Photochemical substitution of halogenopyrrole derivatives

The photochemical behaviour of some iodo substituted pyrroles when they are irradiated in the presence of an aromatic compound is reported, N-Methyl-3,4-diiodopyrrole-2-carbaldehyde and 5-iodopyrrole-2-carbaldehyde are unreactive when they are irradiated in benzene solution. In contrast, ethyl 3,4-dimethyl-5-iodopyrrole-2-carboxylate gives a 1:1 mixture of ethyl 3,4-dlmethyl-5-phenylpyrrole-2-carboxylate and ethyl 3,4-dimethylpyrrole-2-carboxylate in quantitative yields, The same reaction when attempted with acetonitrile as solvent gives ethyl 3,4-dimethylpyrrole-2-carboxylate as the sole product in quantitative yield, Use of 4,5-diiodopyrrole-2-carbaldehyde as substrate and irradiation in benzene gives the corresponding 5-phenyl derivative, The same behaviour is observed with m-xylene, thiophene and 2-chlorothiophene as solvents, With acetonitrile as solvent, the reaction with benzene does not work, With 2-methylthiophene as solvent 2-(5-methyl-2-thienyl)-3-iodopyrrole is obtained as the sole product, The observed behaviour can be explained on the basis of the previously reported data on the mechanism of the photochemical arylation of halogenothiophene derivatives.