Reversible, Room-Temperature, Chiral Ionic Liquids. Amidinium Carbamates Derived from Amidines and Amino-Acid Esters with Carbon Dioxide

Abstract

The properties of a new class of chiral, room-temperature, ionic liquids (RTILs) are described. They are made from easily synthesized, readily available materials and can be transformed reversibly to their nonionic liquid states. The nonionic liquids consist of neat equimolar mixtures of a N‘-alkyl-N,N-dimethylacetamidine (L) and an alkyl ester of a naturally occurring amino acid (n). When exposed to 1 atm of CO2 gas, the L/n solutions become cationic−anionic pairs, amidinium carbamates. Of the 50 L/n combinations examined, all except those involving the methyl ester of tyrosine (which was immiscible with the amidines) form RTIL states under CO2 atmospheres, and several remain liquids to at least −18 °C. Heating the ionic liquids in air at ca. 50 °C or bubbling N2 gas through them at ambient temperatures for protracted periods displaces the CO2 and re-establishes the nonionic L/n states. As an example of the changes effected by cycling between the two liquid states, a spectroscopic probe, 1-(p-dimethylami...