Thermal reactions of 3-Furyl Fulgide and 3-Thienyl Fulgide. Ab initio molecular orbital and CASSCF studies

Abstract

Thermal Reactions of 3-furyl fulgide and 3-thienyl fulgide were examined at ab initio molecular orbital and CASSCF levels. The transition states of EC and EZ-isomerizations were determined by singlet UHF method with 6-31G* basis set, and their activation energies were successively estimated sat CASSCF level. The geometries of the transition states of EC-isomerizations of 3-furyl fulgide and 3-thienyl fulgide are colser to those of C-isomers than those of E-isomers. The EC-isomerization proceeds in a conrotatory mode rather than a disrotatory mode due to steric repulsion of methyl groups, in conflict with the Woodward–Hoffmann selection rule. The activation energies of EC-isomerizations were estimated to be 45 kcal/mol for 3-furyl fulgide and 46 kcal/mol for 3-thienyl fulgide at CASSCF(10,9) level. The activation energies of EZ-isomerizations are less by ∼5 kcal/mol than those of EC-isomerizations, indicating that EZ-isomerizations are thermally preferred rather than EC-isomerizations. Those values are remarkably small compared with 65.6 kcal/mol for rotational barrier of double bond of ethylene molecule at CASSCF(2,2)/6-31G* level. It is, however, shown that smallness of the activation energies of EZ-isomerizations is in harmony with thermal examinations of 3-furyl fulgide and 3-thienyl fulgide at 140 °C.