Photostimulated Reduction of Nitriles by SmI2

Abstract

Despite their high electron-withdrawing strength, nitriles are not good electron acceptors and therefore are hard to reduce. In this work, using photostimulation in the visible region, we examined the reactivity of aliphatic and aromatic, mono- and dicyano compounds in reaction with SmI(2). A proton donor that complexes efficiently with SmI(2) must be used. Maximum yield was obtained at ca.0.2 M MeOH. Aromatic nitriles were more reactive than aliphatic nitriles, which exhibited negligible yields. Phenylacetonitrile presents an intermediate reactivity. The mechanism of the reaction involves coordination of the SmI(2) to the lone pair of the nitrile nitrogen followed by an inner sphere electron transfer. Surprisingly, m-dicyanobenzene was less reactive than the monocyano derivative benzonitrile. This was traced to the lower ability of the dicyano compound to coordinate to the SmI(2) due to, as was shown by quantum mechanical calculations, its lone pair having an energy significantly lower than that of benzonitrile. It is noteworthy that at the SmI(2) initial concentration used (0.04M), light penetrates only the 0.4 mm outer layer of the reaction mixture. Therefore the photostimulation effect observed was due to irradiation of only 4% of the total reaction volume, implying that under optimal conditions the effect should be 25 times larger.