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-dimethylaminophenyl)-2-nitroethylene, senses a polarity like that of toluene before a mixture of N‘-octyl-N,N-dimethylacetamidine/isoleucine methyl ester is exposed to CO2 and a polarity like that of N,N-dimethylformamide afterward; whereas a low-polarity solvent, decane, is solublized readily by the nonionic L/n mixture, it is immiscible with the RTIL. Thermal and spectroscopic properties of both the nonionic and ionic phases are reported and compared. Several possible applications for these RTILs can be envisioned because, unlike many other ionic liquids, these need not be prepared and handled under scrupulously dry conditions and they can be cycled repeatedly between high- and low-polarity states