THE EFFECT OF META-CYANO OR PARA-CYANO SUBSTITUTION ON THE REACTIVITY OF THE RADICAL CATIONS OF ARYLALKENES AND ALKANES - RADICAL IONS IN PHOTOCHEMISTRY .34.

The effect of electron-withdrawing substituents, meta- or para-cyano, on the reactivity of the radical cation of arylalkenes and alkanes has been determined. The radical cations were generated by single electron transfer (set) to an electron-accepting photosensitizer. Three reactions were studied: (i) the addition of nucleophile to the radical cation of arylalkenes, (ii) cleavage of the benzylic carbon-carbon bond of the radical cation of arylalkanes; and (iii) the deprotonation of the benzylic carbon-hydrogen bond of the radical cation of arylalkanes. The radical cations of 4-(1-phenylethenyl)benzonitrile (1b), 3-(1-phenylethenyl)benzonitrile (1c), 4(2-methoxy- 1-phenylethyl)benzonitrile (2b), 3-(2-methoxy- l-phenylethyl)benzonitrile (2c), cis- and trans-5-cyano-2-methoxy- 1-phenylindane (6b-cis and -trans), and 6-cyano-3-phenylindene (7b) were generated, by single electron transfer to the lowest excited singlet state of 1,4-dicyanobenzene (3), in acetonitrile-methanol. The radical cations of 1b, 1c, and 7b react with methanol to yield the anti-Markovnikov adducts (2b, 2c, and 6b-cis and 6b-trans). The radical cations of 2b, 2c, and 6b-trans cleave at the benzylic carbon-carbon bond to give products derived from the radical and carbocation fragments. The radical cation of 6b-cis deprotonates from the benzylic position with subsequent formation of the diastereomer, 6b-trans. This behaviour can be explained/predicted on the basis of the proposed mechanisms for these reactions. Molecular orbital calculations (AM1) support the conclusions.