PHOTOCHEMICAL COUPLING BETWEEN HALOGENOHETEROCYCLIC AND HETEROCYCLIC-DERIVATIVES

The photochemical reaction between 5-iodo-heteroaryl derivatives 1a-d and both 3-heteroaryl allylic alcohols (2a-b) and acetates (3a-b) have been investigated. The presence of an alcoholic function was not comp atible with photochemical coupling since compounds 1a-d were photoredu ced in the presence of an alcohol. In contrast, the acetates 3a, b gav e the expected coupling products. The presence of a weak electron-with drawing group on the alkene induced an inverted regiochemistry giving only aryl-aryl coupling products. The same behaviour was observed usin g 2-(3-acetoxyprop-1-ynyl)thiophene 8, in which case only the product deriving from aryl-aryl coupling was again observed. When 2-prop-1-yny lthiophene 11 was used as starting material, the some product resultin g from attack on the methyl group 13 was observed. Photochemical coupl ing between 2-thienylacetonitrile (14) and halogenothienyl derivatives 1b, c gave no reaction product. In contrast, the irradiation of methy l 2-thienylacetate 15 and methyl 2-(2-thienyl)propionate (17) in the p resence of 1b, c did give the corresponding coupling products.