Photochemical synthesis of phenyl-2-thienyl derivatives

The irradiation (in benzene solution) of 5-bromo- and 5-iodo-thiophene-2- carbaldehyde or the corresponding methyl ketones furnishes the corresponding 5-phenyl derivatives. The same reactivity for other halogenothiophenes is reported: 3,5-dibromothiophene-2-carbaldehyde furnishes the 3-bromo-5-phenyl-2- thienyl derivative while the corresponding di-iodo compound yields 3,5-diphenylthiophene-2-carbaldehyde. In contrast, 5-acetyl-2,3-di-iodothiophene furnishes only the photosubstitution product at C-5. Generally the iodine-containing compounds are more reactive and more stable under the reaction conditions than ones bearing bromine, in agreement with previous reports on the corresponding furan photochemistry.