Polyhalogenoaromatic compounds. Part 41. Photochemical dehalogenation and arylation reactions of polyhalogenoaromatic and polyhalogenoheteroaromatic compounds

Photolysis of pentachloro- and pentabromo-pyridine in diethyl ether or methanol leads to loss of β-halogen. A product (9) derived from attack on diethyl ether was also identified. 4-Bromotetrachloropyridine also undergoes loss of bromine and tetrachloro-4-iodopyridine loses iodine exclusively. Photodehalogenation of some perhalogenothiophens, tetrachloropyrimidine, and hexachlorobenzene is also described. Photolysis of pentachloroiodobenzene, tetrachloroiodopyridines, and trichloro-5-iodothiophen in benzene gives the corresponding polychloroaryl- or polychloroheteroaryl-benzenes.Photolysis of tetrachloro-4-(phenylthio)pyridine (3) gives 1,3,4-trichloro[1]benzothieno[3,2-c]pyridine (6), and the analogous perchloro(phenylthio)pyridine (37) gives the corresponding perchlorobenzothienopyridine (61). The scope of this type of photocyclisation has been explored; starting materials investigated include various arylthiopolyhalogenopyridines, some arylamino- and aryloxy-tetrachloropyridines, and 4-anilinotrichloropyrimidine.