Evaluation of nickel(II) bis[α-(heptafluorobutanoyl)-terpeneketonates] as chiral stationary phases for the enantiomer separation of alkyl-substituted cyclic ethers by complexation gas chromatography

Abstract

A method is described which permits the determination of thermodynamic data for molecular association, as well as the enantioselectivity — Δ R,S (Δ G°), from relative retention data by complexation gas chromatography. Thus, the solute—solvent association equilibria between two achiral, and fifteen chiral alkyl-substituted cyclic ethers and twelve non-racemic nickel(II) bis[α-(heptafluorobutanoyl)terpeneketonates] in squalane have been measured at 60°C. The selectivity of the solute—solvent association between alkyl-substituted oxiranes and the twelve nickel terpeneketonates follows a common trend which is rationalized in terms of opposing electronic and steric effects of the Lewis bases. The origin of the striking influence of ring size of cyclic ethers on the association strength with the twelve nickel terpeneketonates, which varies by two orders of magnitude, is unknown. Improved chiral stationary phases for the enantiomer separation of alkyl-substituted cyclic ethers have been found. The highest enantiomeric bias on racemic oxiranes is induced by nickel(II) bis[3-(hepta-fluorobutanoyl)-(1 R,2 S)-pinan-4-onate], containing a bicyclic terpene structure, and by nickel(II) bis[5-(heptafluorobutanoyl)-( S)-carvonate], containing a monocyclic terpene structure, respectively. The enantioselectivity, — Δ R,S (Δ G°), is generally high when the solute—solvent interaction is intermediate. For chiral alkyl-substituted oxiranes a consistent relationship between molecular configuration and the order of elution is observed for almost all of the twelve nickel terpeneketonates used as solvent. The synthesis of these solvents is described in detail.