A Pyrrole Formation During the Synthesis of Novel Pyrrolo[1,2‐ a ]Imidazole via Cycloaddition Reaction of Imidazolium Salts With Electron Deficient Acetylenes

An unusual pyrrole by‐product formation has been discovered during the synthesis of pyrrolo[1,2‐ a ]imidazole from imidazolium bromides and electron‐deficient acetylenes. The methodology proceeds smoothly under mild conditions, affording 5‐benzoyl‐pyrrolo[1,2‐ a ]imidazole derivatives in moderate‐to‐good yields. Interestingly, the study revealed an unusual side reaction: moisture‐mediated ring opening of the imidazole nucleus, leading to pyrrole derivatives through an enamine hydrolysis pathway. Substrate scope investigations highlighted a striking selectivity while phenacyl bromides furnished both pyrrolo[1,2‐ a ]imidazoles and pyrroles, 2‐bromoacetonitrile derived imidazolium bromide ( 9 ) exclusively yielded pyrrole by‐product as a major ( 12b and 13b) suggesting the nitrile group plays a decisive role in product distribution.