Synthesis and structural characterization of l-prolinol derived chiral eutectic mixtures as sustainable solvents in asymmetric organocatalysis

Abstract

The present study focuses on the synthesis, characterization, and examination of the organocatalytic properties of a series of structurally novel l-prolinol-based chiral solvents. Different l-prolinol/TBAB and l-prolinol/glycolic acid (GA) mixtures have been characterized and evaluated for their structural characteristics using ATR-FTIR spectroscopy, differential scanning calorimetry (DSC), and NMR techniques including diffusion and NOESY NMR measurements. The structural characteristics of the l-prolinol-based chiral solvents enabled these novel materials to display organocatalytic activities in the asymmetric conjugate addition of ketones to nitroolefins, with the chiral liquid l-prolinol/GA 1/1 showing the highest yields and selectivities, which are similar or superior to those displayed by l-prolinol in VOC or under neat conditions. Moreover, the new chiral liquid was successfully recovered and reused with minimal loss of performance over several cycles. Regarding the reaction mechanism, a rapid formation of the oxazoline intermediate has been detected by NMR spectroscopy.