In order to overcome the disadvantages of excess water and many by-products in the conventional process of epoxide hydration to 1,2-diols, 2-carboxylated imidazolium salts were initially used as efficient catalysts for the one-step hydration of epoxides to 1,2-diols. By regulating the cation chain lengths, different steric structures of 2-carboxylated imidazolium salts with chain lengths from C1 to C4 were produced. The shortest chain chain salt (DMIC) exhibited better thermal stability and catalytic performance for hydration, achieving an ethylene oxide (EO) conversion of almost 100% and an ethylene glycol (EG) selectivity of almost 100% at 120 ° C , 0.5 h with only 5 x the molar ratio of H2O to EO. Such a tendency is further confirmed and explained by both the XPS analysis and DFT calculations. Compared to other salts with longer chains, DMIC has a stronger interaction of CO2 Anions and imidazolium cations, which have a lower tendency to release CO. exhibit2 and form HO – CO2that can attack nucleophilically and synergistically activate the ring opening of epoxides with imidazolium cations. The ring-opening transition state with a strong steric dynamic structure slows down the side reaction, and both cations and anions stabilize the transition state imidazolium-EG-HO-CO2both of which could avoid excessive hydration to byproducts, which explains the high 1,2-diol yield. Building on this, the cation-anion synergism mechanism is then proposed.

Graphic summary: Sterically controlling 2-carboxylated imidazolium salts for the one-step efficient hydration of epoxides to 1,2-diols

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