Gas chromatographic enantioseparation of unfunctionalized chiral alkanes: A challenge in separation science (overview, state of the art, and perspectives)

Abstract

The chromatographic enantioseparation of small unfunctionalized chiral alkanes C*HR(1)R(2)R(3) (R = alkyl) represents a challenge in separation science. Because of the lack of any functional groups, enantiorecognition in the presence of a chiral selector is solely based upon weak enantioselective Van der Waals forces. Racemic alkanes containing seven and eight carbon atoms, i.e. 3-methylhexane (C7), 2,3-dimethylpentane (C7), 3-methylheptane (C8), 3,4-dimethylhexane (C8), 2,4-dimethylhexane (C8), 2,3-dimethylhexane (C8), and 2,2,3-trimethylpentane (C8) have been gas chromatographically enantioseparated on different modified cyclodextrins. The substitution pattern and cavity size of the cyclodextrin selectors have a pronounced effect on the degree of enantiorecognition observed. Thermodynamic parameters of enantiorecognition between four chiral alkanes and octakis(6-O-methyl-2,3-di-O-pentyl)-gamma-cyclodextrin (Lipodex G) have been determined. The possible role of molecular inclusion is indicated by the complete loss of enantioselectivity when the cyclodextrins are replaced by the corresponding linear dextrins ("acyclodextrins"). The enantioseparations of all seven chiral C7-C8 alkanes, six of them simultaneously, has been achieved on mixed binary selector systems whereby two different modified cyclodextrins are present in one gas chromatographic column. The smallest chiral (nonisotopically labeled) allene, i.e., 2,3-pentadiene, has been resolved gas chromatographically on a cyclodextrin selector.