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 1butylsulfonate-3-methylimidazolium trifluoromethanesulfonate ([SO 3 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 [SO 3 H-BMIm][OTf] could form three strong Hbonds 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 Liquids with Multi‐Active Sites Synergistically Catalyzed Metal‐Free Transformation of Alcohols Using Dimethyl Carbonate as an Environmental Solvent

“…According to our results and the previous report, [24–28,44] we proposed a possible mechanism involving carbocation and ethers as the intermediates and synergistically promoted effect of hydrogen bonding (Scheme 6). Initially, we thought that the activities may be significantly influenced by the acidities of the ionic liquids.…”

“…And trifluoromethylsulfonate was also essential for the reaction after testing several anions. Moreover, Liu et al proved the hydrogen bonding effect between ionic liquids and ethers using NMR analysis and DFT calculation [44] . Therefore, we proposed cation and anion of the ionic liquids would synergistically catalyze the reaction in our developed system through the effect of hydrogen bonding based on the phenomena.…”

Hydrogen-bond donor and acceptor cooperative catalysis strategy for cyclic dehydration of diols to access O-heterocycles

“…S4), suggesting its high stability under the experimental conditions (fig. S5) ( 30 , 34 ). Thus, the [HO-EtMIm][OTf] catalyst shows a superior waste balance with economy factors (E-factors; that is the mass ratio of waste/isolated product) down to 0.40, indicating that this method has promising application potential ( 35 , 36 ).…”

“…Basic IL 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) could efficiently catalyze one-pot oxidative transformation of alcohols into esters by hydrogen bond ( 29 ). The imidazolium ILs functionalized with -SO 3 H could catalyze the ring-closing metathesis of aliphatic ethers through three strong hydrogen bonds ( 30 ). Obviously, the hydrogen bond interactions in IL systems play important roles in these reactions.…”

Hydrogen‐Binding‐Initiated Activation of O−H Bonds on a Nitrogen‐Doped Surface for the Catalytic Oxidation of Biomass Hydroxyl Compounds

“…[3] Interestingly,h owever,t his characteristic also endows HBs with the ability to facilitate activation of O À Hbonds,through forming strengthened HBs with other proton acceptors such as electric-rich heteroatoms and halogen ions. [4] Thedecreased electron cloud density near the proton nucleus might weaken OÀHbonds,thus enhancing the reactivity of hydroxyl groups.Recently,some explorative studies have been performed in utilizing HBs for promoting the conversion of inert hydroxyl substrates and guiding pathways in purpose.F or example,t he selective activation of the primary hydroxyl group in diol could be accomplished via forming OÀH•••O=SHBs with DMSO as aproton acceptor, which was in favor of the selective alkoxyl radical transformation, rather than occurring b-scission due to the unselective activation of both two hydroxyl groups. [5] Han et al [6] presented that the HBs between the alcohol hydroxyl groups and ionic liquid [EMIM]OAc could accelerate the rate-determining step of hydroxyl groups activation, thus significantly promoting the oxidative esterification of inert aliphatic alcohols.I nt he case of heterogeneous catalysis, researchers also found that the surface-grafted nitrogencontaining ligands on EPy/Pt/Al 2 O 3 catalyst could form HBs with specific hydroxyl groups.…”