Phenylcarbamates, chiral recognition

The phenylcarbamate derivative 23j also showed chiral recognition for many racemates in CDCI3 (Figure 3.38). Figure 3.39 shows the 1H NMR spectra of ( )-2-butanol in the absence and presence of 23j. In the case of secondary alcohols, such as 2-butanol, 2-heptanol, and 2-octanol, the methyl protons at the end of the longer chain and remote from the stereogenic center were enantiomerically separated in the presence of 23j, and the methine and... 

The chiral recognition ability of a series of cellulose phenylcarbamate derivatives has been evaluated extensively [18,43]. The introduction of an electron-donating methyl group or an electron-withdrawing halogen at the 3-and/or 4-position improved the resolution ability, but substituents at position 2 showed poor chiral resolution capacity. The derivatives with heteroatom substi-... 

Polysaccharide-based CSPs also exhibit a chiral recognition for alcohols and a large number of resolutions have been reported. Chiral alcohols can usually be directly resolved with hexane containing a small amount of an alcohol as the eluent. For aliphatic alcohols, which cannot be directly resolved, their resolution is often efficiently attained as phenylcarbamate or benzoate derivatives on OD (Figure 17).85 For example, 2-butanol and 2-pentanol are completely resolved with a very high selectivity on OD as their phenylcarbamates. The derivatization of alcohols to phenylcarbamates and benzoates can be easily achieved by the reaction with phenyl isocyanates and benzoyl chlorides, respectively. In most cases, the phenylcarbamates are better resolved than the benzoates. For chiral compounds bearing phenolic hydroxy groups, the addition of a small amount of an acid to an eluent is recommended to depress its dissociation. 

Enomoto, N., Furukawa, S., Ogasawara, Y., Akano, H., Kawamura, Y., Yashima, E., and Okamoto, Y. (1996) Preparation of silica gel-bonded amylose through enzyme-catalyzed polymerization and chiral recognition ability of its phenylcarbamate derivative in HPLC, Anal. Chem. 68, 2798-2804. 

Amylose has also been derivatized into various phenylcarbamates and evaluated as CSPs in HPLC [48,49]. Again, 3,5-dimethylphenylcarbamate (32, Fig. 15) is one of the most useful derivatives and has been commercialized as Chiralpak AD from Daicel. Chiral recognition of the amylose derivative is rather complimentary to that of 31x, and many compounds that are difficult to resolve on 31x can be resolved on the amylose derivative 32. With these two 3,5-methylphenylcarbamates, nearly 80% of 500 racemates have been resolved by my group [43]. 

Structural Analysis of Phenylcarbamate Derivatives of Cellulose and Amylose Chiral Recognition Mechanism. 

Compared to the non-substituted cellulose derivative 6f, the phenylcarbamates bearing electron-withdrawing substituents, such as halogens, or electron-donating substituents, such as alkyl groups, exhibit better chiral recognitions. These substituents appear to affect the polarity of the carbamate group via an inductive effect and alter the interaction mode between the cellulose derivatives and the racemates. 

---