Determination of enantiomeric excess and concentration of chiral compounds using a 1,8-diheteroarylnaphthalene-derived fluorosensor

Article abstract of DOI:10.1016/j.tetlet.2006.09.012

Enantioselective fluorosensing using a rigid C₂-symmetric 1,8-diacridylnaphthalene N,N′-dioxide sensor allows accurate determination of both the enantiomeric composition and concentration of several analytes capable of chiral hydrogen bonding.

Full text of DOI:10.1016/j.tetlet.2006.09.012

Tetrahedron Letters 47 (2006) 7901–7904
Determination of enantiomeric excess and concentration of chiral
compounds using a 1,8-diheteroarylnaphthalene-derived
fluorosensor
Xuefeng Mei and Christian Wolf*
Department of Chemistry, Georgetown University, Washington, DC 20057, USA
Received 9 August 2006; accepted 1 September 2006
Available online 25 September 2006
Abstract—Enantioselective fluorosensing using a rigid C2-symmetric 1,8-diacridylnaphthalene N,N0-dioxide sensor allows accurate
determination of both the enantiomeric composition and concentration of several analytes capable of chiral hydrogen bonding.
ꢀ 2006 Elsevier Ltd. All rights reserved.
The development of enantioselective fluorescence assays
suitable to high-throughput screening of asymmetric
reactions has recently received increasing attention.1
Fluorescence spectroscopy combines several attractive
features such as different detection modes (fluorescence
quenching, enhancement, and lifetime measurements),
high sensitivity, low cost of instrumentation, waste
reduction, and time-efficiency.2 The potential use of
fluorescence sensing for rapid determination of the
enantioselectivity of asymmetric reactions has been
demonstrated with the titanium tartrate-catalyzed silyl-
cyanation of an immobilized aldehyde and the kinetic
resolution of trans-1,2-diaminocyclohexane using Can-
dida Antarctica.3 In both cases, fluorescence measure-
ments provided accurate ee’s and proved superior over
laborious and time-consuming chromatographic
methods. We have developed a synthetic entry to highly
congested 1,8-diquinolyl- and 1,8-diacridylnaphthalenes
and shown the use of this class of compounds for enan-
tioselective recognition of chiral hydrogen bond donors.
The unique geometry and rigid structure of C2-symmet-
ric 1,8-diheteroarylnaphthalenes and their N,N0-di-
oxides result in a well-defined chiral cleft that places
hydrogen bond interactions into a highly stereoselective
environment. Previous X-ray and NMR spectroscopic
studies conducted in our laboratories have shown that
1,8-diheteroarylnaphthalenes are highly congested, rigid
structures that possess remarkable one-dimensional flex-
ibility.4 The two cofacial heteroaryl rings reside perpen-
dicular to the naphthalene ring and undergo little
splaying. However, the torsional angle, s, can change
over a range of 50ꢁ, in particular upon binding to a
hydrogen bond donor (Fig. 1).5 We believe that this
one-dimensional conformational flexibility facilitates
accommodation of substrates of varying size in the C2-
symmetric pocket of these fluorescent probes. Herein,
we report the usefulness of 1,8-bis(3-(30,50-dimethyl-
phenyl)-9-acridyl)naphthalene N,N0-dioxide, 1, for the
τ
τ
R
R
N
O
N
O
N
N
N
N
R
1
O
R
O
O
O
τ = -25o
τ = 25o
Figure 1. Structure of 1 and one-dimensional flexibility of 1,8-diheteroarylnaphthalenes.
Keywords: Fluorescence spectroscopy; Enantioselective sensing; Chiral recognition; 1,8-Diacridylnaphthalene N,N0-Dioxides.
*
Corresponding author. Tel.: +1 202 687 3468; fax: +1 202 687 6209; e-mail: cw27@georgetown.edu
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.09.012

7902
X. Mei, C. Wolf / Tetrahedron Letters 47 (2006) 7901–7904
Br
N
CO2H
aniline
CO2H
Cl
B(OH)2
Pd(PPh3)4
96%
CO2H
POBr3
>99%
Cu/Cu2O
NH
+
89%
Br
Cl
4
6
3
2
5
Br Br
1. BuLi, -78 ºC
SnBu3
2. Bu3SnCl
m-CPBA
75%
8
98%
N
Pd(PPh3)4
68%
7
N
O
N
O
N
N
9
1
Scheme 1. Synthesis of 1,8-bis(3-(30,50-dimethylphenyl)-9-acridyl)naphthalene N,N0-dioxide 1.
determination of both enantiomeric excess and concen-
tration of several chiral compounds.
11 was used. However, when (ꢀ)-1 was employed in the
same titration experiment, fluorescence quenching sub-
stitution occurred more readily when (R)-11 was added
thus proving that the observed fluorescence changes
are indeed a result of enantioselective recognition and
not due to impurities that could have a strong quenching
effect and be present in only one of the enantiomeric
analyte samples. In general, (R)-carboxylic acids 12–15
were found to be more effective fluorescence quenchers
than the corresponding (S)-enantiomers. The enantio-
mers of diamine 10 selectively enhance the fluorescence
intensity of 1. The fluorescence enhancement observed
in the presence of diaminocyclohexane 10 may be a con-
sequence of increased rigidity of 1 upon complexation to
10 while enantioselective fluorescence quenching of ex-
cited N,N0-dioxide 1 by acids 11–15 may be attributed
to quenching via non-radiative relaxation of diastereo-
meric hydrogen-bond adducts. While all analytes show
linear Stern–Volmer plots indicating formation of equi-
molar diastereomeric hydrogen-bond adducts, we ob-
tained a non-linear relationship with 2-chloromandelic
acid 14, which can be attributed to both static and
dynamic quenching interactions.5c
The synthesis of 1,8-bis(3-(30,50-dimethylphenyl)-9-acr-
idyl)naphthalene N,N0-dioxide, 1, involves Suzuki cou-
pling of 4-bromo-2-chlorobenzoic acid, 2, and 3,5-
dimethylphenylboronic acid, 3, to afford biaryl 4 in al-
most quantitative amounts, Scheme 1. Copper-catalyzed
amination of 4 using a procedure previously reported
from our laboratories followed by ring closure of
anthranilic acid 5 with phosphorus oxybromide gives
9-bromoacridine 6 in excellent yields.6 Lithiation and
subsequent stannylation of 6 using tributylstannyl chlo-
ride yields stannane 7, which is then employed in
Pd(PPh3)4-catalyzed Stille coupling with 1,8-dibromo-
naphthalene to afford 1,8-diacridylnaphthalene 9. The
corresponding N,N0-dioxide 1 is then obtained by oxida-
tion with m-chloroperbenzoic acid.
1,8-Diacridylnaphthalene N,N0-dioxide 1 is fluorescent
with an emission maximum at 571 nm (excitation wave-
length: 490 nm). In order to reveal the usefulness of 1 for
enantioselective sensing we employed chiral analytes
10–15 in fluorescence titration experiments (Fig. 2).
We then decided to develop a fluorescence method that
allows accurate measurements of both the total amount
and the enantiomeric composition of chiral compounds.
We found that this can be accomplished by combination
of two assays. We observed that addition of either enan-
tiomer of substrate 13 to a solution of the racemic sensor
gives superimposable fluorescence titration curves. We
therefore expected that the total concentration of a chi-
ral substrate can be determined by fluorescence sensing
using racemic N,N0-dioxide 1 while employment of the
enantiopure sensor in literally the same fluorescence
titration experiment would allow analysis of the enan-
tiomeric composition. Six samples of 13 exhibiting dif-
ferent concentrations and enantiopurities ranging from
10% to 95% were prepared and subjected to fluorescence
analysis. First, the fluorescence signal of the racemic
sensor in the presence of each sample was measured
and compared to a calibration curve for determination
of the individual concentrations (Table 1). Having deter-
We were pleased to find that micromolar concentrations
of (+)-1 suffice for differentiation of the enantiomers of
all substrates tested (Fig. 3). Titration experiments with
t-Boc-protected valine showed more effective quenching
of the fluorescence of (+)-1 when the (S)-enantiomer of
O
O
NH
NH
2
OH
OH
O
N
H
OH
2
O
10
11
12
15
Cl
O
O
O
OH
OH
OAc
OH
HO
OH
O
14
13
Figure 2. Analytes employed in enantioselective sensing studies with 1.

X. Mei, C. Wolf / Tetrahedron Letters 47 (2006) 7901–7904
7903
2
1
0.8
0.6
0.4
1.8
(S,S)-10
(S)-11
1.6
1.4
1.2
1
(R,R)-10
(R)-11
0
2
4
6
8
10
0
5
10
15
10-3 M
10-3 M
2
1.8
1.6
1.4
1.2
1
2
1.8
1.6
1.4
1.2
1
(R)-13
(R)-12
(S)-12
(S)-13
0
5
10
10-3 M
15
20
0
0.3
0.6
10-2 M
0.9
1.2
2.5
2
1.6
1.4
1.2
1
(R)-15
(R)-14
(S)-15
1.5
1
(S)-14
0
1
2
3
0
10
20
10-3 M
30
40
10-1 M
Figure 3. Stern–Volmer plots of 3.5 · 10ꢀ5 M of (+)-1 in the presence of the enantiomers of analytes 12–15 in acetonitrile. Excitation wavelength:
490 nm; emission wavelength: 571 nm.
sensing method were in good agreement with actual
amounts and enantiomeric compositions. The data dem-
onstrate the high reproducibility and accuracy of this
approach, which is applicable to the screening of sam-
ples exhibiting a wide range of enantiopurity and excess
of either enantiomer.
Table 1. Concentration and enantiomeric composition of nine samples
of 13 determined by fluorosensing
Sample
Actual concn
10ꢀ2 [M]
Actual
% (R)
Calcd concn
10ꢀ2 [M]a
Calc.
(%) (R)a
A
B
C
D
E
F
1.00
1.00
1.00
0.70
0.70
0.70
95
40
10
90
35
5
0.96
1.05
1.10
0.68
0.73
0.69
91
33
3
80
26
3
In summary, we have prepared diacridylnaphthalene
N,N0-dioxide 1 exhibiting a C2-symmetric cleft designed
for chiral recognition of hydrogen bond donors such as
10–15. The use of 1 for quantitative stereoselective fluo-
rescence analysis of both yield and enantiomeric compo-
sition of chiral compounds has been demonstrated. We
have developed a practical method that first utilizes
racemic N,N0-dioxide 1 and then its pure enantiomer
in two facile fluorescence sensing assays. We believe that
this approach combines several attractive features: it
overcomes the difficulty in the determination of both
a Average of three fluorescence measurements at 571 nm.
mined sample concentrations, we were then able to
reveal the enantiomeric composition of each sample
using enantiopure (+)-1 (Fig. 4 and Supplementary
data). In all cases, results obtained by our fluorescence

7904
X. Mei, C. Wolf / Tetrahedron Letters 47 (2006) 7901–7904
Supplementary data
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
y = 0.5136x + 1.3368
R2 = 0.9978
Synthesis and characterization of 1 and actual measure-
ments of the concentration and enantiopurity of several
samples. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.tetlet.
2006.09.012.
% of (R)-enantiomer
actual measurements
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0
0.2
0.4
0.6
0.8
1
(R )/[(R)+(S)]
Figure 4. Fluorescence response of (+)-1 as
a function of the
enantiomeric composition of 13 and determination of actual % (R)-
13 of three samples. The concentration of (+)-1 was 2.50 · 10ꢀ5 M and
the total concentration of 13 was 1.00 · 10ꢀ2 M in acetonitrile. Results
are given in Table 1. Excitation (emission) wavelength: 490 nm
(571 nm).
concentration and enantiomeric composition by the use
of one sensor (in its racemic and enantiopure form), it
depends on two simple assays that provide accurate
values with high reproducibility, it eliminates the need
for substrate derivatization, and it utilizes a cost-effective
and sensitive technique (fluorescence spectroscopy) that
minimizes production of solvent waste.
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Acknowledgements
Funding from the National Science Foundation
(CAREER Award, CHE-0347368) and the Petroleum
Research Fund administered by the ACS (PRF40897-
G4) is gratefully acknowledged.