phenanthridine and further reduction of the radical, then protonation, giving 1-(4-fluorophenyl)-5-phenyltetrazole. Substitution predominates but reduction and protonation becomes a more competing reaction when the halogen is Br or I. Photochemical reaction of the title compounds shows competition between carbon–halogen bond cleavage to give 7-fluorotetrazolo[1,5-f]phenanthridine and loss of nitrogen followed by cyclization to give 2-halogenophenyl-5-fluorobenzimidazole. Carbon–halogen bond cleavage predominates and becomes the only reaction when the halogen is I. The fluorine substituent allows the determination of product yields by ¹⁹F NMR spectroscopy.