EFFECT OF BROMIDES ON THE ANOMALOUS PHOTOARYLATION OF 5-HALO-1,3-DIMETHYLURACILS

Photolysis of 5-chloro- and 5-fluoro-1,3-dimethyluracil in toluene in the presence of a bromide (e.g., p-dibromobenzene, 1,2-dibromoethane, and benzyl bromide) and an acid (trifluoroacetic acid, HBr) afforded 1 ,3-dimethyl-5-tolyluracils and the 6-isomers as the isomeric mixture. 5-Arylation occurred most favorably at ortho, while meta-isomers were preferential products in 6-arylation. The 5-isomers are suggested to b e derived from uracils excited in the singlet states, while the 6-isom ers are presumed to result from the uracils excited in the triplet sta tes.