Use of a racemic derivatizing agent for measurement of enantiomeric excess by circular dichroism spectroscopy

Article abstract of DOI:10.1016/S0040-4039(01)01707-5

Enantiomeric excesses of several alcohols and an amine have been determined by derivatization with racemic 2′-methoxy-1,1′-binaphthyl-2-carbonyl chloride rac-1, with no requirement for kinetic resolution in the acylation step. The necessary information is provided instead by CD and UV spectra of the derived esters and amides, the chiral signal being dominated by the binaphthyl component.

Full text of DOI:10.1016/S0040-4039(01)01707-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8015–8018
Use of a racemic derivatizing agent for measurement of
enantiomeric excess by circular dichroism spectroscopy
Tetsutaro Hattori,a,* Yuji Minato,a Sulan Yao,b M. G. Finnb,* and Sotaro Miyanoa
aDepartment of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Aramaki-Aoba 07, Aoba-ku,
Sendai 980-8579, Japan
bDepartment of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA
Received 7 August 2001; revised 5 September 2001; accepted 7 September 2001
Abstract—Enantiomeric excesses of several alcohols and an amine have been determined by derivatization with racemic
2%-methoxy-1,1%-binaphthyl-2-carbonyl chloride rac-1, with no requirement for kinetic resolution in the acylation step. The necessary
information is provided instead by CD and UV spectra of the derived esters and amides, the chiral signal being dominated by the
binaphthyl component. © 2001 Elsevier Science Ltd. All rights reserved.
Dramatic progress in preparation methods for optically
active compounds calls for efficient ways to determine
their optical purities. Although chiral HPLC and GC
analyses have become the standard methods for deter-
mining enantiomeric purity, the physical separation of a
pair of enantiomers is generally time-consuming and
cannot be accomplished in some cases. Therefore, alter-
native techniques which do not require enantiomer
separation are useful. Circular dichroism (CD) spec-
troscopy, as well as polarimetry, can measure the con-
centration difference between the enantiomer pair in a
mixture and has been used to determine the enan-
tiomeric excess in conjunction with UV spectroscopy,
which can measure the total concentration of the two
enantiomers.1,2 A general drawback of this method is
its poor applicability to compounds having low CD
intensities. This has been addressed in several reported
cases by derivatization with achiral chromophoric
reagents.3,4 Induced CD exhibited by the ligand
exchange reaction of a metal complex and by the
formation of an inclusion compound have also been
used for this purpose,5 but the nonlinear relationship
between the CD intensity and the enantiomeric purity
often makes the method complicated.6 Herein, we
describe a new method employing a racemic chro-
mophoric reagent for the determination of enantiomeric
excesses of alcohols and amines by the combination of
CD and UV spectroscopies.7 Although optically pure
derivatizing agents have been widely used in a variety
of spectroscopic or chromatographic methods,8 this
may be the first example of the use of a racemic
derivatizing agent.
One of us has recently reported a method to determine
enantiomeric excesses of alcohols and amines by their
mass spectroscopic analyses after derivatization with a
large excess of a pair of pseudo-enantiomeric acylating
agents, in which enantiomeric information is coded by
mass.9 The method requires a certain amount of kinetic
resolution in the acylation, and the enantiomeric com-
position of the starting substrate is calculated from the
relative amount of the derivatives of different molecular
weights, which can be measured by mass spectroscopy.
We noticed that CD spectroscopy offers the opportu-
nity to perform a similar experiment using a truly
racemic acylating agent, since CD allows the decoding
of enantiomeric information without a tag of any sort.
aS-COCl
aR-COCl
aS-CO2R
aS-CO2S
ROH
aR-CO2R
aR-CO2S
k1
k2
x
aS-COCl
aR-COCl
SOH
1–x
k2
k1
Scheme 1. Generalized reaction of a partially optically active
alcohol with a racemic acyl chloride. Legends: x, molar ratio
of ROH (05x51); k1, k2, the rate constants of each reaction.
* Corresponding authors. Tel.: +81-22-217-7263; fax: +81-22-217-
7293 (T.H.); tel.: +1-858-784-8845; fax.: +1-858-784-8850 (M.G.F.);
e-mail: hattori@orgsynth.che.tohoku.ac.jp; mgfinn@scripps.edu
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01707-5

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T. Hattori et al. / Tetrahedron Letters 42 (2001) 8015–8018
Furthermore, as discussed below, the method is appli-
cable even if kinetic resolution does not occur in the
derivatization process.
mophores. In addition, the acid can be readily prepared
in both racemic and optically pure forms by using an
ester-mediated SNAr protocol.10,11 Fig. 1 shows the CD
and UV spectra of the optically pure (aS,R)- and
(aS,S)-1-phenylethyl esters, revealing this to be an
example of case (b) above. Both spectra show the
positive first and the negative second Cotton effects
corresponding to the clockwise twist of the binaphthyl
axis, but the spectra are not completely identical due to
participation of the chirality of the 1-phenylethyl moi-
ety in the exciton interaction. On the other hand, their
UV spectra are identical within experimental errors.
The s value can be experimentally derived from the
ratio of peaks obtained in HPLC analysis of the esters
derived from the racemic acyl chloride on an achiral
column, showing [aS-CO2R]+[aR-CO2S] versus [aS-
CO2S]+[aR-CO2R].
Thus, an optically active alcohol, the enantiomeric
components of which are designated ROH and SOH, is
esterified with a large excess of a racemic acyl chloride,
abbreviated as aS-COCl and aR-COCl, to give two sets
of diastereomeric esters (Scheme 1). Assuming that
these reactions are first order in the acyl chloride and
the alcohol, the molar ratio of the esters is given by Eq.
(1), where s is the kinetic resolution selectivity factor.
Assuming no intermolecular interactions which perturb
their CD behavior, the molar CD observed for the ester
mixture is given by the weighted average of the molar
CDs of its components, and a pair of the enantiomers
show the same CD with opposite signs. Therefore, the
molar CD of the ester mixture is the product of the
enantiomeric excess of the starting alcohol and the
molar CD of the ester mixture derived from (R)- [or
(S)-] alcohol of 100% ee, as shown in Eq. (2).
Thus, 1-phenylethanol of known enantiomeric excess
listed in Fig. 2 was esterified with 20 equiv. of the acyl
chloride rac-1 and the product ester mixture was ana-
lyzed by HPLC after purification by column chro-
matography.12 The s value of the acylation reaction was
found to be 1.03–1.06.14 Fig. 2 compares the measured
CD spectra with the calculated ones assuming that
s=1.0. The close correspondence of the curves validates
the assumptions of the method and shows that the
technique is not very sensitive to the magnitude of s for
cases in which kinetic resolution selectivity is poor.
Such cases will be in the majority, since effective kinetic
resolution of this kind is difficult to achieve.
[aR-CO2R]:[aS-CO2R]:[aS-CO2S]:[aR-CO2S]=
x
sx 1−x s(1−x)
k
ꢀ ꢁ
1
:
:
:
s=
(1)
1+s 1+s 1+s 1+s
k2
s
1
1+s
!
"
(2)
Dmobs=(2x−1)
1+s
Dm(aS-CO2R)+
Dm(aR-CO2R)
Dm100
% ee
100
=
The method is most generally applicable with acyl
derivatizing agents having characteristically strong CD
signals, such that the CD of their derived esters are
dominated by the acyl component. Two situations are
then possible. (a) If the CD spectra of the esters are due
only to the acyl component, then the CD spectra of
aS-CO2R and aS-CO2S are the same (and exactly
opposite to the spectra of aR-CO2R and aR-CO2S).
Some level of kinetic resolution is therefore necessary
(s"1.0), and the situation is directly analogous to the
mass spectrometric method. (b) If the CD spectra of the
esters are dominated by the acyl component but also
influenced to some extent by the alcohol component,
then the CD spectra of aS-CO2R and aS-CO2S, while
of the same direction, will not be identical. It should
then be possible to deduce the enantiomeric ratio of
starting alcohols from the CD spectra of the derived
esters for any s value, including situations when no
kinetic resolution is obtained in the acylation step
(s=1.0). As with the MS methodology, a calibration
measurement is necessary using a sample of known
enantiomeric excess, preferably a pure enantiomer. In
contrast to the MS technique, the concentration of the
product esters must be taken into account, conveniently
done with the UV–vis absorption spectrum obtained
during the CD measurement.
As the molar absorptivities of the diastereomeric esters
are identical as shown in Fig. 1, a dual detector to
We have employed 2%-methoxy-1,1%-binaphthyl-2-car-
bonyl chloride, rac-1, as the derivatizing agent because
of the intense Cotton effects originating from exciton
interaction between the two naphthalene chro-
Figure 1. CD and UV spectra of (R)-1-phenylethyl ester of
(aS)-1 (solid curves) and those of (S)-1-phenylethyl ester of
(aS)-1 (dotted curves). Solvent: ethanol–1,4-dioxane (9:1).

T. Hattori et al. / Tetrahedron Letters 42 (2001) 8015–8018
8017
Figure 2. Measured (solid curves) and calculated (dotted
curves) CD spectra of the ester mixtures derived from rac-1
and 1-phenylethanol samples of the indicated optical purities.
The minus sign denotes a sample enriched in the (S)-alcohol.
Solvent: ethanol–1,4-dioxane (9:1).
Figure 3. Plots of the actual versus measured enantiomeric
excesses. Legend: 2 (ꢀ); 3 ("); 4 (ꢁ); 5 (ꢂ).
need to accomplish derivatization in an enantioselective
manner. Instead, the necessary information is provided
by a combination of the dominating CD signal of each
enantiomer of the racemic ‘reporter’ structure (binaph-
thyl acyl in this case) and a perturbation of that signal
provided by each enantiomer of the analyte (alcohol or
amine). By derivatizing with a potent chiral chro-
mophore, the inherent optical rotation of the analyte
does not matter, and the measurement is made far more
sensitive than standard polarimetry, thereby extending
the optical determination of enantiomeric purity to
samples that do not exhibit strong optical rotation. A
practical disadvantage of the method relative to the
related MS technique is that CD measurements must be
performed on purified samples so that the relative
absorption values are accurate. Further studies to
improve the scope and sensitivity of the method are in
progress.
gather both CD and standard absorption data is advan-
tageous.1 With such instrumentation, a practical proce-
dure to determine enantiomeric excess is as follows.12
First, the CD and UV spectra are measured for the
calibration sample derived from an alcohol of 100% ee
and the maximum value of the elliptic angle is divided
by the maximum value of the absorbance. The same
operations are then repeated for the analyte of
unknown % ee and concentration. The enantiomeric
excess of the analyte is calculated by substituting the
two quotients in Eq. (3). According to this procedure,
compounds 2–5 of different enantiomeric compositions
were analyzed (Fig. 3). In every case, the measured
value fell within 5% ee of the actual value.15 Interest-
ingly, the procedure failed for the analysis of enan-
tiomerically enriched 2-octylamine, because its
diastereomeric amides showed exactly the same CD
spectra. This is therefore an example of case (a) above,
with a kinetic resolution selectivity (s=0.94) insufficient
to allow a determination of ee to be made.16
Acknowledgements
qobs % ee q100
Aobs 100 A100
=
·
(3)
This work was partly supported by grants from the
Ministry of Education, Science and Culture, Japan
(No. 10208202 and 10555318), and The Skaggs Institute
for Chemical Biology.
Circular dichroism spectroscopy therefore frees us from
the need to code enantiomeric information into the
derivatizing agent9 or the substrate,17 and from the

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T. Hattori et al. / Tetrahedron Letters 42 (2001) 8015–8018
7, 645; (d) Fukushi, Y.; Yajima, C.; Mizutani, J. Tetra-
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12. Typical procedure for the determination of enantiomeric
excess: To a solution of acyl chloride rac-1 (69.4 mg, 200
mmol) in dry benzene (10 mL) was added a solution of
1-phenylethanol (20 mM in benzene; 500 mL, 10 mmol),
pyridine (100 mL) and N,N-dimethyl-4-aminopyridine
(12.1 mg, 99.0 mmol) and the mixture was stirred at room
temperature for 24 h. After dilution with benzene, the
mixture was washed successively with 2 M HCl, 1 M
Na2CO3 and water, dried over MgSO4 and evaporated.
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ethyl acetate (5:1) as the developer to give the corre-
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spectroscopies using a 1 mm cell. The concentration of
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13. As Eq. (1) applies to any point in the derivatization
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14. The HPLC analysis was carried out on a system equipped
with a silica gel column and a UV detector set at 254 nm,
the eluent being 1% 2-propanol in hexane. The observed
peak ratio was corrected for the molar response, which
was reported to be (aS,R)-ester:(aS,S)-ester=1.03:1 (see
Ref. 11a).
15. The s values of the other analytes were also determined:
menthol, 1.1; 2-octanol, 1.1; 1-phenylethylamine, 0.94.
The values are uncorrected for the molar response.
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