Designing Ternary Chiral DES to Enhance Enantioselectivity

Deep eutectic solvents (DES) are mixtures of two or three components that have a freezing point lower than that of the components. The properties of DES are tunable through the choice and ratio of the components. Three-component or ternary DES increase the flexibility to control the properties compared to binary DES. Two chiral ternary DES composed of tetraoctylammonium bromide (TOABr), tetrabutylammonium bromide (TBABr), (R)-ethyl mandelate ((R)-EM) or methyl-(2R)-2-hydroxy-3-phenylpropanoate ((R)-MHPP) were studied. Solid-liquid equilibrium measurements showed that only TOABr:TBABr:(R)-MHPP with 0.66 mole fraction of (R)-MHPP were liquid at room temperature, but TOABr:TBABr:(R)-EM mixtures were liquid over a wider range, 0.6-0.8, mole fraction of (R)-EM. The density, viscosity, conductivity, and polarity were measured for three TOABr:TBABr:(R)-MHPP and six TOABr:TBABr:(R)-EM mixtures and compared to the TBABr:(R)-MHPP and TBABr:(R)-EM. The enantioselectivity of the ternary DES was measured using circularly polarized luminescence (CPL) spectroscopic measurements of a luminescent lanthanide complex dissolved in the solvents. The ternary DES, TOABr:TBABr:(R)-MHPP and TOABr:TBABr:(R)-EM, exhibit higher enantioselectivity than the corresponding binary DES, TBABr:(R)-MHPP, TBABr:(R)-EM, and TOABr:(R)-EM.