Isomerization of cis-1,2-diarylethylenes. Catalysis by selenium, methanesulphonic acid, and potassium t-butoxide

Second-order rate constants and activation parameters for the isomerization of cis-X-substituted-diphenylethylenes (X =p-OCH3, p-CH3, H, p-Cl, m-Cl, or p-NO2), cis-2-styrylfuran, and cis-2-styrylthiophen have been measured in decahydronaphthalene using selenium (Se), methanesulphonic acid (MSA), and potassium t-butoxide (PTB) as catalysts. The catalyst effectiveness decreases in the order Se > MSA > PTB. Acid-catalysed isomerization is enhanced by electron-donating substituents, base-catalysed one is favoured by electron-withdrawing groups, and selenium-catalysed isomerization is not affected by substituents. The heterocyclic derivatives react faster than stilbene with all catalysts. A stepwise mechanism including addition and elimination of the catalysts is proposed.