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), (<i>R</i>)-ethyl mandelate ((<i>R</i>)-EM) or methyl-(2<i>R</i>)-2-hydroxy-3-phenylpropanoate ((<i>R</i>)-MHPP) were studied. Solid-liquid equilibrium measurements showed that only TOABr:TBABr:(<i>R</i>)-MHPP with 0.66 mole fraction of (<i>R</i>)-MHPP were liquid at room temperature, but TOABr:TBABr:(<i>R</i>)-EM mixtures were liquid over a wider range, 0.6-0.8, mole fraction of (<i>R</i>)-EM. The density, viscosity, conductivity, and polarity were measured for three TOABr:TBABr:(<i>R</i>)-MHPP and six TOABr:TBABr:(<i>R</i>)-EM mixtures and compared to the TBABr:(<i>R</i>)-MHPP and TBABr:(<i>R</i>)-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:(<i>R</i>)-MHPP and TOABr:TBABr:(<i>R</i>)-EM, exhibit higher enantioselectivity than the corresponding binary DES, TBABr:(<i>R</i>)-MHPP, TBABr:(<i>R</i>)-EM, and TOABr:(<i>R</i>)-EM.