Electrophilic Heteroaromatic Reactions. 3. The α-Side-Chain Bromination of Some Polysubstituted α-Methylpyrroles in the Dark. Evidence for the Formation of Intermediate ? Adducts

Article abstract of DOI:10.1021/jo01298a009

The bromination of some ethyl 3,5-dimethyl-4-R-pyrrole-2-carboxylates (R = alkyl, Br, Cl, CO2Et) with molecular bromine in dichloroethane solution, in the dark, occurs smoothly at room temperature to yield the corresponding 5-bromomethyl derivatives.The course of the reaction was followed by titrimetric, spectrophotometric, and NMR methods.The reaction can be regarded as an extreme case of behavior in the interaction of aromatic systems with electrophiles, as it consists of two separately observable processes.Upon addition of bromine to the solution of the pyrrole, a number of cationic ? adducts form as a result of attack on all carbon positions of the pyrrole ring to varying extents.The adducts then decompose relatively slowly to a single substitution product.Each of the 4-R groups appears to affect the rates of the two main steps in opposite ways.This is interpreted in terms of stabilization of the reaction intermediate.