2H-Azirines: Recent Progress in Synthesis and Applications

2H-arizines are important three-membered heterocycles in organic chemistry. Recently, many advances in the synthesis and functionalization of 2H-arizines have been reported. Neber rearrangement, isomerization of isoxazole, oxidation of enamine, C-H bond activation, decomposition of vinyl azide, functionalization of alkyne, and multi-step synthesis have been developed for the efficient assembly of 2H-azirines. As versatile and highly strained three-membered unsaturated heterocycles, 2H-azirines can be used as distinctive building blocks towards significant functional groups, and have attracted extensive attention for the fabrication of numerous heterocycles. Recent studies have focused on [3+n] ring-expansion reactions, nucleophilic addition of C=N double bond or continuous nucleophilic addition followed by cyclization, ring-opening of 2H-azirines to acyclic compounds, and substitution of sp(3)-C-H of 2H-azirines. One of the most critical challenges is seeking for a selective opening of the specific three bonds of the strained azirine circle, some of what can be achieved on the basis of metal catalyst, photocatalysis, or combination of metal catalyst and photocatalysis. In this review, recent advances involving the synthesis and reactivity of 2H-arizines accompanied with challenges and breakthroughs are summarized. The review mainly covers the period from 2019 to 2023.