Electrophilic Heteroaromatic Substitutions. 2. Mechanism of the Side-Chain Halogenation of Polysubstituted α-Methylpyrroles with Molecular Chlorine

Article abstract of DOI:10.1021/ja00524a025

The mechanism of the α-side-chain chlorination with molecular chlorine in dichloromethane or chloroform solution at low temperatures and in the dark of polysubstituted α-methylpyrroles 1-4 has been investigated with regard to the nature of the reagent and to the fate of the attacking as well as the substituent halogen in the reaction products.Careful product analysis was carried out by radiochemical measurements (chlorine exchange), NMR spectra, standard halide ion titration, and preparative layer chromatography (PLC) techniques.The results suggest that the overall process consists of two main steps, i.e., the electrophilic nuclear attack and the subsequent rearrangement of the halogen to the side chain.Halogen migration from nucleus to side chain may occur from either the adjacent β or the vinylogous α' position.The present data provide a considerable extension of the concept of nonconventional electrophilic halogenation of aromatic molecules into the field of pyrroles.