Hydrogen-Bonding Catalyzed Ring-Closing C-O/C-O Metathesis of Aliphatic Ethers over Ionic Liquid under Metal-Free Conditions.

O-heterocycles have wide applications, and their efficient and green synthesis is very interesting. Herein, we report hydrogen-bonding catalyzed ring-closing metathesis of aliphatic ethers to O-heterocycles over ionic liquid (IL) catalyst under metal- and solvent-free conditions. The IL 1-butylsulfonate-3-methylimidazolium trifluoromethanesulfonate ([SO3 H-BMIm][OTf]) is discovered to show outstanding performance, better than the reported catalysts. An interface effect plays an important role in mediating the reaction rate due to the immiscibility between the products and the IL catalyst, and the products can be spontaneously separated. NMR analysis and DFT calculation suggest that a pair of cation and anion of [SO3 H-BMIm][OTf] could form three strong H-bonds with an ether molecule, which catalyze the ether transformation via a cyclic oxonium intermediate. A series of O-heterocycles including tetrahydrofurans, tetrahydropyrans, morpholines and dioxane can be obtained from their corresponding ethers in excellent yields (e.g., >99 %). This work opens an efficient and metal-free way to produce O-heterocycles from aliphatic ethers.