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

Abstract

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.