Intramolecular Dehydration of Cyclic Alcohols to Cyclic Alkenes via Cation- and Anion-Confined Catalysis over Ionic Liquids

Catalytic intramolecular dehydration of cyclic alcohols to cyclic alkenes is very interesting but still challenging due to low selectivity and difficulty in separation. Herein, we report a cation- and anion-confined catalysis strategy for the synthesis of cyclic alkenes from selective dehydration of biomass-derived cyclic alcohols over Lewis acid imidazolium-Zn based ionic liquids (ILs) (e.g., [BMIm][Zn2Cl5]) under mild conditions. It is found that [BMIm][Zn2Cl5] displays extraordinary catalytic performance, affording a series of cyclic alkenes in excellent yields with a selectivity of >99% in most cases. Mechanistic studies reveal that the interactions between the cation and anion of [BMIm][Zn2Cl5] provide a confined microenvironment for barely exposing Zn2+ to coordinate with hydroxyl O of a cyclic alcohol, thus achieving exclusively intramolecular dehydration of cyclic alcohols to cyclic alkenes. Moreover, the products could be readily separated from the reaction system by phase separation due to their immiscibility with the IL, and the IL could be reused without activity loss for seven runs. This strategy provides a promising alternative to produce cyclic alkenes from renewable biomass-derived alcohols in a green manner.