Cis-trans isomerization and oxidation of radical cations of stilbene derivatives

Isomerization from cis stilbene derivatives (c-S (S = RCH=CHC6H5: 1, R = C6H5; 2, R = 4-CH3C6H4; 3, R = 4-CH3OC6H4 (= An); 4, R = 2,4-(CH3O)(2)C6H3; 5, R = 3,4-(CH3O)(2)C6H3; 6, R = 3,5-(CH3O)(2)C6H3; 7, AnCH=C(CH3)C6H5; 8, AnCH=CHAn)) to traits isomers (t-S) and oxidation of S with O-2 were studied in gamma-ray radiolyses of c-S in Ar-saturated 1,2-dichloroethane (DCE) and of S in O-2-saturated DCE, respectively. On the basis of product analyses, it is suggested that a smaller barrier to c-t unimolecular isomerization for c-3(.+)-5(.+) and 8(.+) than for c-1(.+), 2(.+), and 6(.+) due to the single bond character of the central C=C double bond for c-3(.+)-5(.+) and 8(.+) with a p-methoxyl group but not for c-1(.+), 2(.+), and 6(.+) without a p-methoxyl group because of the contribution of a quinoid-type structure induced by charge-spin separation. The isomerization proceeds via chain reaction mechanisms involving c-t unimolecular isomerization and endergonic hole transfer or dimerization and decomposition. The isomerization of c-3(.+) to t-3(.+) is catalyzed by addition of 1,4-dimethoxybenzene but terminated by triethylamine. The regioselective formation of 3d in oxidation of 3(.+) with O-2 is explained by spin localization on the p-olefinic carbon in 3(.+). The results of product analyses are compared with the rate constants of the unimolecular isomerization and the oxidation for S-.+ measured with pulse radiolyses.