Tris(indolyl)methene molecule as an anion receptor and colorimetric chemosensor: Tunable selectivity and sensitivity for anions

Article abstract of DOI:10.1039/c0ob00472c

Simple tris(indolyl)methene receptors 1-3 containing conjugated bisindole skeletons have been designed and synthesized. The anion binding and sensing properties have been studied using UV-vis spectroscopy and ¹H NMR titration technique. Compared with 3,3′-bis-indolyl phenylmethene (4), tris(indolyl)methene receptors could highly selectively detect F^- based on two stages of proton transfer, along with stepwise drastic color changes. The introduction of the electron withdrawing or donating groups into indole unit, which tunes the acidities of the hydrogen bond sites, partially enhanced or inhibited the occurrence of the deprotonation of receptor and has a positive effect on the selectivity and sensitivity of such "proton- transfer" chemosensors for anions.

Full text of DOI:10.1039/c0ob00472c

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Tris(indolyl)methene molecule as an anion receptor and colorimetric
chemosensor: tunable selectivity and sensitivity for anions†‡
Litao Wang,a,c Xiaoming He,a,c Yong Guo,a Jian Xua and Shijun Shao*a,b
Received 22nd July 2010, Accepted 7th October 2010
DOI: 10.1039/c0ob00472c
Simple tris(indolyl)methene receptors 1–3 containing conjugated bisindole skeletons have been
designed and synthesized. The anion binding and sensing properties have been studied using UV-vis
spectroscopy and 1H NMR titration technique. Compared with 3,3¢-bis-indolyl phenylmethene (4),
tris(indolyl)methene receptors could highly selectively detect F- based on two stages of proton transfer,
along with stepwise drastic color changes. The introduction of the electron withdrawing or donating
groups into indole unit, which tunes the acidities of the hydrogen bond sites, partially enhanced or
inhibited the occurrence of the deprotonation of receptor and has a positive effect on the selectivity and
sensitivity of such “proton-transfer” chemosensors for anions.
and AcO- at high analyte concentration. As an extension of our
Introduction
work, we have designed tris(3-indolyl)methene compounds 1–3, by
modifying the BIM skeleton to control the acidity and steric array
of the H-bond sites, thus to tune the selectivity and sensitivity of
such “proton-transfer” chemosensors for anions. Herein, we wish
to report the synthesis of the chemosensors, and demonstrate their
particular abilities in anion recognition and sensing.
The search for new host molecules for anion recognition and
sensing through hydrogen bonding interaction has been an active
research area,1 due to the importance of anions in a wide range
of medicinal, environmental and biological processes.2 One of the
most attractive approaches in this field involves the construction
of colorimetric chemosensors since naked eye detection can offer
qualitative and quantitative information.3 Most of the known
colorimetric chemosensors are based on the synthetic receptors
generally containing some combination of anion binding unit and
signal-reporting group (chromophore), either covalently attached
or intermolecularly linked.4 Recently, indole-based receptors have
attracted considerable attention.5 The acidity of the pyrrole NH
group is expected to enhanced by conjugation with benzene,6
which would lead to higher binding affinity for anions.
Previously, we presented a simple chemosensor 4 utilizing bis(3-
indolyl)methene (BIM) skeleton as building block, which allows
for the colorimetric detection of F- and AcO- in organic aprotic
solution based on the proton transfer signaling mode.7 The large
conjugated bis(3-indolyl)methene skeleton could not only act as
a color-reporting group, but also provide as an acidic H-bond
donor moiety and a basic H-bond acceptor moiety for anion
binding. In addition, the bis(indolyl)methene skeleton has also
been demonstrated to be an efficient chromogenic and fluorescent
moiety for metal ion sensing by Kim and coworkers.8 However,
this system is proved to be inefficient in discriminating between F-
Results and discussion
The synthesis of compounds 1–3 was shown in Scheme 1.
Compounds 1a–3a were synthesized easily using iodine as the
catalyst according to the procedure describing in the previous
literatures in 85%, 81%, and 80% yield, respectively.9 Compounds
aKey Laboratory of Chemistry of Northwestern Plant Resources and Key
Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,
P. R. China. E-mail: sjshao@licp.cas.cn
bState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou
730000, P. R. China
cGraduate School of the Chinese Academy of Sciences, Beijing 100039,
P. R. China
† This publication is part of the web themed issue on fluorine chemistry.
‡ Electronic supplementary information (ESI) available: Characterization
data of all compounds. Details concerning the titration curves, changes
of UV-vis spectroscopy. The stoichiometry ratio of the complexes and the
fitting method for equilibrium constants. See DOI: 10.1039/c0ob00472c
Scheme 1 Synthesis of compounds 1–3.
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1–3 were formed by treating the precursor 1a–3a with DDQ
in acetonitrile or acetone in about 25%, 36%, and 40% yield,
respectively.
absorption band centered at 513 nm evolved until reaching its
maximum upon the addition of 30 equiv of F-, accompanied
by one clearly isosbestic point at 467 nm. We attributed these
spectral changes to the deprotonation of one NHmoiety, due to the
high stability of complex [HF2]-.10 It is the mono-deprotonation
that is responsible for the first pronounced color change. A red-
shift with a similar great extent (from 423 to 517 nm) has been
observed on the interaction of bis(indolyl)methene compound 4
with F-.7 On addition of a further excess of F-, the band at 513 nm
gradually decreased and a new blue-shift band at 477 nm evolved,
while the color of the solution turned from light-pink to orange.
We ascribed this new band to the double deprotonated form of
compound 1. Perhaps due to the weak ability of electron acceptor
in decentralizing the increased electron density of the conjugated
system, compound 1 was forced to undergo a conformational
change, in which three aromatic rings were less coplanar in the
form of [L]2- than that of [HL]-, leading to a moderate blue-shift
in the absorption band.
Comparing with bis(indolyl)methene 4, compound 1 substi-
tutes a larger indolyl group for the phenyl group. Structurally,
compound 1 could be deemed as the combination of two BIM
skeletons, possessing two H-bond donor sites, which may lead to
double deprotonation process in a certain condition. This spec-
ulation was validated by the observation that the color changes
drastically upon addition of F- (Fig. 1). Furthermore, substitution
of a larger group results in deleterious effects on coplanarity of
three aromatic rings, which disfavors the propagation of electron
onto the p-conjugated skeleton. The selectivity and sensitivity of
such conjugated bisindole-based chemosensor for anions may be
tuned by modifying the skeleton.
Titration with the strong base [Me4N]OH, which definitely
leads to deprotonation, also induced the same color and spectral
changes of 1 as those observed with F- anions (Fig. S1, ESI‡).
Furthermore, the F--induced two-step changes in color and
spectra are reversible, progressive additions of protic solvent (such
as methanol or water) result in turning the orange solution to
light-pink, then to yellow. Correspondingly, the absorption band
recovers from 477 nm to 513 nm, then finally to 449 nm (see Fig. S2,
ESI‡). This phenomenon may be due to that protic solvents could
compete for F- with the NH moieties, disfavoring the formation
of [L]2- and [HL]-.
Previous study showed that the deprotonation of compound 4
was also induced by adding large excess of AcO- (up to 100 equiv),
and the color and spectral changes were similar to the case of F-.7
Different from 4, compound 1 has advantage in discriminating
AcO- and F- at high concentration, partly due to the weakening
of the acidity of the indole NH groups of 1 through structural
modification. Upon addition of large amount of AcO-, the
absorption band at 437 nm decreased and underwent a moderate
red-shift, while a less strong shoulder peak at 513 nm appeared
and only increased to a certain extent (Fig. 3a), which could
not induce obvious color change. The result suggested that the
basicity of AcO- is insufficient to induce complete deprotonation
of compound 1, and the system should be in an incipient state of
mono-deprotonation of 1.
Fig. 1 Color changes of 1 (2.5 ¥ 10-5 M in CH3CN) upon addition of 0,
25, 100 equiv of F-.
The interaction of compound 1 with F- was studied in detail
using UV-vis spectroscopic technique and interesting spectral
behaviors were observed, as shown in Fig. 2. Compound 1
was characterized by a strong band (l = 457 nm, emax = 2 ¥
104 dm3 mol-1 cm-1) which is assigned to p–p* transitions of
conjugated bisindole skeleton. In the presence of 0–1 equiv of
F-, the absorption at 457 nm of 1 decreases and undergoes a
moderate blue-shift to 437 nm, presumably due to the formation
of the H-bond complex with F- and the distortion of the planarity
of the conjugated system. With adding excessive amount of F-,
two stages of distinct changes in color and spectra were observed.
In the presence of 1–30 equiv of F-, the solution color gradually
turned from yellow to light-pink. Correspondingly, the intensity
of the band at 437 nm reduced gradually and a new intense
Fig. 2 The changes in UV-vis spectra of 1 recorded in CH3CN (2.5 ¥ 10-5 M) after addition of: (a) 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 equiv of
F-; (b) 1, 2, 3, 5, 7, 9, 15, 20, 30 equiv of F-; (c) 30, 40, 50, 60, 70, 80, 100, 125, 150, 175, 200 equiv of F-.
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by two stages of proton transfer, with the limit of detection
(LOD) of 9.28 ppm for F-, 459 ppm for AcO-, and 558 ppm for
H2PO4 , respectively,11 which further shows that 1 could obviously
-
distinguish F- from other anions through spectral behaviors.
Similar anion binding and sensing properties were also observed
in the more polar DMSO (Fig. S5, ESI‡).
The interaction of 1 and F- was also monitored using 1H NMR
titration in DMSO-d6 (solubility problems prevented the studies
in CD3CN solution), as shown in Fig. 4. The shift at 13.3 ppm
that pertains to the NH groups disappeared and the aromatic
proton signals underwent continuous upfield shifts along with the
stepwise deprotonation. Such effect can be due to the negative
electron transfer onto the p-conjugated framework, acting as a
shielding effect and cause an upfield shift.12 During the titration
experiment, it is found that there exist two titration pause, which
may relate to the two states of proton transfer. In addition, a
signal of [HF2]- at 16.1 ppm was obtained during the titration
process of compound 2 with F- (Fig. S6, ESI‡), which indicated
the deprotonantion of receptor and the formation of [HF2]-
species.3c,10,13
Fig. 3 The changes in UV-vis spectra of 1 recorded in CH3CN (2.5 ¥
10-5 M) after addition of (a) 0, 10, 20, 30, 40, 50, 60, 70, 80 100, 120, 140,
160, 180, 200 equiv of AcO-; (b) 0, 1, 2, 3, 4, 5, 10, 15, 20, 30, 40, 50, 75,
-
100, 200 equiv of H2PO4
.
Fig. 4 1H NMR spectra of 1 (1.0 ¥ 10-2 M) in DMSO-d6 upon addition
of various quantities of tetrabutylammonium fluoride.
Additionally, the absorption spectral changes of 1 on addition of
H2PO4- are shown in Fig. 3b. Upon addition of excessive amount
-
of H2PO4 to the CH3CN solution of 1, a red-shifted absorption
Similar to the case of 1, compound 2, bearing electron-donating
methoxy groups on indole units, also exhibited selective binding
towards F- over other anions tested (Fig. S7, ESI‡). However,
the introduction of methoxy groups into tris(indolyl)methene
skeleton, which leads to the decreased acidity of the indole
NH proton, decreased the affinity of compound 2 towards
anions. Compared with compound 1, the F--induced reversible
deprotonation of compound 2 occurred at high F- concentration,
and the absorption bands related to the mono- and double-
deprotonation forms were centered at 509 nm and 483 nm (Fig. S8,
ESI‡), respectively, two distinct drastic color changes from orange
solution to light-pink, then to yellow were also observed in this
band appeared at 471 nm rather than 513 nm, which is similar to
that witnessed in the process of F--induced double deprotonation,
along with isosbestic points at 370 nm, 401 nm and 446 nm.
-
The results led us to suggest that H2PO4 , with bulky size, could
interact with two NH sites of 1 to form multiple hydrogen bonds
complex by 1 : 1 binding model, which may be in an incipient state
-
of the double deprotonation of 1, and the weak basicity of H2PO4
suppresses the deprotonation process.
The anion binding ability of 1 with other anions (Cl-, Br-,
I-, ClO4 and HSO4 added as their tetrabutylammonium salts)
were also evaluated through UV-vis method (Fig. S3, ESI‡). No
significant spectral and color changes were observed, even in large
excess amount because of the weak coordination interaction.
-
-
process. As for AcO- or H2PO4 , the titration spectra of compound
-
2 showed only a slight red-shift at high anion concentration (up
-
Furthermore, compound 1 is inefficient in detecting HSO4
to 200 equiv) (Fig. S9 and S10, ESI‡), which indicated that the
-
through protonation mode in aqueous system compared with
compound 4 (Fig. S4, ESI‡),7 presumably due to the weakening
of the basicity of the H-bond acceptor by steric modification.
As a new kind of chromogenic molecules based on BIM
skeleton, tris(indolyl)methene 1 could highly selectively sense F-
H-bond interaction of 2 with AcO- or H2PO4 was obviously
weakened.
On the other hand, the anion binding and sensing properties
of compound 3 were observed using UV-vis spectroscopy tech-
nique. The introduction of electron-withdrawing nitro groups into
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tris(indolyl)methene skeleton, which leads to the increase in the
acidity of the indole NH proton, has a positive effect on the affinity
and selectivity of compound 3 towards anions. As shown in Fig. 5,
compound 3 exhibited selective binding and colorimetric sensing
towards F-, AcO- and H2PO4 over other anions tested. The
-
addition of 2 equiv of F- induced the fully mono-deprotonation
process, and further double deprotonation evolved until reaching
equilibrium when 20 equiv of F- was added. The absorption
bands related to the mono- and double-deprotonation forms were
centered at 546 nm and 523 nm (Fig. S11, ESI‡), respectively.
Fig. 5 UV-vis spectra of 3 recorded in CH3CN (2.5 ¥ 10-5 M) after
-
addition of 25 equiv of various anions (none, F-, AcO-, Cl-, Br-, I-, HSO4
ClO4-, H2PO4-).
,
Due to the increase in the acidity of indole NHs, the AcO--
induced mono-deprotonation of compound 3 was observed
(Fig. 6a). The new absorption bands at 546 nm, completely similar
to the mono-deprotonation induced by F-, continuously increases
in intensity, reaching a maximum upon addition of 10 equiv of
this anion. However, no further double deprotonation occurred
even in the presence of large excess of AcO- (1000 equiv), due
to the basicity of AcO-. Fig. 6b shows the results of absorption
Fig. 6 The changes in UV-vis spectra of 3 recorded in CH3CN (2.5 ¥
10-5 M) after addition of (a) 0, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2, 4, 6, 8, 10,
15, 20, 50, 100 equiv of AcO-; (b) 0, 1, 2, 4, 6, 8, 10, 15, 20, 30, 40, 60, 80,
-
100 equiv of H2PO4
.
Table 1 Equilibrium constants for the interaction of receptors 1–3 with
F-, AcO- and H2PO4 at 25 ◦
C
-
-
-
titration of 3 with H2PO4 . Addition of H2PO4 leads to decrease
in the 458 nm absorption band, along with mild increase in the
546 nm and 503 nm bands with the effect that the solution changed
color from yellow to pink. The spectral change almost stops
Receptor
Anion
F-
1
2
3
-
upon addition of 10 equiv of H2PO4 . The results indicate that
6.68 ¥ 103 697b
1.79 ¥ 103 47c
3.98 ¥ 102 14a
1.18 ¥ 103 59a
5.69 ¥ 103 437b
1.34 ¥ 103 36c
2.91 ¥ 102 8a
3.19 ¥ 102 7a
>107b
-
2.68 ¥ 104 3158c
1.49 ¥ 105 5310b
8.42 ¥ 103 1744b
the deprotonation of 3 by H2PO4 anion is incomplete and the
AcO-
H2PO4
interaction may involve multiple hydrogen-bonded complex.
The above results show significant differences in anion binding
and optical properties among receptors 1, 2, and 3 although
only slight structural modification was made by introducing
electron-donating methoxy groups and electron-withdrawing nitro
groups, which tuned the binding abilities and optical responses of
-
a The association constant. b The mono-deprotonated dissociation con-
stant. c The double-deprotonated dissociation constant.
tris(indolyl)methene skeleton towards F-, AcO-, and H2PO4 an-
and 2, due to the increasing in the acidity of indole NHs caused by
the electron withdrawing nitro group. Receptors 1–3 all show the
double deprotonation by F-, and the first dissociation constants
are bigger than the second dissociation constants, which was
probably due to decreasing the NH proton acidity after mono-
deprotonation. Based on hydrogen bond interaction, receptors
-
ions. On the other hand, the definite stoichiometric ratio between
-
receptors 1–3 and AcO- or H2PO4 was determined to be 1 : 1 by
Job’s-plot experiments, the equilibrium constants of receptors 1–3
with three basic anions were evaluated through nonlinear least
squares fitting by origin software,14 and summarized in Table 1.
A further insight into Table 1 reveals that receptor 3 shows much
higher binding affinity for F-, AcO-, and H2PO4- than receptors 1
-
1 and 2 give much higher binding affinity for H2PO4 than for
AcO-. This selectivity will be attributed to the fitness of size and
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shape of dihydrogen phosphate anion towards binding sites of
tris(indolyl)methene by multiple hydrogen bonds.5g,15
Yield 80%, mp: >300 ◦C. IR (KBr) 3360, 1621, 1511, 1472, 1323,
1
1213, 1080, 743 cm-1. H NMR (400 MHz, DMSO–d6), (ppm):
11.67 (s, 3H, NH), 8.45 (d, 3H), 7.99 (dd, 3H), 7.57 (d, 3H), 7.28
(s, 3H), 6.61 (s, 1H, Ar3CH). MS (ESI) m/z: 495.7 ([M - H]-).
Conclusion
To sum up, tris(indolyl)methene, a new kind of unexplored
chromogenic molecules based on BIM skeleton, proved an ex-
cellent chemosensor for F- by two stages of proton transfer,
3,3¢-((3H-Indol-3-ylidene)methylene)bis(1H-indole) (1)
Compound 1a (0.36 g, 1 mmol) was dissolved in acetonitrile
(25 mL), DDQ (0.34 g, 1.5 mmol) solution of acetonitrile was
dropwise and slowly added to the solution. This reaction was
allowed for 1 h and gave a dark red precipitate, which was filtered,
washed with CH3CN, and recrystallized from ethanol. Yield 25%,
mp: >300 ◦C. IR (KBr) 3375, 3117, 2983, 2215, 1699, 1620, 1566,
1476, 1409, 1307, 1213, 1127, 1009, 831, 743 cm-1. 1H NMR
(400 MHz, DMSO-d6), (ppm): 8.41 (s, 3H), 7.69 (d, 3H), 7.34
(t, 3H), 7.07 (t, 3H), 6.94 (s, 3H). 13C NMR (400 MHz, DMSO-d6)
d: 160.36, 143.53, 138.79, 130.02, 124.64, 124.46, 123.52, 123.08,
120.80, 120.56, 118.09, 115.38, 113.99, 113.76. HRMS (ESI) m/z
([M+H]+) calcd for C25H17N3: 360.1495; found, 360.1490.
and could also distinguish AcO- and H2PO4 through spectral
-
behaviors. The introduction of the electron donating group into
the tris(indolyl)methane skeleton decrease the binding ability of
the hydrogen bond site, which partially inhibit the occurrence
of the deprotonation, and improve the selectivity. In contrast,
the introduction of the electron withdrawing group increases
notably the proton acidity of the hydrogen bond donor moiety,
and promotes the occurrence of the reversible deprotonation,
while improving anion affinity and response sensitivity of the
receptor, and reduced the anion selectivity. Consequently, as a
promising building block, the BIM skeleton seems to be general
for developing more anion receptors and sensors through some
modification.
3,3¢-((5-Methoxy-3H-indol-3-ylidene)methylene)bis(5-methoxy-
1H-indole) (2)
Experimental section
Compound 2a (0.45 g, 1 mmol) was dissolved in acetonitrile
(25 mL), DDQ (0.34 g, 1.5 mmol) solution of acetonitrile was
dropwise and slowly added to the solution. This reaction was
allowed for 1 h and gave a dark red precipitate, and then the
precipitate was directly purified by column chromatography and
eluted with EtOAc–methanol (v/v 3 : 1) to afford the product.
Yield 36%, mp: >300 ◦C. IR (KBr) 3362, 3135, 2974, 2892, 2210,
Tris(1H-indol-3-yl)methane (1a)
Compound 1a was prepared according to the literature method.9a
Iodine (0.02 g, 0.1 mmol) was added to a mixture of the indole
(0.23 g, 2 mmol) and orthoformate (0.10 g, 0.66 mmol) in ace-
tonitrile (4 mL), and the reaction was stirred at room temperature
for 5 min. the solvent was removed under reduced pressure, and
the residue was directly purified by column chromatography and
eluted with n-hexane–EtOAc (v/v, 3 :◦1) to afford the product
(white solid). Yield 85%, mp: 238–240 C. IR (KBr) 3399, 3046,
1
1623, 1593, 1461, 1212, 1050, 793 cm-1. H NMR (400 MHz,
DMSO-d6) d: 8.34 (s, 3H), 7.60 (d, 3H), 6.99 (dd, 3H), 6.38 (s,
3H). 13C NMR (400 MHz, DMSO-d6) d: 159.24, 155.94, 142.15,
141.94, 133.30, 127.15, 126.91, 118.09, 114.43, 113.59, 103.80,
54.94 HRMS (ESI) m/z (M+H+) calcd for C28H23N3O3: 450.1812;
found, 450.1806.
1605, 1448, 1417, 1331, 1088, 744 cm-1. H NMR (400 MHz,
1
DMSO-d6) d: 10.70 (s, 3H, NH), 7.39 (d, 3H), 7.32 (d, 3H), 7.01
(t, 3H), 6.92 (d, 3H), 6.84 (t, 3H), 6.04 (s, 1H, Ar3CH). MS (ESI)
m/z: 359.9 ([M - H]-).
3,3¢-((5-Nitro-3H-indol-3-ylidene)methylene)bis(5-nitro-1H-
indole) (3)
Tris(5-methoxy-1H-indol-3-yl)methane (2a)
Compound 2a was prepared in a similar manner to compound
1a. the reaction was stirred at room temperature for 20 min, and
purification by silica column chromatograph and eluted with n-
hexane–EtOAc (v/v, 3 : 2) to afford the product (white solide).
Yield 81%, mp: 218–220 ◦C. IR (KBr) 3404, 3048, 2991, 2937,
2830, 1621, 1582, 1483, 1436, 1219, 1168, 1042, 800 cm-1. 1H NMR
(400 MHz, DMSO–d6), (ppm): 10.56 (s, 3H, NH), 7.22 (d, 3H),
6.94 (s, 3H), 6.85 (d, 3H), 6.67 (dd, 3H), 5.91 (s, 1H, Ar3CH), 3.58
(s, 9H). MS (ESI) m/z: 452.0 ([M+H]+).
Compound 3a (0.5 g, 1 mmol) was dissolved in acetone (25 mL),
DDQ (0.34 g, 1.5 mmol) solution of acetone was dropwise and
slowly added to the solution. This reaction was allowed for 6 h
and gave a dark brown precipitate, which was filtered, washed
with acetone. Yield 40%. mp: >300 ◦C. IR (KBr) 3378, 3099, 2871,
2210, 1703, 1526, 1453, 1343, 1211, 1130, 1057, 829, 740 cm-1. 1H
NMR (400 MHz, DMSO-d6) d: 8.91 (s, 3H), 8.28 (dd, 3H), 8.00
(d, 3H), 7.83 (s, 3H). 13C NMR (400 MHz, DMSO-d6) d: 174.25,
171.97, 171.02, 160.48, 144.59, 143.57, 142.29, 136.17, 126.09,
120.23, 117.97, 117.03, 114.99. HRMS (ESI) m/z ([M+H]+) calcd
for C25H14N6O6: 495.1048; found, 495.1053.
Tris(5-nitro-1H-indol-3-yl)methane (3a)
Compound 3a was prepared according to the literature method.9b
Iodine (0.02 g, 0.1 mmol) was added to a mixture of the 5-
nitroindole (0.32 g, 2 mmol) and orthoformate (0.10 g, 0.66 mmol)
in a closed vessel, and the reaction was stirred at room temperature
for 20 min. and then the residue was directly purified by
column chromatography and eluted with petroleum ether (b.p.
60–90 ◦C)/EtOAc (v/v, 1 : 2) to afford the product (yellow solide).
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 20672121) and the open fund of State
Key Laboratory of Oxo Synthesis & Selective Oxidation (No.
OSSO2008kjk6).
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8 J. W. Lee, S. Y. Park, B. K. Cho and J. S. Kim, Tetrahedron Lett., 2007,
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