A study of the ionic salt effect upon the photosubstitution reaction between 1,4-dicyanobenzene and 2,3-dimethyl-2-butene

Abstract

The photosubstitution reaction between 1,4-dicyanobenzene and 2,3-dimethyl-2-butene was re-investigated with respect to three aspects: irradiation in the presence of the photosensitizer biphenyl; irradiation in the presence of magnesium perchlorate; and irradiation in the presence of both biphenyl and magnesium perchlorate. The photosensitization of this reaction mixture by biphenyl was observed to increase the efficiency and the yield of the products, 1-(4-cyanophenyl)-2,3- dimethyl-2-butene and 3-(4-cyanophenyl)-2,3-dimethyll-butene. The ratio of the products which normally is 1:2 in favour of the terminal alkene changed drastically to 1:18 upon the addition of magnesium perchlorate to the reaction mixture. This dramatic change in product ratio was attributed to the lack of protonation to the 1,4- dicyanobenzene radical anion, a key step in a previously proposed mechanism. It is proposed that this is a result of coordination of the radical anion to the magnesium cation.