Syntheses of cyclic carbonates with amidinium halide catalysts in reusable, reversible, room-temperature ionic liquids or acetonitrile

Abstract

In situ-prepared, reversible, room-temperature ionic liquids (RTILs), amidinium carbamates, have been used as media for the syntheses of cyclic carbonates by cycloaddition of CO₂ to epoxides. The amidinium carbamates were prepared by exposing equimolar mixtures of an easily synthesized amidine and a primary amine to CO₂ gas. For comparison purposes, amidinium dithiocarbamates were also employed as the RTILs in some experiments. Reaction between CO₂ and four epoxides in the RTILs, occurs in good yields (> 90% in many cases) at room temperature or 50 °C in the presence of an amidinium halide catalyst. Product and any unreacted epoxide were extracted easily upon addition of an immiscible, lower density liquid to an RTIL reaction mixture. This process was repeated three times with the same RTIL without any obvious decrease in catalytic activity; presumably, additional transformations could have been conducted. The influences of the type of catalyst, CO₂ pressure, reaction time, and temperature on the reaction yields have been investigated. The relatively mild reaction conditions and ease of separation of products, as well as the ability of the amidinium carbamates to be reused in the presence or absence of water make this an attractive alternative to other procedures for the efficient syntheses of cyclic carbonates.