Reaction of 1-substituted pyrroles with N-fluorodibenzenesulfonimide: Effect of halogen on electrophilic substitution by addition-elimination

The reaction of 1-substituted pyrroles (1-methyl, 1-ethyl and 1-phenyl) with N-fluorodibenzenesulfonimide (NFSi) gave N-(1-substituted 1H-pyrrol-2-yl)dibenzenesulfonimides as products by addition elimination. Defluorination of NFSi was the only competing reaction. In contrast the reaction of 1-t-butylpyrrole with NFSi gave a mixture of fluoropyrroles. This is attributed to a steric interaction between the t-butyl group and the phenyl groups on NFSi. N-Chlorodibenzenesulfonimide was found to dechlorinate under all conditions studied and it was not possible to compare its reactivity with that of NFSi. The results of this study support our hypothesis that the observation of addition-elimination in pyrroles is related to the π-electron donating ability of the halogen initially introduced into the ring.