Palladium(II)-N-Heterocyclic Carbene Complexes: Efficient Catalysts for the Direct C-H Bond Arylation of Furans with Aryl Halides

Article abstract of DOI:10.1002/aoc.4399

This paper contains the synthesis and characterization of the seven new benzimidazolium salts and their corresponding new palladium(II)-NHC complexes with the general formula [PdX₂(NHC)₂], (NHC?=?N-heterocyclic carbene, X?=?Cl or Br)

Full text of DOI:10.1002/aoc.4399

Received: 2 March 2018
DOI: 10.1002/aoc.4399
Revised: 28 March 2018
Accepted: 29 March 2018
F U L L P A P E R
Palladium(II)‐N‐Heterocyclic Carbene Complexes: Efficient
Catalysts for the Direct C‐H Bond Arylation of Furans with
Aryl Halides
Murat Kaloğlu1,2
| İsmail Özdemir1,2
1 Department of Chemistry, İnönü
University, Faculty of Science and Arts,
44280 Malatya, Turkey
2 Catalysis Research and Application
Center, İnönü University, 44280 Malatya,
Turkey
This paper contains the synthesis and characterization of the seven new
benzimidazolium salts and their corresponding new palladium(II)‐NHC com-
plexes with the general formula [PdX2(NHC)2], (NHC = N‐heterocyclic
carbene, X = Cl or Br), and also their catalytic activity in direct C‐H bond
arylation of 2‐substituted furan derivatives with aryl bromides and aryl chlo-
rides. Under the optimal conditions, these palladium(II)‐NHC complexes
showed the good catalytic performance for the direct C‐H bond arylation of
2‐substituted furans with (hetero)aryl bromides, and with readily available
and inexpensive aryl chlorides. The C‐H bond arylation regioselectively pro-
duced C5‐arylated furans by using 1 mol% of the palladium(II)‐NHC catalysts
in moderate to high yields.
Correspondence
İsmail Özdemir, İnönü University, Faculty
of Science and Arts, Department of
Chemistry, 44280 Malatya, Turkey; İnönü
University, Catalysis Research and
Application Center, 44280 Malatya,
Turkey.
Email: ismail.ozdemir@inonu.edu.tr
Funding information
KEYWORDS
Technological and Scientific Research
Council of Turkey (TÜBİTAK), Grant/
Award Number: 109T605; İnönü Univer-
sity Research Fund (İ.Ü. B.A.P.), Grant/
Award Number: 2015/41
benzimidazolium salt, direct C‐H bond arylation, furan, N‐Heterocyclic carbene, palladium
1 | INTRODUCTION
various methods have been developed for the direct
C‐H bond arylation of a variety of (hetero)arenes.[4,5]
Bi(hetero)aryl compounds are important structural moie-
ties with an extensive history of diverse applications in a
variety of fields, such as biology or material sciences,
since, numerous economically important pharmaceuti-
cals or agrochemicals have bi(hetero)aryl units as indis-
pensable substructures.[1–3] The traditional method for
the construction of bi(hetero)aryl moieties is transition
metal‐catalyzed cross‐coupling between aryl(pseudo)
halides and aryl organometallic reagents. However, the
syntheses of aryl organometallic reagents require a
number of steps from the arene and generate undesired
byproducts. Therefore, the direct coupling between
aryl(pseudo) halides and (hetero)arenes is advantageous
with respect to a minimization of reaction steps and a
reduction of byproduct formation. In the recent years,
Compared to the classical cross‐coupling reactions, direct
C‐H bond arylation not only initiate new stream for C‐C
bonds construction but also comply with the require-
ments of ‘green’ and ‘sustainable’ development in chemis-
try (Scheme 1). To date, palladium catalysis is the most
widely developed method for the direct C‐H bond
arylation reactions,[6] but several contributions have been
reported using ruthenium catalysis,[7] or other transitions
metals.[8] Transition metal‐free conditions for such cross‐
couplings have been also described.[9] Early examples of
palladium‐catalyzed direct arylation were reported by
Ames et al.[10] Later development in this area focused
on intermolecular direct arylation and expanded the sub-
strate scope to include various (hetero)arenes containing
directing groups and electron‐deficient arenes. In recent
Appl Organometal Chem. 2018;e4399.
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https://doi.org/10.1002/aoc.4399

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KALOĞLU AND ÖZDEMIR
SCHEME 1 Comparison of classical
cross‐coupling reactions and direct C‐H
bond arylation
years, a wide variety of studies in this field focused on the
developments of new C‐H bond transformations. After
this development, various methods for direct arylation
of other valuable heteroarenes such as pyrroles, azoles,
(benzo)thiophenes and (benzo)furans have been devel-
oped.[11–32]
The furans are very important backbones in organic
chemistry, and many of their derivatives possess medici-
nal and biological activities. Consequently, to develop
an efficient strategy for the synthesis of furan derivatives
is of great importance.[33] As selected examples,
Dantrolene is a muscle relaxant commonly used for the
treatment of life‐threatening complications during anes-
thesia,[34] Guanidine exhibits high Na+/H+ exchange iso-
form‐1 inhibitory activity,[35] and Bioymifi is a compound
that binds to the extracellular domain of the DR 5 recep-
tors and induces their aggregation[36] (Figure 1).
The direct arylation of furans generally gives the C2‐
and C5‐arylated products with high regioselectivity, since,
the C2‐ and C5‐positions of furan are the most electron‐
rich as a consequence of the resonance. The C3‐ and
C4‐positions of furan are neither electron‐rich nor elec-
tron poor and as such will only react under forcing
conditions when the C2‐ and C5‐positions are blocked
(Figure 2). Therefore, the formation of mixtures of C2‐
arylated furans and C2,C5‐diarylated furans was gener-
ally observed with unsubstituted furan. The use of a
blocking group at the C2‐position allowed the selective
production of C5‐monoarylated furans.[37]
The palladium‐catalyzed direct arylation of furans
with aryl halides at low catalyst loadings would provide
an economically and environmentally attractive proce-
dure for the preparation of such compounds. The first
example of palladium‐catalyzed direct arylation of furans
was reported in 1990 by Ohta et al. Using Pd(PPh3)4 as
the catalyst and KOAc as the base, the reaction of 4‐
bromobenzaldehyde with furan gave the C2‐arylated
furan in a low yield of 40%.[38] To date, series of elec-
tron‐rich furan derivatives have been activated success-
fully, often by using palladium species to catalyze the
direct arylation. Especially, Doucet and co‐workers con-
ducted an intense and fruitful activity in the area of the
palladium‐catalyzed intermolecular arylations of furan
derivatives with (hetero)aryl halides since 2009.[39] But,
only a few examples of palladium(II)‐NHC‐catalyzed
direct arylation of furans were found in the literature to
date.[40] In this connection, recently we have also
reported direct arylation of furans with aryl halides
FIGURE 1 Examples of biologically active furan derivatives
FIGURE 2 Most favourable positions of
furans for direct C‐H bond arylations

KALOĞLU AND ÖZDEMIR
3 of 15
catalyzed by palladium(II)‐NHC complexes.[41] In the
present article, we now described the synthesis and char-
acterization of seven new benzimidazolium salts (2a‐g)
and their corresponding seven new palladium(II)‐NHC
complexes (3a‐g) of the general formula [PdX2(NHC)2]
(Figure 3). All new compounds were characterized by
1H NMR, 13C NMR, FT‐IR spectroscopy and elemental
analysis techniques. The palladium(II)‐NHC complexes
were tested in the direct C5‐arylation of 2‐substituted
furans with aryl halides in moderate to high yields.
column of 30 m length, 0.32 mm diameter, and 0.25 μm
film thickness.
2.2 | General procedure for the
preparation of N‐(4‐phenoxybutyl)
benzimidazole, (1)
For the preparation of N‐(4‐phenoxybutyl)benzimidazole
(1), benzimidazole (5.90 g, 50.0 mmol) and potassium
hydroxide (2.80 g, 50.0 mmol) were dissolved in ethyl
alcohol (50 ml), the reaction mixture was stirred at room
temperature for 1 h. Then, 4‐phenoxybutyl bromide
(11.45 g, 50.0 mmol) was slowly added, and the solution
was heated to reflux for 5 h. The mixture cooled to room
temperature, the precipitated potassium bromide was
removed by filtration. The solvent was removed by distil-
lation. The crude product was then distilled under
vacuum.
2 | EXPERIMENTAL
2.1 | Materials
All manipulations were carried out under argon using
standard Schlenk line techniques. The solvents used were
purified by distillation over the drying agents indicated
and were transferred under argon. Elemental analyses
were performed by İnönü University Scientific and Tech-
nological Research Center (Malatya, Turkey). Melting
points were measures in open capillary tubes with an
Electrothermal‐9200 melting points apparatus. FT‐IR
spectra were recorded on ATR unit in the range of
400–4000 cm−1 with Perkin Elmer Spectrum 100
2.2.1 | N‐(4‐Phenoxybutyl)benzimidazole,
(1)
1
Yield: 9.72 g, 73%; H NMR (400 MHz, CDCl3, 25 °C): δ
(ppm) = 1.84 (pent, J = 5.9 Hz, 2H, CH2CH2CH2
CH2OC6H5); 2.13 (pent,
J
=
7.4 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.00 (t, J = 6.0 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.30 (t, J = 7.1 Hz, 2H,
CH2CH2CH2CH2OC6H5); 6.90 (d, J = 7.1 Hz, 2H, arom
CHs, CH2CH2CH2CH2OC6H4); 6.98 (t, J = 7.4 Hz, 1H,
arom. CH, CH2CH2CH2CH2OC6H4); 7.29–7.36, 7.45–7.46
and 7.84–7.86 (m, 6H, arom. CHs, NC6H4N and
CH2CH2CH2CH2OC6H4); 7.96 (s, 1H, NCHN). 13C NMR
1
Spectrofotometer. H NMR and 13C NMR spectra were
recorded using Bruker Avance AMX and Bruker Avance
III spectrometer operating at 300, 400 and 500 MHz (1H
NMR) and at 75, 100 and 125 MHz (13C NMR) in CDCl3.
The NMR studies were carried out in high‐quality 5 mm
NMR tubes. The chemical shifts (δ) are reported in ppm
relative to CDCl3. Coupling constants (J values) are given
in hertz. NMR multiplicities are abbreviated as follows:
s = singlet, d = doublet, t = triplet, pent = pentet,
m = multiplet. 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). All catalytic reactions were
monitored on an Agilent 6890 N GC and Schimadzu
2010 Plus GC–MS system by GC‐FID with an HP‐5
(100 MHz, CDCl3, 25 °C):
δ
(ppm)
=
26.5
(CH2CH2CH2CH2OC6H5); 27.0 (CH2CH2CH2CH2OC6H5);
44.9 (CH2CH2CH2CH2OC6H5); 67.0 (CH2CH2CH2
CH2OC6H5); 109.7, 114.4, 120.4, 120.9, 122.2, 123.0,
129.6, 133.8, 142.9, 158.7 (arom. Cs, NC6H4N and
CH2CH2CH2CH2OC6H5); 143.8 (NCHN). Elemental anal-
ysis calcd. (%) for C17H18N2O (Mr = 266.3): C 76.66, H
6.81, N 10.52; found (%): C 76.67, H 6.86, N 10.54.
2.3 | General procedure for the
preparation of benzimidazolium salts,
(2a‐g)
For the preparation of benzimidazolium salts (2a‐g), N‐
(4‐phenoxybutyl)benzimidazole, 1, (1.33 g, 5.0 mmol)
was dissolved in degassed dimethylformamide, (DMF),
(10 ml) and alkyl halide (5.0 mmol) was added at room
temperature. The reaction mixture was stirred at 80 °C
for 36 h. After completion of the reaction, the solvent
was removed by vacuum and diethyl ether (15 ml) was
added to obtain a white crystalline solid, which was
FIGURE 3 Palladium(II)‐NHC complexes 3a‐g synthesised and
assessed in the present study

4 of 15
KALOĞLU AND ÖZDEMIR
filtered off. The solid was washed with diethyl ether
(3 × 10 ml) and dried under vacuum. The crude product
was recrystallized from ethanol/diethyl ether mixture
(1:2, v/v) and completely dried under vacuum.
2.3.3 | 1,3‐Di(4‐phenoxybutyl)
benzimidazolium Bromide, (2c)
1
Yield: 2.30 g, 93%; H NMR (500 MHz, CDCl3, 25 °C): δ
(ppm)
=
1.98 (pent,
J
=
5.8 Hz, 4H,
CH2CH2CH2CH2OC6H5); 2.31 (pent, J = 7.5 Hz, 4H,
CH2CH2CH2CH2OC6H5); 4.06 (t, J = 5.8 Hz, 4H,
CH2CH2CH2CH2OC6H5); 4.74 (t, J = 7.5 Hz, 4H,
CH2CH2CH2CH2OC6H5); 6.83–7.76 (m, 14H, arom. CHs,
NC6H4N and CH2CH2CH2CH2OC6H5); 11.54 (s, 1H,
NCHN). 13C NMR (125 MHz, CDCl3, 25 °C): δ
2.3.1 | 1‐(4‐Phenoxybutyl)‐3‐(methyl)
benzimidazolium Bromide, (2a)
1
Yield: 1.53 g, 85%; H NMR (300 MHz, CDCl3, 25 °C): δ
(ppm)
=
1.89 (pent,
J
=
5.8 Hz, 2H,
(ppm)
=
26.1
(CH2CH2CH2CH2OC6H5);
26.4
CH2CH2CH2CH2OC6H5); 2.24 (pent, J = 7.2 Hz, 2H,
CH2CH2CH2CH2OC6H5); 3.96 (t, J = 5.7 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.20 (s, 3H, CH3); 4.67 (t,
J = 7.4 Hz, 2H, CH2CH2CH2CH2OC6H5); 6.73–7.70 (m,
9H, arom. CHs, NC6H4N and CH2CH2CH2CH2OC6H5);
11.21 (s, 1H, NCHN). 13C NMR (75 MHz, CDCl3,
25 °C): δ (ppm) = 26.2 (CH2CH2CH2CH2OC6H5); 26.4
(CH2CH2CH2CH2OC6H5); 47.3 (CH2CH2CH2CH2OC6H5);
66.7 (CH2CH2CH2CH2OC6H5); 113.2, 114.3, 120.8, 127.2,
129.5, 131.3, 158.5 (arom. Cs, NC6H4N and
CH2CH2CH2CH2OC6H5); 142.6 (NCHN). Elemental anal-
ysis calcd. (%) for C27H31BrN2O2 (Mr = 495.4): C 65.45, H
6.31, N 5.65; found (%): C 65.47, H 6.33, N 5.67.
(CH2CH2CH2CH2OC6H5);
33.9
(CH3);
47.4
(CH2CH2CH2CH2OC6H5); 66.7 (CH2CH2CH2CH2OC6H5);
113.0, 113.1, 114.4, 120.8, 127.3, 129.5, 131.1, 132.0, 158.5
(arom. Cs, NC6H4N and CH2CH2CH2CH2OC6H5); 142.8
(NCHN). Elemental analysis calcd. (%) for C18H21BrN2O
(Mr = 361.3): C 59.84, H 5.86, N 7.75; found (%): C
59.86, H 5.88, N 7.77.
2.3.4 | 1‐(4‐Phenoxybutyl)‐3‐(3,5‐
dimethylbenzyl)benzimidazolium Bro-
mide, (2d)
1
Yield: 2.07 g, 89%; H NMR (500 MHz, CDCl3, 25 °C): δ
(ppm)
=
1.99 (pent,
J
=
5.8 Hz, 2H,
CH2CH2CH2CH2OC6H5); 2.27 (s, 6H, CH2C6H3(CH3)2–
3,5); 2.35 (pent, J = 7.4 Hz, 2H, CH2CH2CH2CH2OC6H5);
4.05 (t, J = 5.8 Hz, 2H, CH2CH2CH2CH2OC6H5); 4.78 (t,
J = 7.4 Hz, 2H, CH2CH2CH2CH2OC6H5); 5.76 (s, 2H,
CH2C6H3(CH3)2–3,5); 6.82–7.76 (m, 12H, arom. CHs,
NC6H4N, CH2CH2CH2CH2OC6H5 and CH2C6H3(CH3)2–
3,5); 11.61 (s, 1H, NCHN). 13C NMR (125 MHz, CDCl3,
25 °C): δ (ppm) = 21.2 (CH2C6H3(CH3)2–3,5); 26.2
(CH2CH2CH2CH2OC6H5); 26.4 (CH2CH2CH2CH2OC6H5);
47.5 (CH2CH2CH2CH2OC6H5); 51.5 (CH2C6H3(CH3)2–
3,5); 66.7 (CH2CH2CH2CH2OC6H5); 113.1, 113.9, 114.4,
125.9, 127.2, 129.5, 130.9, 131.2, 131.5, 132.4, 139.1,
158.5 (arom. Cs, NC6H4N, CH2CH2CH2CH2OC6H5 and
CH2C6H3(CH3)2–3,5); 142.7 (NCHN). Elemental analysis
calcd. (%) for C26H29BrN2O (Mr = 465.4): C 67.10, H
6.28, N 6.02; found (%): C 67.12, H 6.30, N 6.05.
2.3.2 | 1‐(4‐Phenoxybutyl)‐3‐(2,2‐
diethoxyethyl)benzimidazolium Bromide,
(2b)
1
Yield: 2.08 g, 90%; H NMR (300 MHz, CDCl3, 25 °C): δ
(ppm) = 1.07 (t, J = 7.2 Hz, 6H, CH2CH(OCH2CH3)2);
1.90 (pent, J = 5.7 Hz, 2H, CH2CH2CH2CH2OC6H5);
2.26 (pent, J = 7.3 Hz, 2H, CH2CH2CH2CH2OC6H5);
3.61 and 3.71 (qq, J = 7.2, 1.6 Hz, 4H, CH2CH
(OCH2CH3)2); 3.98 (t,
J
=
5.6 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.63 (t, J = 7.4 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.72 (d, J = 4.2 Hz, 2H,
CH2CH(OCH2CH3)2); 4.96 (t,
J
=
4.2 Hz, 1H,
CH2CH(OCH2CH3)2); 6.76–7.79 (m, 9H, arom. CHs,
NC6H4N and CH2CH2CH2CH2OC6H5); 11.24 (s, 1H,
NCHN). 13C NMR (75 MHz, CDCl3, 25 °C): δ (ppm) = 15.2
(CH2CH(OCH2CH3)2); 26.1 (CH2CH2CH2CH2OC6H5);
26.3 (CH2CH2CH2CH2OC6H5); 47.3 (CH2CH2CH2CH2
2.3.5 | 1‐(4‐Phenoxybutyl)‐3‐(2,3,5,6‐
tetramethylbenzyl)benzimidazolium Chlo-
ride, (2e)
OC6H5);
50.0
(CH2CH(OCH2CH3)2);
64.7
1
(CH2CH(OCH2CH3)2); 66.7 (CH2CH2CH2CH2OC6H5);
100.1 (CH2CH(OCH2CH3)2); 112.5, 114.4, 114.9, 120.9,
126.8, 126.9, 129.5, 130.9, 132.5, 158.5 (arom. Cs,
NC6H4N and CH2CH2CH2CH2OC6H5); 143.4 (NCHN).
Elemental analysis calcd. (%) for C23H31BrN2O3
(Mr = 463.4): C 59.61, H 6.74, N 6.05; found (%): C
59.63, H 6.76, N 6.08.
Yield: 1.77 g, 79%; H NMR (500 MHz, CDCl3, 25 °C): δ
(ppm) = 1.95 (pent, J = 5.8 Hz, 2H, CH2CH2CH2
CH2OC6H5); 2.22 (pent,
J
=
7.4 Hz, 2H,
CH2CH2CH2CH2OC6H5); 2.25 and 2.26 (s, 12H,
CH2C6H(CH3)4–2,3,5,6); 4.04 (t, J = 5.8 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.84 (t, J = 7.4 Hz, 2H,
CH2CH2CH2CH2OC6H5); 5.88 (s, 2H, CH2C6H(CH3)4–

KALOĞLU AND ÖZDEMIR
5 of 15
2,3,5,6); 6.82 and 7.74 (d, J = 7.9 Hz, 4H, arom. CHs,
CH2CH2CH2CH2OC6H4); 7.25 (t, J = 7.9 Hz, 1H, arom.
CH, CH2CH2CH2CH2OC6H4); 7.08 (s, 1H, arom. CH,
CH2C6H(CH3)4–2,3,5,6); 6.94, 7.29, 7.48 and 7.59 (t,
J = 7.8 Hz, 4H, arom. CHs, NC6H4N); 11.34 (s, 1H,
NCHN). 13C NMR (125 MHz, CDCl3, 25 °C): δ
(ppm) = 16.2 and 20.6 (CH2C6H(CH3)4–2,3,5,6); 26.1
(CH2CH2CH2CH2OC6H5); 26.5 (CH2CH2CH2CH2OC6H5);
47.3 (CH2CH2CH2CH2OC6H5); 47.8 (CH2C6H(CH3)4–
2,3,5,6); 66.7 (CH2CH2CH2CH2OC6H5); 113.0, 113.7,
114.3, 120.8, 127.1, 127.7, 129.5, 131.5, 131.6, 133.6,
134.1, 135.1, 158.5 (arom. Cs, NC6H4N, CH2C6H(CH3)4–
2,3,5,6 and CH2CH2CH2CH2OC6H5); 143.4 (NCHN). Ele-
mental analysis calcd. (%) for C28H33ClN2O (Mr = 449.0):
C 74.90, H 7.41, N 6.24; found (%): C 74.92, H 7.43, N
6.26.
CH2CH2CH2CH2OC6H5 and CH2C6H4(Cl)‐2); 11.53 (s,
1H, NCHN). 13C NMR (125 MHz, CDCl3, 25 °C): δ
(ppm)
=
26.1
(CH2CH2CH2CH2OC6H5);
26.4
(CH2CH2CH2CH2OC6H5); 47.5 (CH2CH2CH2CH2OC6H5);
48.7 (CH2C6H4(Cl)‐2); 66.7 (CH2CH2CH2CH2OC6H5);
113.1, 113.8, 114.4, 120.8, 127.2, 127.3, 128.1, 129.5,
130.1, 130.2, 131.0, 131.2, 131.3, 131.4, 133.6, 158.5 (arom.
Cs, NC6H4N, CH2CH2CH2CH2OC6H5 and CH2C6H4(Cl)‐
2); 143.3 (NCHN). Elemental analysis calcd. (%) for
C24H24ClBrN2O (Mr = 471.8): C 61.10, H 5.13, N 5.94;
found (%): C 61.13, H 5.14, N 5.95.
2.4 | General procedure for the
preparation of palladium(II)‐NHC
complexes, (3a‐g)
All benzimidazolium salts were converted, with moder-
ated yields, into the palladium(II)‐NHC complexes (3a‐
g). A suspension of the benzimidazolium salt (1.0 mmol)
and Pd(OAc)2 (0.50 mmol) in degassed DMSO (3 mL) was
heated with vigorous stirring at 100 °C for 24 h. Volatiles
were removed in vacuo, and the residue was washed with
n‐pentane (2 × 5 ml). The crude product was dissolved
with CH2Cl2 then filtered through a pad of celite and sil-
ica gel to remove the unreacted Pd(OAc)2 and
benzimidazolium salt. Next, the crude complex was crys-
tallized from dichloromethane/diethyl ether mixture (1:2,
v/v) at room temperature and, completely dried under
vacuum. All palladium complexes were isolated as air‐
and moisture‐stable yellow solids and were isolated in
46–69% yields. All the new compounds were prepared
according to general reaction pathway depicted in
Scheme 2.
2.3.6 | 1‐(4‐Phenoxybutyl)‐3‐(3,4,5‐
trimethoxybenzyl)benzimidazolium
Bromide, (2f)
1
Yield: 2.11 g, 80%; H NMR (500 MHz, CDCl3, 25 °C): δ
(ppm)
=
1.96 (pent,
J
=
5.8 Hz, 2H,
CH2CH2CH2CH2OC6H5); 2.31 (pent, J = 7.2 Hz, 2H,
CH2CH2CH2CH2OC6H5); 3.85 and 3.79 (s, 9H,
CH2C6H2(OCH3)3–3,4,5); 4.03 (t, J = 5.8 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.74 (t, J = 7.4 Hz, 2H,
CH2CH2CH2CH2OC6H5); 5.78 (s, 2H, CH2C6H2(OCH3)3–
3,4,5); 6.81–7.72 (m, 11H, arom. CHs, NC6H4N,
CH2CH2CH2CH2OC6H5 and CH2C6H2(OCH3)3–3,4,5);
11.76 (s, 1H, NCHN). 13C NMR (125 MHz, CDCl3,
25 °C): δ (ppm) = 26.2 (CH2CH2CH2CH2OC6H5); 26.4
(CH2CH2CH2CH2OC6H5); 47.3 (CH2CH2CH2CH2OC6H5);
51.6
(CH2C6H3(CH3)2–3,5);
56.7
and
60.8
(CH2C6H2(OCH3)3–3,4,5); 66.7 (CH2CH2CH2CH2OC6H5);
106.1, 113.1, 113.7, 114.4, 120.9, 127.1, 128.4, 129.5, 131.3,
131.4, 138.5, 153.8, 158.5 (arom. Cs, NC6H4N,
CH2CH2CH2CH2OC6H5 and CH2C6H2(OCH3)3–3,4,5);
143.5 (NCHN). Elemental analysis calcd. (%) for
C27H31BrN2O4 (Mr = 527.4): C 61.48, H 5.92, N 5.31;
found (%): C 61.49, H 5.93, N 5.33.
2.4.1 | cis/trans‐Dibromo‐bis[1‐(4‐
phenoxybutyl)‐3‐(methyl)benzimidazol‐2‐
ylidene]palladium(II), (3a)
1
Yield: 0.206 g, 50%; H NMR (400 MHz, CDCl3, 25 °C): δ
(ppm) = 1.89 and 1.97 (pent, J = 5.9 Hz, 4H,
CH2CH2CH2CH2OC6H5);
2.42
and
2.44
(pent,
J = 7.2 Hz, 4H, CH2CH2CH2CH2OC6H5); 3.91 and 4.03
(t, J = 6.1 Hz, 4H, CH2CH2CH2CH2OC6H5); 4.17 and
4.30 (s, 6H, CH3); 4.79 and 4.85 (t, J = 7.5 Hz, 4H,
CH2CH2CH2CH2OC6H5); 6.78–6.80, 6.85–6.89, 7.15–7.23
and 7.29–7.35 (m, 18H, arom. CHs, NC6H4N and
CH2CH2CH2CH2OC6H5). 13C NMR (100 MHz, CDCl3,
2.3.7 | 1‐(4‐Phenoxybutyl)‐3‐(2‐
chlorobenzyl)benzimidazolium Bromide,
(2 g)
1
Yield: 1.95 g, 83%; H NMR (500 MHz, CDCl3, 25 °C): δ
(ppm)
=
1.99 (pent,
J
=
5.8 Hz, 2H,
25
°C):
δ
(ppm)
=
25.7
and
and
25.8
26.0
CH2CH2CH2CH2OC6H5); 2.34 (pent, J = 7.2 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.05 (t, J = 5.9 Hz, 2H,
CH2CH2CH2CH2OC6H5); 4.78 (t, J = 7.3 Hz, 2H,
CH2CH2CH2CH2OC6H5); 5.99 (s, 2H, CH2C6H4(Cl)‐2);
(CH2CH2CH2CH2OC6H5);
(CH2CH2CH2CH2OC6H5); 33.6 and 33.8 (CH3); 47.0 and
47.1 (CH2CH2CH2CH2OC6H5); 66.2 and 66.3
25.9
(CH2CH2CH2CH2OC6H5); 113.4, 113.5, 113.6, 113.7,
6.83–7.76
(m,
13H,
arom.
CHs,
NC6H4N,
119.6, 119.7, 121.9, 122.0, 128.4, 128.5, 133.5, 133.6,

6 of 15
KALOĞLU AND ÖZDEMIR
SCHEME 2 Synthesis of palladium(II)‐NHC complexes (3a‐g)
134.1, 134.2, 157.8, 157.9 (arom. Cs, NC6H4N and
CH2CH2CH2CH2OC6H5); 180.2 and 180.3 (Pd‐Ccarbene).
Elemental analysis calcd. (%) for C36H40Br2N4O2Pd
(Mr = 827.0): C 52.29, H 4.88, N 6.78; found (%): C
52.31, H 4.90, N 6.79.
(100 MHz, CDCl3, 25 °C): δ (ppm) = 15.3 and 15.4
(CH2CH(OCH2CH3)2); 26.6 and 26.7 (CH2CH2CH2
CH2OC6H5); 26.9 and 27.0 (CH2CH2CH2CH2OC6H5);
48.1 and 48.2 (CH2CH2CH2CH2OC6H5); 51.6 and 51.7
(CH2CH(OCH2CH3)2);
64.2
and
64.9
(CH2CH
(OCH2CH3)2); 67.2 and 67.4 (CH2CH2CH2CH2OC6H5);
102.1 and 102.5 (CH2CH(OCH2CH3)2); 109.9, 110.0,
112.6, 113.0, 114.5, 114.6, 120.6, 120.7, 122.6, 122.8,
129.4, 129.5, 134.4, 135.6, 158.8, 158.9 (arom. Cs, NC6H4N
and CH2CH2CH2CH2OC6H5); 181.5 and 181.7 (Pd‐
2.4.2 | cis/trans‐Dibromo‐bis[1‐(4‐
phenoxybutyl)‐3‐(2,2‐diethoxyethyl)
benzimidazol‐2‐ylidene] palladium(II), (3b)
1
Yield: 0.237 g, 46%; H NMR (400 MHz, CDCl3, 25 °C): δ
Ccarbene).
Elemental
analysis
calcd.
(%)
for
(ppm)
=
0.97 and 1.01 (t,
J
=
7.0 Hz, 12H,
C46H60Br2N4O6Pd (Mr = 1031.2): C 53.58, H 5.86, N
5.43; found (%): C 53.60, H 5.87, N 5.45.
CH2CH(OCH2CH3)2); 1.92 and 1.97 (pent, J = 5.7 Hz,
4H, CH2CH2CH2CH2OC6H5); 2.43 and 2.44 (pent,
J = 7.3 Hz, 4H, CH2CH2CH2CH2OC6H5); 3.36–3.49 and
3.65–3.75 (m, 8H, CH2CH(OCH2CH3)2); 3.92 and 3.99 (t,
J = 6.1 Hz, 4H, CH2CH2CH2CH2OC6H5); 4.84 and 4.85
(t, J = 7.3 Hz, 4H, CH2CH2CH2CH2OC6H5); 4.86 and
4.87 (d, J = 5.7 Hz, 4H, CH2CH(OCH2CH3)2); 5.49 and
5.54 (t, J = 5.6 Hz, 2H, CH2CH(OCH2CH3)2); 6.78–6.88,
7.15–7.22, 7.30–7.35 and 7.56–7.62 (m, 18H, arom. CHs,
NC6H4N and CH2CH2CH2CH2OC6H5). 13C NMR
2.4.3 | Dibromo‐bis[1,3‐di(4‐phenoxybutyl)
benzimidazol‐2‐ylidene]palladium(II), (3c)
1
Yield: 0.377 g, 69%; H NMR (500 MHz, CDCl3, 25 °C): δ
(ppm)
=
1.90 (pent,
J
=
6.5 Hz, 8H,
CH2CH2CH2CH2OC6H5); 2.43 (pent, J = 7.3 Hz, 8H,
CH2CH2CH2CH2OC6H5); 3.91 (t, J = 6.0 Hz, 8H,
CH2CH2CH2CH2OC6H5); 4.83 (t, J = 7.5 Hz, 8H,

KALOĞLU AND ÖZDEMIR
7 of 15
CH2CH2CH2CH2OC6H5); 6.77–6.87, 7.14–7.21 and 7.33–
NC6H4N, CH2CH2CH2CH2OC6H4 and CH2C6H(CH3)4–
7.36
(m,
28H,
arom.
CHs,
NC6H4N
and
2,3,5,6). 13C NMR (100 MHz, CDCl3, 25 °C):
δ
CH2CH2CH2CH2OC6H5). 13C NMR (125 MHz, CDCl3,
25 °C): δ (ppm) = 26.7 (CH2CH2CH2CH2OC6H5); 27.1
(CH2CH2CH2CH2OC6H5); 48.2 (CH2CH2CH2CH2OC6H5);
67.2 (CH2CH2CH2CH2OC6H5); 110.5, 114.5, 120.7, 122.9,
129.5, 134.7, 158.8 (arom. Cs, NC6H4N and
CH2CH2CH2CH2OC6H5); 181.2 (Pd‐Ccarbene). Elemental
analysis calcd. (%) for C54H60Br2N4O4Pd (Mr = 1095.3):
C 59.21, H 5.52, N 5.12; found (%): C 59.23, H 5.55, N
5.14.
(ppm) = 16.7 and 20.6 (CH2C6H(CH3)4–2,3,5,6); 27.1
(CH2CH2CH2CH2OC6H5); 29.7 (CH2CH2CH2CH2OC6H5);
47.9 (CH2CH2CH2CH2OC6H5); 50.9 (CH2C6H(CH3)4–
2,3,5,6); 67.3 (CH2CH2CH2CH2OC6H5); 112.0, 114.4,
114.5, 120.6, 120.7, 122.9, 129.4, 129.5, 134.3, 134.8,
134.9, 135.0, 158.9 (arom. Cs, NC6H4N, CH2C6H(CH3)4–
2,3,5,6 and CH2CH2CH2CH2OC6H5); 181.9 (Pd‐Ccarbene).
Elemental analysis calcd. (%) for C56H64Cl2N4O2Pd
(Mr = 1002.5): C 67.09, H 6.43, N 5.59; found (%): C
67.15, H 6.48, N 5.62.
2.4.4 | cis/trans‐Dibromo‐bis[1‐(4‐
phenoxybutyl)‐3‐(3,5‐dimethylbenzyl)
benzimidazol‐2‐ylidene] palladium(II), (3d)
2.4.6 | cis/trans‐Dibromo‐bis[1‐(4‐
phenoxybutyl)‐3‐(3,4,5‐trimethoxybenzyl)
benzimidazol‐2‐ylidene] palladium(II), (3f)
1
Yield: 0.238 g, 46%; H NMR (400 MHz, CDCl3, 25 °C): δ
1
(ppm) = 1.61 and 1.69 (pent, J = 6.0 Hz, 4H,
CH2CH2CH2CH2OC6H5); 2.11 and 2.18 (s, 12H,
CH2C6H3(CH3)2–3,5); 2.26 and 2.28 (pent, J = 7.3 Hz,
4H, CH2CH2CH2CH2OC6H5); 3.66 and 3.94 (t,
J = 6.0 Hz, 4H, CH2CH2CH2CH2OC6H5); 4.76 and 4.88
(t, J = 7.2 Hz, 4H, CH2CH2CH2CH2OC6H5); 5.79 and
6.00 (s, 4H, CH2C6H3(CH3)2–3,5); 6.73–6.77, 6.85–6.88,
7.01–7.18 and 7.20–7.24 (m, 24H, arom. CHs, NC6H4N,
CH2CH2CH2CH2OC6H5 and CH2C6H3(CH3)2–3,5). 13C
NMR (100 MHz, CDCl3, 25 °C): δ (ppm) = 20.1 and
Yield: 0.278 g, 48%; H NMR (400 MHz, CDCl3, 25 °C): δ
(ppm) = 1.68 and 1.95 (pent, J = 6.0 Hz, 4H,
CH2CH2CH2CH2OC6H5);
2.32
and
2.47
(pent,
J = 7.2 Hz, 4H, CH2CH2CH2CH2OC6H5); 3.68, 3.72, 3.73
and 3.74 (s, 18H, CH2C6H2(OCH3)3–3,4,5); 3.84 and 3.95
(t, J = 6.0 Hz, 4H, CH2CH2CH2CH2OC6H5); 4.82 and
4.91 (t, J = 7.4 Hz, 4H, CH2CH2CH2CH2OC6H5); 5.89
and 6.04 (s, 4H, CH2C6H2(OCH3)3–3,4,5); 6.75–6.88,
7.08–7.20 and 7.31–7.36 (m, 22H, arom. CHs, NC6H4N,
CH2CH2CH2CH2OC6H5 and CH2C6H2(OCH3)3–3,4,5).
13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) = 25.7 and
20.2
(CH2C6H3(CH3)2–3,5);
25.6
and
25.7
(CH2CH2CH2CH2OC6H5); 25.9 and 26.0 (CH2CH2CH2
CH2OC6H5); 47.1 and 47.2 (CH2CH2CH2CH2OC6H5);
51.5 and 51.7 (CH2C6H3(CH3)2–3,5); 66.1 and 66.3
(CH2CH2CH2CH2OC6H5); 109.2, 109.3, 113.4, 113.5,
121.7, 121.8, 124.3, 124.4, 124.5, 124.7, 124.8, 124.9,
128.3, 128.4, 128.5, 128.6, 133.4, 133.5, 133.8, 133.9,
134.2, 134.3, 137.0, 137.1, 137.2, 137.3, 138.2, 138.3,
157.9, 158.0 (arom. Cs, NC6H4N, CH2CH2CH2CH2OC6H5
and CH2C6H3(CH3)2–3,5); 180.7 and 180.8 (Pd‐Ccarbene).
Elemental analysis calcd. (%) for C52H56Br2N4O2Pd
(Mr = 1035.3): C 60.33, H 5.45, N 5.41; found (%): C
60.35, H 5.47, N 5.43.
25.9
(CH2CH2CH2CH2OC6H5);
26.0
and
26.2
(CH2CH2CH2CH2OC6H5); 46.8 and 46.9 (CH2CH2
CH2CH2OC6H5); 51.7 and 51.8 (CH2C6H3(CH3)2–3,5);
55.4, 55.6, 59.7 and 59.8 (CH2C6H2(OCH3)3–3,4,5); 66.1
and 66.2 (CH2CH2CH2CH2OC6H5); 103.6, 103.7, 109.4,
110.3, 113.4, 113.5, 119.6, 119.7, 122.1, 122.2, 128.4,
128.5, 130.2, 130.4, 133.0, 133.2, 133.6, 133.7,
152.4,152.7, 157.8, 157.9 (arom. Cs, NC6H4N,
CH2CH2CH2CH2OC6H5 and CH2C6H2(OCH3)3–3,4,5);
180.6 and 180.7 (Pd‐Ccarbene). Elemental analysis calcd.
(%) for C54H60Br2N4O8Pd (Mr = 1159.3): C 55.95, H
5.22, N 4.83; found (%): C 55.97, H 5.23, N 4.85.
2.4.5 | Dichloro‐bis[1‐(4‐phenoxybutyl)‐3‐
(2,3,5,6‐tetramethylbenzyl)benzimidazol‐2‐
ylidene] palladium(II), (3e)
2.4.7 | cis/trans‐Dibromo‐bis[1‐(4‐
phenoxybutyl)‐3‐(2‐chlorobenzyl)
benzimidazol‐2‐ylidene] palladium(II),
(3 g)
1
Yield: 0.275 g, 55%; H NMR (400 MHz, CDCl3, 25 °C): δ
1
(ppm)
=
1.93 (pent,
J
=
6.0 Hz, 4H,
Yield: 0.241 g, 46%; H NMR (500 MHz, CDCl3, 25 °C): δ
CH2CH2CH2CH2OC6H5); 2.17 and 2.24 (s, 24H,
CH2C6H(CH3)4–2,3,5,6); 2.47 (pent, J = 7.2 Hz, 4H,
CH2CH2CH2CH2OC6H5); 3.94 (t, J = 5.8 Hz, 4H,
CH2CH2CH2CH2OC6H5); 4.96 (t, J = 7.1 Hz, 4H,
CH2CH2CH2CH2OC6H5); 6.28 (s, 4H, CH2C6H(CH3)4–
2,3,5,6); 6.31–6.35 and 6.70–7.29 (m, 20H, arom. CHs,
(ppm) = 1.72 and 2.04 (pent, J = 5.9 Hz, 4H,
CH2CH2CH2CH2OC6H5);
2.35
and
2.57
(pent,
J = 7.4 Hz, 2H, CH2CH2CH2CH2OC6H5); 3.78 and 4.05
(t, J = 6.0 Hz, 4H, CH2CH2CH2CH2OC6H5); 4.84 and
4.97 (t, J = 7.5 Hz, 4H, CH2CH2CH2CH2OC6H5); 6.05
and 6.32 (s, 4H, CH2C6H4(Cl)‐2); 6.87 and 6.92 (d,

8 of 15
KALOĞLU AND ÖZDEMIR
J
=
7.8
CH2CH2CH2CH2OC6H5 and CH2C6H4(Cl)‐2); 6.97 (t,
7.3 Hz, 2H, arom. CHs, NC6H4N,
Hz,
8H,
arom.
CHs,
NC6H4N,
1493 cm−1 for the ‐C=N‐ bond vibration of 1. (For the
NMR and FT‐IR spectrum of 1, see SI, pp. S1–S2).
Obtained spectroscopic values are consistent with those
found in the literature for other N‐(alkyl)benzimidazole
compounds.[42]
J
=
CH2CH2CH2CH2OC6H5 and CH2C6H4(Cl)‐2); 7.14–7.47
(m, 16H, arom. CHs, NC6H4N, CH2CH2CH2CH2OC6H5
and CH2C6H4(Cl)‐2). 13C NMR (125 MHz, CDCl3,
25
°C):
δ
(ppm)
=
26.7
and
and
and
26.8
27.1
49.4
3.2 | Synthesis of benzimidazolium salts
(CH2CH2CH2CH2OC6H5);
(CH2CH2CH2CH2OC6H5);
26.9
48.2
The benzimidazolium salts 2a‐g were synthesized by the
reaction of N‐(4‐phenoxybutyl)benzimidazole (1) with
different alkyl halides in anhydrous dimethylformamide
(DMF) at 80 °C for 36 h. All compounds were isolated
as air‐ and moisture‐stable crystalline solids in high yields
(79–93%). The structures of the 2a‐g were determined by
their characteristic spectroscopic data and elemental
analyses. In the 13C NMR spectra of 2a‐g, the characteris-
tic peak of the benzimidazolium C(2)‐carbon, (NCHN),
resonance were detected as typical singlets at between δ
(CH2CH2CH2CH2OC6H5); 49.0 and 49.4 (CH2C6H4(Cl)‐
2); 67.1 and 67.3 (CH2CH2CH2CH2OC6H5); 110.4, 110.5,
110.9, 111.0, 120.5, 120.7, 123.2, 123.3, 127.2, 127.4,
128.9, 129.0, 129.3, 129.4, 129.5, 129.6, 132.1, 132.2,
132.7, 132.8, 134.2, 134.4, 134.7, 134.9, 158.8, 158.9 (arom.
Cs, NC6H4N, CH2CH2CH2CH2OC6H5 and CH2C6H4(Cl)‐
2); 182.2 and 182.5 (Pd‐Ccarbene). Elemental analysis calcd.
(%) for C48H46Br2Cl2N4O2Pd (Mr = 1048.0): C 55.01, H
4.42, N 5.35; found (%): C 55.03, H 4.45, N 5.37.
1
= 142.6–143.5 ppm. The H NMR spectra of the 2a‐g fur-
ther supported the assigned structures, the resonances for
benzimidazolium C(2)H‐proton (NCHN) were observed
as sharp singlets at between δ = 11.21–11.76 ppm. The
formation of the 2a‐g were also evident through their
FT‐IR spectra, which showed peaks v(C=N) at between
1556–1565 cm−1 for the ‐C=N‐ bond vibration of 2a‐g.
(For the NMR and FT‐IR spectrum of 2a‐g, see SI, pp.
S3‐S16). Obtained spectroscopic values are consistent
with those found in the literature for other
benzimidazolium salts.30–32, 41a
2.5 | General procedure for the direct
arylation of 2‐substituted furan derivatives
An oven dried 10 ml Schlenk tube was charged with
palladium(II)‐NHC complex (0.01 mmol), 2‐substituted
furan derivative (2.0 mmol), aryl halide (1.0 mmol),
KOAc (2.0 mmol), and DMAc (2 mL) under argon. The
Schlenk tube was placed in a preheated oil bath at
120 °C, and the reaction mixture was stirred for different
durations, as given in Table 3 and Table 4. Completion of
the reaction, the solvent was removed under vacuum and
the residue was charged directly onto a micro silica gel
column. The products were eluted by using n‐hexane/
diethyl ether mixture (5:1, v/v). The chemical character-
izations of the products were made by GC–MS spectrom-
etry. The conversions were based on the aryl halide by
GC and GC–MS.
3.3 | Synthesis of palladium(II)‐NHC
complexes
The palladium(II)‐NHC complexes 3a‐g were synthesized
by the reaction of benzimidazolium salts, (2a‐g), with
Pd(OAc)2 in degassed dimethyl sulfoxide (DMSO) at
100 °C for 24 h. The palladium(II)‐NHC complexes were
obtained as light yellow solids in 46–69% yields. The air‐
and moisture‐stable palladium(II)‐NHC complexes were
soluble in organic solvents such as acetone, dichloro-
methane, chloroform, DMF, ethanol and acetonitrile.
The structures of the 3a‐g complexes were also deter-
mined by their characteristic spectroscopic data and ele-
3 | RESULTS AND DISCUSSION
3.1 | Synthesis of N‐(4‐phenoxybutyl)
benzimidazole
The N‐(4‐phenoxybutyl)benzimidazole (1) were synthe-
sized by the reaction of benzimidazole with 4‐
phenoxybutyl bromide in ethyl alcohol at 78 °C for 5 h.
Compound 1 was isolated as a viscous liquid in moderate
yield (73%). The compound 1 was fully characterised by
elemental analysis, 1H and 13C NMR and FT‐IR spectros-
copies. In the 13C NMR spectra of 1, the characteristic
peak of the imino carbon, (NCHN), resonance was
detected as typical singlet at δ = 143.8 ppm. The signal
of the NCHN proton was also detected as singlet at δ =
7.96 ppm for 1. FT‐IR spectra showed a broad band at
1
mental analyses. In the H NMR and 13C NMR spectra
of the all palladium(II)‐NHC complexes, loss of the
benzimidazolium
C(2)H‐proton
(NCHN)
and
benzimidazolium C(2)‐carbon (NCHN) signal suggests
the formation of the palladium(II)‐NHC complexes. The
characteristic peak of the Pd‐Ccarbene resonance for all
palladium(II)‐NHC complexes were detected at between
δ = 180.2–182.5 ppm, however, NMR studies showed that
all palladium(II)‐NHC complexes except 3c and 3e were a
mixture of cis/trans isomers in an approximate 40:60

KALOĞLU AND ÖZDEMIR
9 of 15
ratio. The results of the elemental analysis were in good
agreement with the theoretical values. Palladium(II)‐
NHC complexes exhibit a characteristic v(NCN) band typi-
cally at between 1395–1411 cm−1. (For the NMR and FT‐
IR spectrum of 3a‐g, see SI, pp. S17–S30). The spectro-
scopic data are similar to those found for other
reaction time and reaction temperature are given in
Table 2. The chemical characterizations of the products
were made by GC–MS spectrometry. The conversions
were based on the aryl halide by GC and GC–MS.
The
arylation
of
2‐furaldehyde
with
4‐
bromoacetophenone was first carried out at 150 °C for
2 h without the addition of 3c complex in order to exam-
ine the effect of catalyst on the reaction. As attempt, no
products were formed without the addition of 3c complex
(Table 2, entry 1). In the presence of 2 mol% of 3c 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 full conver-
sion with 91% isolated yield (Table 2, entry 2). When the
amount of 3c was decreased to 1 mol%, similar yield of
product was observed (Table 2, entry 3). Decreasing the
reaction temperature from 150 °C to 120 °C or 90 °C
had a detrimental effect on the conversion (Table 2,
entries 4 and 5). When the reaction time was reduced
from 2 h to 1 h at 120 °C, no noticeable effect on the con-
version with 83% isolated yield was observed (Table 2,
entry 6), but when the reaction time was reduced to
0.5 h, the conversion dropped to 43% with 31% isolated
yield (Table 2, entry 7). Finally, the best conditions
leading to 94% conversion (with 83% isolated yield) of
4‐bromoacetophenone with high selectivity in favor of
the C5‐arylated product were obtained, when the
reaction was carried out in DMAc/KOAc combination
in the presence of 1 mol% 3c catalyst at 120 °C for 1 h
(Table 2, entry 6).
41a
palladium(II)‐NHC complexes in the literature.30,
The analytical data are in good agreement with the com-
positions proposed for all the new compounds we pre-
pared, and are summarized in the Table 1.
3.4 | Direct arylation of 2‐substituted
furan derivatives
Our first objective was to determine the most suitable
reaction conditions using our palladium(II)‐NHC com-
plexes. For this purpose, we selected the complex 3c as
the model catalyst, and the 2‐furaldehyde as the model
heteroaromatic substrate with a blocked C2‐position.
We
used
the
4‐bromoacetophenone
or
4‐
chloroacetophenone as the model coupling partner. Also,
we selected DMAc as the solvent, and KOAc as the base,
because the DMAc/KOAc combination has been com-
monly used for the direct arylation of 5‐membered het-
erocycles.[39] We focused on the direct arylation at the
C5‐position of 2‐furaldehyde. The coupling reactions
were regioselective, and in almost all cases, only the C5‐-
arylated products were formed, namely, 5‐(4‐
acetylphenyl)‐2‐furaldehyde. The results of varying the
other reaction conditions including catalyst loading,
TABLE 1 Physical and spectroscopic properties of the new compoundsa
1
Compound Molecular formula Isolated yield [%] M.p. [oC] v(CN) [cm−1
]
H(2) H NMR [ppm] C(2) 13C NMR [ppm]
1
C17H18N2O
73
85
90
93
89
79
80
83
50
46
69
46
55
48
‐
1493
1563
1565
1559
1560
1556
1558
1557
1404
1407
1407
1404
1395
1411
1409
7.96
143.8
2a
2b
2c
2d
2e
2f
C18H21BrN2O
108–109
127–128
82–83
11.21
142.8
C23H31BrN2O3
C27H31BrN2O2
C26H29BrN2O
11.24
143.4
11.54
142.6
162–163
176–177
116–117
158–159
190–192
155–157
171–173
241–243
128–130
214–216
138–140
11.61
142.7
C28H33ClN2O
11.34
143.4
C27H31BrN2O4
C24H24ClBrN2O
C36H40Br2N4O2Pd
C46H60Br2N4O6Pd
C54H60Br2N4O4Pd
C52H56Br2N4O2Pd
C56H64Cl2N4O2Pd
C54H60Br2N4O8Pd
11.76
143.5
2 g
3a
3b
3c
3d
3e
3f
11.53
143.3
‐
‐
‐
‐
‐
‐
‐
180.2 and 180.3
181.5 and 181.7
181.2
180.7 and 180.8
181.9
180.6 and 180.7
182.2 and 182.5
3 g
C48H46Cl2Br2N4O2Pd 46
aAs previously reported by several groups,[43] NMR data showed that all complexes except 3c and 3e were cis/trans mixtures.

10 of 15
KALOĞLU AND ÖZDEMIR
TABLE 2 Influence of the reaction conditions for palladium(II)‐NHC‐catalyzed direct C5‐arylation of 2‐furaldehyde with 4‐
chloroacetophenone and 4‐bromoacetophenonea
Entry
3c [mol‐%]
X
Time [h]
Temperature [oC]
Conversion(Yield)b,c [%]
1
No
2
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
150
150
150
120
90
‐
2
2
100(91)
100(91)
97(87)
65(53)
94(83)
43(31)
‐
3
1
2
4
1
2
5
1
2
6
1
1
120
120
120
120
120
120
120
120
120
7
1
0.5
1
8
1
9
1
2
7(−)
10
11
12
13
14
1
4
25(12)
50(36)
65(47)
82(68)
87(70)
1
8
1
16
20
24
1
1
aConditions: 2‐Furaldehyde (2.0 mmol), aryl halide (1.0 mmol), KOAc (2.0 mmol), DMAc (2 ml).
bConversions were calculated with respect to aryl halide from the results of GC and GC–MS spectrometry.
cIsolated yields were shown in parentheses.
When the less reactive 4‐chloroacetophenone was
used as substrate in the presence of 1 mol% of 3c catalyst
and at 120 °C, the yields increased depending on the reac-
tion time (Table 2, entries 9, 10, 11, 12), while no conver-
sion was observed after lower reaction time such as 1 h
(Table 2, entry 8). Interestingly, up to 68% yield were
obtained, but for this, the reaction required a longer reac-
tion time of 20 h (Table 2, entry 13). When the reaction
time was increased from 20 h to 24 h at 120 °C, the very
close yield of 4‐chloroacetophenone was obtained
(Table 2, entry 14). The conditions are commonly used
for the direct arylation of 5‐membered heterocycles. For
this reason, the conditions are also consistent with the
literature.[39]
Encouraged by the above successful results, we tried
to evaluate the scope and limitations of the
palladium(II)‐NHC complexes 3a‐g for the direct
arylation of 2‐furaldehyde and 2‐n‐butylfuran with differ-
ent aryl halides (Table 3 and Table 4). All reactions
worked smoothly to give the desired C5‐arylated products
in moderate to high yields. A survey of coupling of (het-
ero)aryl bromides with 2‐furaldehyde and 2‐n‐butylfuran,
KOAc as the base in DMAc at 120 °C for 1 h is provided
in Table 3. A variety of functional groups on (hetero)aryl
bromide are tolerated. Quite similar yields were obtained
(69–82% for 2‐furaldehyde and 61–77% for 2‐n‐
butylfuran) using the neutral aryl bromide such as
bromobenzene (Table 3, entries 1–7). When the reaction
of 2‐furaldehyde and 2‐n‐butylfuran with electron‐rich
aryl bromide such as 4‐bromotoluene was investigated,
yields at between 69–82% and 58–78%, respectively
(Table 3, entries 8–14). The coupling of 2‐furaldehyde
and 2‐n‐butylfuran with electron‐poor aryl bromide such
as 4‐bromobenzaldehyde also proceeds nicely. 4‐
Bromobenzaldehyde gave the C5‐arylated furan with
67–85% and 60–75% yields, respectively (Table 3, entries
15–21). The electron‐poor 4‐bromoacetophenone was also
a good substrate to afford the corresponding products 5‐
(4‐acetylphenyl)‐2‐furaldehyde and 5‐(4‐acetylphenyl)‐2‐
n‐butylfuran at between 63–83% and 71–83% yields,
respectively (Table 3, entries 22–28). With 2‐furaldehyde
and 2‐n‐butylfuran, even the sterically hindered 3‐
bromoquinoline led to the expected coupling products
with 58–79% and 68–78% yields, respectively (Table 1,
entries 24–27).
Then, the scope and limitation of this reaction were
examined by using a number of aryl chlorides (Table 4).
A survey of coupling of aryl chlorides with 2‐furaldehyde

KALOĞLU AND ÖZDEMIR
11 of 15
TABLE 3 Palladium(II)‐NHC‐catalyzed direct C5‐arylation of 2‐substituted furan derivatives by using (hetero)aryl bromidesa
Conversion(Yield)b,c [%]
(Hetero)aryl
bromide
Entry
Catalyst
Product
R = CHO
R = n‐Bu
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
95(74)
90(71)
98(78)
97(75)
98(82)
90(77)
88(69)
81(68)
88(73)
90(77)
80(69)
75(62)
88(72)
78(61)
3 g
8
9
3a
3b
3c
3d
3e
3f
91(68)
92(77)
98(82)
97(70)
94(73)
90(74)
85(69)
89(74)
85(70)
90(78)
79(65)
89(72)
83(68)
71(58)
10
11
12
13
14
3 g
15
16
17
18
19
20
21
3a
3b
3c
3d
3e
3f
89(73)
95(82)
100(85)
91(73)
86(69)
95(70)
90(67)
86(74)
79(66)
87(75)
74(60)
80(65)
77(63)
81(68)
3 g
22
23
24
25
26
27
28
3a
3b
3c
3d
3e
3f
91(70)
84(77)
94(83)
78(63)
87(73)
88(72)
74(67)
90(79)
90(75)
97(83)
92(80)
84(71)
87(74)
91(79)
3 g
29
30
31
32
33
34
35
3a
3b
3c
3d
3e
3f
80(58)
84(68)
91(77)
85(71)
90(79)
88(72)
80(65)
90(77)
81(69)
95(78)
92(74)
90(72)
83(68)
87(71)
3 g
aConditions: Pd‐NHC, 3a‐g (0.01 mmol), furan derivative (2.0 mmol), (hetero)aryl bromide (1.0 mmol), KOAc (2.0 mmol), DMAc (2 ml),
120 °C, 1 h.
bConversions were calculated with respect to (hetero)aryl bromide from the results of GC spectrometry.
cIsolated yields were shown in parentheses.
and 2‐n‐butylfuran, KOAc as the base in DMAc at 120 °C
for 20 h is provided in Table 4. When the reaction of
2‐furaldehyde with chlorobenzene, 4‐chlorotoluene, 4‐
chlorobenzaldehyde, 4‐chloroacetophenone and 4‐
chlorobenzotrifluoride was investigated, yields at between
51–72%, 52–69%, 50–76%, 53–69% and 60–76% were
observed, respectively. When the reaction of 2‐n‐
butylfuran with corresponding aryl chlorides was also
investigated, yields at between 49–72%, 50–72%, 45–67%,
53–80% and 54–74% were observed, respectively. As can
be seen from Table 4, substituents on the aryl chlorides
had some effect on the reactions. For example, the
electron‐withdrawing groups such as ‐CHO, ‐COCH3 and
‐CF3 (Table 4, entries 15–35) were generally better

12 of 15
KALOĞLU AND ÖZDEMIR
TABLE 4 Palladium(II)‐NHC‐catalyzed direct C5‐arylation of 2‐substituted furan derivatives by using aryl chloridesa
Conversion(Yield)b,c [%]
Aryl
R = n‐
Entry
chloride
Catalyst
Product
R = CHO
Bu
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
80(67)
75(63)
87(72)
69(53)
83(70)
84(70)
65(51)
69(54)
75(63)
85(72)
76(64)
82(68)
83(70)
61(49)
3 g
8
9
3a
3b
3c
3d
3e
3f
70(54)
73(58)
87(69)
70(52)
73(60)
75(62)
71(58)
60(50)
81(67)
86(72)
84(72)
81(69)
68(57)
70(56)
10
11
12
13
14
3 g
15
16
17
18
19
20
21
3a
3b
3c
3d
3e
3f
65(50)
68(53)
89(76)
80(68)
76(57)
84(72)
80(65)
68(55)
69(54)
81(67)
78(63)
73(60)
75(61)
59(45)
3 g
22
23
24
25
26
27
28
3a
3b
3c
3d
3e
3f
68(53)
73(61)
82(68)
72(60)
80(65)
82(69)
75(63)
82(67)
81(64)
95(80)
84(71)
75(62)
76(65)
65(53)
3 g
29
30
31
32
33
34
35
3a
3b
3c
3d
3e
3f
77(63)
79(68)
90(76)
82(69)
78(64)
88(75)
74(60)
78(64)
76(61)
88(74)
80(69)
74(61)
70(57)
67(54)
3 g
aConditions: Pd‐NHC, 3a‐g (0.01 mmol), furan derivative (2.0 mmol), aryl chloride (1.0 mmol), KOAc (2.0 mmol), DMAc (2 mL), 120 °C, 20 h.
bConversions were calculated with respect to aryl chloride from the results of GC spectrometry.
cIsolated yields were shown in parentheses.
substrates, than those neutral aryl chloride such as chloro-
benzene or 4‐chlorotoluene having electron‐donating sub-
stituent such as ‐CH3 group (Table 4, entries 1–14).
Generally, the reactivity of 2‐n‐butylfuran was found
to be less reactive than 2‐furaldehyde. Also, when the
performances of the complexes 3a‐g were compared in
the direct C‐H bond arylation of furans reaction, complex
3c bearing NHC ligands with 1,3‐di(4‐phenoxybutyl)
substituents exhibited better catalytic activity than the
others. We attributed these performance differences to
well‐accordance electronic and steric properties of the
NHC ligand. It is known that oxidative additions of elec-
tron‐withdrawing substrates to electron rich palladium‐
complexes and reductive elimination of the product from
large, sterically hindered palladium‐complexes proceed
more readily. Therefore, the presence of an NHC ligand

KALOĞLU AND ÖZDEMIR
13 of 15
SCHEME 3 Proposed general catalytic
pathway for the C‐H bond arylation of 2‐
substituted furans
bearing a different second donating group such as ether
side chains on the metal may radically increase the cata-
lytic performance of the catalyst. The chelating nature of
these ligands promotes production of highly stable com-
plexes. The hemilabile part of such ligands is capable of
reversible dissociation to produce vacant coordination
sites, allowing complexation of substrates during the cat-
alytic cycle. At the same time the strong‐donor carbene
moiety remains connected to the metal centre.40b In this
direct C‐H bond arylation, we believe that the bulky
and electron‐donor NHC ligands bearing 4‐phenoxybutyl
group in complexes 3a‐g provide the synergetic steric
and electronic effects to confer the metal center the
appropriate properties to make optimum for the key steps
of the catalytic cycles. The proposed general catalytic
pathway according to above explanations is shown in
Scheme 3.
When the proposed catalytic pathway is examined,
initially, oxidative addition of aryl halides to Pd(0) species
affords a Pd(II)‐aryl intermediate A. We believe that in
the presence of electron‐donor NHC ligands bearing 4‐
phenoxybutyl substituent, oxidative addition step more
readily takes place and this is might be a key step. Then,
followed by exchange of X ligand with KOAc to give B.
The nature of the base used in this step is very important.
Then, intermediate B reacts with 2‐substituted furan to
give C by C‐H activation. In this step, the chelating
nature of 4‐phenoxybutyl substituent promotes produc-
tion of highly stable complexes. Finally, the reductive
elimination of intermediate C produces the desired C5‐
arylated furan products. It is clear that, reductive elimina-
tion of the product from large and sterically hindered
NHC ligands bearing 4‐phenoxybutyl substituent proceed
more readily.
4 | CONCLUSIONS
In conclusion, seven new benzimidazolium salts and
their corresponding new palladium(II)‐NHC complexes
1
were successfully synthesized and characterized by H
NMR, 13C NMR and FT‐IR spectroscopy, and microanal-
ysis techniques. The catalytic activities of these new
palladium(II)‐NHC complexes were investigated as a
broadly applicable catalysts in the direct C‐H bond
arylation of 2‐substituted furan derivatives with different
aryl halides. Under the optimal conditions, 2‐substituted
furans could be arylated solely in the C5‐position with
(hetero)aryl bromides. In addition, various substituents
such as neutral, electron‐donating and electron‐with-
drawing substrates could be tolerated, affording a effi-
cient methodology for the direct C5‐arylation of furans
with economic and easily available aryl chlorides. Only
a minor effect of the NHC ligand on the palladium com-
plex was observed for the coupling of aryl halides with 2‐
substituted furan derivatives. Surprisingly, similar yields
were obtained for the coupling of each aryl halides. We
can say that there is no significant difference between
these complexes on the catalytic activity of direct
arylation of 2‐substituted furan derivatives by aryl
halides. The only significant difference between 3a‐g
complexes indicates that electronic and steric properties
are also playing some role in these processes. Finally, sat-
isfactory results were obtained in presence of low catalyst

14 of 15
KALOĞLU AND ÖZDEMIR
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is economically and environmentally attractive. Also, the
only byproducts are AcOH/KX, (X = Cl or Br), instead of
metallic salts with classical coupling procedures such as
Suzuki, Stille, or Negishi reactions. Moreover, no prepara-
tion of an organometallic derivative is required, reducing
the number of steps and consequently the amount of
waste to prepare these compounds. It has to be empha-
sized that this procedure is environmentally more attrac-
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ACKNOWLEDGEMENTS
This work was financially supported by the Technological
and Scientific Research Council of Turkey TÜBİTAK‐
BOSPHORUS (109 T605) and the İnönü University
Research Fund (İ.Ü. B.A.P. 2015/41).
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ORCID
[18] M. Lafrance, C. N. Rowley, T. K. Woo, K. Fagnou, J. Am. Chem.
Soc. 2006, 128, 8754.
Murat Kaloğlu http://orcid.org/0000-0002-2770-5532
İsmail Özdemir http://orcid.org/0000-0001-6325-0216
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