Chiral recognition based on enantioselectively aggregation-induced emission

Article abstract of DOI:10.1021/jo900527e

(Figure Presented) Novel chiral AIE compounds bearing a tartaric acid group were synthesized. They selectively aggregated with one enantiomer of a number of chiral amines, such that one enantiomer led to strong fluorescence and another enantiomer showed no or only weak fluorescence. This was used for the quantitative analysis of enantiomeric composition.

Full text of DOI:10.1021/jo900527e

pubs.acs.org/joc
5
vapors, heparin, metal ions, chlorine, and immuno
6
7
8
Chiral Recognition Based on Enantioselectively
Aggregation-Induced Emission
9
assays, and even in monitoring the layer-by-layer self-
1
0
assembling process.
Chiral recognition through changes in fluorescence has
attracted much interest because it can provide time-efficient
and sensitive enantiomer determination of chiral reagents,
catalysts, natural products, and drugs. However, to design
and synthesize fluorescent chiral receptors is still a chal-
Yan-Song Zheng* and Yu-Jian Hu
Department of Chemistry, Huazhong University of Science
and Technology, Wuhan 430074, P. R. China
1
1
1
1,12
zyansong@hotmail.com
Received March 19, 2009
lenge.
teryl groups have been evaluated as crystalline light-emitting
Although chiral AIE compounds bearing choles-
1
3
materials, work related to chiral recognition based on
AIE or AIEE effects has not been reported to the best
of our knowledge. Here, we report highly selective chiral
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Novel chiral AIE compounds bearing a tartaric acid
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one enantiomer of a number of chiral amines, such that
one enantiomer led to strong fluorescence and another
enantiomer showed no or only weak fluorescence. This
was used for the quantitative analysis of enantiomeric
composition.
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660 J. Org. Chem. 2009, 74, 5660–5663
Published on Web 06/24/2009
DOI: 10.1021/jo900527e
r 2009 American Chemical Society

Zheng and Hu
JOCNote
SCHEME 1. Synthesis of the Chiral AIE Compounds D-6 and
L-6 and the Structures of the Chiral Amines
-
3
FIGURE 1. Changes in the fluorescence spectra of L-6 (2.0ꢀ10
M) with increasing water in ethanol. λex=320 nm, ex/em slits =
/5 nm. Inset: curve of fluorescence intensity vs water percentage
5
measured at 420 nm.
a
TABLE 1. Interaction Results of L-6 with Chiral Amines in Solvents
amine
state
amine
state
b
(
1S,2R)-7
solution
(1R,2R)-9
(1S,2S)-9
(R)-10
suspension
solution
b
recognition system based on the AIE character of an organic
compound.
(1R,2S)-7
(2R)-8
precipitates
solution
suspension
precipitates
solution
(
2S)-8
a
(S)-10
The chiral compounds D-6 and L-6 bearing tartaric acid
groups were synthesized in excellent yield without the use of
column chromatography (Scheme 1). D-6 and L-6 are soluble
in common organic solvents such as ethanol, THF, and
chloroform but are insoluble in water, methanol, and hex-
ane. Dilute solutions in ethanol or THF had almost no
fluorescence, but as solids they displayed a strong blue
emission under UV irradiation. When water was added in
a solution of L-6 in ethanol or THF, the resulting suspension
also exhibited emission. Figure 1 shows that the fluorescence
intensity of L-6 did not increase very much when the volume
ratio of water vs ethanol was less than 65:35. When the ratio
was more than 70:30, a suspension appeared and the fluore-
scence intensity increased rapidly. At a ratio of 90:10, the
fluorescence intensity of the resultant suspension was 1494
times larger than the solution without water. Therefore, D-6
and L-6 exhibit AIE.
0
.02 M L-6 and 0.02 M amine in 1,2-dichloroethane at approximately
b
°C. 0.01 M L-6 and 0.01 M amine in a mixed solvent of n-hexane and
,2-dichloroethane (volume ratio 3:2).
5
1
same conditions, L-6 and (1R,2S-7) remained in suspension.
This methodology was extended to mixtures of L-6 with (2S)-
8, (1R,2R)-9, or (R)-10, which remained in suspension, and
mixtures with (2R)-8, (1S,2S)-9, or (S)-10 which became
clear. These results are consistent with those in 1,2-dichloro-
ethane, indicating that the interaction of L-6 with chiral
amines was unaffected by the polarity of the solvent used.
The enantioselective aggregation in aqueous ethanol could
not only be visualized with the naked eye, but quantified by
fluorescence measurements. The AIE character of 6 is shown in
Figure 2A-D. A suspension of L-6 and (1R,2S)-7 gave a fluo-
rescence intensity of 419, but the solution of L-6 and (1S,2R)-7
gave only 1.6, proving this system exhibits a high enantiomer
fluorescence intensity ratio of 262 (I(1R,2S)-7/I(1S,2R)-7). For amines
The chiral recognition of amines 7-10 by receptor L-6 was
studied initially. Aggregates formed enantioselectively when
L-6 and chiral amines were mixed in the appropriate sol-
vent (Table 1). When equal volumes of L-6 (0.04 M in
8, 9, and 10, the same methodology with L-6 gave I(2S)-8/I(2R)-8
10, I(1R,2R)-9/I(1S,2S)-9=18, and I(R)-10/I(S)-10=17.
=
To confirm that the enantioselective aggregation was the
result of chiral recognition by the receptor, the same chiral
amines were mixed with D-6 under the same conditions.
When D-6 was mixed with 7, 8, and 9, the expected results
were observed. The mixtures with (1S,2R)-7, (2R)-8, or
(1S,2S)-9 formed suspensions more easily than the mixtures
with (1R,2S)-7, (2S)-8, or (1R,2R)-9. And similarly, the
suspensions had stronger fluorescence than the solutions
(Figures S11-S13, Supporting Information). Again the
fluorescence intensity ratio for the mixture of D-6 with
amine-7 (I(1S,2R)-7/I(1R,2S)-7) was high at 1125 (Figure S11,
Supporting Information). D-6, however, showed a preference
to aggregate with (R)-10 instead of the expected (S)-10. To
1
1
,2-dichloroethane) was mixed with (1R,2S)-7 (0.04 M in
,2-dichloroethane) or (R)-10 a precipitate formed, but the
solutions of L-6 mixed with (1S,2R)-7 or (S)-10 remained
clear after standing for more than 1 day at 5 °C. Under the
same conditions, L-6 mixed with (2R)-8 remained clear, but
L-6 mixed with (2S)-8 led to a suspension. Neither enantio-
mer of amine 9, (1R,2R)-9, or (1S,2S)-9 in 1,2-dichloro-
ethane gave an aggregate with L-6 that could be observed
with the naked eye. Nevertheless, addition of n-hexane to the
solution (n-hexane/1,2-dichloroethane 3:2, 0.01 M) of L-6
with (1R,2R)-9 led to a suspension and that of L-6 with
(1S,2S)-9 remained clear.
Suspensions of L-6 and (1S,2R)-7 produced by adding
1
clarify this result, a H NMR titration (Figures S28 and S29,
water (1.33 mL) to a mixture of the two compounds (molar
ratio 1:1) in ethanol (0.4 mL) could be redissolved by adding
a small amount of additional ethanol (0.26 mL). Under the
Supporting Information) was conducted. This revealed the
binding ratio of 6 to 10 was 1:2. Contrary to the result at the
ratio of 1:1, the mixture of L-6 with (S)-10 precipitated,
J. Org. Chem. Vol. 74, No. 15, 2009 5661

Zheng and Hu
JOCNote
FIGURE 3. Change of fluorescence intensity of mixture of L-6 and
-
5
(
1R,2S)-7 (molar ratio 1:1) with concentrations from 6 ꢀ 10 to 1 ꢀ
-3
1
measured at 420 nm.
0
2
M in H O/EtOH 2:1. Inset: curve of intensity vs concentration
FIGURE 4. Change of fluorescence intensity of the mixture of L-6
-
4
and 7 (molar ratio 1:1) with enantiomer content. [L-6]=5ꢀ10 M.
-
4
Total concentration of two enantiomers of 7 was 5ꢀ10 M in H
2
O/
EtOH 2:1).
with (R)-10 remained a suspension but D-6 mixed with (S)-10
became a solution at a ratio of 1:2. This result demonstrates
that the selectivity of D-6 and L-6 toward the enantiomers of
amine 10 is as expected when the binding ratio reaches
saturation. Therefore, the interaction of chiral receptor 6
with chiral amines is indeed enantioselective.
In addition, the interaction of chiral receptor L-6 with
other chiral amines such as prolinamide 11, 1,2-diaminocy-
clohexane 12, and 2-aminobutanol 13 also resulted in en-
antioselective aggregation. The fluorescence intensity ratio
of two enatiomers of these three chiral amines is ID-11/IL-11=
455, I(1R,2R)-12/I(1S,2S)-12 =47, and ID-13/IL-13 =1.4, respec-
FIGURE 2. Fluorescence spectra of mixture of L-6 and enantio-
mers of amines in aqueous ethanol (concentration of all compounds
was 0.002 M, λex=320 nm, ex/em slits=5/5 nm): (A) of L-6 with 7 in
H
2
O/EtOH 2:1; (B) of L-6 with 8 in H
2
O/EtOH 2.4:1; (C) of L-6 with
9
in H O/EtOH 1.5:1; (D) of L-6 with 10 in H
2
2
O/EtOH 1:1.
tively (Figures S16-S18, Supporting Information).
It was noted that the fluorescence intensity of the suspen-
sions increased with increasing concentration of L-6 and
but the mixture of L-6 with (R)-10 stayed in solution. Similar
results have been found when different catalyst stoi-
chiometries have resulted in inverted enantioselectivity in
Sharpless asymmetric epoxidations. The enantioselective
aggregation was repeated with D-6 (Figures S14 and S15,
Supporting Information) and showed that the mixture of D-6
(
1R,2S)-7 (Figure 3). Valuably, when the concentration
-
4
1
4
was less than 5.5ꢀ10 M, the fluorescence intensity in-
creased linearly with the concentration (inset in Figure 3).
This means that the fluorescence intensity of the suspension
can be used to determine the enantiomer concentration.
-
4
When 5.0ꢀ10 M solutions of 7 with varying enantoime-
(14) Lu, L. D. L.; Johnson, R. A.; Finn, M. G.; Sharpless, K. B. J. Org.
Chem. 1984, 49, 728–731.
ric ratios were tested with L-6 at the same concentration
5
662 J. Org. Chem. Vol. 74, No. 15, 2009

Zheng and Hu
JOCNote
-1
TABLE 2. Association Constant K
a
(M ) of L-6 with Chiral Amines
amine
K
a
amine
K
a
amine
K
a
5
4
3
6
6
(
(
1S,2R)-7
1R,2S)-7
(1.02 ( 0.98) ꢀ 10
(2R)-8
(2S)-8
(1.43 ( 0.18) ꢀ 10
(7.86 ( 0.85) ꢀ 10
(1R,2R)-9
(1S,2S)-9
>10
>10
3
(8.98 ( 0.71) ꢀ 10
(
Figure 4), the fluorescence intensity increased with increas-
dry THF (30 mL) were stirred at room temperature for 0.5 h.
The resulting solution was evaporated to about 2 mL, and 6
was precipitated by addition of methanol. The resultant white
solid was collected by filtration and air-dried (5.2 g, 93%). D-6:
ing molar percentage of (1R,2S)-7. Similarly, the fluores-
cence intensity of the same experiment with D-6 increased
with increasing molar percentage of (1S,2R)-7. As a result,
two standard curves were drawn from which the enantio-
meric purity of chiral amine 7 could be obtained. This is very
important for high-throughput analysis of enantiomer pur-
ity of chiral drugs and reagents. By the same method, the
enantiomer composition of other amines such as 8-10 could
be also analyzed.
2
0
mp 166-168 °C; [R]
295, 2923, 2852, 1739, 1712, 1669, 1525 cm ; H NMR (400
MHz, DMSO-d ) δ 10.82 (s, 1H), 8.07 (d, J=7.6 Hz, 2H), 8.03
D
þ77.8 (c 1.0, EtOH); IR (KBr) λ 3514,
-
1 1
3
6
(
d, J=7.6 Hz, 2H), 7.93 (s, 1H), 7.91 (d, J=8.8 Hz, 2H), 7.72
quintet, J=8.4 Hz, 6H), 7.59 (quintet, J=8.0 Hz, 4H), 7.50 (t,
1
2
(
J=7.6 Hz, 2H), 7.43 (t, J=7.6 Hz, 1H), 6.05, 5.98 (2d, J=2.8
1
3
Hz, 2ꢀ1H); C NMR (100 MHz, DMSO-d ) δ 167.8, 165.4,
6
To further demonstrate the selective interaction of L-6
1
with enantiomers of a chiral amine, H NMR titrations of L-6
165.3, 164.3, 142.7, 140.6, 134.5, 134.4, 130.6, 130.0, 129.9,
129.7, 129.6, 129.5, 129.4, 129.2, 129.1, 126.1, 120.1, 118.5,
1
24 2 7
09.2, 73.4, 72.2; ESI- MS m/z calcd for C33H N O 560 [M],
with different amines were carried out in CDCl . By produ-
3
-
found 559.1 [(M - 1)] . Anal. Calcd for for C H N O : C,
7
mp 167-169 °C; [R]
3
cing a Job plot of the amide NH resonance, it was found that
L-6 bound to 7, 8, and 9 in a 1:1 molar ratio. Compound 10,
however, formed at a 2:1 ratio (10 vs L-6) as previously
described in ethanol (Figures S19-S32, Supporting Infor-
mation). The association constants for 7 and 8 were calcu-
33 24
2
7
0.71; H, 4.32; N, 5.00. Found: C, 70.58; H, 4.37; N, 4.95. L-6:
2
0
D
-81.5 (c 1.0, EtOH); IR (KBr) λ 3516,
292, 2976, 2921, 1740, 1712, 1669, 1523 cm ; H NMR (400
-1 1
MHz, DMSO-d ) δ 10.84 (s, 1H), 8.07 (d, J=7.6 Hz, 2H), 8.03
6
(
7
d, J=7.6 Hz, 2H), 7.93 (s, 1H), 7.90 (d, J=8.8 Hz, 2H), 7.76 -
2
lated by nonlinear curve fitting with relative coefficient R
more than 0.996 (Table 2). The association constant of L-6
.66 (m, 6H), 7.63-7.54 (m, 4H), 7.52 (t, J=7.6 Hz, 2H), 7.42
1
3
(t, J=7.6 Hz, 1H), 6.04, 5.99 (2d, J=2.8 Hz, 2ꢀ1H); C NMR
(100 MHz, DMSO-d ) δ 167.9, 165.5, 165.3, 164.6, 142.7,
140.6, 134.4, 134.3, 130.6, 130.0, 129.9, 129.7, 129.6, 129.5,
29.4, 129.3, 126.1, 120.1, 118.5, 109.1, 73.6, 72.4; ESI-MS m/z
6
with 9 was much larger than 10 M and so could not be
-1
6
1
15
accurately determined by H titration. The difference in the
binding affinities of L-6 with the two enantiomers of 7 and 8
indicated that L-6 exhibited chiral recognition, and this
resulted in enantioselective aggregations.
In conclusion, highly selective chiral recognition has been
achieved with the combination of AIE and chirality in an
organic compound for the first time. This finding provides a
simple method for the synthesis of a chiral receptor and a
new method for the determination of chiral excess through
changes in the fluorescence with high enantioselectivity.
1
-
calcd for C H N O 560 [M], found 559.1 [(M - 1)] . Anal.
Calcd for for C33
C, 70.65; H, 4.35; N, 4.98.
33 24
2
7
H
24
N
2 7
O : C, 70.71; H, 4.32; N, 5.00. Found:
Acknowledgment. We thank the National Natural Science
Foundation of China (No. 20872040) for financial sup-
port and thank the Analytical and Testing Centre at
Huazhong University of Science and Technology for mea-
surements.
Experimental Section
Synthesis of Compound 6. Compound 4 (2.2 g, 0.01 mol),
L-2,3-dibenzoyltartaric acid anhydride 5 (3.7 g, 0.011 mol), and
Supporting Information Available: Experimental details,
spectra of compounds, and calculation of association constants.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(
15) Fielding, L. Tetrahedron 2000, 56, 6151–6170.
J. Org. Chem. Vol. 74, No. 15, 2009 5663