ABSTRACT An unusual pyrrole by‐product formation has been discovered during the synthesis of pyrrolo1,2‐ a imidazole from imidazolium bromides and electron‐deficient acetylenes. The methodology proceeds smoothly under mild conditions, affording 5‐benzoyl‐pyrrolo1,2‐ 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 pyrrolo1,2‐ 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.
Chandrashekharappa et al. (Sun,) studied this question.