RESUMO
Aryl vinyl thioethers 5a and 9a and aryl vinyl ethers 5b and 9b form ylide intermediates following laser irradiation at 308 nm. In benzene, the ylides possess long-lived absorption bands in the 600-800 nm region with a second weaker band at approximately 460 nm. In methanol, a known quencher of zwitterionic species, the lifetimes are reduced significantly. The decay kinetics measured within the long wavelength absorption envelope vary with wavelength, indicating the presence of more than one ylide species. Formation of the ylides occurs via a naphthalene-like triplet state in the case of aryl vinyl ethers, while for the thioethers the multiplicity of the ylide precursor could be either singlet or triplet. Product formation in benzene for 5a and 5b involves ring closure of the ylide to produce dihydrothiophene and dihydrofuran products, respectively. For short periods of irradiation (either lamps or laser) a mixture of cis- and trans-fused products is observed, while for prolonged irradiation only the cis-fused compound is detected, suggesting a photoenolization mechanism for conversion of trans to cis. In addition to products derived from ring closure, 9a provides intramolecular addition product 12. Conversely, the ylide derived from 9b gives rise to the [3 + 2] cycloaddition product 13.