N‐Heterocyclic Carbene Catalyzed Reactions Involving Acetylenic Breslow and/or Acylazolium as Key Intermediates

Abstract

N‐heterocyclic carbene (NHC) organocatalysis has been developed as a powerful tool in modern synthetic chemistry. NHC catalytic activation of ynals and alkynoic acid derivatives provided versatile reactions that involve acetylenic Breslow and/or acylazolium as key intermediates, and diverse transformations have been established for access to molecules with unique skeletons in efficient fashions. Herein we summarize the recent achievements in NHC‐catalyzed reactions involving acetylenic Breslow and/or acylazolium intermediates. Different reactions belonging to three catalytic modes, including (1) conjugate additions to acetylenic Breslow derived α,β‐unsaturated acylazolium intermediates, (2) β‐umpolung of ynals via acetylenic Breslow intermediates, and (3) conjugate additions to acetylenic acylazolium intermediates, are emphasized with examples and plausible mechanisms cited to guide a better understanding.