Synthesis of novel palladium(II) N-heterocyclic carbene complexes and their catalytic activities in the direct C5 arylation reactions

Article abstract of DOI:10.1016/j.ica.2016.07.055

New palladium(II) N-heterocyclic carbene complexes have been easily obtained in good yields from the carbene transfer reaction of silver-NHC complexes with PdCl₂(PhCN)₂and characterized by elemental analysis,¹H NMR and

Full text of DOI:10.1016/j.ica.2016.07.055

Inorganica Chimica Acta 453 (2016) 23–28
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Inorganica Chimica Acta
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Research paper
Synthesis of novel palladium(II) N-heterocyclic carbene complexes and
their catalytic activities in the direct C5 arylation reactions
Murat Yig˘it a, Beyhan Yig˘it a, , Yetkin Gök b
⇑
a Department of Chemistry, Faculty of Science and Art, Adıyaman University, 02040 Adıyaman, Turkey
b Department of Chemistry, Faculty of Science and Art, Inönü University, 44260 Malatya, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
New palladium(II) N-heterocyclic carbene complexes have been easily obtained in good yields from the
carbene transfer reaction of silver-NHC complexes with PdCl2(PhCN)2 and characterized by elemental
analysis, 1H NMR and 13C NMR spectroscopy. These complexes were used as catalysts in the direct C5 ary-
lation of 2-n-butylfuran, 2-n-butylthiophene and 2-n-propylthiazole with various aryl bromides at 130 °C
in N,N-dimethylacetamide (DMAc) for 1 h. The arylation reactions were regioselective, and in all cases,
only the C5-arylated products were formed. The corresponding arylation products were obtained in mod-
erate to good yields by using 0.5 mol% of palladium-NHC complexes.
Received 4 March 2016
Received in revised form 9 June 2016
Accepted 29 July 2016
Available online 30 July 2016
Keywords:
N-heterocyclic carbene
Direct arylation
Palladium complexes
Benzimidazole
Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction
couplings [45–48]. However, these reactions require the
organometallic nucleophilic reagents and provide a stoichiometric
N-heterocyclic carbenes (NHCs) have become popular ligands
for metal-catalyzed reactions due to their ease of synthesis,
thermal and oxidative stability, functional group tolerance and
excellent catalytic activities [1–4]. In 1991, Arduengo and
co-workers successfully isolated and characterized the first stable
free N-heterocyclic carbene [5]. Since then, many N-heterocyclic
carbene-metal complexes have been synthesized and used as cat-
alysts of various transformations such as C–C and C–N arylation
[6–10], olefin metathesis [11,12], transfer hydrogenation [13–16],
hydrosilylation [17–19] and CO-ethylene co-polymerization reac-
tions [20,21]. Imidazole, benzimidazole, imidazoline and triazole
derived NHCs have been reported during the last decade [22–26].
In general, NHC chemistry is dominated by imidazole and imidazo-
line based carbene ligands [27,28]. On the other hand, benzimida-
zol-2-ylidenes have received considerably less interest, though in
recent years this area has been developed by Hahn and others
[29–36]. However, catalytic activity of benzimidazol-2-ylidene
metal complexes in C–C coupling reactions [37–40] and transfer
hydrogenations [41–43] has been reported. In recent years, the
synthesis of arylated heterocycles have received considerable
attention because of their biological and physical properties [44].
Bi(hetero)aryl compounds can be prepared via palladium-
catalyzed reactions such as Stille, Suzuki, Kumada or Negishi
amount of side products. But, extraordinary reactivities of these
reagents as nucleophiles and bases sometimes give rise to the
limitations in the synthesis of multifunctional compounds.
In 1990, Ohta and co-workers have reported direct access to
arylated heterocycles from heteroaromatics (such as thiophenes,
furans or thiazoles) and aryl halides in moderate to good yields
by using [Pd(PPh3)4] as the catalyst [49]. Since then, the palla-
dium-catalyzed direct arylation of heteroaryl derivatives with aryl
halides has become a valuable method for the synthesis of arylated
heterocycles [50–67]. So far, there are only a few report on (NHC)
Pd-catalyzed direct arylation of heterocycles [68–76]. Therefore,
we now report the synthesis and characterization of new palla-
dium(II) complexes with N-heterocyclic carbene ligand from silver
complexes and their application as catalysts for the direct 5-aryla-
tion of heteroaromatics with various aryl halides.
2. Experimental
2.1. Materials and methods
All reactions for the preparation of the palladium(II)-NHC com-
plexes (2a-d) were carried out under argon in flame-dried glass-
ware using standard Schlenk techniques. The solvents used were
purified by distillation over the drying agents indicated and were
transferred under Ar: THF, Et2O (Na/K alloy), CH2Cl2 (P4H10), hex-
ane, toluene (Na). All reagents were purchased from Sigma-Aldrich,
⇑
Corresponding author.
E-mail address: byigit@adiyaman.edu.tr (B. Yig˘it).
http://dx.doi.org/10.1016/j.ica.2016.07.055
0020-1693/Ó 2016 Elsevier B.V. All rights reserved.

24
M. Yig˘it et al. / Inorganica Chimica Acta 453 (2016) 23–28
Merck and Fluka. All 1H and 13C NMR were recorded in CDCI3 using
a Bruker AC300P FT spectrometer operating at 300.13 MHz (1H) or
75.47 MHz (13C). Chemical shifts (d) are given in ppm relative to
TMS, coupling constants (J) in hertz. FT-IR spectra were recorded
as KBr pellets in the range 400–4000 cmÀ1 on a Mattson 1000 spec-
trophotometer (wavenumbers, cmÀ1). GC were measured by
GC-FID on a Agilent 6890N gas chromatograph equipped with an
2.2.3. Bis[1-(1-methyl-2-dimethylaminoethyl)-3-(2,3,5,6-
tetramethylbenzyl)benzimidazol-2-ylidene]-dichloropalladium(II), 2c
This compound was prepared in same way as 2a from chloro-
[1-(1-methyl-2-dimethylaminoethyl)-3-(2,3,5,6-tetramethylben-
zylbenzimidazol-2-ylidene]silver(I) complex (0.31 g, 0.62 mmol)
and PdCl2(PhCN)2 (0.12 g, 0.32 mmol) in dichloromethane
(10 mL). Yield: 80%; m.p. 228–229 °C; IR
m .
(NCN): 1474 cmÀ1 1H
HP-5 column of 30 m length, 0.32 mm diameter and 0.25
l
m film
NMR (300.13 MHz, CDCl3, 25 °C, TMS): d = 1.02 (d, 6H, J: 6.6 Hz,
CH(CH3)CH2N(CH3)2); 2.29 (s, 12H, N(CH3)2); 4.10–4.12 (m, 2H,
CH(CH3)CH2N(CH3)2); 2.33 and 2.36 (s, 24H, CH2C6H(CH3)4-2,
3,5,6); 4.98–5.06 (m, 4H, CH(CH3)CH2N(CH3)2); 5.85 and 5.88 (s,
4H, CH2Ar); 6.31–7.28 (m, 10H, Ar-H). 13C NMR (75.47 MHz, CDCl3,
25 °C, TMS): d = 11.6 (CH(CH3)CH2N(CH3)2); 40.6 (N(CH3)2); 50.0
and 51.1 (CH(CH3)CH2N(CH3)2); 16.5 and 20.6 (CH2C6H2(CH3)3-2,
3,5,6); 59.5 (CH2Ar); 110.7, 111.9, 122.5, 122.8, 130.7, 132.3,
134.2, 134.3, 134.4, 134.7, 134.8 and 135.3 (Ar-C); 181.8 (C-Pd).
Anal. Calcd. For C46H62N6PdCl2: C, 63.06; H, 7.08; N, 9.60. Found:
C, 63.15; H, 7.15; N, 9.68%.
thickness. Melting points were measured in open capillary tubes
with an Electrothermal-9200 melting point apparatus and uncor-
rected. Elemental analyses were performed at Inönü University
research center.
2.2. General procedure for the preparation of Ag(I)-N-heterocyclic
carbene complexes, 1
A
solution of benzimidazolium salt (1.0 mmol), Ag2O
(0.5 mmol) and activated molecular sieves Å in dichloromethane
(20 mL) was stirred for 24 h under exclusion of light at room tem-
perature. The reaction mixture was filtered through celite and
solvent removed under reduced pressure. The crude product was
recrystallized from dichloromethane: hexane (1:2) at room tem-
perature. The resulting white solid was isolated by filtration and
dried in vacuum.
2.2.4. Bis[1-(1-methyl-2-dimethylaminoethyl)-3-(2,3,4,5,6-
pentamethylbenzyl)benzimidazol-2-ylidene]-dichloropalladium(II),
2d
This compound was prepared in same way as 2a from chloro-
[1-(1-methyl-2-dimethylaminoethyl)-3-(2,3,4,5,6-pentamethyl-
benzylbenzimidazol-2-ylidene]silver(I) complex (0.32 g, 0.62 mmol)
and PdCl2(PhCN)2 (0.12 g, 0.32 mmol) in dichloromethane (10 mL).
Yield: 81%; m.p. 254–255 °C; IR
m .
(NCN): 1476 cmÀ1 1H NMR
2.2.1. Bis[1-(1-methyl-2-dimethylaminoethyl)-3-benzylbenzimidazol-
2-ylidene]-dichloropalladium(II), 2a
(300.13 MHz, CDCl3, 25 °C, TMS): d = 1.13 (d, 6H, J: 6.6 Hz, CH
(CH3)CH2N(CH3)2); 2.27 (s, 12H, N(CH3)2); 4.09–4.14 (m, 2H, CH
(CH3)CH2N(CH3)2); 2.22, 2.29 and 2.34 (s, 30H, CH2C6(CH3)5-2,
3,4,5,6); 5.09–5.14 (m, 4H, CH(CH3)CH2N(CH3)2); 5.83 and 5.86
(s, 4H, CH2Ar); 6.41–7.79 (m, 8H, Ar-H). 13C NMR (75.47 MHz,
CDCl3, 25 °C, TMS): d = 11.5 (CH(CH3)CH2N(CH3)2); 40.6 (N
(CH3)2); 50.4 and 51.7 (CH(CH3)CH2N(CH3)2); 16.8, 16.9, 17.0,
17.3 and 17.5 (CH2C6(CH3)5-2,3,4,5,6); 59.8 (CH2Ar); 110.8, 111.1,
112.1, 122.8, 123.0, 127.7, 133.1, 133.8, 134.1, 134.2, 135.1 and
135.9 (Ar-C); 181.6 (C-Pd). Anal. Calcd. For C48H66N6PdCl2: C,
63.76; H, 7.31; N, 9.30. Found: C, 63.82; H, 7.37; N, 9.38%.
A solution of chloro-[1-(1-methyl-2-dimethylaminoethyl)-3-
benzylbenzimidazol-2-ylidene]silver(I) complex (0.27 g, 0.62 mmol)
and PdCl2(PhCN)2 (0.12 g, 0.32 mmol) in dichloromethane (20 mL)
was stirred for 24 h under exclusion of light at room temperature.
Then, the solution was filtered through celite, and crystallized from
dichloromethane: diethyl ether (1:2) at room temperature. The
crystals were filtered, washed with diethyl ether (3 Â 10 mL) and
dried under vacuum. Yield: 83%; m.p. 248–249 °C; IR m(NCN)
:
1477 cmÀ1 1H NMR (300.13 MHz, CDCl3, 25 °C, TMS): d = 0.91 (d,
.
6H, J: 6.6 Hz, CH(CH3)CH2N(CH3)2); 2.16 (s, 12H, N(CH3)2); 3.69–
3.74 (m, 2H, CH(CH3)CH2N(CH3)2); 4.60–4.68 (m, 4H, CH(CH3)
CH2N(CH3)2); 5.90 and 5.98 (d, 4H, J: 12.9 Hz, CH2Ar); 7.12–7.38
(m, 18H, Ar-H). 13C NMR (75.47 MHz, CDCl3, 25 °C, TMS): d = 11.0
(CH(CH3)CH2N(CH3)2); 40.4 (N(CH3)2); 51.0 and 52.7 (CH(CH3)
CH2N(CH3)2); 59.0 (CH2Ar); 110.8, 110.9, 111.4, 122.8, 122.9,
127.4, 127.5, 128.8, 128.9, 133.7, 135.4 and 135.7 (Ar-C); 181.6
(C-Pd). Anal. Calcd. For C38H46N6PdCl2: C, 59.73; H, 6.03; N,
11.00. Found: C, 59.78; H, 6.09; N, 11.12%.
2.3. General procedure for the direct C5 arylations
The heteroaryl derivative (2 mmol), aryl halide (1 mmol), Pd
complexes 2a-d (0.005 mmol), KOAc (1 mmol) and DMAc (2 mL)
were added into a Schlenk tube equipped with a magnetic stirring
bar. The Schlenk tube was purged several times with argon and
was placed in a preheated oil bath at 130 °C. The reactants were
stirred for 1 h with aryl bromides. The solvent was removed by
heating the reaction vessel under vacuum. The products were
eluted by using an appropriate ratio of diethyl ether/pentane
(1:3). The reaction mixture was purified by flash chromatography
on silica gel. The purity of compounds was checked by GC and
NMR. Conversions were based on aryl bromides.
2.2.2. Bis[1-(1-methyl-2-dimethylaminoethyl)-3-(2,4,6-
trimethylbenzyl)benzimidazol-2-ylidene]-dichloropalladium(II), 2b
This compound was prepared in same way as 2a from chloro-
[1-(1-methyl-2-dimethylaminoethyl)-3-(2,4,6-trimethylbenzyl-
benzimidazol-2-ylidene]silver(I) complex (0.30 g, 0.62 mmol) and
PdCl2(PhCN)2 (0.12 g, 0.32 mmol) in dichloromethane (10 mL).
Yield: 85%; m.p. 241–242 °C; IR
m
(NCN): 1475 cmÀ1
.
1H NMR
3. Results and discussion
(300.13 MHz, CDCl3, 25 °C, TMS): d = 1.11 (d, 6H, J: 6.6 Hz, CH
(CH3)CH2N(CH3)2); 2.31 (s, 12H, N(CH3)2); 4.00–4.06 (m, 2H, CH
(CH3)CH2N(CH3)2); 2.31 and 2.38 (s, 18H, CH2C6H2(CH3)3-2,4,6);
4.85–4.98 (m, 4H, CH(CH3)CH2N(CH3)2); 6.42 and 6.45 (s, 4H,
CH2Ar); 6.94 (s, 4H, Ar-H); 7.14–7.43 (m, 8H, Ar-H). 13C NMR
(75.47 MHz, CDCl3, 25 °C, TMS): d = 11.2 (CH(CH3)CH2N(CH3)2);
40.6 (N(CH3)2); 50.2 and 51.2 (CH(CH3)CH2N(CH3)2); 20.9 and
21.0 (CH2C6H2(CH3)3-2,4,6); 59.4 (CH2Ar); 110.6, 111.1, 111.7,
122.4, 122.7, 127.9, 129.6, 134.0, 135.4, 138.1 and 138.4 (Ar-C);
181.9 (C-Pd). Anal. Calcd. For C44H58N6PdCl2: C, 62.31; H, 6.85; N,
9.91. Found: C, 62.40; H, 6.81; N, 9.98%.
3.1. Synthesis of Pd(II)-NHC complexes
Various synthetic methods have been described in the literature
for the synthesis of Pd(II)-NHC complexes. One of these methods is
the generation of a silver(I)-NHC complex, followed by transfer of
the carbene unit to palladium metal. This reaction has successfully
been applied to a variety of metals, including ruthenium, rhodium,
iridium, gold and nickel [77]. Our previously reported [78] silver
(I)-NHC complexes, 1 were prepared by reacting the benzimida-
zolium salts with Ag2O in dichloromethane under exclusion of light

M. Yig˘it et al. / Inorganica Chimica Acta 453 (2016) 23–28
25
at room temperature. The palladium(II)-NHC complexes 2a-d were
prepared by treatment of silver(I)-NHC complexes as a carbene
transfer reagent with PdCl2(PhCN)2 in dichloromethane at room
temperature in good yields (Scheme 1). These complexes are stable
both in solution and in solid states against air, light and moisture,
and are soluble in chlorinated solvents. The new complexes were
characterized by 1H NMR, 13C NMR, IR spectroscopy and elemental
analysis techniques, which support the proposed structures. They
NHC complexes resulted in only 1% yields under this reaction con-
ditions. The coupling reactions proceed selectively at C5 position of
heteroaromatic compounds, and in all cases, only the C5-arylated
products were formed. Initially, we investigated the reactions of
2-n-propylthiazole with bromobenzene for the direct C5 arylation
reactions using these new palladium(II)-NHC (2a-d) complexes as
catalyst. The coupling products were obtained with high yields by
Pd-NHC complexes (2b, 2c and 2d) (Table 1, entries 14–16). Then,
activated and deactivated aryl bromides were reacted with 2-n-
propylthiazole in the presence of these complexes (Table 1, entries
1–13). 2-n-Propylthiazole has been arylated in good to excellent
yields using aryl bromides (Table 1, entries 1–16). Of the five dif-
ferent aryl bromides, those bearing electron-donating groups react
with 2-n-propylthiazole to give the coupled products in excellent
yields (Table 1, entries 5–12). For all aryl bromides, the best con-
versions were achieved with complexes (2b, 2c and 2d) in 96–
100% conversions.
Next, we examined the reactivity of 2-n-butylthiophene using
the same aryl bromides in the presence of palladium(II)-NHC
(2a-d) complexes as catalyst under similar reaction conditions
(Table 2). The coupling products were obtained with high yields.
As seen in the reactions of 2-n-propylthiazole with aryl bromides,
substituents on aryl bromides display a minor effect on the reac-
tion of 2-n-butylthiophene with aryl bromides. 4-Bromoacetophe-
none was successfully coupled with 2-n-butylthiophene in
presence of palladium(II)-NHC complexes (2a-d) to give 5-(4-acet-
ylphenyl)-2-n-butylthiophene in 65–77% yields. High conversions
for 2-n-butylthiophene were obtained using the electron-rich 4-
bromoanisole in the presence of palladium(II)-NHC (2a-d) (Table 2,
entries 5–8). The catalytic activity of palladium(II)-NHC complexes
(2a-d) in these reactions were similar to the direct C5 arylation
of 2-n-propylthiazole. The complex 2a catalyses the reaction of
2-n-butylthiophene with aryl bromides in good yields (Table 2,
entries 1, 5, 9 and 13), whereas the use of complexes (2b, 2c and
2d) gave higher yields for these reactions.
show
a characteristic t(NCN) band at 1477, 1475, 1474 and
1476 cmÀ1 for 2a, 2b, 2c and 2d, respectively. NMR analyses of
the complexes showed that the N-heterocyclic carbene ligands
transferred from silver complexes to palladium center. The 13C
NMR spectra of the Pd-NHC complexes exhibit the NCN resonances
between 181.6 and 181.9 ppm, which are consistent with reported
values for [PdCl2(NHC)2] complexes. The geometry of palladium(II)
di-NHC complexes may be trans or cis. Generally, The trans geom-
etry was obtained but palladium(II) complexes with cis oriented
NHC ligands have been described [79–81]. The geometry of these
complexes was not defined, as an appropriate single crystal from
these complexes for X-ray diffraction could not be obtained.
3.2. Catalytic studies: direct arylation of various heteroaromatic
compounds with ary bromides using 2a-d as catalyst
The catalytic activities of palladium complexes as catalysts in
the direct arylation reactions to prepare arylated heterocycles,
which are important chemicals for many applications were exten-
sively investigated. However, only a few examples of Pd-carbene
complexes bearing benzimidazol-2-ylidene ligand for direct aryla-
tion of heteroaromatic compounds have been reported so far
[69,72,76]. Based on our previous results [76], in this study,
N,N-dimethylacetamide (DMAc) was chosen as the solvent and
potassium acetate (KOAc) as the base. The reactions were per-
formed at 130 °C for 1 h in presence of palladium(II)-NHC com-
plexes (2a-d). Under this reaction conditions, three heterocycles
substrates (2-n-butylfuran, 2-n-butylthiophene and 2-n-propylthi-
azole) were reacted with aryl bromides bearing electron-donating
or electron-withdrawing groups at the para position to furnish the
arylated products in moderate to good yields (Tables 1–3). The
reactions between the 4-bromoacetophenone with 2-n-butylfuran,
2-n-butylthiophene or 2-n-propylthiazole without palladium(II)-
Finally, we studied the reaction of 2-n-butylfuran with aryl bro-
mides by using palladium(II)-NHC (2a-d) catalysts in order to
obtain 5-aryl-2-n-butylfurans. The reactions were performed with
4-substituted aryl bromides at 130 °C for 1 h in N,N-dimethylac-
etamide (DMAc) and the results of these reactions are summarized
in Table 3. All of the aryl bromides gave moderate to high yields of
N(CH3)2
N(CH3)2
N
N
PdCl2
PdCl2
N
2
N
2
N(CH3)2
2a
2d
N
AgCl
N
Rn
1
N(CH3)2
N(CH3)2
PdCl2
+
PdCl2(PhCN)2
N
N
N
N
PdCl2
2
2
2b
2c
Scheme 1. The synthesis of palladium(II)-NHC complexes.

26
M. Yig˘it et al. / Inorganica Chimica Acta 453 (2016) 23–28
Table 1
Direct C5 arylation of 2-n-propylthiazole by using aryl bromides.abcd
N
N
Pd-NHC (0.005 mmol)
+
R
Br
R
nPr
nPr
DMAc (2 mL), KOAc (1 mmol), 1 h
S
S
Entry
Aryl bromide
Pd-NHC
Product
Conv. (Yield%)
1
2
6
4
2a
2b
2c
2d
90(68)
99(76)
98(78)
96(74)
N
H3COC
Br
H3COC
H3CO
H3C
nPr
S
5
6
7
8
2a
2b
2c
2d
96(78)
100(89)
100(92)
99(87)
N
H3CO
H3C
Br
Br
nPr
S
9
2a
2b
2c
2d
98(75)
98(86)
99(85)
99(89)
N
Br
10
11
12
nPr
S
13
14
15
16
2a
2b
2c
2d
94(71)
98(76)
99(81)
97(79)
N
nPr
S
a
b
c
Reaction condition:2-n-propylthiazole (2 mmol), aryl bromide (1 mmol), Pd-NHC (2a-d) (0.005 mmol), KOAc (1 mmol), N,N-dimethylacetamide (2 mL), 130 °C, 1 h.
Product purity was checked by GC and NMR.
Conversions were calculated according to aryl bromide.
Isolated yields.
d
Table 2
Direct C5 arylation of 2-n-butylthiophene by using aryl bromides.abcd
Pd-NHC (0.005 mmol)
+
R
Br
R
nBu
DMAc (2 mL), KOAc (1 mmol), 1 h
S
nBu
S
Entry
Aryl bromide
Pd-NHC
Product
Conv. (Yield%)
1
2
6
4
2a
2b
2c
2d
88(65)
94(74)
96(77)
95(76)
H3COC
Br
H3COC
H3CO
H3C
nBu
S
5
6
7
8
2a
2b
2c
2d
93(76)
99(88)
100(90)
97(86)
H3CO
H3C
Br
Br
nBu
S
9
2a
2b
2c
2d
94(74)
98(87)
97(83)
96(81)
Br
10
11
12
nBu
S
13
14
15
16
2a
2b
2c
2d
90(71))
95(79)
97(83)
96(81)
nBu
S
a
b
c
Reaction condition: 2-n-butylthiophene (2 mmol), aryl bromide (1 mmol), Pd-NHC (2a-d) (0.005 mmol), KOAc (1 mmol), N,N-dimethylacetamide (2 mL), 130 °C, 1 h.
Product purity was checked by GC and NMR.
Conversions were calculated according to aryl bromide.
Isolated yields.
d
C5 arylation products in the presence of 0.5 mol% catalysts. The
reaction of 2-n-butylfuran with bromobenzene generated the cor-
responding product (2-n-butyl-5-phenylfuran) in 69–83% yields
using palladium(II)-NHC complexes (2a-d) (Table 3, entries
13–16). High conversions for 2-n-butylfuran were obtained using
4-bromoacetophenone in the presence of palladium(II)-NHC
(2a-d) (Table 3, entries 1–4). The catalytic activities of complexes
with NHC ligands bearing methyl substituents on benzyl group
were quite close to each other. On the other hand, the palladium
(II)-NHC complex (2a) exhibited low catalytic activity compared

M. Yig˘it et al. / Inorganica Chimica Acta 453 (2016) 23–28
27
Table 3
Direct C5 arylation of 2-n-butylfuran by using aryl bromides.abcd
Pd-NHC (0.005 mmol)
+
R
Br
R
nBu
DMAc (2 mL), KOAc (1 mmol), 1 h
nBu
O
O
Entry
Aryl bromide
Pd-NHC
Product
Conv. (Yield%)
1
2
6
4
2a
2b
2c
2d
95(77)
98(88)
99(90)
96(86)
H3COC
Br
H3COC
H3CO
H3C
nBu
O
5
6
7
8
2a
2b
2c
2d
87(73)
94(84)
97(87)
95(85)
H3CO
H3C
Br
Br
nBu
O
9
2a
2b
2c
2d
85(67)
97(87)
93(82)
95(84)
Br
10
11
12
nBu
O
13
14
15
16
2a
2b
2c
2d
88(69)
90(76)
94(79)
97(83)
nBu
O
a
b
c
Reaction condition: 2-n-butylfuran (2 mmol), aryl bromide (1 mmol), Pd-NHC (2a-d) (0.005 mmol), KOAc (1 mmol), N,N-dimethylacetamide (2 mL), 130 °C, 1 h.
Product purity was checked by GC and NMR.
Conversions were calculated according to aryl bromide.
Isolated yields.
d
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4. Conclusion
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In summary, The palladium(II)-NHC complexes 2a-d have been
easily prepared by reaction of silver(I)-NHC complexes as a car-
bene transfer reagent with PdCl2(PhCN)2 in dichloromethane at
room temperature in good yields. The catalytic activity of these
complexes was investigated in direct C5 arylation of thiazole, thio-
phene and furan derivatives in the presence of potassium acetate.
Although 3-, 4- and 5-positions of heteroaromatics are open, the
coupling reactions proceed selectively at C5 position of heteroaro-
matic compounds, and in all cases, only the C5-arylated products
were formed. In this study, all palladium(II)-NHC complexes
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thiazole, thiophene and furan derivatives. Among the tested com-
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