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

Abstract

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 ([SO3H‐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 [SO3H‐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.

Ionic Liquid‐Catalyzed C−C Bond Formation for the Synthesis of Polysubstituted Olefins

“…As we know, the H-bond interaction could activate ether to improve the electrophilicity of the C atom near the oxygen atom. [26] To further explore the mechanism between the catalyst and the substrates, density functional theory (DFT) was used to simulate this system. The detailed mechanism has been studied for the coupling reaction of benzylic ether 1 d and alkene 2 k catalyzed by cation and anion of acidic thiomorpholine 1,1- dioxide ionic liquid (SO 3 H-ILs), leading to tri-substituted olefin 3 k and methanol (Scheme 3).…”

“…As we know, the H‐bond interaction could activate ether to improve the electrophilicity of the C atom near the oxygen atom [26] . To further explore the mechanism between the catalyst and the substrates, density functional theory (DFT) was used to simulate this system.…”

“…Varies interactions, such as hydrogen bonding, and hydrophilic/hydrophobic interaction, exist widely in ILs‐catalyzed systems and are very attractive [23–25] . For instance, SO 3 H‐ILs or HO‐ILs were applied to access O ‐heterocycles from aliphatic ethers and diols via hydrogen bonding interaction [26,27] . In the utilization of CO 2 , a strong hydrogen bonding between ILs and epoxide could increase the C−O bond length, which was benefit for the ring‐opening process of the epoxide [28] …”

“…[23][24][25] For instance, SO 3 H-ILs or HO-ILs were applied to access O-heterocycles from aliphatic ethers and diols via hydrogen bonding interaction. [26,27] In the utilization of CO 2 , a strong hydrogen bonding between ILs and epoxide could increase the CÀ O bond length, which was benefit for the ring-opening process of the epoxide. [28] During the ongoing investigation about the synthesis of polysubstituted olefins, we have developed a method to access polysubstituted olefins from olefins and alcohols catalyzed by sulfonic acid-functionalized ionic liquids.…”

Selective isomerization of α-pinene oxide totrans-carveol by task-specific ionic liquids: mechanistic insightsviaphysicochemical studies

“…[14][15][16] The unique properties of ILs endow them with the opportunity to serve as metal-free catalysts. 17 In addition, the isomerization reaction is one of the best methods for preparing a variety of industrially important products. It has been noticed that basic ILs composed of [N 4,4,4,4 ]Pro (tetrabutyl ammonium proline) and [TMG]Pro (tetramethyl guanidinium proline) have the tendency to efficiently isomerize glucose into fructose.…”