Facile C-S coupling reaction of aryl iodide and thiophenol catalyzed by Cu-grafted furfural functionalized mesoporous organosilica

Article abstract of DOI:10.1039/c0dt01771j

A new functionalized mesoporous organosilica has been designed via Schiff-base condensation of furfural and 3-aminopropyltriethoxy-silane (APTES) followed by its hydrothermal co-condensation with tetraethylorthosilicate (TEOS) in the presence of a cationi

Full text of DOI:10.1039/c0dt01771j

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PAPER
Facile C–S coupling reaction of aryl iodide and thiophenol catalyzed by
Cu-grafted furfural functionalized mesoporous organosilica
John Mondal, Arindam Modak, Arghya Dutta and Asim Bhaumik*
Received 15th December 2010, Accepted 4th March 2011
DOI: 10.1039/c0dt01771j
A new functionalized mesoporous organosilica has been designed via Schiff-base condensation of
furfural and 3-aminopropyltriethoxy-silane (APTES) followed by its hydrothermal co-condensation
with tetraethylorthosilicate (TEOS) in the presence of a cationic surfactant CTAB. Subsequent reaction
of this mesoporous organosilica with Cu(OAc)2 in absolute ethanol leads to the formation of a new
Cu(II)-grafted mesoporous organosilica catalyst 1. Powder XRD, HR TEM, FE SEM, N2 sorption and
FT IR spectroscopic tools are used to characterize the materials. This Cu-anchored mesoporous
material acts as an efficient, reusable catalyst in the aryl-sulfur coupling reaction between aryl iodide
and thiophenol for the synthesis of value added diarylsulfides.
very tedious job due to the more favourable parallel oxidative S–S
coupling reaction and the simultaneous deactivation of the metal
Introduction
catalyst through the coordination with a sulfur heteroatom.15 C–
S bond formation could proceed via the reduction of sulfones
and sulfoxides by using strong reducing agents like LiAlH4.16 The
C–S cross coupling reaction of aryl halides and thiophenol for
the development of new C–S bonds is very important industri-
ally as the cost and environmental pollution of the process is
relatively low.9 Aryl-sulfur coupling reactions can be carried out
by using Pd,17 Fe,18 Co, Ni and Cu19 containing homogeneous
catalysts carrying suitable ligands. The use of Pd-catalysts in the
large-scale processes has been limited due to the high cost of
Pd(OAc)2 and air sensitivity of palladium catalysts. Moreover,
the efficient palladium catalysts are usually prepared by using
organophosphorous or phosphine ligands, which are difficult to
prepare and environmentally unfriendly. A problem arises for the
Ni and Co catalysts since these are toxic in nature and have low
turnover numbers in their respective reactions. On the other hand,
Cu-catalysts are prepared from Cu-salts, which are very cheap
and easily available. The problems traditionally associated Cu-
mediated reactions include very high temperature, long reaction
time, severe reaction conditions and the requirement of greater
amount of Cu-salt than its stoichiometry.6,20 So a successful
strategy for designing an alternative inexpensive, non-air sensitive
and recyclable heterogeneous catalyst anchored by Cu-centres on
a high surface area material is highly desirable to address these
industrial and environmental concerns.
Organically functionalized mesoporous materials have attracted
great attention in recent times due to their huge potential
applications in gas storage,1 sensing,2 light-harvesting,3 catalysis,4
drug-delivery5 and so on. For an efficient and reusable catalyst
strong binding of the active metal centres to the mesopore
surface via coordination/covalent bond is very crucial and this
can eliminate the possibility of leaching of the active metal from
the catalyst surface. Ease of separation of the product from the
reaction mixture, recovery of catalyst by simple filtration and
excellent recycling efficiency makes the heterogeneous catalysts
potentially more useful over their corresponding metal com-
plexes as homogeneous catalysts.6 Among different approaches
for heterogenization, stabilization/incorporation of an active
metal into insoluble zeolitic frameworks,7 mesoporous solids,8 or
related inorganic solids9 having high surface areas are considered
extensively. Mesoporous organosilica, which offers large pores
with a high surface area and donor sites to capture the metal
complex, can stabilize the active metal centres under different
reaction conditions due to the presence of monodentate, bidentate
or tetradentate chelating donor sites at the mesopore surfaces.10
In this context it is pertinent to mention that C–S, C–N and
C–O bond formation reactions offer an indispensable tool for
designing new target molecules in synthetic organic chemistry.11
A
large variety of aryl-sulfur compounds are used for the treatment
of Alzheimer’s and Parkinson’s disease12 and also for cancer.13
Molecules containing C–S bonds are also used as a molecular
precursor for the synthesis of new materials.14 However, transition
metal mediated C(aryl)–S bond formation is less studied than the
corresponding C–N and C–O bonds as C–S bond formation is a
Herein, we report the synthesis of a new furfural functionalized
mesoporous organosilica grafted with Cu(II) (Scheme 1, catalyst
1) and its excellent catalytic activity in the aryl-sulfur coupling
reaction of aryl iodide and thiophenol in the presence of a base.
Functionalized mesoporous organosilica has been prepared by
using the Schiff-base condensation product of furfural and an
organic ligand (3-aminopropyl-triethoxysilane) as precursor. The
Department of Materials Science, Indian Association for the Cultivation of
Science, Jadavpur 700 032, India. E-mail: msab@iacs.res.in
5228 | Dalton Trans., 2011, 40, 5228–5235
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Scheme 1
material is thoroughly characterized by using powder XRD, HR
TEM, FE SEM, N2 sorption, ICP-AES, UV-vis and FT IR
spectroscopic tools. This new mesoporous catalyst remains in a
separate solid phase in the reaction mixture, as a result of which
the recovery of the catalyst can be easily done by simple filtration
and the reaction can proceed without any precaution of inert
atmospheric conditions.
0.0075 mol, 2.73 g) was dissolved in an aqueous solution of tartaric
acid (0.6 g in 20 g of H2O) under vigorous stirring conditions.
The resulting mixture was stirred for about 30 min to obtain
a clear solution. Then dark brown colour viscous gel (1.35 g)
was added dropwise into the surfactant solution under stirring
conditions for about 2 h. After 2 h of stirring 5.245 g of TEOS
(0.025 mol) was added in the reaction mixture and it was stirred
for about 2 h to obtain a gel. After 2 h stirring 2 M NaOH
solution was added dropwise into the gel until the pH reached
ca. 12.0. The resulting gel was kept under stirring for another 12 h
at room temperature and then hydrothermally treated at 353 K
for 72 h under static conditions. The yellow coloured solid was
recovered under filtration, washed several times with water and
dried under vacuum. The template CTAB was removed from the
as-synthesized sample by extracting the solid three times with an
ethanolic solution of HCl at room temperature and this sample is
designated as B.
Experimental
Synthesis of Schiff base ligand:
A solution of furfural (0.01 mol, 0.9609 g) in dry ethanol (20 mL)
was placed under an argon atmosphere in a two neck round bottom
flask equipped with a condenser and an additional funnel. A
solution of APTES (0.01 mol, 2.21 g, Aldrich) in dry ethanol
(5 mL) was added dropwise into the furfural solution over 20 min
under continuous stirring conditions. After the complete addition
of the organic ligand the reaction mixture was kept under reflux
conditions for about 8 h. Then reaction mixture was allowed to
cool at room temperature and excess solvent was evaporated to
dryness, leaving behind a dark brown coloured viscous gel. This
dark brown coloured viscous liquid was designated as A and it
was characterized by 1H and 13C NMR.21
Synthesis of mesoporous organic silica supported Cu(II) catalyst (1)
1 g of the respective dried mesoporous material grafted with
Schiff-base ligand B was suspended in an absolute ethanol
(20 mL) solution of copper(II) acetate (0.5 g) and was kept
under refluxing conditions for about 8 h at 393 K. The colour
of the mesoporous material was slowly changed from yellow to
light green and no further colour change occurred on further
reflux. The reaction mixture was cooled at room temperature and
the resulting mesoporous material was filtered through suction
with thorough washing with ethanol. After washing it was dried
under vacuum. The light green coloured mesoporous material was
designated as Cu-grafted catalyst 1. The outline for the preparation
of this furfural functionalized mesoporous silica supported Cu(II)
catalyst is shown in Scheme 1.
Synthesis of furfural functionalized mesoporous organic hybrid
silica (B):
A was used as an organosilane precursor along with tetraethy-
lorthosilicate (TEOS, Aldrich) in 1 : 5 molar ratio for the synthesis
of organically functionalized mesoporous silica. Cetyltrimethy-
lammonium bromide (CTAB) was used as a structure directing
agent and tartaric acid was used to maintain the initial pH of the
solution. In a typical synthesis procedure CTAB (Loba Chemie,
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Table 1 Unit cell parameters of Cu-loaded catalyst 1
General procedure for aryl-sulfur coupling reaction of Thiophenol
with Aryl iodide:
Phase: Triclinic
Cu-grafted furfural functionalized mesoporous silica 1 (Scheme 1)
was used as a catalyst for the aryl-sulfur coupling of thiophenol
with aryl iodide. To a solution of iodobenzene (204 mg, 1 mmol)
and thiophenol (121 mg, 1.1 mmol) in 3 mL of DMF, K2CO3
(276 mg, 2 mmol) and Cu-grafted catalyst 1 (20 mg) were added.
The mixture was heated at 383 K in a oil bath and the progress
of the reaction was monitored by TLC. After a period of time
the reaction mixture was allowed to cool and the catalyst was
separated by simple filtration. The catalyst was washed and the
filtrate was diluted with Et2O (20 mL) and extracted with Et2O
with the addition of 10% aqueous NaOH solution. The combined
organic layer was washed with water and brine solution. Then the
combined organic layer was dried over Na2SO4 and evaporated to
dryness to give the crude product diphenylsulfide as a colourless
Parameters
Deviations
˚
a =
b =
c =
a =
b =
g =
13.073 A
0.010
0.014
0.006
0.16
0.13
0.16
˚
12.289 A
˚
10.737 A
91.04
91.18
90.25
3
˚
Unit Cell Volume = 1724.411 A
ESD = 2.631
h
1
0
2
0
2
0
1
3
4
0
2
2
1
1
k
0
0
0
2
0
0
0
1
0
4
4
0
-5
-4
l
2h
6.77
8.23
d
0
13.067
10.727
6.533
6.139
5.527
3.576
3.468
3.294
3.144
2.962
2.770
2.462
2.362
2.286
1
0
0
1
13.54
14.46
16.01
24.88
25.66
27.05
28.38
30.10
32.25
36.48
38.05
39.40
1
oil. The isolated crude product was characterized by H and 13C
3
-3
-2
-1
-1
0
4
1
NMR, respectively. The aryl-sulfur coupling reaction between
thiophenol with aryl iodide over catalyst 1 is shown in the
Scheme 2.
Characterization techniques. Powder X-ray diffraction pat-
terns were recorded on a Bruker D-8 Advance SWAX diffractome-
ter operated at 40 kV voltages and 40 mA current. The instrument
was calibrated with a standard silicon sample, using Ni-filtered Cu-
Ka (l = 0.15406 nm) radiation. Nitrogen adsorption/desorption
isotherms are obtained by using a Bel Japan Inc. Belsorp-HP at
77 K. Prior to gas adsorption, samples are degassed for 4 h at 393 K
under high vacuum conditions. A JEOL JEM 6700F field emission
scanning electron microscope is used for the determination of the
morphology of the particles. FT IR spectra of the samples are
recorded using a Nicolet MAGNA-FT IR 750 Spectrometer Series
II. UV-vis diffuse reflectance spectra of materials B and 1 were
obtained by using a Shimadzu UV 2401PC spectrophotometer
with an integrating sphere attachment. A BaSO4 pellet was used as
the background standard. High resolution transmittance electron
microscopic (HR TEM) images were recorded in a Jeol JEM 2010
transmission electron microscope. Cu loading in the sample was
estimated by using a Perkin-Elmer Optima 2100 DV Inductive
-3
peak was reduced and it shifted towards high angle, having a
2q value of 2.79◦, which indicates that the organic template was
removed from the as-synthesized material. The d spacing of the
mesophase (pore centre to pore centre correlation length) for
the as-synthesized (a) and extracted materials (b) are 3.33 nm
and 3.12 nm respectively. The d spacing has been reduced in the
case of the extracted sample on removal of the template, which
suggests a contraction of the pore width after extraction, which
is an usual trend for the mesoporous materials. When copper was
loaded in the mesoporous hybrid material the small angle peak
drastically reduced but an intense peak emerged at a higher value
of 2q, namely 6.89◦ and 8.38◦, respectively (Fig. 1). The wide
angle XRD pattern of the Cu-grafted sample is shown in Fig. 2.
This powder diffraction peaks can be assigned very well with a
new triclinic phase having the following unit cell parameters: a =
1
Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). H
and 13C NMR experiments were carried out on a Bruker DPX-300
NMR spectrometer.
◦
◦
˚
˚
˚
13.073 A, b = 12.289 A, c = 10.737 A; a = 91.04 , b = 91.18
and g = 90.25◦. The unit cell volume for this triclinic Cu-grafted
3
˚
catalyst 1 is V = 1724.411 A and hkl for different planes and the
Results and discussion
respective d spacings are given in Table 1. The PXRD pattern of
the sample has been evaluated with the PXRD pattern calculated
by using Reflex, a software package for crystal determination. The
CELSIZ program was used to obtain the lattice parameters along
with estimated standard deviations (esd, see Table 1). Very small
deviations are in accordance with a good fit of the calculated values
with the experimental data.
The small angle X-ray powder diffraction patterns for the as-
synthesized and template-extracted samples of B are shown in
Fig. 1. The as-synthesized sample exhibits a single intense peak
with maxima at a 2q value of 2.58◦, attributed to the presence
of a disordered mesostructure in the sample. Upon extraction
of the sample with an acid–ethanol mixture the intensity of the
Scheme 2
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Fig. 3 N2 adsorption/desorption isotherm of the furfural functionalized
mesoporous material (a) and Cu-grafted catalyst 1 (b). Respective pore
size distributions, estimated through NLDFT method, are shown in the
inset.
Fig. 1 Small angle powder XRD pattern of the as-synthesized (a) and
acid extracted (b) furfural functionalized mesoporous material and the
Cu-grafted catalyst 1 (c).
The FE SEM image of the sample B (a) and Cu-grafted catalyst
1 (b) are shown in Fig. 4. The as-synthesized sample has spherical
morphology having dimension ca. 120–130 nm and these spherical
particles are self-assembled to form large agglomerates. On the
other hand the SEM image of the catalyst 1 suggests that the
material has rod shaped particle morphology corresponding to
a crystalline material. The change from an amorphous to a
crystalline structure on Cu-grafting, as observed from the powder
XRD results, is clearly observed from this change in particle
morphology. The high-resolution TEM images of the Cu-grafted
Fig. 2 Wide angle powder XRD pattern of the Cu-grafted catalyst 1.
Different planes are indexed.
N2 adsorption/desorption isotherms of the mesoporous func-
tionalized silica and Cu-grafted samples are shown in Fig. 3.
For clarity the Y axis values of the extracted sample (b) have
been multiplied by 2. Both the samples showed a typical type
IV isotherm characteristic of mesoporous materials.22 The BET
surface area for the extracted and Cu-grafted samples are 617 m2g-1
and 132 m2g-1, respectively, with corresponding pore volumes of
0.597 ccg-1 and 0.1629 ccg-1, respectively. A considerable decrease
in the BET surface area and pore volume suggests that Cu-centres
have been anchored on the inner surface of the pores. The pore size
distribution of both the samples are calculated by using non-local
density functional theory (NLDFT) method shown in the inset of
Fig. 3, suggests a uniform distribution of mesopores throughout
the samples with pore dimensions of ca 2.5 nm and 2.17 nm,
respectively, for sample B and catalyst 1. The calculated pore
volume and pore size of the Cu-loaded mesoporous organic hybrid
silica is adequate to carry out the aryl-sulfur coupling reaction.
Fig. 4 FE SEM image of the furfural functionalized mesoporous material
(a) and Cu-grafted catalyst 1 (b).
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Fig. 6 FT-IR spectra of the farfural functionalized mesoporous material
(as-synthesized) (a), extracted material (b), Cu-grafted catalyst 1 (c) and
Cu-grafted catalyst after addition of base (d).
Fig. 5 TEM image of the Cu-grafted catalyst 1 in low (a) and high (b)
magnification.
material suggested the presence of bridging acetate ions (Scheme 1,
1). Further, the FT IR spectrum for the as-synthesized sample
(Fig. 6a) showed two absorption bands at 2924 cm-1 and 2854 cm-1.
These could be attributed to the C–H vibrations of the CTAB
molecules. But these two strong absorption bands are almost
absent in the acid-extracted sample B, as shown in Fig. 6b.
Thus the FT IR spectral analysis revealed that the template
CTAB molecule has been removed almost completely from the
as synthesized sample. The UV spectra of the extracted (a) and
Cu-loaded (b) samples are given in Fig. 7. In the case of the
extracted sample the peak in between 300–400 nm is due to the
p–p* transition of the conjugated system present in the imine
ligand. In the case of the Cu-loaded sample a small hump at
around 340 nm is present and a strong peak that arises around
256 nm can be attributed to the charge transfer band between
Cu+2 and the oxygen of the ligand.26 In addition to this, a very
broad absorption band in between 600–800 nm is observed. This
catalyst 1 are shown in Fig. 5. It is clear from this figure that the
material is composed of particles having rod shaped morphology.
The high magnification image further suggested the existence of
pores of dimension 2.2–2.5 nm randomly distributed throughout
the material 1. This result agrees well with the dimension of
pores obtained from the N2 sorption data. Here the change in
particle morphology during the grafting of Cu could be attributed
to the formation of crystalline Cu-complex at the surface of the
mesopores. This is reflected in the powder XRD pattern of material
1 also. Thus, the change in morphology is due to phase change
(amorphous to triclinic structure) during Cu-grafting and the
origin of reflections belongs to the Cu-complex. The fine particles
(dark spots) observed in the TEM image are neither Cu, Cu-oxides
or related to silica. It could be attributed to the Cu-nanoclusters
formed at the surface of the mesopores due to the network formed
on coordination (chelating sites) of imine-N and furfural-O donor
sites with Cu. Interestingly, as seen from the TEM image these
dark contrast nanocluster/nanoparticle like species are uniformly
distributed at the mouth of the mesopores, having dimensions of
ca. 2–3 nm. Such uniformly distributed fine nanoclusters of the
metal-complex could be responsible for the high catalytic activity
of this functionalized mesoporous material in the C–S coupling
reaction.
The FT IR spectra of the extracted sample (b) and Cu-grafted
catalyst (c) are shown in Fig. 6, from which it is evident that
the peak at 1637 cm-1 for (a) is for C N. For the Cu anchored
mesoporous material the C N stretching frequency is shifted
to lower wavelength at ca.1627 cm-1 indicating the C N bond
is coordinated to copper through the lone pair of electrons of
23
nitrogen. The two strong absorption bands at 1555 cm-1 and
24
1436 cm-1 can be assigned to the asymmetric and symmetric
vibrations, respectively, of the bridging acetate ions and a weak
peak at 418 cm-1 is attributed to the Cu–N coordination.25 Thus,
the presence of these two strong absorption bands at 1555 cm-1 and
1436 cm-1 in the Cu-grafted furfural functionalized mesoporous
Fig. 7 UV-vis diffuse reflectance spectra of furfural functionalized
mesoporous material (a) and Cu-grafted catalyst 1 (b).
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could be attributed to the d–d transition of Cu+2 ions in a pseudo-
octahedral environment.27
This result suggests that our novel Cu-grafted catalyst is a very
efficient catalyst in the C–S coupling reaction.
To test any leaching of the copper into the reaction mixture
during the catalytic reactions a hot filtration test was conducted
taking the coupling reaction between iodobenzene and thiophenol
as a representative case. In this technique, the reaction mixture was
filtered under hot condition from the suspended catalyst after 4 h
of the reaction and with the filtrate the reaction was continued at
the same temperature and for another 4 h. It was observed that
there was almost no increase in product yield compared to the
conversionthat took place after 4h(46.8 and46.6% yield for before
and after hot filtration, respectively). This result suggests that no
leaching of Cu occurred during the course of reaction and the
catalyst is purely heterogeneous in nature. A probable mechanistic
reaction pathway is suggested in Fig. 9. It shows that on addition
of base the active species (I) is generated due to reduction. In
order to prove the presence of active intermediate I (Fig. 9) in the
catalytic reaction we have performed in situ FT IR analysis of the
Cu-grafted mesoporous material when base was added into this
system. The FT IR spectrum of the sample at this stage is shown
in the Fig. 5d, where the two strong absorption bands at 1555 cm-1
and 1436 cm-1 are absent. This FT IR result thus suggests that in
the presence of base the acetate bridges open-up and the active
species I (Cu0) was generated during the C–S coupling reaction.
This active species undergoes oxidative addition with aryl iodide
to give an intermediate (II). In the presence of base, thiophenol
reacts with (II) to afford another intermediate (III) which readily
undergoes reductive elimination to yield the desired product and
the catalyst get regenerated during this process.
Catalysis
The high catalytic activity of different Cu-loaded catalysts28,29
has motivated us to explore the possibility of using catalyst 1
in the C–S coupling reaction of various electron-withdrawing
and electron-donating aryl iodides. The yields of the different
product diarylsulfides obtained in these reactions are high as
shown in Table 2. For both symmetrical as well as unsymmetrical
diarylsulfides the yield varied from 75–88%.30 The catalytic activity
of the Cu-anchored mesoporous material 1 was studied both at
elevated temperature (383 K) as well as at room temperature (298
K). As seen from the table, the Cu-grafted furfural functionalized
mesoporous organosilica catalyst is highly reactive at elevated
temperature and it takes only 7–9 h for the completion of the
reaction, whereas at room temperature the yield of the C–S
coupling product was 35.2% only (Table 2, entry 6). When we have
carried out the coupling reaction of iodobenzene and thiophenol
in the presence of Schiff base material B, before Cu-grafting and in
the absence of any catalyst at 383 K, in both cases not even a trace
of the C–S coupling product was observed. On the other hand,
in a control experiment when homogeneous phase Cu(OAc)2 was
used as the catalyst under optimized conditions at 383 K only a
20.0% yield of the diphenylsulfide (Table 2, entry 5) was obtained.
In Table 2 we have summarized the TOFs for the synthesis of
diarylsulfides over catalyst 1. As seen from the table, for catalyst
1 these TOFs are quite high (varies between 121–183) compared
to that of the homogeneous counterpart (copper acetate, TOF =
36.3). The coupling reaction between iodobenzene and thiophenol
was also taken as a representative case to check the recycling
efficiency of the Cu anchored catalyst. It was found that the
catalyst can be efficiently recycled and reused for five repeating
cycles without significant loss of catalytic activity. The Cu content
in the fresh catalyst and the catalyst obtained after fifth cycle
was estimated by ICP-AES. Cu content in the fresh catalyst is
0.68 mmol L-1, whereas for the recovered catalyst after fifth cat-
alytic cycle it is 0.67 mmol L-1. The yield of the product and the Cu
content of the catalyst has been marginally reduced after the fifth
repetitive catalytic cycle. We have plotted the recycling efficiency
of the catalyst for five consecutive catalytic cycles for the coupling
reaction of iodobenzene and thiophenol, which is shown in Fig. 8.
Fig. 9 Catalytic cycle for the C–S coupling reaction.
Conclusions
We have developed a furfural functionalized mesoporous silica
material and grafted it with Cu(II) to obtain a new and robust
Fig. 8 Recycling efficiency of the Cu-grafted catalyst 1.
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Table 2 C–S coupling reactions over Cu-grafted functionalized mesoporous material 1
Entry
1
ArI
ArSH
Product (Ar–S–Ar)
Time (h)
8
Yield (%)
85.2
TOFa (h-1)
154.5
2
7
88.0
182.8
3
7
87.1
180.7
4
9
75.0
121.1
5b
8
8
20.0
35.2
36.3
44.0
6c
a TOF = Turn over frequency = moles of substrate converted per mole of Cu per h. b Cu(OAc)2 was used as catalyst. c Reaction was carried out at 298 K.
heterogeneous catalyst. Aryl-sulfur coupling reaction takes place
very easily under aerobic condition over this Cu-grafted catalyst to
JM, AM and AD thank CSIR, New Delhi for their respective
obtain different value added symmetrical as well as unsymmetrical
Acknowledgements
senior research fellowships.
bis-thioethers. Good catalytic activity and recycling efficiency in
this C–S cross coupling reaction suggests the potential application
of this Cu-grafted functionalized mesoporous material for the
synthesis of additional organic target molecules. The efficient
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30 1H and 13C NMR chemical shifts for different bis-thioethers reported
in Table 2:Entry 1. 1H NMR (300 MHZ, CDCl3) d = 7.519–7.109 (5H,
m). 13C NMR (300 MHZ, CDCl3) d = 137.61, 131.17, 129.29, 124.94.
1
Entry 2. H NMR (500 MHZ, CDCl3) d = 3.72 (3H, S); 6.594–6.577
(2H, d), 7.469–7.452 (2H, d), 7.412–7.402 (2H, d), 7.22–7.190 (4H, m),
7.152–7.114 (1H, m). 13C NMR (500 MHZ, CDCl3) d = 159.65, 55.4,
116.53, 138.36, 137.23, 129.21, 127.73, 127.31.
1
Entry 3. H NMR (300 MHZ, CDCl3) d = 2.36 (3H, S), 6.952–6.926
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(2H, d), 7.34–7.169 (5H, m). 13C NMR (300 MHZ, CDCl3) d = 21.1,
137.5, 137.18, 132.48, 131.341, 129.934, 127.677, 126.548.
Entry 4. 1H NMR (300 MHZ, CDCl3) d = 7.28 (1H, S), 6.852–
6.850 (2H, m), 6.995–6.927 (2H, d), 7.222–7.202 (2H, m). 13C NMR
(300 MHZ, CDCl3) d = 113.94, 116.95, 125.5, 129.4, 129.75, 130.5,
132.44, 138.5, 163.0.
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