The Mechanisms for Thermal and Photochemical Isomerizations of N-Substituted 2-Halopyrroles:Syntheses of N-Substituted 3-Halopyrroles

摘要

Halopyrroles,N-substituted 2-halopyrroles were prepared by halogenation of N-substituted pyrroles with NBS,NCS,or surfuryl chloride.N-Substituted 3-halopyrroles were synthesized by acid-catalyzed thermal and photochemical isomerization reactions of N-substituted 2-halopyrroles.Both the thermal and photochemical reactions were acid-catalyzed.For the acid-catalyzed isomerization,a mechanism of [1,3] bromine shift followed by deprotonation is operated.For the acid-catalyzed photoisomerization,an excited triplet state of 2-protonated N-benzyl-2-halopyrrole produces an intermediate N-substituted pyrrole complex with halonium ion which is equilibrated with TV-substituted pyrrole plus halonium ion,and then the halonium ion newly adds to 3-position of N-substituted pyrrole followed by deprotonation to afford N-benzyl-3-halopyrrole.