Hydrogen bonding-catalyzed synthesis of 1,4-dioxanes from dehydrative cyclization of vicinal diols in ionic liquids†

1,4-Dioxane and its derivatives are a class of important chemicals, which can be produced via dehydrative cyclization of vicinal diols catalyzed by acid or Lewis acid catalysts, but generally suffer from their inherent shortcomings. Herein, we report a hydrogen-bonding catalyzed approach for the synthesis of 1,4-dioxanes from vicinal diols in ionic liquids under metal-free conditions. The ILs possessing a cation hydrogen-bond donor and an anion hydrogen-bond acceptor (e.g., 1-hydroxyethyl-3-methyl imidazolium trifluoromethanesulfonate, [HO-EtMIm][OTf]) were found to be effective for the reaction. In particular, [HO-EtMIm][OTf] showed the best performance among the tested ILs, even better than H₂SO₄, affording a series of 1,4-dioxanes in high yields and excellent selectivity under mild conditions (e.g., 120 °C). The mechanism investigation indicates that the IL cation and anion as the hydrogen-bond donor and acceptor, respectively, can activate the C–O and O–H bonds of diols via hydrogen-bonding, which synergistically realizes the dehydrative cyclization of vicinal diols to generate 1,4-dioxanes. In addition, this IL could be reused 5 times without activity loss. This work provides a simple, green route to produce 1,4-dioxanes, which may have promising applications.