Photochemical addition of 2,2,2-trifluoroethanol to benzonitrile and p-, m-, and o-methylbenzonitrile

Irradiation of benzonitrile in 2,2,2-trifluoroethanol (TFE) with 254-nm light from low-pressure mercury lamps results in the formation of four addition products, the stereoisomers of 6-cyano-2-(2,2,2-trifluroethoxy)bicyclo[3.1.0]hex-3-ene, 22-25. The proposed mechanism begins by formation of the 6-cyanobicyclo[3.1.0]hex-3-en-2,6-diyl biradical/zwitterion from S-1 followed by both endo and exo protonation by TFE at C6. Deuterium labeling demonstrated that the resulting 6-cyanobicyclo[3.1.0]hex-3-en-2-yl cation underwent a rapid degenerate 1,4-sigmatropic rearrangement with inversion of configuration at the migrating carbon before being trapped by the nucleophilic solvent. Irradiation of p- and m-methylbenzonitrile in the same way gave six major addition products. Three of them, 32, 33, and 35 were 6-cyano-2-(2,2,2-trifluoroethoxy)-4-methylbicyclo [3.1.0]hex-3-enes and the other three, 32, 33, and 35, were 6-cyano-2-(2,2,2-trifluoroethoxy)-2-methylbicyclo[3.1.0]hex-3-enes. The proposed mechanism is again by TFE endo and exo protonation of the first formed biradical/zwitterion followed by trapping of the cations by the solvent. The 1,4-sigmatropic rearrangement of the cations now stops at the most stable structures, the endo- and exo-6-cyano-2-methylbicyclo[3.1.0]hex-3-en-2-yl cations, and all of the products are derived from them.