Role of Lewis‐Base‐Coordinated Halogen(I) Intermediates in Organic Synthesis: The Journey from Unstable Intermediates to Versatile Reagents

Abstract

The widespread use of halogen‐based reagents in organic synthesis is well known. Hypervalent halogens (oxidation states III and V) have been explored to a significant extent. Among them, hypervalent iodine reagents have found major use and applications. In contrast, understanding of the reactivity of halogen(I) species in the presence of Lewis bases, and their reaction mechanisms, are very limited. This microreview sheds light on the importance of halogen(I) species and their applications in organic synthesis. In commercially available halogen(I) precursors such as N‐halosuccinimides, the halogen atom is attached to the nitrogen atom through a covalent bond with low electrophilicity and thus low reactivity. Interestingly, the reactivity increases if the electrophilic halogen(I) atom is attached to a Lewis base through a polarized covalent bond. The halogen‐bonding interaction between the Lewis base and the halonium ion (or halonium ion equivalent) results in highly reactive and thermodynamically stable halogen(I) intermediates that act as versatile reagents. This microreview unravels the evolution of the unstable halogen(I) intermediates to become versatile reagents.