Cis−Trans Isomerization and Oxidation of Radical Cations of Stilbene Derivatives

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Abstract
Isomerization from cis stilbene derivatives (c-S (S = RCHCHC6H5: 1, R = C6H5; 2, R = 4-CH3C6H4; 3, R = 4-CH3OC6H4 (= An); 4, R = 2,4-(CH3O)2C6H3; 5, R = 3,4-(CH3O)2C6H3; 6, R = 3,5-(CH3O)2C6H3; 7, AnCHC(CH3)C6H5; 8, AnCHCHAn)) to trans isomers (t-S) and oxidation of S with O2 were studied in γ-ray radiolyses of c-S in Ar-saturated 1,2-dichloroethane (DCE) and of S in O2-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 CC 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 O2 is explained by spin localization on the β-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.

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