and Gil-Av 1967; Feibush et al. 1972) consisted of carbonyl-bis amino acid esters,
so-called “ureido phases”, which offered the possibility of separating the enantiomers of acylated secondary amines in addition to amino acids. Particularly high
enantioselectivity was achieved by n-dodecanoyl-L-valine-tertbutylamide phases
introduced by Feibush (1971). This diamide possessed only one chiral centre,
however, several centres suitable for the formation of hydrogen bonds. Somewhat
unexpected was the wide range of application of n-dodecanoyl-(S)-α-(1-naphthyl)
ethylamide with only one amide function (Weinstein et al. 1976). On this phase, not
only amino acids and amines, but also α-chiral carboxylic acid amides could be
separated.
A considerable advancement in applicability and practicability of
enantioselective gas chromatography represented the fixation of the chiral selectors
to a polymer, a very innovative new concept suggested by Frank et al. at the end of
the 1970s (Frank et al. 1977, 1978). By co-polymerisation of dimethylsiloxane with
(2-carboxypropyl) methylsiloxane and coupling of the carboxy group with the amino
group of L-valine-tertbutylamide, a polysiloxane with chiral side chains were
obtained. This chiral polymer, commercially available as Chirasil-val
® , exhibited
both increased temperature stability above 200
C, excellent enantioselectivity, and
a wide scope of application. In line with this concept, however, a different approach
was pursued by Sandra and co-workers (Saeed et al. 1979). After hydrolysis of the
cyanoalkyl side chains of the polysiloxanes OV-225 or Silar-10C (for structure
information see Fernandezsanchez et al. 1991, Bose et al. 2006, Moskovkina et al.
2013), they obtained chiral polymers by coupling L-valine-tertbutylamide to the
carboxyl groups. However, according to König (1987), temperature stability and
selectivity for the separation of amino acid enantiomers were not quite as good as in
the case of Chirasil-val.
Similarly, König and Benecke functionalised cyanoalkyl-polysiloxanes by reducing the nitrile group to aminomethyl groups and coupling N-acylated amino acids.
These phases were well suited to separate amino acids, amino alcohols and amines.
Far more versatile in their applications turned out to be chiral polymers prepared by
modifying the polysiloxane XE-60 (König and Benecke 1981; König et al. 1981).
Another type of chiral polymers was introduced by Schomburg as promising
CSPs (Schomburg et al. 1985): Carbowax 20M and acryloyl-L-valine-(S)-αphenylethylamide were crosslinked by treatment with dicumoyl peroxide and at
the same time immobilised on the inner surface of the capillary column.
Diastereomeric association may also take place between chiral molecules and
chiral transition metal complexes as early studies already revealed. An experimental
approach taking advantage of this principle for enantioselective separation was first
described by Schurig (Schurig 1977; Schurig and Bürkle 1982). Since hydrogen
bonding is not essential for chiral recognition in these complexes, a number of
compounds without functional groups of this kind could be separated, such as cyclic
ethers, epoxides and spiroacetals. Thus, complexation gas chromatography can be
considered complementary to enantiomer separation on diamide CSPs. The application of metal-based/inorganic CSPs for chiral separation is currently a major focus
5.1 The Evolution of Chiral Stationary Phases for Capillary Gas Chromatography
77
so-called “ureido phases”, which offered the possibility of separating the enantiomers of acylated secondary amines in addition to amino acids. Particularly high
enantioselectivity was achieved by n-dodecanoyl-L-valine-tertbutylamide phases
introduced by Feibush (1971). This diamide possessed only one chiral centre,
however, several centres suitable for the formation of hydrogen bonds. Somewhat
unexpected was the wide range of application of n-dodecanoyl-(S)-α-(1-naphthyl)
ethylamide with only one amide function (Weinstein et al. 1976). On this phase, not
only amino acids and amines, but also α-chiral carboxylic acid amides could be
separated.
A considerable advancement in applicability and practicability of
enantioselective gas chromatography represented the fixation of the chiral selectors
to a polymer, a very innovative new concept suggested by Frank et al. at the end of
the 1970s (Frank et al. 1977, 1978). By co-polymerisation of dimethylsiloxane with
(2-carboxypropyl) methylsiloxane and coupling of the carboxy group with the amino
group of L-valine-tertbutylamide, a polysiloxane with chiral side chains were
obtained. This chiral polymer, commercially available as Chirasil-val
® , exhibited
both increased temperature stability above 200
C, excellent enantioselectivity, and
a wide scope of application. In line with this concept, however, a different approach
was pursued by Sandra and co-workers (Saeed et al. 1979). After hydrolysis of the
cyanoalkyl side chains of the polysiloxanes OV-225 or Silar-10C (for structure
information see Fernandezsanchez et al. 1991, Bose et al. 2006, Moskovkina et al.
2013), they obtained chiral polymers by coupling L-valine-tertbutylamide to the
carboxyl groups. However, according to König (1987), temperature stability and
selectivity for the separation of amino acid enantiomers were not quite as good as in
the case of Chirasil-val.
Similarly, König and Benecke functionalised cyanoalkyl-polysiloxanes by reducing the nitrile group to aminomethyl groups and coupling N-acylated amino acids.
These phases were well suited to separate amino acids, amino alcohols and amines.
Far more versatile in their applications turned out to be chiral polymers prepared by
modifying the polysiloxane XE-60 (König and Benecke 1981; König et al. 1981).
Another type of chiral polymers was introduced by Schomburg as promising
CSPs (Schomburg et al. 1985): Carbowax 20M and acryloyl-L-valine-(S)-αphenylethylamide were crosslinked by treatment with dicumoyl peroxide and at
the same time immobilised on the inner surface of the capillary column.
Diastereomeric association may also take place between chiral molecules and
chiral transition metal complexes as early studies already revealed. An experimental
approach taking advantage of this principle for enantioselective separation was first
described by Schurig (Schurig 1977; Schurig and Bürkle 1982). Since hydrogen
bonding is not essential for chiral recognition in these complexes, a number of
compounds without functional groups of this kind could be separated, such as cyclic
ethers, epoxides and spiroacetals. Thus, complexation gas chromatography can be
considered complementary to enantiomer separation on diamide CSPs. The application of metal-based/inorganic CSPs for chiral separation is currently a major focus
5.1 The Evolution of Chiral Stationary Phases for Capillary Gas Chromatography
77