Recent Advances in Phosphine-Promoted (4+1) Annulation Reactions

Over the past two decades, the nucleophilic phosphine catalysis has effected many powerful annulation reactions and emerged as an intriguing platform for the generation of highly functionalized carbo- and heterocycles. In addition to the well-established (3 + 2) annulation, the phosphine-promoted (4 + 1) annulation has received increasing attention and developed rapidly in recent years, which complements the (3 + 2) annulation and also provides an alternative approach to structurally diverse five-membered carbo- and heterocycles. This minireview summarizes the recent advances in phosphine-promoted (4 + 1) annulation reactions according to the type of substrates used in the annulations.