The phototransposition in acetonitrile and the photoaddition of 2,2, 2-trifluoroethanol to the six isomers of dimethylbenzonitrile

Abstract

The six dimethylbenzonitriles can be divided into two independent triads in their photochemical reactivity. The first triad is comprised of the 2,3-dimethyl, 3,4-dimethyl, and 2,6-dimethyl isomers (11-2,3, 11-3,4, and 11-2,6, respectively); the second triad is comprised of the 2,4-dimethyl, 2,5-dimethyl, and 3,5-dimethyl isomers (11-2,4, 11-2,5, and 11-3,5, respectively). In acetonitrile, phototransposition converts the members of one triad to other members of the same triad, although only 11-3,4 was reactive enough to have significant conversion approaching a steady-state composition. Irradiation in 2,2,2-trifluoroethanol (TFE) resulted in the formation of addition products, 6-cyano-X,Y-dimethylbicyclo[3.1. 0]hex-3-en-2-yl 2,2,2,-trifluoroethyl ethers, but in significant yield only from 11-3,4 of the first triad and 11-2,4 of the second triad. The 11-3,4 isomer gave seven major regio- and stereoisomers; the 11-2,4 isomer gave three different regio- and stereoisomers. These addition products were all explained by formation of bicyclo[3. 1.0]hex-3-en-1-yl cations resulting from protonation by TFE at C6 followed by nucleophilic trapping by TFE. From these and previous results on aromatic nitriles, a consistent mechanistic picture is obtained where the critical carbon in determining the products of the phototransposition and photoaddition reactions is the cyano substituted one.