CYCLIZATIONS .1. ELECTROCHEMICAL AND PHOTOCHEMICAL-REACTIONS OF 1-(4-FLUOROPHENYL)-5-(2-HALOGENOPHENYL)TETRAZOLES

Electrochemical reduction of the title compounds, where the halogen su bstituent is Cl, Br or I. leads to cleavage of the carbon-halogen bond to leave a phenyl radical. Competition then follows between intramole cular radical substitution giving 7-fluorotetrazolo1,5-f!phenanthridi ne and further reduction of the radical, then protonation, giving 1-(4 -fluorophenyl)-5-phenyltetrazole. Substitution predominates but reduct ion and protonation becomes a more competing reaction when the halogen is Br or I. Photochemical reaction of the title compounds shows compe tition between carbon-halogen bond cleavage to give 7-fluorotetrazolo 1,5-f!phenanthridine and loss of nitrogen followed by cyclization to g ive 2-halogenophenyl-5-fluorobenzimidazole. Carbon-halogen bond cleava ge predominates and becomes the only reaction when the halogen is 1. T he fluorine substituent allows the determination of product yields by F-19 NMR spectroscopy.