Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 7: methanol, conjugated dienes, and 1,4-dicyanobenzene

Abstract

The scope of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include conjugated dienes: 1,3-butadiene (9), 2-methyl-1,3-butadiene (10), 2,3-dimethyl-1,3-butadiene (11), and 2,5-dimethyl-2,4-hexadiene (12). Acetonitrile–methanol solutions of the dienes 9,10, and 11, and 1,4-dicyanobenzene (1), with and without codonor (biphenyl (5)), were irradiated with a medium-pressure mercury vapour lamp through Pyrex. Both 1,2- and 1,4-addition products were formed in approximately equal amounts (combined yields of photo-NOCAS products, 50–65%). In marked contrast, when an acetonitrile–methanol solution of 2,5-dimethyl-2,4-hexadiene (12), 1, and 5 was irradiated, only the 1,4-addition product, trans-2-(4-cyanophenyl)-5-methoxy-2,5-dimethyl-3-hexene (22,82%), was obtained. This photolysate also contained a small amount of another 1,4-addition product, that which had incorporated cyanide ion instead of methanol, trans-2-(4-cyanophenyl)-2,2,5-trimethyl-3-hexenenitrile (23, 2%). Irradiation of an acetonitrile solution (no methanol) of 12,1, and 5 gave 23 in good yield (68%). An excellent yield (80%) of 23 was obtained upon irradiation of an acetonitrile solution of 1,12, 5, potassium cyanide, and 18-crown-6. Addition of 2,4,6-trimethylpyridine (collidine, 25), a mild, non-nucleophilic base, to the reaction mixture diverts the reaction involving 12 from photo-NOCAS products to 1:1 substitution products; 3-(4-cyanophe-nyl)-2,5-dimethyl-1,4-hexadiene (26), trans-5-(4-cyanophenyl)-2,5-dimethyl-1,3-hexadiene (27), (Z)-1-(4-cyanophenyl)-2,5-dimethyl-2,4-hexadiene (28), and (E)-1-(4-cyanophenyl-2,5-dimethyl-2,4-hexadiene (29) were formed. The mechanisms of these reactions are discussed and an explanation for the observed regio- and stereoselectivity is given.