PHOTOCHEMISTRY OF ARYL VINYL SULFIDES AND ARYL VINYL ETHERS - EVIDENCE FOR THE FORMATION OF THIOCARBONYL AND CARBONYL YLIDES

Aryl vinyl thioethers 5a and 9a and aryl vinyl ethers 5b and 9b form y lide intermediates following laser irradiation at 308 nm. In benzene, the ylides possess long-Lived absorption bands in the 600-800 nm regio n with a second weaker band at similar to 460 nm. In methanol, a known quencher of zwitterionic species, the lifetimes are reduced significa ntly. The decay kinetics measured within the long wavelength absorptio n envelope vary with wavelength, indicating the presence of more than one ylide species. Formation of the ylides occurs via a naphthalene-Li ke triplet state in the case of aryl vinyl ethers, while for the thioe thers the multiplicity of the ylide precursor could be either singlet or triplet. Product formation in benzene for 5a and 5b involves ring c losure of the ylide to produce dihydrothiophene and dihydrofuran produ cts, respectively. For short periods of irradiation (either lamps or l aser) a mixture of cis- and trans-fused products is observed, while fo r prolonged irradiation only the cis-fused compound is detected, sugge sting a photoenolization mechanism for conversion of trans to cis, In addition to products derived from ring closure, 9a provides intramolec ular addition product 12. Conversely, the ylide derived from 9b gives rise to the [3 + 2] cycloaddition product 13.