Nonclassical Mechanism in the Cyclodehydration of Diols Catalyzed by a Bifunctional Iridium Complex

1,4‐ and 1,5‐diols undergo cyclodehydration upon treatment with cationic N‐heterocyclic carbene (NHC)–IrIII complexes to give tetrahydrofurans and tetrahydropyrans, respectively. The mechanism was investigated, and a metal‐hydride‐driven pathway was proposed for all substrates, except for very electron‐rich ones. This contrasts with the well‐established classical pathways that involve nucleophilic substitution.
Hydrogen transfer vs. acid catalysis: Mechanistic studies on the iridium‐catalyzed cyclodehydration of a series of diols are carried out. Hammett and Hammett–Brown analyses of the reactivity data are compared, along with kinetic isotope effect and crossover experiments. This analysis enabled the elucidation of the reaction mechanisms, which are found to be dependent on the electronic properties of the substrates (see picture).