Remarkable enantioselectivity enhancement of the extractors with nonaxial chirality in liquid–liquid extraction of underivatized amino acids by introducing t‐butyl group

Abstract

A class of carbonyl extractors, (R)‐3, (R)‐4, and (R)‐5, with nonaxial chirality containing asymmetric carbons has been synthesized and studied for their efficiencies in enantioselective liquid–liquid extraction for underivatized amino acids. The bulky t‐butyl ketone extractors, (R)‐4 and (R)‐5, showed the stereoselectivities ranging 5.4–9.4 of l/d ratio much better than those of the aldehyde extractor, (R)‐3, ranging 2.4–5.2. The imine formation rates and yields of the t‐butyl ketones were not significantly affected by their bulkiness and even in the absence of resonance‐assisted hydrogen bond. This work confirms that a bulky t‐butyl ketone can be a good choice in the development of an extractor not only with axial chirality but also with nonaxial chirality for the enantioselective extraction of unprotected amino acids.

A new oxidatively stable ligand for the chiral functionalization of amino acids in Ni(II)–Schiff base complexes

“…Additionally, this bulky group may significantly alter the whole steric construction of a molecule and give rise to additional interactions which would increase the stereocontrolling properties. This idea efficiently works in the enantioselective extraction of the unprotected amino acids [ 38 – 39 ]. In the case of the Ni–Schiff base complexes, the t -Bu group may give rise to additional dispersion interactions with the phenyl ring in the proline auxiliary, making the Schiff base complexes more conformationally rigid, thus increasing the stereochemical outcome of the functionalized amino acids as compared to the parent ligand L1 .…”