Palladium(II) N-heterocyclic carbene complexes as catalysts for the direct arylation of pyrrole derivatives with aryl chlorides

Article abstract of DOI:10.1016/j.ica.2017.05.047

Silver(I) N-heterocyclic carbene complexes were synthesized in good yields by the reactions of 1,3-dialkylperhydrobenzimidazolyum salts with silver(I) oxide and used as transmetallation reagents for the synthesis of palladium(II) N-heterocyclic carbene co

Full text of DOI:10.1016/j.ica.2017.05.047

Accepted Manuscript
Research paper
Palladium(II) N-heterocyclic carbene complexes as catalysts for the direct ary-
lation of pyrrole derivatives with aryl chlorides
Beyhan Yiğit, Nevin Gürbüz, Murat Yiğit, Zeynep Dağdeviren, İsmail Özdemir
PII:
S0020-1693(16)31020-9
http://dx.doi.org/10.1016/j.ica.2017.05.047
ICA 17617
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
10 December 2016
25 April 2017
22 May 2017
Please cite this article as: B. Yiğit, N. Gürbüz, M. Yiğit, Z. Dağdeviren, I. Özdemir, Palladium(II) N-heterocyclic
carbene complexes as catalysts for the direct arylation of pyrrole derivatives with aryl chlorides, Inorganica Chimica
Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.05.047
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Palladium(II) N-heterocyclic carbene complexes as catalysts for the direct arylation of pyrrole
derivatives with aryl chlorides
Beyhan Yiğit,1 Nevin Gürbüz,2,3 Murat Yiğit,1* Zeynep Dağdeviren,1 İsmail Özdemir2,3
1 Department of Chemistry, Faculty of Science and Art, Adıyaman University, 02040 Adıyaman, Turkey
2 Department of Chemistry, Faculty of Science and Art, Inönü University, 44280 Malatya, Turkey
3Catalysis Research and Application Center, Inönü University, 44280 Malatya, Turkey
Abstract
Silver(I) N-heterocyclic carbene complexes were synthesized in good yields by the reactions of 1,3-
dialkylperhydrobenzimidazolyum salts with silver(I) oxide and used as transmetallation reagents for
the synthesis of palladium(II) N-heterocyclic carbene complexes. All of the complexes were
1
characterized using elemental analysis, H NMR and 13C NMR spectroscopy. The catalytic activity of
Pd(II)-NHC complexes in the direct arylation of pyrrole derivatives was investigated. These Pd(II)-NHC
complexes showed the good catalytic performance for the direct arylation of pyrrole derivatives with
readily available and inexpensive aryl chlorides. The arylation reactions regioselectively produced C2-
or C5-arylation products in moderate to good yields by using 1 mol % of the palladium complex.
1. Introduction
N-Heterocyclic carbenes (NHC) have become one of the most widely used and investigated
classes of ligands in organometallic chemistry, especially for the preparation of new metal-based
catalysts since isolation of the first stable NHC species in 1991 by Arduengo [1, 2]. N-Heterocyclic
carbenes are strong σ-donating and weak π-accepting ligands, and they form stable NHC complexes
with transition metals [3-5]. A variety metal complexes of N-Heterocyclic carbenes have been
synthesized and used as catalysts for many chemical transformations [6-15]. In particular, their
palladium complexes have been successfully employed as highly active precatalysts for C-C and C-N
coupling, and direct arylation reactions [16-21].
In recent years, the synthesis of arylated heterocycles have received considerable attention
because of their biological and physical properties [22]. The arylation of heteroarenes with aryl

halides is one of the most valuable synthetic proceses for the preparation of arylated heteroarenes
as it eliminates the preactivation of coupling components used in conventional methods such as
Stille, Suzuki, Kumada or Negishi couplings [23-26]. In 1985, Ohta and co-workers reported that the
arylation of heteroaromatics with aryl halides proceed in moderate to good yields using [Pd(PPh3)4]
as the catalyst [27, 28]. Since then, the palladium-catalyzed direct arylation of heteroaryl derivatives
via C-H bond activation using aryl halides has become a valuable method for the synthesis of arylated
heterocycles [29-37].
Pyrroles are an important class of heterocyclic compounds because of their biological
pharmacological activities [38, 39]. They have also been found in many drugs such as Atorvastatin,
Fendosal and Tanaproget. Therefore, preparation of pyrrole derivatives has attracted increasing
attention and several effective strategies have been developed for the synthesis of arylated pyrroles
[40-50]. The direct arylation of pyrroles with aryl halides is one convenient methods for the
introduction of aryl groups at C2 or C5 positions of pyrroles [51-60]. In most cases, palladium-
catalyzed direct arylation of pyrroles were performed with aryl iodies and bromides [61-70].
Howewer, only a few examples on arylation of pyrroles using aryl chlorides have been reported,
despite their lower cost and more easy availability [71-75]. For example, Daugulis and co-workers
have reported the C2 or C5 arylation of pyrrole derivatives with various aryl chlorides using 5 mol %
of Pd(OAc)2 associated to 10 mol % of Cy2P-o-biphenyl as the catalyst. The coupling products were
obtained in 42-78% yields [71]. Despite the efficiency of palladium-N-Heterocyclic carbene systems in
C-C and C-N coupling reactions, use of these complexes in direct arylation of heteroaromatics are
relatively rare. So far, a few examples of (NHC)Pd-catalyzed direct arylation of nitrogen containing
heterocycles have been reported. In 2006, Sames and co-workers have reported the direct arylation
of pyrroles, indoles, imidazoles and imidazo[1,2-a]pyridines with bromobenzene and aryl iodides
using palladium complexes with both phosphine and carbene ligands as catalyst [76]. Later, use of
Pd-NHC complexes for the direct arylation of pyrroles, imidazoles and benzimidazoles with aryl
halides was reported by the groups of Lee, Shao, Huynh and Liu [77-81]. To the best our knowledge,
only one example of the direct arylation of pyrroles with aryl chlorides using N-heterocyclic carbene
ligands has been reported by Doucet to date [75]. However, palladium complexses bearing a
perhydrobenzimidazol-2-ylidene ligand have not yet been employed as catalyst for the direct
arylation of pyrrole derivatives with aryl chlorides. Therefore, we now report the synthesis and
characterization of new silver(I) and palladium(II) complexes with perhydrobenzimidazol-2-ylidene
ligand and use of palladium complexes as catalysts for the direct C2- or C5-arylation of pyrrole
derivatives with various aryl chlorides.

2. Experimental
2.1. Materials and methods
All reactions for the preparation of the silver(I) and palladium(II)-NHC complexes were carried out
under argon in flame-dried glassware 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), hexane, toluene (Na). All reagents were purchased from Sigma-Aldrich,
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 (δ) 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 cm-1
on a Mattson 1000 spectrophotometer (wavenumbers, cm-1). GC were measured by GC-FID on a
Agilent 6890N gas chromatograph equipped with an HP-5 column of 30 m length, 0.32 mm diameter
and 0.25 µm film thickness. Melting points were measured in open capillary tubes with an
Electrothermal-9200 melting point apparatus and uncorrected. Elemental analyses were performed
at Inönü University research center.
2.1. Synthesis of perhydrobenzimidazolium salts, 1
A mixture of N,N’-dialkyl-1,2-diaminocyclohexane (4.2 mmol), NH4Cl (4.2 mmol) and triethyl
°
orthoformate (10 mL) was heated for 12 h at 110 C. Upon cooling to room temperature, colorless
crystals were obtained. These were filtered off, washed with diethyl ether (3 × 15 mL) and dried
under vacuum. The crude product was recrystallized from EtOH / Et2O.
2.2. Synthesis of silver(I)-NHC complexes, 2
A solution of the appropriate perhydrobenzimidazolium chloride (1.08 mmol), Ag2O (0.54 mmol) and
activated molecular sieves 4Å in dichloromethane (25 mL) was stirred for 24 h at room temperature
in the dark conditions, covered with aluminum foil under argon. The reaction mixture was filtered
through celite, and the solvent was removed under reduced pressure. The crude product was
recrystallized from dichloromethane:hexane (1:2) at room temperature. The resulting white solid
was isolated by filtration and dried in vacuum.
2.2.1. Chloro-1,3-bis(4-ethylbenzyl)perhydrobenzimidazol-2-ylidenesilver(I), 2a

Yield: 0.47 g, 87%; en: 151-152 oC. 1H NMR (CDCl3) δ: 1.18-1.24 (m, 4H,
NCHCH2CH2CH2CH2CHN), 1.73-1.80 and 2.01-2.05 (m, 4H, NCHCH2CH2CH2CH2CHN), 2.88-2.99 (m, 2H,
NCHCH2CH2CH2CH2CHN), 1.25 (t, 6H, J = 7.56 Hz, CH2C6H4CH2CH3-4), 2.65 (q, 4H, J = 7.42 Hz,
CH2C6H4CH2CH3-4), 4.41, 4.62, 4.90 and 5.08 (d, 4H, J = 15.0 Hz, CH2C6H4CH2CH3-4), 7.19 and 7.25 (d,
8H, J = 7.4 Hz, CH2C6H4CH2CH3-4). 13C NMR (CDCl3) δ: 23.8 (NCHCH2CH2CH2CH2CHN), 28.5
(NCHCH2CH2CH2CH2CHN), 53.3 (NCHCH2CH2CH2CH2CHN), 15.4 (CH2C6H4CH2CH3-4), 16.6
(CH2C6H4CH2CH3-4), 66.4 (CH2C6H4CH2CH3-4), 127.4, 128.3, 132.2 and 144.3 (CH2C6H4CH2CH3-4). Anal.
Calc. for C25H33AgClN2: C, 59.46; H, 6.54; N, 5.55. Found: C,59.55; H, 6.62; N, 5.48%.
2.2.2. Chloro-1,3-bis(2,4-dimethylbenzyl)perhydrobenzimidazol-2-ylidenesilver(I), 2b
Yield: 0.46 g, 85%; en: 184-185 oC. 1H NMR (CDCl3) δ: 1.11-1.38 (m, 4H,
NCHCH2CH2CH2CH2CHN), 1.70-1.79 and 1.86-1.90 (m, 4H, NCHCH2CH2CH2CH2CHN), 3.04-3.16 (m, 2H,
NCHCH2CH2CH2CH2CHN), 2.32 and 2.33 (s, 12H, CH2C6H3(CH3)2-2,4), 4.50, 4.72, 4.90 and 5.02 (d, 4H, J
= 15.1 Hz, CH2C6H3(CH3)2-2,4), 6.87-7.28 (m, 6H, CH2C6H3(CH3)2-2,4). 13C NMR (CDCl3) δ: 23.4
(NCHCH2CH2CH2CH2CHN), 28.4 (NCHCH2CH2CH2CH2CHN), 48.2 (NCHCH2CH2CH2CH2CHN), 20.9 and
21.0 (CH2C6H3(CH3)2-2,4), 67.7 (CH2C6H3(CH3)2-2,4), 126.9, 128.7, 130.2, 131.1, 135.6 and 137.7
(CH2C6H3(CH3)2-2,4). Anal. Calc. for C25H33AgClN2: C, 59.46; H, 6.54; N, 5.55. Found: C,59.54; H, 6.59;
N, 5.51%.
2.2.3. Chloro-1,3-bis(3,4-dimethoxybenzyl)perhydrobenzimidazol-2-ylidenesilver(I), 2c
Yield: 0.46 g, 82%; en: 186-187 oC. 1H NMR (CDCl3) δ: 1.08-1.28 (m, 4H,
NCHCH2CH2CH2CH2CHN), 1.72-1.78 and 2.04-2.08 (m, 4H, NCHCH2CH2CH2CH2CHN), 2.90-2.93 (m, 2H,
NCHCH2CH2CH2CH2CHN), 3.88 and 3.89 (s, 12H, CH2C6H3(OCH3)2-3,4), 4.57 and 4.85 (d, 4H, J = 15.0 Hz,
CH2C6H3(OCH3)2-3,4), 6.82-6.88 (m, 6H, CH2C6H3(OCH3)2-3,4). 13C NMR (CDCl3) δ: 23.8
(NCHCH2CH2CH2CH2CHN), 28.1 (NCHCH2CH2CH2CH2CHN), 53.3 (NCHCH2CH2CH2CH2CHN), 55.9 and
56.1 (CH2C6H3(OCH3)2-3,4), 66.4 (CH2C6H3(OCH3)2-3,4), 110.9, 111.2, 120.2, 127.5, 148.9 and 149.3
(CH2C6H3(OCH3)2-3,4). Anal. Calc. for C25H33AgClN2O4: C, 52.77; H, 5.80; N,4.92. Found: C,52.82; H,
5.74; N, 4.96%.
2.2.4. Chloro-1,3-bis(2,4,5-trimethoxybenzyl)perhydrobenzimidazol-2-ylidenesilver(I), 2d

Yield: 0.71 g, 81%; en: 173-175 oC. 1H NMR (CDCl3) δ: 1.33-1.48 (m, 4H,
NCHCH2CH2CH2CH2CHN), 1.68-1.72 and 1.76-1.81 (m, 4H, NCHCH2CH2CH2CH2CHN), 3.57-3.62 (m, 2H,
NCHCH2CH2CH2CH2CHN), 3.81, 3.88 and 3.90 (s, 18H, CH2C6H2(OCH3)3-2,4,5), 4.50 ve 4.82 (d, 4H, J =
14.7 Hz, CH2C6H2(OCH3)3-2,4,5), 6.49 and 6.84 (s, 4H, CH2C6H2(OCH3)3-2,4,5). 13C NMR (CDCl3) δ: 23.2
(NCHCH2CH2CH2CH2CHN), 28.5 (NCHCH2CH2CH2CH2CHN), 44.3 (NCHCH2CH2CH2CH2CHN), 56.2, 56.6
and 56.9 (CH2C6H2(OCH3)3-2,4,5), 58.8 (CH2C6H2(OCH3)3-2,4,5), 97.4, 113.7, 115.1, 143.0, 149.7 and
151.9 (CH2C6H2(OCH3)3-2,4,5). Anal. Calc. for C27H37AgClN2O6: C, 51.55; H, 5.88; N,4.45. Found: C,
51.59; H, 5.84; N, 4.48%.
2.2.5. Chloro-1,3-bis(2,3,4-trimethoxybenzyl)perhydrobenzimidazol-2-ylidenesilver(I), 2e
Yield: 0.61 g, 79%; en: 160-162 oC. 1H NMR (CDCl3) δ: 1.15-1.36 (m, 4H,
NCHCH2CH2CH2CH2CHN), 1.67-1.71 and 1.74-1.78 (m, 4H, NCHCH2CH2CH2CH2CHN), 3.80-3.84 (m, 2H,
NCHCH2CH2CH2CH2CHN), 3.86, 3.87 and 3.90 (s, 18H, CH2C6H2(OCH3)3-2,3,4), 4.42 and 4.93 (d, 4H, J =
15.0 Hz, CH2C6H2(OCH3)3-2,3,4), 6.62-6.67 and 6.96-7.02 (m, 4H, CH2C6H2(OCH3)3-2,3,4). 13C NMR
(CDCl3)
δ:
24.0
(NCHCH2CH2CH2CH2CHN),
28.3
(NCHCH2CH2CH2CH2CHN),
45.6
(NCHCH2CH2CH2CH2CHN), 56.0, 58.7 and 60.8 (CH2C6H2(OCH3)3-2,3,4), 67.0 (CH2C6H2(OCH3)3-2,3,4),
107.2, 121.2, 123.8, 142.1, 151.9 and 153.8 (CH2C6H2(OCH3)3-2,3,4). Anal. Calc. for C27H37AgClN2O6: C,
51.55; H, 5.88; N,4.45. Found: C, 51.61; H, 5.82; N, 4.50%.
2.3. Synthesis of palladium(II)-NHC complexes, 3
A solution of required silver(I)-NHC complex (0.82 mmol) and PdCl2(PhCN)2 (0.41 mmol) in
dichloromethane (20 mL) was stirred for 24 h at room temperature in the dark. Then, the resulting
mixture was filtered through celite, and the solvent was removed under reduced pressure. The crude
product was recrystallized from dichloromethane:diethyl ether (1:2) at room temperature. The white
crystals were filtered off, washed with diethyl ether (3 x 10 mL) and dried under vacuum.
2.3.1. Bis[1,3-bis(4-ethylbenzyl)perhydrobenzimidazol-2-ylidene]dichloropalladium(II), 3a
o
1
Yield: 0.31 g, 85%, mp 255 C. IR, υ: 1514 cm-1 (NCN). H NMR (CDCI3) δ: 0.98-1.16 (m, 8H,
NCHCH2CH2CH2CH2CHN), 1.58-1.60 and 1.84-1.86 (m, 8H, NCHCH2CH2CH2CH2CHN), 2.88-2.90 (m, 4H,
NCHCH2CH2CH2CH2CHN), 1.21 (t, 12H, J = 7.8 Hz, CH2C6H4CH2CH3-4), 2.61 (q, 8H, J = 7.5 Hz,
CH2C6H4CH2CH3-4), 4.96, 5.08, 5.41 and 5.54 (d, 8H, J = 15.3 Hz, CH2C6H4CH2CH3-4), 7.07 and 7.48 (d,

16H, J = 7.8 Hz, CH2C6H4CH2CH3-4). 13C NMR (CDCI3) δ: 23.9 (NCHCH2CH2CH2CH2CHN), 28.5
(NCHCH2CH2CH2CH2CHN), 51.9 (NCHCH2CH2CH2CH2CHN), 15.4 (CH2C6H4CH2CH3-4), 18.9
(CH2C6H4CH2CH3-4), 66.3 (CH2C6H4CH2CH3-4), 127.7, 128.4, 133.6 and 143.1 (CH2C6H4CH2CH3-4), 203.8
(Pd-C). Anal. Calc. for C50H64N4PdCI2: C, 66.85; H, 7.13; N, 6.23. Found: C, 66.89; H, 7.17; N, 6.21%.
2.3.2. Bis[1,3-bis(2,4-dimethylbenzyl)perhydrobenzimidazol-2-ylidene]dichloropalladium(II), 3b
o
1
Yield: 0.32 g, 84%, mp 290-292 C. IR, υ: 1497 cm-1 (NCN). H NMR (CDCI3) δ: 0.97-1.28 (m,
8H, NCHCH2CH2CH2CH2CHN), 1.55-1.64 (m, 8H, NCHCH2CH2CH2CH2CHN), 2.96-2.99 (m, 4H,
NCHCH2CH2CH2CH2CHN), 2.08 and 2.27 (s, 24H, CH2C6H3(CH3)2-2,4), 4.83, 4.95, 5.13 and 5.21 (d, 8H, J
= 16.2 Hz, CH2C6H3(CH3)2-2,4), 6.8 (s, 4H, CH2C6H3(CH3)2-2,4), 6.97 and 7.45 (d, 8H, J = 7.5 Hz and, J =
7.8 Hz, CH2C6H3(CH3)2-2,4). 13C NMR (CDCI3) δ: 23.9 (NCHCH2CH2CH2CH2CHN), 28.2
(NCHCH2CH2CH2CH2CHN), 48.7 (NCHCH2CH2CH2CH2CHN), 19.3 and 20.9 (CH2C6H3(CH3)2-2,4), 67.4
(CH2C6H3(CH3)2-2,4), 126.6, 128.3, 130.5, 131.6, 135.0 and 136.1 (CH2C6H3(CH3)2-2,4), 205.2 (Pd-C).
Anal. Calc. for C50H64N4PdCI2: C, 66.85; H, 7.13; N, 6.23. Found: C, 66.87; H, 7.18; N, 6.25%.
2.3.3. Bis[1,3-bis(3,4-dimethoxybenzyl)perhydrobenzimidazol-2-ylidene]dichloropalladium(II), 3c
o
1
Yield: 0.53 g, 79%, mp 249-250 C. IR, υ: 1513 cm-1 (NCN). H NMR (CDCI3) δ: 0.98-1.26 (m,
8H, NCHCH2CH2CH2CH2CHN), 1.59-1.62 and 1.86-1.90 (m, 8H, NCHCH2CH2CH2CH2CHN), 2.82-2.88 (m,
4H, NCHCH2CH2CH2CH2CHN), 3.84 and 3.87 (s, 24H, CH2C6H3(OCH3)2-3,4), 4.87, 5.02, 5.44 and 5.65 (d,
8H, J = 15.0 Hz, CH2C6H3(OCH3)2-3,4), 6.65-6.92 and 7.28-7.43 (m, 12H, CH2C6H3(OCH3)2-3,4). 13C NMR
(CDCI3)
δ:
23.9
(NCHCH2CH2CH2CH2CHN),
28.1
(NCHCH2CH2CH2CH2CHN),
51.9
(NCHCH2CH2CH2CH2CHN), 55.7 and 56.5 (CH2C6H3(OCH3)2-3,4), 66.1 (CH2C6H3(OCH3)2-3,4), 110.4,
111.4, 120.2, 128.9, 148.3 and 149.2 (CH2C6H3(OCH3)2-3,4), 203.3 (Pd-C). Anal. Calc. for
C50H64N4O8PdCI2: C, 58.50; H, 6.24; N, 5.46. Found: C, 58.55; H, 6.27; N, 5.49%.
2.3.4. Bis[1,3-bis(2,4,5-trimethoxybenzyl)perhydrobenzimidazol-2-ylidene]dichloropalladium(II), 3d
o
1
Yield: 0.35 g, 75%, mp 258-260 C. IR, υ: 1511 cm-1 (NCN). H NMR (CDCI3) δ: 0.98-1.16 (m,
8H, NCHCH2CH2CH2CH2CHN), 1.47-1.58 and 1.81-1.85 (m, 8H, NCHCH2CH2CH2CH2CHN), 3.45-3.46 (m,
4H, NCHCH2CH2CH2CH2CHN), 3.72, 3.84 and 3.85 (s, 36H, CH2C6H2(OCH3)3-2,4,5), 5.03 and 5.53 (d, 8H,
J = 15.3 Hz, CH2C6H2(OCH3)3-2,4,5), 6.39 and 7.55 (s, 8H, CH2C6H2(OCH3)3-2,4,5). 13C NMR (CDCI3) δ:

23.6 (NCHCH2CH2CH2CH2CHN), 28.0 (NCHCH2CH2CH2CH2CHN), 43.9 (NCHCH2CH2CH2CH2CHN), 56.0,
56.6 and 56.8 (CH2C6H2(OCH3)3-2,4,5), 58.6 (CH2C6H2(OCH3)3-2,4,5), 97.1, 113.8, 117.0, 143.5, 148.5
and 150.6 (CH2C6H2(OCH3)3-2,4,5), 204.1 (Pd-C). Anal. Calc. for C54H72N4O12PdCI2: C, 56.56; H, 6.28; N,
4.88. Found: C, 56.59; H, 6.31; N, 4.92%.
2.3.5. Bis[1,3-bis(2,3,4-trimethoxybenzyl)perhydrobenzimidazol-2-ylidene]dichloropalladium(II), 3e
o
1
Yield: 0.48 g, 73%, mp 224-225 C. IR, υ: 1495 cm-1 (NCN). H NMR (CDCI3) δ: 0.89-1.13 (m,
8H, NCHCH2CH2CH2CH2CHN), 1.57-1.66 and 1.87-1.89 (m, 8H, NCHCH2CH2CH2CH2CHN), 3.50-3.55 (m,
4H, NCHCH2CH2CH2CH2CHN), 3.79, 3.81 and 3.82 (s, 36H, CH2C6H2(OCH3)3-2,3,4), 4.87 and 5.65 (d, 8H,
J = 15.3 Hz, CH2C6H2(OCH3)3-2,3,4), 6.59-6.66 and 7.65-7.70 (m, 8H, CH2C6H2(OCH3)3-2,3,4). 13C NMR
(CDCI3)
δ:
24.0
(NCHCH2CH2CH2CH2CHN),
27.9
(NCHCH2CH2CH2CH2CHN),
44.6
(NCHCH2CH2CH2CH2CHN), 55.8, 58.7 and 60.7 (CH2C6H2(OCH3)3-2,3,4), 66.8 (CH2C6H2(OCH3)3-2,3,4),
107.6, 122.7, 125.0, 141.5, 151.3 and 152.7 (CH2C6H2(OCH3)3-2,3,4), 204.8 (Pd-C). Anal. Calc. for
C54H72N4O12PdCI2: C, 56.56; H, 6.28; N, 4.88. Found: C, 56.60; H, 6.33; N, 4.91%.
2. 4. General procedure for the direct arylations
The heteroaryl derivative (2 mmol), aryl chloride (1 mmol) and KOAc (2 mmol) were
introduced in a Schlenk tube equipped with a magnetic stirring bar. Pd complex (0.01 mmol) and
DMAc (2 mL) were added and the Schlenk tube was purged several times with argon. The Schlenk
tube was placed in a preheated oil bath at 150 oC, the reaction mixture was stirred for 20 h. Then, the
reaction mixture was analysed by gas chromatography to determine the conversion of the aryl
chloride. The solvent was removed by heating the reaction vessel under vacuum and the residue was
charged directly onto a silica-gel column. The products were eluted by using an appropriate ratio of
diethyl ether and pentane.
3. Results and discussion
3.1. Synthesis of Ag(I) and Pd(II)-NHC complexes
The symmetrical 1,3-dialkylperhydrobenzimidazolium salts as NHC precursors were obtained
in high yields from the cyclization of the N,N’-dialkylcyclohexan-1,2-diamines with triethyl
orthoformate and ammonium chloride [82, 83]. Treatment of the perhydrobenzimidazolium salts

with Ag2O in dichloromethane at room temperature in the dark afforded the expected silver
complexes Ag(I)-NHC 2a-e (Scheme 1). The silver-NHC complexes 2a-e were obtained in high yields as
white solids, soluble in halogenated solvents. The formation of Ag(I)-NHC complexes 2a-e were
confirmed by the disappearance of the resonance signals of perhydrobenzimidazolium C2 proton in
1
the H NMR spectra and C2 carbon in 13C NMR spectra. In the 2a-e complexes, the resonances for
carbene carbon were not detected, which has also been mentioned in the literature and given as a
reason for the fluxional behavior of the NHC complexes [84, 85].
R
R
R
R
R
[PdCl2(PhCN)2], CH2Cl2
RT, 24 h
N
N
N
N
Ag2O, CH2Cl2
RT, 24 h
N
-
AgCl
PdCl2
+) Cl
N
2
R
3a-e
2a-e
1
MeO
a, R=
d, R=
OMe
OMe
b, R=
c, R=
MeO
e, R=
OMe
OMe
OMe
OMe
Scheme 1. The synthesis of palladium(II)-NHC complexes.
Several methods are available for the synthesis of Pd(II)-NHC complexes, which have been
described in the literature. Among them, a facile and more general method 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 [86]. The palladium(II)-NHC complexes 3a-e were prepared by treating PdCl2(PhCN)2 with
silver(I)-NHC complexes as a carbene transfer reagent in dichloromethane at room temperature
(Scheme 1). An instant precipitation of AgCl indicated successfully the carbene transfer onto

palladium to form palladium(II)-NHC complexes 3a-e. These complexes were obtained in high yields
and appeared to be stable both in solution and in solid states against air, light and moisture. The new
complexes were characterized by 1H NMR, 13C NMR, IR spectroscopy and elemental analysis
techniques, which support the proposed structures. They show a characteristic υ(NCN) band at 1514,
1497, 1513, 1511 and 1495 cm-1 for 3a, 3b, 3c, 3d and 3e 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 exhibited a singlet resonance of
the C2 carbon at 203.8, 205.2, 203.3, 204.1 and 204.8 ppm respectively for 3a-e. This is consistent
with reported values for [PdCl2(NHC)2] complexes. The results obtained from the elemental analysis
of the complexes 2a-e and 3a-e are in agreement with the theoretical requirements of their
structures. The geometry of these complexes was not defined, as suitable single crystals of these
complexes for X-ray diffraction studies could not be obtained.
3.2. Catalytic studies: Direct arylation of pyrroles with ary chlorides using 3a-e as catalyst
The arylation of pyrroles is an important field of research in organic synthesis due to their
biological and physical properties, and it provides a cost-effective and environmentally attractive
procedure for the preparation of arylpyrroles. The first examples of direct arylation of pyrroles by
using aryl chlorides as the coupling partners and Pd-NHC complexes as the catalyst was reported by
Doucet and co-workers [75]. They found that the treatment of electron-deficient aryl chlorides with
pyrroles derivatives in the presence of 1mol% a Pd-NHC complex gave the corresponding arylation
products in moderate to good yields. Based on previous results, in this study, N,N-dimethylacetamide
(DMAc) and potassium acetate (KOAc) were selected as the solvent and base. The catalytic reactions
o
were performed at 150 C for 20 h under argon in the presence of complexes 3a-e. Under these
reaction conditions, pyrrole derivatives were reacted with aryl chlorides bearing electron-
withdrawing groups at the para or ortho position to produce the arylated products in moderate to
good yields (Tables 1, 2). A wide range of functional groups on both pyrroles and aryl chlorides such
as aldehyde, acetyl, ester, cyano and trifluoromethyl were well tolerated during the course of this
reaction.
Table 1. Direct arylation of pyrrole derivatives with aryl chlorides.a

7-9
4-6
10-14
Entry
1
Pyrrole
ArCl
Pd-NHC
3a
Product
Yield(%)b
77
2
3b
3c
50
3
47
4
7
8
9
10
11
12
4
3d
3e
60
5
62
6
3a
74
7
3b
3c
76
8
80
4
9
3d
3e
68
10
11
12
13
14
15
16
17
18
75
3a
81
3b
3c
65
68
4
3d
3e
51
46
3a
96
3b
3c
93
93

19
20
3d
3e
93
92
5
7
13
21
22
23
24
25
3a
3b
3c
3d
3e
94
91
90
89
72
6
7
14
aReaction conditions: Pd-NHC (0.01 mmol), aryl chloride (1 mmol), pyrroles (2 mmol), KOAc (2 mmol),
DMAc (3 mL), 20 h, 150 oC. bGC yields using dodecane as internal standard in an average of two runs
and stucture of compound was checked by NMR.
Initially, the reaction of 1-methylpyrrole-2-carboxaldehyde (4) with 4-chlorobenzonitrile (7)
was examined in the presence of complexes 3a-e as catalysts and KOAc as base in N,N-
o
dimethylacetamide at 150 C. Regioselective arylation on only the C5 position of 1-methylpyrrole-2-
carboxaldehyde (4) was observed, because the hydrogen at the 2- and 5-position of pyrrole is more
reactive than that of 3- and 4-position [69], and good yields of the coupling product (10) were
obtained with complexes 3a and 3e (Table 1, entries 1 and 5). Then, 2-chlorobenzonitrile (8) and 4-
(trifluoromethyl)chlorobenzene (9) were reacted with 1-methylpyrrole-2-carboxaldehyde (4) in the
presence of complexes 3a-e (Table 1, entries 6-15). When the 2-chlorobenzonitrile (8) was used as
the coupling partners, the best yields (76% and 80%) were achieved with catalysts 3b and 3c to give
11. 4-(Trifluoromethyl)chlorobenzene (9) also gave the product 12 in good yields (Table 1, entries 11-
15). Next, we examined the reactions of 4-chlorobenzonitrile (7) with 2-acetyl-1-methylpyrrole (5)
and methyl 1-methylpyrrole-2-carboxylate (6) under the same reaction conditions (Table 1). The
reaction of 2-acetyl-1-methylpyrrole (5) with 4-chlorobenzonitrile (7) gave the expected product 13
in 92-96% yields (Table 1, entries 16-20). Methyl 1-methylpyrrole-2-carboxylate (6) also reacts with 4-
chlorobenzonitrile (7) to give 14 in 72-94% yields with complexes 3a-e (Table 1, entries 21-25).
Table 2. Direct arylation of 1-methylpyrrole and 1-phenylpyrrole with aryl chlorides.a

15, 16
7, 9
17-19
Entry
1
Pyrrole
ArCl
Pd-NHC
3a
Product
Yield(%)b
91
2
3b
3c
94
3
92
15
7
17
18
19
4
3d
3e
94
5
97
6
3a
83
7
3b
3c
76
8
79
7
16
9
3d
3e
80
10
11
12
13
14
15
94
3a
18
3b
3c
52
44
9
16
3d
3e
35
28
aReaction conditions: Pd-NHC (0.01 mmol), aryl chloride (1 mmol), pyrroles (2 mmol), KOAc (2 mmol),
DMAc (3 mL), 20 h, 150 oC. bGC yields using dodecane as internal standard in an average of two runs
and stucture of compound was checked by NMR.

Finally, we also investigated the arylation of the 1-methylpyrrole (15) and 1-phenylpyrrole
(16) with aryl chlorides. Under the same reaction conditions, only C2-arylated products were
obtained in good yields (Table 2, entries 1-15), bisarylated products were not isolated. 1-
Methylpyrrole was efficiently arylated with 4-chlorobenzonitrile in 91-97% yields, but slightly lower
yields (76-94%) were observed for 1-phenylpyrrole (Table 2, entries 6-10). Among the tested
complexes, palladium complexes (3c-e) with methoxy substituents on benzyl group exhibited high
catalytic activities compared to that bearing methyl or ethyl substituents (3a, b) for the arylation of
pyrrole derivatives and high yields were obtained with complexs 3c.
4. Conclusion
In summary, silver(I) and palladium(II) NHC complexes were successfully synthesized and
characterized by elemental analysis and spectroscopic methods. The catalytic activities of the
palladium complexes were investigated in direct C2 or C5 arylation of pyrrole derivatives in the
presence of potassium acetate. Although aryl chlorides possesed poor reactivity, arylation of pyrrole
derivatives proceeded efficiently with Pd(II)-NHC complexes, and all complexes exhibited high
catalytic activities in these reactions. Furthermore, the catalyst loading was lower, compared to
previous reports. This method is cost-effective and environmentally attractive for the preparation of
arylpyrroles because it is use inexpensive aryl chlorides and KOAc as the coupling partners and the
base, reduces the number of reaction steps and produces less waste. It should be noted that these
arylation reactions are very selective, such that in all cases, only the C2 or C5-arylated products were
formed; the 3- or 4-arylated products were not detected by GC analysis of the reaction mixtures.
Acknowledgements
We thank the Adıyaman University Research Fund (FEFMAP/2015-0004) and İnönü University
Research Fund (İÜBAP: 2015/41-Güdümlü) for financial support of this work
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• The new Ag(I) and Pd(II)-NHC complexes were prepared.
• These complexes were characterized by elemental analysis, IR, 1H, 13C NMR
pectroscopy.
• The new PdII complexes were tested as catalysts in direct arylation of pyrrole
derivatives.

ArCl
R
N
R1
Ar
Ar
R1
N
N
PdCl2
N
2
R
N
N
R2
R2
ArCl