PHOTO-CHEMICAL REACTION OF IMIDAZOLES WITH UNSATURATED NITRILES - CHEMISTRY OF ENCOUNTER COMPLEX AND ION-PAIR

The photochemical reactions of various N-unsubstituted (la, b) and N-substituted (2a-g) imidazoles with nitriles such as acrylonitrile (AN) and 2-cyanopyridine (CP) were remarkably sensitive to the nature of imidazoles, nitriles, and solvents employed. The N-unsubstituted imidazole, e.g., 2, 4, 5-triphenylimidazole (lb), reacted with AN, giving 2-[2-(2, 4, 5-triphenyl-2H-imidazolyl)]propionitrile (3b) both in ethanol and acetonitrile, whereas the N-substituted imidazole, e.g., l-methyl-2, 4, 5-triphenylimidazole (2a), was led to the [2 + 2] cvcloaddition products 4a and 4a' in ethanol and l-methyl-2, 4, 4-triphenyl-Δ2-imidazolin-5-one (5a) in acetonitrile. On the other hand, 2a and CP underwent a novel type of regiospecific addition to give 5-cyano-l-methyl-4α-pyridyl-2, 4, 5-triphenyl-Δ2-imidazoline (7a). Simultaneously 2a initiated the condensation of CP to yield 2, 4ʹ-bipyridine-2ʹ-carbonitrile (8) and a terpyridinecarbonitrile 9 by the elimination of hydrogen cyanide. Photolvses of other imidazoles with AN and CP gave similar results. From fluorescence quenching studies, quantum yield measurements, and effects of the solvents and the identity of nitriles on the photoreactions, it is concluded that encounter complexes (or exciplexes)and ion pairs are the key intermediates in these reactions. © 1979, American Chemical Society. All rights reserved.