Cis‐trans isomerization of styrylpyrroles by stray light

초록

A series of substituted N‐methylstyrylpyrroles (H, p‐Br, p‐CN, o,p‐diCl, m‐CH<inf>3</inf>, m‐CN, m‐NO<inf>2</inf>) were prepared via the Wittig reaction of 1‐methylpyrrole‐2‐carboxaldehyde and substituted benzyltriphenylphosphonium bromides. Both cis and trans isomers were found to be present in the reaction mixture and they were separable by column chromatography in a few cases (H, p‐Br, m‐CN, m‐NO<inf>2</inf>). Photochemical isomerizations of cis‐styrylpyrroles to trans isomers were observed when the substituents were o,p‐diCl and m‐NO<inf>2</inf>, while the opposite was the case with compounds having H, p‐Br, m‐CH<inf>3</inf>, m‐CN. It was difficult to separate the cis and trans mixture of p‐cyanostyrylpyrroles and the equilibrium ratio did not change under similar photochemical reaction conditions. Copyright © 1993 Journal of Heterocyclic Chemistry