Effect Of Ortho-Substituent On Structure And Chiral Recognition Of Polysaccharide Phenylcarbamate Derivatives

Polysaccharide-based chiral stationary phases (CSPs) are well-known to exhibit high chiral recognition to more than 90% of racemic compounds for high-performance liquid chromatography (HPLC),especially those utilizing phenylcarbamate and benzoate derivatives of cellulose and amylose. The chiral recognition abilities of cellulose derivatives can significantly change depending on the substituent on the phenl group, and are absolutely low when bearing a farily polar group. Therefore, in order to investigate the influence of a farily polar substitution and ortho-substituted position on the structure and chiral recognition ability,a series of 3,5-dimethylphenylcarbamate derivatives of cellulose bearing an amount of 2-nitrophenylcarbamate at 6- position were synthesized in this paper, and their chiral recognition abilities on the molecular level are further explored compared with the substituent of 2-methyl and 2-fluoro.Firstly, a variety of 3,5-dimethylphenylcarbamate derivatives of cellulose and amylose bearing a mount of orthosubstitution, 2-nitro, 2-fluoro and 2-methyl, at 6-position were synthesized by the regioselective procedures, and their structures were characterized by 1H NMR and IR spectroscopy. Then,the CSPs based on the obtained polysaccharide derivatives were prepared by slurry method after coating on the aminopropyl silica gel. The coating ratio of CSPs was analyzed by thermogravimetry analysis (TGA). Finally, the chiral recognition abilities of the polysaccharide derivatives were evaluated through enantioseparation of nine racemates based on the novel CSPs by HPLC. The effect of electronic negativity and steric hindrance of ortho-substituent on the chiral recognition abilities of derivatives were further investigated.It can be found that the chiral recognition abilities of polysaccharide derivatives bearing different groups at 6-position could be significantly influenced by the amount of ortho-substituted phenylcarbamates and will change depending on the structures of racemates.Polysaccharide derivatives bearing the same amount of 2-methyl or 2-fluoro as 2-nitro phenylcarbamates at 6-position exhibited better chiral recognitions. The position and steric hindrance of the polar ortho-substitution of phenylcarbamate derivatives of cellulose and amylose could change the higher-order structures of the derivatives and enhance nonenantioselective interactions between CSPs and chiral compounds, as the results of lower chiral recognition abilities.