Cation-anion confined hydrogen-bonding catalysis strategy for ring-closing C-O/O-H metathesis of alkoxy alcohols under metal-free conditions

• Published in: Green chemistry : an international journal and green chemistry resource : GC 2023-10, Vol.25 (21), p.8791-8797
• Main Authors: Wang, Huan, Zhao, Zhi-Hao, Zhao, Yanfei, Zhang, Fengtao, Xiang, Junfeng, Han, Buxing, Liu, Zhimin
• Format: Article
• Language: English
• Subjects: Alcohols; Anions; Catalysis; Catalysts; Cations; Chemical bonds; Ethers; Green chemistry; Hydrogen bonding; Hydrogen bonds; Ionic liquids; Metathesis; Microenvironments
• ISSN: 1463-9262; 1463-9270
• DOI: 10.1039/d3gc03041e

Ring-closing metathesis (RCM) reactions of multiple bonds have seen considerable progress; however, RCM reactions involving single bonds, especially two different single bonds are scarce and extremely challenging. Herein, we present a cation-anion confined hydrogen bonding catalysis strategy for catalyzing the ring-closing C-O/O-H metathesis of alkoxy alcohols to O-heterocycles under metal-free conditions. Assisted with theoretical computation, the effective ionic liquid catalysts were first predicted. [HO-EtMIm][OTf] was found to display the highest activity, consistent with the predicted results. This catalyst could afford a series of O-heterocycles, including tetrahydrofurans, tetrahydropyrans, dioxanes, and some complex ethers that are difficult to access via conventional routes. Moreover, it was recyclable and reusable without activity loss after 5 recycles. Comprehensive investigations endorse that [HO-EtMIm] + cation and [OTf] − anion selectively form hydrogen bonds with the ether O atom and hydroxyl H atom of alkoxy alcohol in opposite directions, respectively, which cooperatively catalyze the reaction in the cation-anion confined ionic microenvironment. The strategy presented here provides a novel and green route to access cyclic ethers. Cation-anion confined hydrogen-bonding-catalyzed ring-closing C-O/O-H metathesis of alkoxy alcohols to O-heterocycles is realized over OH-functionalized ionic liquids under metal-free and mild conditions (≥70 °C).