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.