PEPPSI-Type Palladium–NHC Complexes: Synthesis, Characterization, and Catalytic Activity in the Direct C5-Arylation of 2-Substituted Thiophene Derivatives with Aryl Halides

Article abstract of DOI:10.1002/ejic.201601452

Six benzimidazolium salts, having two nitrogen atoms substituted by various alkyl groups, have been synthesized in high yields. The benzimidazolium salts readily converted into the corresponding PEPPSI-type palladium–NHC complexes (PEPPSI = pyridine-enhan

Full text of DOI:10.1002/ejic.201601452

A Journal of
Accepted Article
Title: Palladium-NHC-PEPPSI Complexes: Synthesis, Characterization
and Catalytic Activity in the Direct C5-Arylation of 2-Substituted
Thiophene Derivatives with Aryl Halides
Authors: Murat Kaloğlu, İsmail Özdemir, Vincet Dorcet, Christian
Bruneau, and Henri Doucet
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601452
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10.1002/ejic.201601452
European Journal of Inorganic Chemistry
FULL PAPER
Palladium-NHC-PEPPSI Complexes: Synthesis, Characterization
and Catalytic Activity in the Direct C5-Arylation of 2-Substituted
Thiophene Derivatives with Aryl Halides
Murat Kaloğlu,[a,b] İsmail Özdemir*[a,b] Vincent Dorcet,[c] Christian Bruneau,[c] and Henri Doucet[c]
Abstract: Benzimidazolium salts (2a-f) having their two nitrogen
atoms substituted by different alkyl groups have been synthesized in
high yields. The benzimidazolium salts were readily converted into
the corresponding PEPPSI-type palladium-NHC complexes (3a-f)
(PEPPSI= pyridine-enhanced precatalyst preparation, stabilisation,
and initiation). The structures of all compounds were characterized
C(sp2) bonds in contemporary organic synthesis because of the
numerous applications of heteroaromatic compounds as
biologically active compounds and functional materials.[29,30]
Thiophene derivatives show valuable biological activities and
present considerable interest in pharmaceutical chemistry. As
selected examples, Canagliflozin[31a] is a drug for treatment of
type-2 diabetes, Evista[31b] is used for the prevention and
treatment of osteoporosis, Saviprazole[31c] is a gastric proton
pump inhibitor, Tiflucarbine[31d] displays antidepressant
properties and Motapizone[31e] is used against platelet
aggregation. Because of these properties, the discovery of
simple and direct accesses to thiophene derivatives remains an
important challenge for organic chemists.
1
by H NMR, 13C NMR, IR and elemental analysis techniques, which
supported the proposed structures. The molecular structure of the
complex 3f was determined by single-crystal X-ray diffraction. The
catalytic activity of PEPPSI-type palladium-NHC complexes was
evaluated in the direct C5-arylation of 2-substituted thiophene
derivatives with various aryl halides. This arylation occured efficiently
and selectively at the C5-position of 2-substituted thiophene
derivatives.
Introduction
The use of N-heterocyclic carbenes (NHCs) as ligands for
transition metals was first described in 1968 by Öfele[1] and
Wanzlick.[2] The development of metal-NHC complexes by
Lappert[3] in the early 1970s and the isolation of the first stable
free NHC by Arduengo and co-workers[4] in 1991 set the scene
for an ever-growing interest and advancement in the field of
NHC chemistry. Shortly thereafter, NHCs have been utilized
extensively as ligands of transition metal complexes in
organometallic chemistry and homogeneous catalysis.[5-17]
Since the initial work by Ohta and co-workers, [18] transition
metal-catalyzed direct arylation reactions have been rapidly
developing for the synthesis of a wide range of heteroarenes
and it still receives much interest from academic and industrial
research groups.[19-28]
Figure 1. Examples of biologically active thiophene derivatives.
Palladium-catalyzed direct arylation of C-H bonds of
thiophenes with aryl halides is known to occur preferentially at
the α-positions to the sulfur atom (C2 and/or C5) following the
typical reactivity profiles of the thiophene ring (Scheme 1). [32]
The direct arylation of heteroarenes with aryl halides has
become the most valuable method for the formation of C(sp2)-
[a]
Department of Chemistry, Faculty of Science and Art, İnönü
University.
44280 Malatya, Turkey
E-mail: ismail.ozdemir@inonu.edu.tr
https://www.inonu.edu.tr/
Catalysis Research and Application Center, İnönü University.
44280 Malatya, Turkey
Scheme 1. Pd-catalyzed direct C5-arylation of 2-substituted thiophenes with
aryl halides.
[b]
[c]
UMR 6226 CNRS-Université de Rennes 1, Institut des
Sciences Chimiques de Rennes, Campus de Beaulieu.
35042 Rennes, France
A number of palladium complexes with a single NHC
ligand have proven to be useful catalysts in cross-coupling
reactions of aryl halides.[33,34] Among the most popular catalysts
for such reactions are PEPPSI-type palladium-NHC complexes
Supporting information for this article is given via a link at the
end of the document.
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10.1002/ejic.201601452
European Journal of Inorganic Chemistry
FULL PAPER
chloroform, dichloromethane and diethylether. The absence of
the characteristic signals of the imino carbon (143-144 ppm) and
of the type [PdX2(NHC)(pyridine)], (X= halide, PEPPSI=
Pyridine-Enhanced Precatalyst Preparation, Stabilization and
Initiation), which have gained real practical importance in
numerous catalytic processes.[35] In 2006, Organ et al.
reported[36] easily-handled, air and moisture-stable Pd-NHC
complexes through the PEPPSI method that featured Pd(II)
species bearing an NHC ligand, two halides, and a labile ligand
such as 3-chloropyridine. A lot of work has been done by many
groups since 2006 on this type of complexes,[37] but only a few
examples deal with the direct arylation of heteroarenes.[38] In
view of the above information and the growing interest of
PEPPSI-type palladium-NHC complexes in catalysis we decided
to investigate the catalytic activity of new members of this family
in the direct C5-arylation of 2-substitued thiophenes.
1
the acidic imino proton (10-12 ppm) in 13C NMR and H NMR,
which were present in the salts (2a-f) suggested the formation of
the NHC-carbenes and their coordination to form the PEPPSI-
type palladium-NHC complexes. In addition, the characteristic
carbenic carbons in compounds 3a-f appeared in the 13C NMR
spectra as deshielded singlets at 163.6, 162.2, 163.1, 163.1,
163.1 and 162.0 ppm, respectively. The IR data also clearly
indicated the presence of (CN) vibration at 1408, 1408, 1425,
1405, 1412 and 1410 cm-1 for 3a-f. The formation of carbenes is
correlated by a shift of the (CN) vibration from 1556-1594 cm-1 in
the benzimidazolium salts to 1408-1425 cm-1 in the coordinated
carbenes.
We now describe the synthesis and characterization of the
benzimidazolium salts (2a-f) as NHC species, and their
corresponding PEPPSI-type palladium-NHC complexes (3a-f).
1
These compounds were characterized by H and 13C NMR, IR
and elemental analysis. The structure of the trans-dibromo[1-(3-
methoxypropyl)-3-(3,4,5-trimethoxybenzyl)benzimidazol-2-ylide-
ne](pyridine)palladium(II) complex (3f) was determined by
single-crystal X-ray diffraction. We then examined the activity of
the PEPPSI-type palladium-NHC complexes in the direct C5-
arylation of 2-substituted thiophene derivatives with various aryl
bromides and aryl chlorides as coupling partners.
Results and Discussion
Preparation of benzimidazolium salts: The benzimidazolium
salts (2a-f) were prepared by reacting N-(3-methoxypropyl)-
o
benzimidazole (1) with various alkyl halides in DMF at 80 C for
36 h (Scheme 2). The benzimidazolium salts (2a-f) were air- and
moisture-stable both in the solid state and in solution. The
structures of the salts were determined by their characteristic
spectroscopic data and elemental analyses. In the 13C NMR
spectra of 2a-f, the characteristic signals of the imino carbon,
(NCHN) were detected as typical singlets at 143.8, 143.8, 143.2,
Scheme 2. Synthesis of the benzimidazolium salts (2a-f).
1
143.8, 143.7 and 143.8 ppm, respectively. The H NMR signals
of the C(2)-H protons were observed as sharp singlets at
chemical shifts of 11.82, 11.49, 10.86, 11.83, 11.55 and 11.68
ppm, respectively for 2a-f, and further supported the assigned
structures. These NMR values were in line with those found for
other benzimidazolium salts of the literature.[39] The formation of
the benzimidazolium salts were also evidenced by their IR
spectra, which showed (CN) bond absorption at 1560, 1557,
1556, 1557, 1558 and 1594 cm-1 for the respective CN bond
vibration of 2a-f.
Preparation of the PEPPSI-type palladium-NHC complexes:
The general procedure for the preparation of PEPPSI-type
palladium-NHC complexes (3a-f) according to the method
reported by Organ[36] is shown in Scheme 3. Benzimidazolium
salts (2a-f) were incorporated into the PEPPSI-type palladium-
NHC complexes (3a-f) by reaction with PdCl2 in refluxing
pyridine in the presence of K2CO3 as a base and a large excess
of KBr for 16 h. These complexes, which were stable both in
solution and in solid state against air, light and moisture, were
soluble in different solvents such as dimethylsulfoxide,
Scheme 3. Synthesis of the PEPPSI-type palladium-NHC complexes (3a-f).
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10.1002/ejic.201601452
European Journal of Inorganic Chemistry
FULL PAPER
Structural characterization of complex 3f: Single crystals of
complex 3f suitable for diffraction study were obtained by slow
diffusion of n-pentane into a dichloromethane solution of
complex 3f. The molecular structure of complex 3f has been
confirmed by X-ray single-crystal analyses. This complex
crystallizes in a centrosymmetric monoclinic P 1 21/c 1 system,
and adopts a square planar geometry. The carbene and the
pyridine ligands are in trans-position with respective distances to
the palladium centre of 1.963(7) Å and 2.109(6) Å. The
molecular structure of complex 3f is shown in Figure 1, and
selected bond lengths and angles are summarized in Table 1.
Table 1. Selected bond lengths [Å] and angles [°] for complex 3f.
Pd1-C6
Pd1-N1
1.963(7)
2.109(6)
2.4180(9)
2.4265(9)
177.0(3)
87.70(19)
89.17(19)
N1-Pd1-Br1
N1-Pd1-Br2
Br1-Pd1-Br2
C5-N1-Pd1
C1-N1-Pd1
N2-C6-Pd1
N3-C6-Pd1
92.03(16)
91.21(16)
176.27(4)
123.0(5)
118.9(5)
125.7(5)
127.4(5)
Pd1-Br1
Pd1-Br2
C6-Pd1-N1
C6-Pd1-Br1
C6-Pd1-Br2
Direct C5-arylation of 2-substituted thiophene derivatives: In
order to screen the experimental conditions, we selected the
complex 3f as the catalyst. In all of complexes, methoxypropyl
group linked to the nitrogen atom of the benzimidazole ring has
an oxygen atom oriented toward the metal center. This
orientation has also been proven for complex 3f by X-ray
analysis (see Figure 2). For this reason, we think that this group
could be hamilable effect throughout the catalytic cycle. We also
selected the 2-acetylthiophene as model heteroaromatic
substrate with a blocked C2-position (Scheme 4). The results of
the reaction parameters including solvent, base, temperature
and catalyst loading are gathered in Table 2.
Figure 2. Perspective view of the molecular structure of 3f.
Scheme
4.
Pd-NHC-PEPPSI-catalyzed
direct
C5-arylation
of
2-acetylthiophene with 4-chloroacetophenone and 4-bromoacetophenone.
Table 2. Influence of the reaction conditions for Pd-NHC-PEPPSI catalyzed direct C5-arylation of 2-acetylthiophene with 4-chloro- and 4-bromoacetophenone.[a]
Pd-NHC-PEPPSI
(mol %)
Temp.
(oC)
Conversion[b]
(%)
Yield[c]
(%)
Entry
X
Solvent
Base
Time (h)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (0.5)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
3f (1)
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
DMAc
DMF
Toluene
DMAc
DMF
Toluene
DMAc
DMF
Toluene
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
NaOAc
NaOAc
NaOAc
K2CO3
K2CO3
K2CO3
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
2
2
2
2
2
2
2
2
2
2
2
2
1
2
5
120
120
120
120
120
120
120
120
120
130
150
150
150
150
150
150
150
150
120
130
150
61
34
47
31
19
24
86
46
58
91
100
81
84
14
24
43
68
87
64
71
90
54
30
41
22
11
18
71
37
49
84
91
78
78
8
12
34
58
79
36
51
81
10
15
20
20
20
25
[a] Conditions: 2-acetylthiophene (2 mmol), aryl halide (1 mmol), base (2 mmol), solvent (2 mL). [b] Conversions were calculated according to aryl halide
by GC and GC-MS. [c] Isolated yields.
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10.1002/ejic.201601452
European Journal of Inorganic Chemistry
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When DMF or toluene were used as solvent, the reaction gave
low conversion of only 19-58% with NaOAc, K2CO3 or KOAc as
base after 2 h at 120 oC (Table 2, entries 2, 3, 5, 6, 8, 9). DMAc
150 °C to 120 °C had a detrimental effect on the conversion
(Table 2, entries 19 and 20). When the reaction time was
increased from 20 h to 25 h at 150 °C, the very close conversion
of 4-chloroacetophenone was obtained (Table 2, entry 21).
Finally, the scope of the direct C5-arylation of two
2-substituted thiophene substrates was investigated with various
aryl halides, including five (hetero)aryl bromides (Tables 3 and
5) and four aryl chlorides (Tables 4 and 6) applying our best
experimental conditions. Only a minor effect of the carbene
ligand on the Pd-PEPPSI complex was observed for the
coupling of aryl bromides with thiophene derivatives. In all cases,
o
proved to be the best tested solvent after 2 h at 120 C. In this
solvent decreasing the reaction temperature from 150 °C to
120 °C had a detrimental effect on the conversion (Table 2,
entries 1, 4, 7, 10, 11). In the presence of 0.5 mol% of 3f as the
catalyst, KOAc as the base, DMAc as the solvent and
4-bromoacetophenone as the coupling partner at 150 °C for 2 h,
the C5-arylated product was obtained in 78% isolated yield
(Table 2, entry 12). When the reaction time was reduced to 1 h,
the reaction gave low conversion of only 84% (Table 2, entry 13). except in a few cases with complex 3f, high conversions of the
Finally, the best conditions leading to full conversion of
4-bromoacetophenone with high selectivity in favor of the C5-
arylated product were obtained when the reaction was carried
out in DMAc in the presence of 2 equiv. of KOAc at 150 C for 2
h (Table 2, entry 11).
aryl bromide were observed, and the coupling products were
isolated in high yields. At elevated temperature, the oxidative
addition of the aryl bromide to palladium is generally easy and
does not require the use of very specific ligands. A more
important effect of the nature of the ligand was expected for the
coupling of aryl chlorides with the thiophenes derivatives (Tables
4 and 6).[32j] Indeed, again in all cases the coupling products
were produced in good yields. Surprisingly, similar yields were
obtained for the coupling of electron-deficient aryl chlorides
(Tables 4 and 6, entries 13-24) as with chlorobenzene (Table 4
and 6, entries 1-6). In this case, the presence of the NHC-
ligands on palladium likely favors the oxidative addition step.
o
When the less reactive 4-chloroacetophenone was used
as substrate in the presence of 1 mol% of catalyst 3f and KOAc
o
as the base at 150 C, the conversions increased depending on
the reaction time (Table 2, entries 14, 15, 16, 17). Interestingly,
up to 87% conversion with 79% isolated yield were obtained, but
for this the reaction required a longer reation time of 20 h
(Table 2, entry 18). The reaction temperature decreasing from
Table 3. Pd-NHC-PEPPSI-catalyzed direct C5-arylation of 2-acetylthiophene with aryl bromides.[a]
Entry
Aryl bromide
Catalyst
3a
3b
3c
3d
Product
Conversion[b] (%)
Yield[c] (%)
1
2
3
4
5
6
79
83
90
96
85
90
71
75
88
85
80
87
3e
3f
7
8
9
10
11
12
3a
3b
3c
3d
3e
3f
91
83
100
97
96
100
82
79
92
90
84
90
13
14
15
16
17
18
3a
3b
3c
3d
3e
3f
76
98
93
87
86
70
88
87
84
81
91
100
19
20
21
22
23
24
3a
3b
3c
3d
3e
3f
86
83
100
94
91
100
82
74
93
91
83
85
25
26
27
28
29
30
3a
3b
3c
3d
3e
3f
91
87
98
100
93
98
84
81
87
92
88
87
[a] Conditions: Pd-NHC-PEPPSI (0.01 mmol), 2-acetylthiophene (2 mmol), aryl bromide (1 mmol), KOAc (2 mmol), DMAc (2 mL), 150 °C, 2 h.
[b] Conversions were calculated according to aryl bromide by GC. [c] Isolated yields.
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European Journal of Inorganic Chemistry
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Table 4. Pd-NHC-PEPPSI catalyzed direct C5-arylation of 2-acetylthiophene by using aryl chlorides.[a]
Entry
Aryl chloride
Catalyst
3a
3b
3c
3d
Product
Conversion[b] (%)
Yield[c] (%)
1
2
3
4
5
6
87
59
51
63
74
83
78
54
44
57
71
76
3e
3f
7
8
9
10
11
12
3a
3b
3c
3d
3e
3f
71
63
59
68
78
79
64
57
51
66
69
61
13
14
15
16
17
18
3a
3b
3c
3d
3e
3f
78
86
50
76
64
87
76
79
41
69
60
79
19
20
21
22
23
24
3a
3b
3c
3d
3e
3f
80
94
66
88
73
81
74
88
61
80
64
72
[a] Conditions: Pd-NHC-PEPPSI (0.01 mmol), 2-acetylthiophene (2 mmol), aryl chloride (1 mmol), KOAc (2 mmol), DMAc (2 mL), 150 °C, 20 h.
[b] Conversions were calculated according to aryl chloride by GC. [c] Isolated yields.
Table 5. Pd-NHC-PEPPSI catalyzed direct C5-arylation of 2-cyanomethylthiophene by using aryl bromides.[a]
Entry
Aryl bromide
Catalyst
Product
Conversion[b] (%)
Yield[c] (%)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
3a
3b
3c
3d
3e
3f
3a
3b
3c
3d
3e
3f
3a
3b
3c
3d
3e
3f
88
78
85
74
89
79
100
90
100
86
90
84
89
88
84
93
97
73
94
76
83
84
81
61
93
87
77
88
84
75
84
71
79
69
85
72
91
86
88
80
71
78
81
77
77
87
89
68
89
71
78
79
78
56
86
84
74
82
77
73
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
3a
3b
3c
3d
3e
3f
[a] Conditions: Pd-NHC-PEPPSI (0.01 mmol), 2-cyanomethylthiophene (2 mmol), aryl bromide (1 mmol), KOAc (2 mmol), DMAc (2 mL), 150 °C, 2 h.
[b] Conversions were calculated according to aryl bromide by GC. [c] Isolated yields.
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Table 6. Pd-NHC-PEPPSI catalyzed direct C5-arylation of 2-cyanomethylthiophene by using aryl chlorides.[a]
Entry
1
2
3
4
5
6
7
8
Aryl chloride
Catalyst
3a
3b
3c
3d
3e
3f
3a
3b
3c
3d
3e
3f
3a
3b
3c
3d
3e
3f
3a
3b
3c
3d
3e
3f
Product
Conversion[b] (%)
Yield[c] (%)
64
73
71
78
71
83
70
62
57
49
67
71
64
85
78
74
88
74
70
69
63
76
75
75
78
65
74
68
77
68
58
54
44
64
63
59
79
76
70
84
60
61
65
58
72
70
73
74
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
[a] Conditions: Pd-NHC-PEPPSI (0.01 mmol), 2-cyanomethylthiophene (2 mmol), aryl chloride (1 mmol), KOAc (2 mmol), DMAc (2 mL), 150 °C, 20 h.
[b] Conversions were calculated according to aryl chloride by GC. [c] Isolated yields.
Conclusions
Experimental Section
In summary, we have prepared a series of new benzimidazolium
salts from N-substituted benzimidazole. These benzimidazolium
salts were metallated with PdCl2 in pyridine to give easily-
handled, air and moisture stable new PEPPSI-type palladium-
NHC complexes. Then, all benzimidazolium salts and PEPPSI-
type palladium-NHC complexes were characterized using
different spectroscopic and analytical techniques. The structure
of complex 3f was determined by single-crystal X-ray diffraction.
The PEPPSI-type palladium-NHC complexes were used in the
direct C5-arylation of 2-substituted thiophenes.
The catalytic systems generated from these PEPPSI-type
palladium-NHC complexes in the presence of KOAc as the base
and DMAc as the solvent at 150 oC were very efficient at 1 mol%
catalyst loading for the selective C-C bond formation from aryl
halides. When the catalytic studies were evaluated, it was found
that all of the complexes were suitable for the direct C5-arylation
of 2-substituted thiophene derivatives with aryl bromides. With
these catalysts, even non-activated aryl chlorides such as
chlorobenzene or 4-chlorotoluene were coupled with thiophenes
derivatives in good yields.
General Methods: All manipulations were performed in Schlenk type
flasks under argon. All chemical reactants were obtained from
commercial sources. The solvents used were purified by distillation over
the drying agents indicated and were transferred under argon. Pd-NHC-
PEPPSI complexes were prepared according to known methods in the
literature.[37] DMAc analytical grade (99%) was not distilled before use.
KOAc (99%) was employed. 1H NMR (used 300 and 500 MHz) and 13C
NMR (used 75 and 125 MHz) spectra were recorded in CDCl3. Chemical
shifts (δ) are reported in ppm relative to CDCl3. 1H NMR spectra are
referenced to residual protiated solvents (δ= 7.26 ppm for CDCl3), 13C
chemical shifts are reported relative to deuteriated solvents (δ = 77.16
ppm for CDCl3). IR spectra were recorded on ATR unit in the range of
400-4000 cm-1 with Perkin Elmer Spectrum 100 Spectrofotometer.
Melting points were determined in glass capillaries under air with an
Electrothermal-9200 melting point apparatus. Elemental analyses were
performed by İnönü University Scientific and Technological Research
Center (İBTAM). All catalytic reactions were monitored on an Agilent
6890N GC and Schimadzu 2010 Plus GC-MS system by GC-FID with an
HP-5 column of 30 m length, 0.32 mm diameter, and 0.25 μm film
thickness.
General procedure for the preparation of benzimidazolium salts: For
the preparation of N-(3-methoxypropyl)benzimidazole (1), benzimidazole
(1.0 mmol) and potassium hydroxide (1.0 mmol) were dissolved in ethyl
alcohol (50 mL). 3-Methoxypropyl chloride (1.0 mmol) was slowly added
and the reaction mixture was stirred at room temperature for 1 h. The
solution was refluxed for 5 h, then cooled to room temperature and the
precipitated potassium chloride was removed by filtration. The solvent
was removed by distillation. The crude product was then distilled under
vacuum. For the preparation of the benzimidazolium salts (2a-f),
N-(3-methoxypropyl)benzimidazole (1), (1.0 mmol) was dissolved in dried
dimethylformamide (3 mL) and the alkyl halide (1.0 mmol) was added
It is clear that, the use of PEPPSI-type palladium-NHC
complexes as catalysts in the direct arylation of thiophenes was
very limited in the literature. Also, this study has some
advantages such as low catalyst loading and less reaction time
when compered with the previously reported studies on the
arylation of thiophene derivatives.[29f,30j,32c,38c]
This article is protected by copyright. All rights reserved.

10.1002/ejic.201601452
European Journal of Inorganic Chemistry
FULL PAPER
slowly. The reaction mixture was stirred at 80 °C for 36 h under argon.
After completion of the reaction, all dimethylformamide was removed by
vacuum and diethylether (15 mL) was added to obtain a white crystalline
solid, which was filtered off. The solid was washed with diethylether (3 ×
10 mL) and dried under vacuum. The crude product was recrystallized
from an ethanol/diethylether mixture (1:2, v/v).
NCH2CH2CH2OCH3); 3.44 (t, J= 5.4 Hz, 2H, NCH2CH2CH2OCH3); 3.75 (s,
3H, NCH2C6H4(OCH3)-3); 4.72 (t, J= 6.8 Hz, 2H, NCH2CH2CH2OCH3);
5.83 (s, 2H, NCH2C6H4(OCH3)-3); 6.79-7.74 (m, 8H, NC6H4N,
NCH2C6H4(OCH3)-3); 11.55 (s, 1H, NCHN). 13C NMR (125 MHz, CDCl3,
25 oC): δ 29.6 (NCH2CH2CH2OCH3); 45.0 (NCH2CH2CH2OCH3); 51.2
(NCH2C6H4(OCH3)-3);
55.5
(NCH2C6H4(OCH3)-3);
58.7
(NCH2CH2CH2OCH3); 68.9 (NCH2CH2CH2OCH3); 113.0, 113.7, 113.9,
114.6, 120.3, 127.0, 130.3, 131.1, 131.7, 134.5, 160.2 (NC6H4N,
NCH2C6H4(OCH3)-3); 143.7 (NCHN). IR (cm-1) ν(CN): 1558; Anal. Calcd.
for C19H23ClN2O2: C, 65.79; H, 6.68; N, 8.08. Found: C, 65.78; H, 6.62; N,
8.01.
1-(3-Methoxypropyl)-3-(4-methylbenzyl)benzimidazolium
chloride
(2a): Yield: 1.76 g, 91%; mp: 130-131 C; H NMR (500 MHz, CDCl3, 25
oC):
2.30 (s, 3H, NCH2C6H4(CH3)-4); 2.36 (p, J= 5.6 Hz, 2H,
o
1
δ
NCH2CH2CH2OCH3); 3.27 (s, 3H, NCH2CH2CH2OCH3); 3.49 (t, J= 5.4 Hz,
2H, NCH2CH2CH2OCH3); 4.74 (t, J= 7.0 Hz, 2H, NCH2CH2CH2OCH3);
5.84 (s, 2H, NCH2C6H4(CH3)-4); 7.15-7.76 (m, 8H, NC6H4N,
NCH2C6H4(CH3)-4); 11.82 (s, 1H, NCHN). 13C NMR (125 MHz, CDCl3, 25
1-(3-Methoxypropyl)-3-(3,4,5-trimethoxylbenzyl)benzimidazolium
chloride (2f): Yield: 2.15 g, 91%; mp: 121-122 oC; 1H NMR (500 MHz,
CDCl3, 25 oC): δ 2.34 (p, J= 5.4 Hz, 2H, NCH2CH2CH2OCH3); 3.24 (s, 3H,
NCH2CH2CH2OCH3); 3.46 (t, J= 5.3 Hz, 2H, NCH2CH2CH2OCH3); 3.79
and 3.85 (s, 9H, NCH2C6H2(OCH3)3-3,4,5); 4.72 (t, J= 6.4 Hz, 2H,
NCH2CH2CH2OCH3); 5.80 (s, 2H, NCH2C6H2(OCH3)3-3,4,5); 7.57-7.75
(m, 6H, NC6H4N, NCH2C6H2(OCH3)3-3,4,5); 11.68 (s, 1H, NCHN). 13C
NMR (125 MHz, CDCl3, 25 oC): δ 29.6 (NCH2CH2CH2OCH3); 44.9
(NCH2CH2CH2OCH3); 51.7 (NCH2C6H2(OCH3)3-3,4,5); 56.6 and 60.8
oC):
(NCH2CH2CH2OCH3);
δ
21.2 (NCH2C6H4(CH3)-4); 29.7 (NCH2CH2CH2OCH3); 45.0
51.3 (NCH2C6H4(CH3)-4); 58.8
(NCH2CH2CH2OCH3); 68.9 (NCH2CH2CH2OCH3); 113.0, 113.7, 126.9,
128.3, 129.9, 130.0, 130.1, 131.0, 131.8, 139.1 (NC6H4N,
NCH2C6H4(CH3)-4); 143.8 (NCHN). IR (cm-1) ν(CN): 1560; Anal. Calcd. for
C19H23ClN2O: C, 68.97; H, 7.01; N, 8.47. Found: C, 68.78; H, 7.02; N,
8.49.
(NCH2C6H2(OCH3)3-3,4,5);
58.7
(NCH2CH2CH2OCH3);
68.8
(NCH2CH2CH2OCH3); 106.1, 113.1, 113.6, 127.0, 128.6, 131.1, 131.7,
138.5, 153.8 (NC6H4N, NCH2C6H2(OCH3)3-3,4,5); 143.8 (NCHN). IR (cm-
1) ν(CN): 1594; Anal. Calcd. for C21H27ClN2O4: C, 61.99; H, 6.69; N, 6.88.
Found: C, 62.03; H, 6.72; N, 6.92.
1-(3-Methoxypropyl)-3-(4-tert-butylbenzyl)benzimidazolium bromide
1
(2b): Yield: 2.12 g, 87%; mp: 161-162 oC; H NMR (300 MHz, CDCl3, 25
oC): δ 1.19 (s, 9H, NCH2C6H4(C(CH3)3)-4); 2.29 (p, J= 5.6 Hz, 2H,
NCH2CH2CH2OCH3); 3.18 (s, 3H, NCH2CH2CH2OCH3); 3.42 (t, J= 5.4 Hz,
2H, NCH2CH2CH2OCH3); 4.67 (t, J= 7.0 Hz, 2H, NCH2CH2CH2OCH3);
5.76 (s, 2H, NCH2C6H4(C(CH3)3)-4); 7.32-7.70 (m, 8H, NC6H4N,
NCH2C6H4(C(CH3)3)-4); 11.49 (s, 1H, NCHN). 13C NMR (75 MHz, CDCl3,
25 oC): δ 29.6 (NCH2CH2CH2OCH3); 31.2 (NCH2C6H4(C(CH3)3)-4); 34.6
General procedure for the preparation of the PEPPSI-type
palladium-NHC complexes: The PEPPSI-type palladium-NHC
complexes (3a-f) were obtained in moderate to good yields using the
standard procedure initially developed by Organ.[37] A mixture of K2CO3
(5.0 mmol), pyridine (3 mL), PdCl2 (1.1 mmol), benzimidazolium salt (1.0
mmol), and KBr (10.0 mmol) was heated at 80 °C for 16 h. The reaction
mixture was then filtered through a pad of celite and silica gel to remove
the unreacted PdCl2 and benzimidazolium salt. The solvent in the
reaction medium was then removed. The resulting complexes were
washed with n-pentane (3 × 5 mL) and dried under vacuum. The crude
products were recrystallized from dichloromethane/n-pentane (1:2, v/v).
(NCH2C6H4(C(CH3)3)-4);
(NCH2C6H4(C(CH3)3)-4);
45.0
58.7
(NCH2CH2CH2OCH3);
(NCH2CH2CH2OCH3);
51.0
68.9
(NCH2CH2CH2OCH3); 113.0, 113.7, 126.2, 126.3, 127.0, 127.1, 128.1,
129.8, 131.1, 131.7, 152.4 (NC6H4N, NCH2C6H4(CH3)-4); 143.8 (NCHN).
IR (cm-1) ν(CN): 1557; Anal. Calcd. for C22H29BrN2O: C, 63.31; H, 7.00; N,
6.71. Found: C, 63.38; H, 7.12; N, 6.79.
1-(3-Methoxypropyl)-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazol-
ium chloride (2c): Yield: 1.55 g, 84%; mp: 143-144 oC; 1H NMR (500
MHz, CDCl3, 25 oC): δ 2.20 (p, J= 5.3 Hz, 2H, NCH2CH2CH2OCH3); 2.23,
2.26 and 2.28 (s, 15H, NCH2C6(CH3)5-2,3,4,5,6); 3.20 (s, 3H,
NCH2CH2CH2OCH3); 3.44 (t, J= 5.5 Hz, 2H, NCH2CH2CH2OCH3); 4.82 (t,
J= 6.9 Hz, 2H, NCH2CH2CH2OCH3); 5.84 (s, 2H, NCH2C6(CH3)5-
2,3,4,5,6); 7.33-7.79 (m, 4H, NC6H4N); 10.86 (s, 1H, NCHN). 13C NMR
(125 MHz, CDCl3, 25 oC): δ 17.0, 17.1 and 17.3 (NCH2C6(CH3)5-
2,3,4,5,6); 29.7 (NCH2CH2CH2OCH3); 45.1 (NCH2CH2CH2OCH3); 48.1
trans-Dibromo[1-(3-methoxypropyl)-3-(4-methylbenzyl)benzimidazo-
le-2-ylidene](pyridine)palladium(II) (3a): Yield: 0.12 g, 54%; mp: 79-80
1
oC; H NMR (300 MHz, CDCl3, 25 oC): δ 2.24 (s, 3H, NCH2C6H4(CH3)-4);
2.53 (p, J= 7.1 Hz, 2H, NCH2CH2CH2OCH3); 3.29 (s, 3H,
NCH2CH2CH2OCH3); 3.44 (t, J= 5.6 Hz, 2H, NCH2CH2CH2OCH3); 4.95 (t,
J= 7.1 Hz, 2H, NCH2CH2CH2OCH3); 6.10 (s, 2H, NCH2C6H4(CH3)-4);
7.04-7.73 and 8.92-8.95 (m, 13H, NC6H4N, NCH2C6H4(CH3)-4 and
NC5H5). 13C NMR (75 MHz, CDCl3, 25 oC): δ 21.2 (NCH2C6H4(CH3)-4);
(NCH2C6(CH3)5-2,3,4,5,6);
58.7
(NCH2CH2CH2OCH3);
69.0
(NCH2CH2CH2OCH3); 113.1, 113.6, 125.1, 127.0, 131.3, 131.9, 133.6,
133.9, 137.3 (NC6H4N, NCH2C6(CH3)5-2,3,4,5,6); 143.2 (NCHN). IR (cm-
1) ν(CN): 1556; Anal. Calcd. for C23H31ClN2O: C, 71.39; H, 8.07; N, 7.24.
Found: C, 71.78; H, 8.12; N, 7.29.
30.1
(NCH2CH2CH2OCH3);
58.6
45.1
(NCH2CH2CH2OCH3);
(NCH2CH2CH2OCH3);
53.0
69.1
(NCH2C6H4(CH3)-4);
(NCH2CH2CH2OCH3); 110.5, 111.4, 123.1, 123.2, 124.5, 127.9, 129.5,
132.0, 134.0, 135.2, 137.8, 138.2, 151.2 (NC6H4N, NCH2C6H4(CH3)-4
and NC5H5); 163.6 (Pd-Ccarbene). IR (cm-1) ν(CN): 1408; Anal. Calcd. for
C24H27Br2N3OPd: C, 45.06; H, 4.25; N, 6.57. Found: C, 45.08; H, 4.27; N,
6.59.
1-(3-Methoxypropyl)-3-(4-chlorobenzyl)benzimidazolium
chloride
(2d): Yield: 1.89 g, 90%; mp: 125-126 oC; H NMR (500 MHz, CDCl3, 25
oC): δ 2.35 (p, J= 5.4 Hz, 2H, NCH2CH2CH2OCH3); 3.26 (s, 3H,
NCH2CH2CH2OCH3); 3.47 (t, J= 5.4 Hz, 2H, NCH2CH2CH2OCH3); 4.71 (t,
J= 7.0 Hz, 2H, NCH2CH2CH2OCH3); 5.96 (s, 2H, NCH2C6H4(Cl)-4); 7.29-
7.76 (m, 8H, NC6H4N, NCH2C6H4(Cl)-4); 11.83 (s, 1H, NCHN). 13C NMR
1
trans-Dibromo[1-(3-methoxypropyl)-3-(4-tert-butylbenzyl)benzimida-
zole-2-ylidene](pyridine)palladium(II) (3b): Yield: 0.19 g, 74%; mp:
109-110 oC; 1H NMR (300 MHz, CDCl3, 25 oC): δ 1.21 (s, 9H,
NCH2C6H4(C(CH3)3)-4); 2.54 (p, J= 5.6 Hz, 2H, NCH2CH2CH2OCH3);
3.30 (s, 3H, NCH2CH2CH2OCH3); 3.45 (t, J= 5.3 Hz, 2H,
NCH2CH2CH2OCH3); 4.93 (t, J= 7.1 Hz, 2H, NCH2CH2CH2OCH3); 6.06 (s,
2H, NCH2C6H4(C(CH3)3)-4); 7.00-7.70 and 8.94-8.98 (m, 13H, NC6H4N,
NCH2C6H4(C(CH3)3)-4 and NC5H5). 13C NMR (75 MHz, CDCl3, 25 C): δ
28.8
(NCH2C6H4(C(CH3)3)-4);
(NCH2C6H4(C(CH3)3)-4);
(NCH2CH2CH2OCH3); 109.4, 110.5, 122.0, 122.1, 123.4, 123.5, 124.7,
126.7, 126.8, 130.9, 133.1, 137.0, 151.0 (NC6H4N, NCH2C6H4(CH3)-4
(125 MHz, CDCl3, 25 oC):
(NCH2CH2CH2OCH3); 50.6
δ
29.6 (NCH2CH2CH2OCH3); 45.1
(NCH2C6H4(Cl)-4); 58.8
(NCH2CH2CH2OCH3); 68.8 (NCH2CH2CH2OCH3); 113.1, 113.6, 127.1,
129.5, 129.6, 129.9, 130.9, 131.6, 131.7, 135.2 (NC6H4N, NCH2C6H4(Cl)-
4); 143.8 (NCHN). IR (cm-1) ν(CN): 1557; Anal. Calcd. for C18H20Cl2N2O: C,
61.55; H, 5.74; N, 7.97. Found: C, 61.78; H, 5.82; N, 7.99.
o
(NCH2CH2CH2OCH3);
30.3
44.3
57.6
(NCH2C6H4(C(CH3)3)-4);
(NCH2CH2CH2OCH3);
(NCH2CH2CH2OCH3);
33.5
51.8
68.1
1-(3-Methoxypropyl)-3-(3-methoxylbenzyl)benzimidazolium chloride
(2e): Yield: 1.46 g, 74%; mp: 95-96 oC; 1H NMR (500 MHz, CDCl3, 25
oC): δ 2.31 (p, J= 5.5 Hz, 2H, NCH2CH2CH2OCH3); 3.22 (s, 3H,
This article is protected by copyright. All rights reserved.

10.1002/ejic.201601452
European Journal of Inorganic Chemistry
FULL PAPER
and NC5H5); 162.2 (Pd-Ccarbene). IR (cm-1) ν(CN): 1408; Anal. Calcd. for
C27H33Br2N3OPd: C, 47.56; H, 4.88; N, 6.16. Found: C, 47.60; H, 4.92; N,
6.19.
substituted thiophene derivative (2.0 mmol), Pd-NHC-PEPPSI complexes
3a-f (1.0 mol%). Then degassed DMAc (2 mL) was added under an
argon stream. The Schlenk flask was placed in a preheated oil bath at
150 °C, and the reaction mixture was stirred for different durations given
in the specific tables. At the end of the reaction, the solvent was removed
under vacuum, and the residue was charged directly onto a silica gel
column. The products were eluted from n-hexane/diethylether mixture
(5:1, v/v). The chemical characterizations of the products were made by
GC and GC-MS. The convertions were based on aryl halides.
trans-Dibromo[1-(3-methoxypropyl)-3-(2,3,4,5,6-pentamethylbenzyl)
benzimidazole-2-ylidene](pyridine)palladium(II) (3c): Yield: 0.21 g,
63%; mp: 204-205 oC; 1H NMR (300 MHz, CDCl3, 25 oC): δ 2.25, 2.31
and 2.33 (s, 15H, NCH2C6(CH3)5-2,3,4,5,6); 2.61 (p, J= 6.1 Hz, 2H,
NCH2CH2CH2OCH3); 3.38 (s, 3H, NCH2CH2CH2OCH3); 3.52 (t, J= 6.2 Hz,
2H, NCH2CH2CH2OCH3); 5.03 (t, J= 7.1 Hz, 2H, NCH2CH2CH2OCH3);
6.27 (s, 2H, NCH2C6(CH3)5-2,3,4,5,6); 6.40-7.83 and 8.98-9.00 (m, 9H,
NC6H4N and NC5H5). 13C NMR (75 MHz, CDCl3, 25 oC): δ 16.9, 17.3 and
17.4 (NCH2C6(CH3)5-2,3,4,5,6); 30.1 (NCH2CH2CH2OCH3); 45.2
X-ray Crystallographic Data: Single crystals of complex 3f suitable for
X-ray analysis were obtained by slow diffusion of n-pentane into a
dichloromethane solution of the complex 3f. Empirical formula=
C26 H31 Br2 N3 O4 Pd; M= 715.76 g.mol-1; crystal color= yellow; crystal
system= monoclinic; space group= P 1 21/c 1 system; a= 7.4450(3), b=
45.231(2), c= 8.8912(5) Å, β= 111.745(2) °, V= 2781.0(2) Å3. Z= 4, d=
1.710 g.cm-3, μ= 3.577 mm-1. F(000)= 1424; T= 150(2) K. The crystal
size (0.340 × 0.200 × 0.150 mm) was studied with an D8 VENTURE
Bruker AXS diffractometer. Mo-Kα radiation (λ= 0.71073 Å). The
structure was solved by direct methods using the SIR97 program,[40] and
then refined with full-matrix least-square methods based on F2 (SHELXL-
97).[41] All non-hydrogen atoms were refined with anisotropic atomic
(NCH2CH2CH2OCH3);
51.2
(NCH2C6(CH3)5-2,3,4,5,6);
58.6
(NCH2CH2CH2OCH3); 69.2 (NCH2CH2CH2OCH3); 110.2, 111.5, 122.6,
123.0, 124.4, 127.8, 133.1, 134.6, 135.0, 135.9, 138.0, 151.2 (NC6H4N,
NCH2C6(CH3)5-2,3,4,5,6 and NC5H5); 163.1 (Pd-Ccarbene). IR (cm-1) ν(CN)
:
1425; Anal. Calcd. for C28H35Br2N3OPd: C, 48.33; H, 5.07; N, 6.04.
Found: C, 48.34; H, 5.07; N, 6.09.
trans-Dibromo[1-(3-methoxypropyl)-3-(4-chlorobenzyl)benzimidazo-
le-2-ylidene](pyridine)palladium(II) (3d): Yield: 0.10 g, 44%; mp: 92-93
oC; 1H NMR (300 MHz, CDCl3, 25 oC): δ 2.52 (p, J= 7.0 Hz, 2H,
NCH2CH2CH2OCH3); 3.30 (s, 3H, NCH2CH2CH2OCH3); 3.44 (t, J= 5.5 Hz,
2H, NCH2CH2CH2OCH3); 4.93 (t, J= 7.3 Hz, 2H, NCH2CH2CH2OCH3);
6.11 (s, 2H, NCH2C6H4(Cl)-4); 7.20-7.75 and 8.92-8.94 (m, 13H, NC6H4N,
NCH2C6H4(Cl)-4 and NC5H5). 13C NMR (75 MHz, CDCl3, 25 oC): δ 29.0
(NCH2CH2CH2OCH3); 44.2 (NCH2CH2CH2OCH3); 51.4 (NCH2C6H4(Cl)-
4); 57.6 (NCH2CH2CH2OCH3); 68.0 (NCH2CH2CH2OCH3); 109.7, 110.1,
122.3, 122.4, 123.6, 128.1, 128.3, 132.5, 132.8, 133.0, 134.1, 137.2,
150.2 (NC6H4N, NCH2C6H4(Cl)-4 and NC5H5); 163.1 (Pd-Ccarbene). IR (cm-
1) ν(CN): 1405; Anal. Calcd. for C23H24Br2ClN3OPd: C, 41.85; H, 3.66; N,
6.37. Found: C, 41.87; H, 3.72; N, 6.39.
displacement parameters.
H atoms were finally included in their
calculated positions. A final refinement on F2 with 6350 unique intensities
and 329 parameters converged at ωR(F2) = 0.1438 (R(F)= 0.0645) for
5558 observed reflections with I > 2σ(I). w= 1 / [σ(Fo ) + aP2 + bP]
2
2
where P= [2Fc2 + MAX(Fo , 0)] /3.
CCDC reference number is 1510296 (for complex 3f) contain the
supplementary crystallographic data for this paper. This data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre.
Supporting Information (see footnote on the first page of this article):
for the 1H NMR, 13C NMR and IR spectra of benzimidazolium salts (2a-f)
and PEPPSI-type palladium-NHC complexes (3a-f).
trans-Dibromo[1-(3-methoxypropyl)-3-(3-methoxylbenzyl)benzimid-
azole-2-ylidene](pyridine)palladium(II) (3e): Yield: 0.18 g, 94%; mp:
138-139 oC; 1H NMR (300 MHz, CDCl3, 25 oC): δ 2.53 (p, J= 6.0 Hz, 2H,
NCH2CH2CH2OCH3); 3.30 (s, 3H, NCH2CH2CH2OCH3); 3.45 (t, J= 5.6 Hz,
2H, NCH2CH2CH2OCH3); 3.66 (s, 3H, NCH2C6H4(OCH3)-3); 4.91 (t, J=
7.2 Hz, 2H, NCH2CH2CH2OCH3); 6.06 (s, 2H, NCH2C6H4(OCH3)-3); 6.06-
7.69 and 8.96-8.99 (m, 13H, NC6H4N, NCH2C6H4(OCH3)-3 and NC5H5).
13C NMR (75 MHz, CDCl3, 25 oC): δ 29.6 (NCH2CH2CH2OCH3); 45.5
Acknowledgements
This work was financially supported by the Technological
Scientific Research Council of Turkey TÜBİTAK-BOSPHORUS
(France) [109T605] and İnönü University Research Fund (İ .Ü.
B.A.P: 2015/41-Güdümlü).
(NCH2CH2CH2OCH3);
(NCH2C6H4(OCH3)-3);
53.7
58.6
(NCH2C6H4(OCH3)-3);
(NCH2CH2CH2OCH3);
55.7
69.2
(NCH2CH2CH2OCH3); 110.5, 111.4, 113.0, 114.7, 120.4, 123.0, 123.1,
123.2, 124.6, 129.7, 134.2, 135.4, 136.5, 137.9, 152.6, 160.2 (NC6H4N,
NCH2C6H4(OCH3)-3 and NC5H5); 163.1 (Pd-Ccarbene). IR (cm-1) ν(CN)
:
Keywords: N-heterocyclic carbenes • benzimidazolium salts •
1412; Anal. Calcd. for C24H27Br2N3O2Pd: C, 43.96; H, 4.15; N, 6.41.
Found: C, 43.95; H, 4.15; N, 6.40.
PEPPSI-type palladium-NHC complexes
• heteroarenes •
substituted thiophenes • aryl halides • direct arylation
trans-Dibromo[1-(3-methoxypropyl)-3-(3,4,5-trimethoxylbenzyl)benz-
imidazole-2-ylidene](pyridine)palladium(II) (3f): Yield: 0.26 g, 78%;
mp: 176-177 oC; 1H NMR (300 MHz, CDCl3, 25 oC): δ 2.62 (p, J= 7.2 Hz,
2H, NCH2CH2CH2OCH3); 3.39 (s, 3H, NCH2CH2CH2OCH3); 3.53 (t, J=
5.6 Hz, 2H, NCH2CH2CH2OCH3); 3.84 (s, 9H, NCH2C6H2(OCH3)3-3,4,5);
5.00 (t, J= 7.2 Hz, 2H, NCH2CH2CH2OCH3); 6.10 (s, 2H,
NCH2C6H2(OCH3)3-3,4,5); 6.87-7.83 and 9.06-9.08 (m, 11H, NC6H4N,
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Palladium-NHC-PEPPSI Complexes:
Efficient Catalsts For The Direct C5-
Arylation
of
2-Substituted
Thiophenes
M. Kaloğlu, İ. Özdemir* V. Dorcet,
C. Bruneau, H. Doucet
PEPPSI-type palladium-NHC complexes have been synthesized and the catalytic
activity of the complexes have been evaluated in the direct C5-arylation of 2-
substituted thiophene derivatives with various aryl bromides and aryl chlorides
1 – 10
Palladium-NHC-PEPPSI Complexes:
Synthesis,
Characterization
and
Catalytic Activity in the Direct C5-
Arylation of 2-Substituted Thiophene
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