GC separation of enantiomers of alkyl esters of 2-bromo substituted carboxylic acids enantiomers on 6-tbdms-2,3-di-alkyl- β- And γ-cyclodextrin stationary phases

Article abstract of DOI:10.1002/chir.22310

The gas chromatographic separation of enantiomers of 2-Br carboxylic acid derivatives was studied on four different 6-TBDMS-2,3-di-O-alkyl- β- and -γ-CD stationary phases. The differences in thermodynamic data {ΔH and -ΔS} for the 15 structurally related

Full text of DOI:10.1002/chir.22310

CHIRALITY 26:279–285 (2014)
Short Communication
GC Separation of Enantiomers of Alkyl Esters of 2-Bromo Substituted
Carboxylic Acids Enantiomers on 6-TBDMS-2,3-di-alkyl- β- and
γ-Cyclodextrin Stationary Phases
1
1
1
IVAN ŠPÁNIK, DARINA KAČERIAKOVÁ, JAN KRUPČÍK, AND DANIEL WAYNE ARMSTRONG2
*
1Institute of Analytical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, Bratislava, Slovak Republic
2Department of Chemistry and Biochemistry, University of Texas, Arlington, TX, USA
ABSTRACT
The gas chromatographic separation of enantiomers of 2-Br carboxylic acid
derivatives was studied on four different 6-TBDMS-2,3-di-O-alkyl- β- and -γ-CD stationary phases.
The differences in thermodynamic data {ΔH and –ΔS} for the 15 structurally related racemates
were evaluated. The influence of structure differences in the alkyl substituents covalently
attached to the stereogenic carbon atom, as well as in the ester group of the homologous
analytes, and the selectivity of modified β- and γ- cyclodextrin derivatives was studied in detail.
The cyclodextrin cavity size, as well as elongation of alkyl substituents in positions 2 and 3 of
6-TBDMS-β-CD, also affected their selectivity. The quality of enantiomeric separations is
influenced mainly by alkyl chains of the ester group of the molecule and this appears to be inde-
pendent of the CD stationary phase used. In some cases the separations occur as the result of
external adsorption rather than inclusion complexations with the chiral selector. It was found
that the temperature dependencies of the selectivity factor were nonlinear. Chirality 26:279–285,
2014. © 2014 Wiley Periodicals, Inc.
KEY WORDS: capillary GC; chiral recognition; 6-TBDMS-2,3-alkyl- β- and γ-cyclodextrins;
enantiomer separation
INTRODUCTION
in chiral discrimination are affected by functional groups in
CD substituents and also by structure of the functional
The native cyclodextrins are cyclic glucose oligomers in
which each glucose unit contains three hydroxyl groups
bonded to the 2, 3, and 6 carbon atoms. They were introduced
as stationary phases in gas chromatography by Smolková-
Keulemansová and Soják for separation of xylene isomers.1
Since that time, various substituents such as alkyl, acyl, or
tert-butyldimethylsilyl were introduced into CD molecule by
substitution of hydrogen atoms in hydroxyl groups.2–8 Cyclo-
dextrin derivatives (CDs) are currently the most frequently
used stationary phases for separation of enantiomers in gas
chromatography (GC) and other separation methods. How-
ever, in many cases the chiral recognition process is still
not fully understood.9–11 The GC separation of enantiomers is
generally influenced by several parameters. Apart from the
nonspecific ones, which generally influence the GC separa-
tion (i.e., column characteristics, mobile phase flow rate,
and working conditions), the enantiomer separation depends
particularly on the nature of the immobilized chiral selector
and its immediate environment.12–14 The type of CD and its
substituents, the degree of substitution and location of the
substituents, as well as the concentration of a given CD
derivative and the polarity of an achiral solvent,15 belong to
the basic parameters which determine the quality of enantio-
meric separations. It is assumed that inclusion into the CD
cavity, interactions with inner or outer CD surface, as well
as substituents attached to CD derivatives by hydrogen-
bonding, dispersion forces, dipole–dipole interactions, and
electrostatic interactions are a prerequisite for a successful
chiral recognition process.9 The types of interactions involved
© 2014 Wiley Periodicals, Inc.
groups of the separated enantiomers.9,16
Indeed, in one of the first articles dealing with studies of
the chiral separation mechanism, Venema et al.17 reported
that the enantioselectivity of alkylated CDs is not influenced
by the permanent dipole of the chiral compounds (which
depends on the electronegativity of the X substituent) in
2-X-alkyl derivatives (where X = Cl, Br, I, OH, O-CH3, CN
and NH2), but rather by the alkyl chain length of the sub-
stituent bonded to the stereogenic carbon atom. Similar
conclusions have been reached using 2-alkanoic acids esters
that have α-substituted with Cl or Br atoms.18 In another
work, Špánik et al. studied interactions of ethyl esters of
α-substituted propionic acid derivatives substituted by Cl, Br,
I, CN, OH, O-C2H5, O-C6H5, and NH-COCF3 on alkylated
β- and γ-CDs.19 It was found that the contribution of particular
substituents to the overall chemical interaction (ΔH) varies
from 10% to 43%, depending mostly on the polarity of the sub-
stituent. The highest contribution was observed for phenoxy
and the lowest for the Cl substituent on all of the evaluated
alkylated cyclodextrin stationary phases. The contribution of
a particular substituent to inclusion into the CD cavity (ΔS)
*Correspondence to: Ivan Špánik, Institute of Analytical Chemistry, Faculty of
Chemical and Food Technology, Slovak University of Technology, Bratislava,
Slovak Republic. E-mail: ivan.spanik@stuba.sk
Received for publication 24 November 2013; Accepted 23 January 2014
DOI: 10.1002/chir.22310
Published online 21 April 2014 in Wiley Online Library
(wileyonlinelibrary.com).

280
ŠPÁNIK ET AL.
polysiloxane matrix (6-TBDMS-2,3-DiMe-β-CD). The column was ob-
tained from MEGA (Capillary Column Laboratory, Legnano, Italy).
Column B. A 25-m capillary column with 0.25 mm i.d. was coated with a
0.25 μm film thickness of a 6-TBDMS-2,3-di-O-ethyl-β-CD dissolved in a
polysiloxane matrix (6-TBDMS-2,3-DiEt-β-CD). The column was obtained
from MEGA.
varies from 0.5% to 33%. From the presented data, it followed
that the 2-NHCOCF3 derivative would create the most stable
inclusion complexes on all studied CD derivatives.19 The
role of alkyl chain in the chiral recognition process on
permethylated-β-CD was studied by separation of N-TFA-
O-alkyl amino acid derivatives with linear, branched, and
cyclic alkyl chains attached at the stereogenic center, as
well as the ester part of the molecule. The separation of
enantiomers was influenced mainly by the type of the sub-
stituent at the stereogenic center. Short and linear alkyl
groups bonded to the stereogenic center were found to
be favorable for the separation of enantiomers, while bran-
ching of the alkyl chain caused a change in the elution
order.20 In a further study, the GC separation of enantio-
mers of seven amino acid N-TFA-O-alkyl derivatives on four
capillary columns coated with 2,3,6-tri-O-methyl-, and 2,6-di-
O-methyl-3-O-pentyl-β- and γ-CD stationary phases was
examined. The influence of the alkyl substituents bonded
to the stereogenic center (R1) and/or the ester group
(R2) of the N-TFA-O-alkyl amino acid derivatives showed
that the enantioselectivity was mainly influenced by the
alkyl substituents attached to the stereogenic carbon atom
R1.21 The effect of cavity size, as well as the 3-O-alkyl chain
length of 2,6-di-O-methyl-3-O-alkyl-β- and γ-cyclodextrins,
also had an impact on the resolution of enantiomers.21 In
contrast, the resolution of N-trifluoroacetyl-O-alkyl nipecotic
acid ester enantiomers on the permethylated-β-CD station-
ary phase is influenced by structure of alkyl chain in the
ester part of molecule. The n-alkyl esters provided stronger
interactions with permethylated-CD than esters containing
branched alkyl groups. Despite having weaker interactions
with the CD chiral selectors, esters containing branched
alkyl groups exhibited higher enantioselectivity than the
corresponding n-alkyl esters. This indicated that there were
different types of enantioselective interactions for the linear
alkyl chain and branched alkyl chain esters.22 As can be
seen, most studies were performed on alkylated cyclodex-
trins, but there are limited data for other substituted CD
derivatives.
Column C. A 30-m capillary column with 0.25 mm i.d. was coated with a
0.25 μm film thickness of a 6-TBDMS-2,3-di-O-propyl-β-CD dissolved in
a
polysiloxane matrix (6-TBDMS-2,3-DiPr-β-CD). The column was
obtained from RESTEK (Bellefonte, PA) .
Column D. A 25-m capillary column with 0.25 mm i.d. was coated with a
0.25 μm film thickness of a 6-TBDMS-2,3-di-O-ethyl-γ-CD dissolved in a
polysiloxane matrix (6-TBDMS-2,3-DiEt-γ-CD). The column was obtained
from MEGA.
Chemicals
Racemic mixtures of 2-Br-propionic acid, 2-Br-butyric acid, 2-Br-pentanoic
acid, and 2-Br-hexanoic acid were purchased from Sigma-Aldriche; Chemie
(Munich, Germany). Methanol, ethanol, propanol, butanol, and n-hexane
for chromatography was purchased from Merck (Darmstadt, Germany).
Analytes
The methyl, ethyl, n-propyl, and n-butyl esters of all racemic mixtures
of 2-bromo carboxylic acids were prepared by esterification of the
corresponding 2-bromo carboxylic acid with 20% (v/v) solution of
acetylchloride in the corresponding alcohol at elevated temperature
(from 90–120 °C depending on the type of alcohol used) in a vial with a
Teflon cap. After a 30-min reaction the excess reagent was removed with
a stream of nitrogen. The general chemical structure of prepared deriva-
tives is shown in Figure 1.
RESULTS AND DISCUSSION
The overall free energy of transfer (ΔG) of the chiral mole-
cule to the cyclodextrin stationary phase can be represented
as a sum of partial interaction energies (δGi) proportional to
individual partial interactions:23
n
X
ΔG ¼
δGi
(1)
i¼1
where n is the number of individual interactions of a
selectand with a selector. This energy is related to the ther-
modynamic parameters enthalpy (ΔH) and entropy (ΔS):
In this work, the enantiomers of alkyl esters of 2-Br-carboxylic
acids were separated by GC on four capillary columns coated
with 6-TBDMS-2,3-di-O-alkyl-β-CD and 6-TBDMS-2,3-di-O-ethyl-
γ-CD stationary phases. The separation of enantiomers was
evaluated in terms of the nonpolar interactions of the alkyl
substituents bonded to the stereogenic carbon (R1) and/or the
ester group (R2) as well as the 2,3-O-alkyl chain length
of 6-TBDMS-2,3-di-O-alkyl-β-CD.
ꢀΔH ΔS
ΔG¼
þ
(2)
RT
R
where R is a universal gas constant and T is the absolute tem-
perature in Kelvin.
An overall interaction energy of a selectand with a selector
in a chromatographic column can be calculated from the chro-
matographic experiment by:
MATERIALS AND METHODS
Instruments
ΔG¼ ꢀ RT ln k ꢀ RT ln β
(3)
Agilent 7890 gas chromatograph and Chemstation software were used
for recording and evaluating signals for all separations. The split-splitless
injector and flame ionization detector (FID) were utilized, both with
temperatures of 250°C. Helium was used as a carrier gas with an optimal
flow rate of 25–30 cm s-1. Methane was used to determine the hold time.
Sample volumes of 1 μl were injected into the column by split injection
with a ratio of 100:1. The measurements were performed at isothermal
conditions in the temperature range 30–160°C at 10°C increments. All
measurements were repeated three times and statistically evaluated.
where k is the retention factor and β is the ratio of volumes
(V) in the column ( β = Vm /Vs where m denotes mobile and
s stationary phase).
Columns
Column A. A 25-m capillary column with 0.25 mm i.d. was coated with a
0.25 μm film thickness of 6-TBDMS-2,3-di-O-methyl-β-CD dissolved in a
Fig. 1. General formula of studied esters of 2-Br-carboxylic acid.
Chirality DOI 10.1002/chir

GC SEPARATION OF ENANTIOMERS
281
A combination of eqs. 2 and 3 leads to:
of 2-Br-propionic acid has weaker interactions than methyl
ester of 2-Br-butanoic acid on all studied CD derivatives.
The smallest differences in ΔH and ΔS values between those
two studied molecules were observed on 6-TBDMS-2,3-DiEt-
γ-CD. This indicates that a similar retention and separation
mechanism is employed in separation of the studied γ-CD
derivative independently on position of the alkyl substituents
in the studied molecule. This is also confirmed by the
obtained similar selectivity factors α, (α = k2/k1 and k is the
retention factor of the first and second eluted enantiomer,
respectively) obtained for enantiomers of the 2-Br-carboxylic
acid derivatives on all studied CD stationary phases. Table 1
gives the obtained enthalpies and entropies that characterize
interactions of enantiomers with the cyclodextrin-based chiral
stationary phase derivatives. It must be noted that the
obtained enthalpies and entropies are assumed to be indepen-
dent of T in the ranges used in GC experiments. The ethyl
ester of 2-Br-propionic acid had the weakest interaction which
resulted in the least retention of all studied CD derivatives. As
discussed previously, selected compounds represent homolo-
gous series that differ by one methylene group either in the
alkyl chain attached on asymmetric the carbon atom or in
the ester group of the molecule. The methyl esters represent
the smallest molecule of all the compounds studied in this
work. From the data given in Figure 2 it follows that methyl
esters always show the smallest retention on all studied CD
derivatives. Elongation of the alkyl chain in both R1 and R2
groups increases the retention on all studied CD derivatives
in the order Methyl, Ethyl, Propyl, and Butyl independently
of their position in the molecule. This is also supported by
ΔH ΔS
ln k ¼ ꢀ
þ
ꢀ ln β
(4)
RT
R
which indicates a linear dependence of ln k on the inverse of
temperature (1/T).
Esters of 2-Br-carboxylic acid used in this study can be
illustrated as follows:
R1 ꢀ CBrH ꢀ CO ꢀ OR2
where R1 and R2 are the alkyl substituents bonded to the
stereogenic carbon or to the carboxylate group, respectively.
Since the 2-Br carboxylic acid moiety is identical for all of the
studied 2-Br carboxylic acid derivatives, the contribution of
interactions of the alkyl substituents R1 and R2 with a cyclo-
dextrin stationary phase can be studied independently. Van’t
Hoff plots (semilogarithmic dependencies of the retention
factor on the inverse of absolute temperature) constructed
for the first eluted enantiomer using the data obtained from
the separation of the studied racemates on 6-TBDMS-2,3-di-
O-alkyl cyclodextrin stationary phases are shown in Figure 2.
The linear van’t Hoff dependencies (correlation coefficient
higher than 0.99) also were obtained for the second eluted
enantiomer. The obtained linear dependencies confirm the
validity of eq. 4, thus the slopes of these plots can be used
to determine enthalpies (ꢀΔH/R) terms and the intercepts
for the entropic (ΔS/R) contributions. The comparison of
the retention behavior of 2-Br carboxylic acid derivatives with
equal molecular weight shows that enantiomers of ethyl ester
Fig. 2. van’t Hoff plots for the first eluted enantiomer found from data obtained by the separation of the studied compounds on 6-TBDMS-2,3-DiMe-β-CD (A), 6-
TBDMS-2,3-DiEt-β-CD (B), 6-TBDMS-2,3-DiPr-β-CD (C), and 6-TBDMS-2,3-DiEt-γ-CD (D) columns.
Chirality DOI 10.1002/chir

282
ŠPÁNIK ET AL.
TABLE 1. ΔH values in kJ mol-1 and –ΔS in J mol-1 K-1 found for enantiomers of all studied compounds from of van´t Hoff plots
6-TBDMS-2,3-DiMe-β-CD 6-TBDMS-2,3-DiEt-β-CD 6-TBDMS-2,3-DiEt-γ-CD 6-TBDMS-2,3-DiPr-β-CD
Molecular
weight
Compound*
ΔH
–ΔS
ΔH
–ΔS
ΔH
–ΔS
ΔH
–ΔS
ET-Pro 1
2
PR-Pro 1
2
BU-Pro 1
2
ME-But 1
2
ET-But 1
2
PR-But 1
2
BU-But 1
2
ME-Pen 1
2
ET-Pen 1
2
PR-Pen 1
2
BU-Pen 1
2
ME-Hex 1
2
ET-Hex 1
2
PR-Hex 1
2
BU-Hex 1
2
181.03
195.05
209.08
181.03
195.05
209.08
223.11
195.05
209.08
223.11
237.14
209.08
223.11
237.14
251.16
44.87
48.07
48.59
50.72
52.38
54.18
47.11
50.19
48.21
51.53
51.70
54.00
56.21
57.42
50.85
54.44
51.43
54.80
54.68
57.00
58.55
60.02
54.18
57.88
55.02
58.16
58.59
60.64
66.14
67.15
112.70
119.93
117.62
122.41
122.47
126.54
117.00
123.61
117.47
125.07
121.83
127.10
128.59
131.04
122.77
130.80
121.85
129.58
125.57
130.92
130.51
133.89
126.58
134.76
126.28
133.37
130.73
135.44
145.66
147.87
56.72
63.59
60.78
64.62
60.60
63.65
59.27
70.21
58.74
68.35
60.53
67.26
62.26
67.03
60.23
70.64
57.56
65.48
61.50
67.08
63.90
67.43
58.90
66.93
61.38
68.36
64.02
67.68
68.47
71.21
145.19
161.55
151.83
161.13
146.3
42.53
43.75
47.48
48.64
51.74
52.05
43.07
43.95
47.25
48.90
50.62
52.47
53.71
54.73
48.26
49.07
51.59
53.46
53.52
55.61
56.38
58.00
51.56
52.29
54.28
55.69
56.83
58.37
59.20
60.32
112.44
115.32
120.46
123.25
126.73
127.52
112.59
114.70
120.24
124.15
124.40
128.83
127.75
130.20
121.50
123.41
127.04
131.51
127.51
132.48
130.54
134.43
125.37
127.16
129.42
132.79
131.65
135.37
133.24
135.93
46.40
51.94
48.42
50.12
50.15
51.51
47.09
57.57
47.07
53.59
48.68
51.82
52.57
54.99
49.59
59.03
49.92
55.78
51.96
54.68
54.57
55.13
52.71
59.75
52.03
55.61
54.91
55.87
58.72
58.89
118.00
131.73
118.72
122.97
118.45
121.83
118.23
143.59
115.72
131.84
115.43
123.23
120.65
126.68
120.48
143.41
118.82
133.34
119.66
126.46
121.72
123.09
123.82
140.98
119.78
128.59
122.61
124.98
127.74
128.15
153.67
149.75
175.41
146.80
169.93
147.50
164.05
147.21
158.89
148.53
172.85
139.66
158.45
145.93
159.63
147.33
155.94
140.54
159.04
144.69
144.69
147.64
156.56
154.27
160.95
*First two letters express the abbreviation for an ester part: ME - Methyl, ET- Ethyl, PR - Propyl and BU - Butyl; the second three letters express the abbreviation for
2-Br-carboxylic acid: Pro - 2-Br propionic acid, But - 2-Br butyric acid, Pen- 2-Br pentanoic acid and Hex - 2-Br hexanoic acid.
the data in Table 2 (ΔH) obtained on 6-TBDMS-2,3-DiMe-β-
CD 6-TBDMS-2,3-DiEt-γ-CD. However, the data in Table 2
shows that the methyl ester of the studied 2-Br carboxylic
acids provide stronger interactions than the corresponding
ethyl derivatives on 6-TBDMS-β-CD derivatives with ethyl
and propyl attached in positions 2 and 3. McGachy et al.
explain this behavior by a higher number of interactions
inside the CD cavity and suggest that the chiral molecule
gains additional interactions from other parts of the CD
derivative.22 Indeed, the CD moiety in 6-TBDMS-2,3-alkyl-β-
CD derivatives decreases with elongation of alkyl substitu-
ents in positions 2 and 3 from methyl to propyl. Considering
the fact that the chiral recognition process includes inclusion
of molecules into the CD cavity, the chiral molecules would
be faced with a stronger interaction with substituents at
position 3 in the CD derivative since those are oriented into
the CD cavity. The 6-TBDMS-2,3-DiEt-γ-CD derivative has a
larger cavity size due to higher number of glucopyranose
units compared to corresponding β-CD derivative.
The calculated enthalpies are higher on (6-TBDMS-2,3-
DiEt)-β-CD in comparison to the corresponding γ-CD
derivative. An interesting behavior was observed on the
6-TBDMS-2,3-DiPr-β-CD, where the calculated entropies for
all studied esters of 2-Br-propionic acid, 2-Br-butyric acid,
and methyl and ethyl esters of 2-Br-pentanoic acid showed
similar values. This indicates that interactions on the outer
surface of this CD derivative are preferred in chiral
Chirality DOI 10.1002/chir
recognition and separation for the columns studied under
those conditions. From the data in Table 1 it follows that the
decrease of enthalpic terms is compensated for by an in-
crease in the entropic term on all studied CD derivatives
except for 6-TBDMS-2,3-DiPr-β-CD. This compensation,
however, is not proportional, since inclusion as well as inter-
actions of enantiomers with the CD derivative surface affect
the overall entropic term. It is supposed that the chiral recog-
nition process is not driven by one only type of separation
mechanism, but involves many types of intramolecular
interactions such as hydrogen bonding, nonpolar interac-
tions, dipole–dipole interactions, or electrostatic interactions
(generally, the sum of those interactions is expressed by
the enthalpic term23) and inclusion phenomena connected
with loss of freedom in molecular motion (generally, ex-
pressed as entropic part23).
Figure 3 shows the dependencies of lnα = f(1/T). It was
expected that the dependencies in Figure 3 would be linear;
however, all the dependencies are nonlinear, which indicates
that the chiral recognition mechanism on the 6-TBDMS-2,3-
di-O-alkyl-β and γ-CD stationary phases probably shows non-
additive contributions of the individual interactions of solute
enantiomers. Table 2 shows selectivity factors obtained for
the racemic analytes evaluated at 80 °C on all CD stationary
phases studied. The highest α values were observed on the
6-TBDMS-2,3-DiEt-β-CD phase for all analytes. The smallest
α values were obtained on the CD derivative with the largest

GC SEPARATION OF ENANTIOMERS
283
TABLE 2. The selectivity factors (α) obtained for compounds
studied evaluated at 80 °C on all CDs stationary phases used in
this study
of enantiomers decreases with an increase in the number of
carbon atoms in the alkyl chain of the ester part of the mole-
cule R2 on all β-CD stationary phases used in this study. The
opposite effect was observed on the γ-CD stationary phase,
where elongation of the R2 alkyl chain from methyl to ethyl
either slightly improved or had little effect on the enantio-
meric separation of 2-Br-carboxylic acid derivatives. The
further elongation of the alkyl chain in the ester part of
molecule from ethyl to butyl decreased the enantiomeric
resolution. Table 2 and Figure 4 show that methyl esters
provide the highest selectivity factors regardless of the type
of CD stationary phase. A further increase from methyl to
ethyl in the ester group caused a significant loss of enan-
tioselectivity on almost all studied CD derivatives. However,
the most evident decrease in enantioselectivity was observed
on 6-TBDMS-2,3-DiPr-β-CD. This CD derivative has the
smallest cavity from all studied CD derivatives that are suffi-
cient to form the most stable inclusion complexes with
methyl esters. The small cavity size, however, will prevent
deeper inclusion of a molecule into the CD cavity and interac-
tions with outer the CD surface must also be considered in
the chiral recognition process. This conclusion is supported
also by similar entropic terms obtained on this CD derivative
presented in Table 1.
6-TBDMS-
2,3-DiMe-
β-CD
6-TBDMS- 6-TBDMS- 6-TBDMS-
2,3-DiEt-β-
2,3-DiEt-γ-
2,3-DiPr-β-
Compound*
CD
CD
CD
ET-Pro
PR-Pro
BU-Pro
ME-But
ET-But
PR-But
BU-But
ME-Pen
ET-Pen
PR-Pen
BU-Pen
ME-Hex
ET-Hex
PR- Hex
BU- Hex
1.24
1.16
1.13
1.29
1.24
1.16
1.12
1.29
1.24
1.15
1.10
1.34
1.25
1.15
1.10
1.45
1.19
1.17
1.93
1.62
1.32
1.25
1.91
1.60
1.29
1.21
1.79
1.50
1.22
1.16
1.07
1.05
1.00
1.05
1.09
1.08
1.05
1.04
1.09
1.10
1.08
1.03
1.07
1.08
1.06
1.16
1.04
1.03
1.65
1.23
1.08
1.04
1.54
1.18
1.03
1.00
1.38
1.11
1.02
1.00
*See footnote to Table 1.
cavity size (6-TBDMS-2,3-DiEt-γ-CD) for a smaller methyl and
ethyl esters of the studied 2-Br carboxylic acids and BU-Pro.
On the contrary, propyl and butyl esters (except for BU-Pro)
showed the lowest α values on a CD derivative with the
smallest cavity size (6-TBDMS-2,3-DiPr-β-CD). Figure 4
shows the dependencies of α-values on the carbon atom num-
ber of the alkyl chains R1 (Fig. 4A) and R2 (Fig. 4B) obtained
on all of the studied stationary phases at 80°C. The resolution
A further increase of alkyl chain length to propyl and butyl
causes another significant drop in α values or complete loss of
enantioselectivity. The elongation of the alkyl chain attached
to the stereogenic center, R1 did not influence the enan-
tioselectivity as dramatically as the alkyl chain in the ester
group. This may lead to the conclusion that the quality of
enantiomer separation alkyl esters of 2-Br carboxylic acids
on the studied 6-TBDMS-2,3-Di-alkyl-CD is mostly affected
Fig. 3. Dependencies of ln α on the reverse of temperature (1/T) found from data obtained by the separation of the studied compounds on 6-TBDMS-2,3-DiMe-β-
CD (A), 6-TBDMS-2,3-DiEt-β-CD (B), 6-TBDMS-2,3-DiPr-β-CD (C), and 6-TBDMS-2,3-DiEt-γ-CD (D) columns.
Chirality DOI 10.1002/chir

284
ŠPÁNIK ET AL.
different 6-TBDMS-2,3-di-O-alkyl- β- and -γ-CD stationary
phases. The linear temperature dependencies of the retention
factors (ln k on 1/T) of enantiomers of all the studied com-
pounds on all of the studied CD stationary phases allowed
us to compare the basic thermodynamic data {ΔH and –ΔS}
in order to characterize the overall interactions of the studied
enantiomers with the stationary phases. On the contrary, the
temperature dependencies of selectivity factors, ln α on 1/T,
were nonlinear as a consequence of the nonadditive contri-
butions of the individual interactions in the chiral recognition
process. The chiral recognition process of alkyl esters of 2-Br-
carboxylic acid derivatives on 6-TBDMS-2,3-alkyl-β and
6-TBDMS-2,3-ethyl-γ-CD derivatives is substantially affected
by both properties of the chiral stationary phase (CD cavity
size, alkyl chain length in position 2 and 3 in the CD
derivative) as well as the structure of the separated enantio-
mers. It was shown that the alkyl chain in ester group 2-Br
carboxylic acids influences the overall retention and also
plays a more significant role in the chiral recognition process
on 6-TBDMS-2,3-di-O-alkyl-CD derivatives. The length of the
alkyl chain attached to the asymmetric carbon did not have
a significant influence on the quality of enantiomer separa-
tion. In some cases, chiral recognition and separation is
the result of external adsorption on the chiral selector
rather than inclusion complex formation. The prolongation
of the alkyl chain in 6-TBDMS-2,3-di-O-alkyl-β-CD influences
obtained α values, while the best separation was achieved
on 6-TBDMS-2,3-di-O-ethyl-β-CD. The cyclodextrin cavity
size, as well as elongation of alkyl substituents in positions
2 and 3 of 6-TBDMS-β-CD, also strongly influenced the sep-
aration of enantiomers.
ACKNOWLEDGMENT
The authors thank the Slovak Grant Agency for Grant 1/0573/14.
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