Electrophilic aromatic addition reaction: electrophilic attack at an aromatic H substituent position

reaction: see text] We report and propose a mechanism for an unusual electrophilic aromatic addition reaction (Ad(E)()Ar). During our preparation of 5,7-dibromo-8-methoxyquinaldine as a key intermediate in the synthesis of 7-bromoquinaldine-5,8-dione, direct bromination in either acidic or neutral conditions led only to the formation of 5-bromo-8-methoxyquinaldine. Under basic methanolic conditions, however, we unexpectedly obtained the 5,7-dibromo-8,8-dimethoxy-7,8-dihydroquinaldine adduct 2a. This result not only allows for the functionalization of aromatic compounds via the addition adducts, but also introduces the possibility of an alternate mechanism for electrophilic substitution reactions.