Electrochemical reactions. Part XVII. Selective dehalogenation of styrylpyrazoline and styrylpyrazole derivatives

Abstract

Selective monodehalogenation of some dihalogeno-aromatic compounds has been achieved by controlled potential electrochemical reduction in dimethylformamide. 5-(4-Chlorophenyl)-3-(4-chlorostyryl)-1-phenyl-Δ²-pyrazoline and 3-(4-chlorophenyl)-5-(4-chlorostyryl)pyrazole were dehalogenated at the styryl 4-position, and the corresponding bromo-derivatives behaved in a similar manner. The selectivity of these reactions is discussed with respect to the correlation previously noted between the rate of carbon–halogen bond fragmentation and the free-electron density distribution in the first formed radical-anion. Only 1,5-diphenyl-3-styrylpyrazole could be isolated from reduction of 5-(4-chlorophenyl)-3-(4-chlorostyryl)-1-phenylpyrazole or the corresponding dibromoderivative. Reduction in the presence of 1% deuterium oxide resulted in no deuterium being incorporated into the product, in accord with the mechanism previously proposed for the reaction.