Metal-free synthesis of heterocycles via activation of alkynes by hypervalent iodine

In recent years, hypervalent iodine reagents have gained increasing popularity in metal-free oxidative transformation due to their low toxicity, easy handling, transition-metal-like reactivity, and other benefits. However, although metal-free oxidative transformations of alkynes using iodine (III) reagents have been well studied for the preparation of alkynyl (aryl) iodonium and alkenyl (aryl) iodonium salts, stepwise methods including preparation step of these iodonium species have been commonly used in the synthesis of heterocycles from alkynes. In this account, our two approaches to the metal-free and single-step synthesis of heterocycles via an activation of alkynes by iodine (III) are mainly described. One is iodine (III)-mediated/catalyzed oxidative cycloisomerization reactions of propargyl compounds for the preparation of heterocycles bearing various functional group at their side chains. In these reactions, iodine (III) species works as an activator of carbon-carbon triple bonds as well as a donor of various heteroatomic functional groups. This methodology can be extended to iodine (III)-mediated/catalyzed oxidative annulation of alkynes and nitriles as another approach, in which heteroatoms on iodine (III) species are incorporated in the azole rings. In addition, molecular iodine-catalyzed cyclization of o-alkylanilines via iodocyclization/proto-deiodination sequence is described as an iodine catalysis without terminal oxidants. © 2021 Society of Synthetic Organic Chemistry. All rights reserved.