Covalent anchoring of a cellulose per(phenyl carbamate) chiral selector onto silica gel through alkyne-azide click chemistry and its utilization in HPLC

High-performance liquid chromatography is a powerful tool for enantioseparation, based on chiral stationary phases as separation media. Cellulose-based chiral selectors are among the most successful ones used for the preparation of chiral separation materials, exploiting the inherent chirality of the homopolymer. Compared to initial coating-type chiral stationary phases solely deposited onto silica as the chromatographic support, covalently immobilized selectors exhibit a significantly broader scope of applicable eluents, but appropriate synthetic strategies are still scarce. In this work, we present the application of the Cu(I)-catalyzed Huisgen alkyne-azide click reaction as a means to covalently immobilize a cellulose 3,5-dichlorophenyl carbamate-type chiral selector to a silica-based chromatographic support. Cellulose was first functionalized with 3,5-dichlorophenyl carbamate groups (DS = 2.35) and 4-propargyloxy-3,5-dichlorophenyl carbamate groups (propargyl carbamate DS = 0.45, overall DS = 2.80), then clicked to 3-azidopropyl-functionalized silica gel as the chromatographic support affording a 9 wt.% covalently functionalized chiral stationary phase. The chiral selector was comprehensively characterized by means of ATR-FTIR and NMR spectroscopy, and elemental analysis. The degrees of substitution of both, overall functionalization and propargyl-anchor substitution, were estimated by NMR spectroscopy. Additionally, coating-type chiral stationary phases with 9 and 20 wt.% loadings were prepared. All chiral phases were tested with regard to their separation performance using a representative set of racemic analytes under usual normal-phase conditions. Solvent compatibility and thus the chemical robustness of the immobilized stationary phase was studied using higher shares of stronger, more polar solvents in the eluent, i.e., ethyl acetate, tetrahydrofuran and chloroform. The covalently linked selectors performed very favorably with regard to separation performance and stability.