Preparation of cellulose derivative bearing bulky 4-(2-benzothienyl)phenylcarbamate substituents as chiral stationary phase for enantioseparation

Abstract

A novel cellulose derivative bearing bulky 4-(2-benzothienyl)phenylcarbamate substituents (Cel-1) was readily synthesized by carbamoylation followed by Suzuki-Miyaura coupling reaction. The corresponding coated-type chiral stationary phase (CSP) was prepared on basis of the derivative, and its chiral recognition ability was then evaluated by high-performance liquid chromatography (HPLC). The chiral recognition ability of the cellulose derivative was greatly influenced by introduction of the bulky benzothienyl pendants on the aromatic moieties of phenylcarbamates, compared with its analog with smaller groups. Many racemates, including the metal tris(acetylacetonate) complexes, chiral drug, and the analyte with axial chirality, were sufficiently separated with good enantioselectivities on Cel-1. Some of them were even higher than those on the commercially powerful Chiralcel OD, which is also a coated-type CSP derived from cellulose phenylcarbamate derivative containing smaller 3,5-dimethyl pendants. The ¹ H NMR and circular dichroism (CD) spectra of Cel-1 indicated that the obtained derivative possessed a regular higher order structure, and a strong cotton effect was observed within the absorption range of π-conjugated pendant at 350-500 nm. Impressively, the cellulose derivative bearing the bulky 4-(2-benzothienyl)phenylcarbamates exhibited good enantioselective fluorescence quenching behavior to the enantiomer pair of 1-phenylethylamine, probably suggesting its potential for the application as a chiral fluorescent sensor with high efficiency. The combination of the arrangement of bulky π-conjugated benzothienyl pendants on the phenylcarbamate moieties surrounding the helical backbone and the regular higher order structure of the polymer itself probably played a key role for this high chiral fluorescent recognition ability of Cel-1. The interaction sites of bulky 4-(2-benzothienyl)phenylcarbamate pendants in its excited state can exhibit higher enantioselective discrimination via fluorescent response to the chiral compound Q1, whereas the chiral recognition ability of Cel-1 to the same compound in the ground state had no clear improvement.

Keywords: cellulose 4-(2-benzothienyl)phenylcarbamate; chiral stationary phase; enantioselective fluorescence quenching; enantioseparation; high-performance liquid chromatography.