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

Selective intramolecular dehydrationof cyclic alcoholsto cyclic alkenes via cation- and anion-confinedcatalysis over ionic liquids. Catalytic intramolecular dehydration of cyclic alcoholsto cyclicalkenes is very interesting but still challenging due to low selectivityand difficulty in separation. Herein, we report a cation- and anion-confinedcatalysis strategy for the synthesis of cyclic alkenes from selectivedehydration of biomass-derived cyclic alcohols over Lewis acid imidazolium-Znbased 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 selectivityof >99% in most cases. Mechanistic studies reveal that the interactionsbetween the cation and anion of [BMIm][Zn2Cl5] provide a confined microenvironment for barely exposing Zn2+ to coordinate with hydroxyl O of a cyclic alcohol, thusachieving exclusively intramolecular dehydration of cyclic alcoholsto cyclic alkenes. Moreover, the products could be readily separatedfrom the reaction system by phase separation due to their immiscibilitywith the IL, and the IL could be reused without activity loss forseven runs. This strategy provides a promising alternative to producecyclic alkenes from renewable biomass-derived alcohols in a greenmanner.