PHOTOCHEMICAL SUBSTITUTION OF HALOGENOPYRROLE DERIVATIVES

The photochemical behaviour of some iodo substituted pyrroles when the y are irradiated in the presence of an aromatic compound is reported, N-Methyl-3,4-diiodopyrrole-2-carbaldehyde and 5-iodopyrrole-2-carbalde hyde are unreactive when they are irradiated in benzene solution. In c ontrast, ethyl 3,4-dimethyl-5-iodopyrrole-2-carboxylate gives a 1:1 mi xture of ethyl 3,4-dlmethyl-5-phenylpyrrole-2-carboxylate and ethyl 3, 4-dimethylpyrrole-2-carboxylate in quantitative yields, The same react ion when attempted with acetonitrile as solvent gives ethyl 3,4-dimeth ylpyrrole-2-carboxylate as the sole product in quantitative yield, Use of 4,5-diiodopyrrole-2-carbaldehyde as substrate and irradiation in b enzene gives the corresponding 5-phenyl derivative, The same behaviour is observed with m-xylene, thiophene and 2-chlorothiophene as solvent s, With acetonitrile as solvent, the reaction with benzene does not wo rk, With 2-methylthiophene as solvent 2-(5-methyl-2-thienyl)-3-iodopyr role is obtained as the sole product, The observed behaviour can be ex plained on the basis of the previously reported data on the mechanism of the photochemical arylation of halogenothiophene derivatives.