Catalytic activities in direct arylation of novel palladium N-heterocyclic carbene complexes

Article abstract of DOI:10.1002/aoc.3220

Eight novel palladium N-heterocyclic carbene (Pd-NHC) complexeswere synthesized by the reaction of chloro 1,3-dialkylbenzimidazolin-2-ylidene silver(I) complexes with bis(benzonitrile)palladium(II) chloride in dichloromethane. These eight Pd-NHC complexes

Full text of DOI:10.1002/aoc.3220

Full Paper
Received: 22 July 2014
Accepted: 24 August 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3220
Catalytic activities in direct arylation of novel
palladium N-heterocyclic carbene complexes
Senem Akkoça* and Yetkin Gökb
Eight novel palladium N-heterocyclic carbene (Pd-NHC) complexes were synthesized by the reaction of chloro 1,3-dialkylbenzimidazolin-
2-ylidene silver(I) complexes with bis(benzonitrile)palladium(II) chloride in dichloromethane. These eight Pd-NHC complexes
are as follows: bis[1-phenyl-3-(2,4,6-trimethylbenzyl)benzimidazol-2-ylidene]dichloropalladium(II), bis[1-phenyl-3-(2,3,5,
6-tetramethylbenzyl)benzimidazol-2-ylidene]dichloropalladium(II), bis[1-phenyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazol-2-yl-
idene]dichloropalladium(II), bis[1-phenyl-3-(3,4,5-trimethoxybenzyl)benzimidazol-2-ylidene]dichloropalladium(II), bis[1-(2-
diethylaminoethyl)-3-(3-methylbenzyl)benzimidazol-2-ylidene]dichloropalladium(II), bis[1-(2-diethylaminoethyl)-3-(2,3,5,
6-tetramethylbenzyl)benzimidazol-2-ylidene]dichloropalladium(II), bis[1-(2-morpholinoethyl)-3-naphthalenomethylbenzimidazol-
2-ylidene]dichloropalladium(II) and bis[1-(2-morpholinoethyl)-3-(2-methylbenzyl)benzimidazol-2-ylidene]dichloropalladium(II).
Also, these synthesized complexes were fully characterized using Fourier transform infrared, 1H NMR and 13C NMR spectroscopic
methods and elemental analysis techniques. These synthesized novel Pd-NHC complexes were tested as catalysts in the direct
arylation of 2-n-butylthiophene, 2-n-butylfuran and 2-isopropylthiazole with various aryl bromides at 130°C for 1 h. The
complexes showed very good catalytic activities in these reactions. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: N-heterocyclic carbene; palladium complex; direct arylation; aryl bromide
dichloromethane at room temperature. Their structures were char-
acterized using appropriate spectroscopic methods (NMR, FT-IR)
Introduction
N-heterocyclic carbenes (NHCs) have been widely used in both
organometallic chemistry and coordination chemistry for years.
Much attention has been focused on these compounds as ancillary
ligands for a number of transition metal complexes since the
discovery of stable NHCs in 1991.[1,2] Various metal complexes have
been applied in many areas such as catalysis, medicine, magneto-
structural chemistry, the dye and polymer industry, analytical
chemistry, agriculture and enzyme modelling.[3–7] Among these
metal complexes, NHC metal complexes which have high stability,
strong σ-donating and weak π-accepting properties[8,9] are signifi-
cant structural motifs in medicinal chemistry.[10–14] Also, these com-
plexes are very significant for catalytic applications. Numerous
saturated and unsaturated 1,3-disubstituted benzimidazol-2-
ylidene palladium NHC (Pd-NHC) complexes have been successfully
applied in various reactions in the area of catalytic C–C bond
formation such as direct arylation and Suzuki–Miyaura, Mizoroki–
Heck, Sonogashira, Hiyama and Kumada–Tamao–Corriu coupling
reactions.[15–21] In homogeneous catalysis, NHCs as alternatives to
phosphines[22,23] have also been used in olefin metathesis[24,25]
and C–N coupling reactions.[26,27]
A significant aim in chemistry is to find ways for using less raw
material and energy so that the use of renewable resources in-
creases while the use of hazardous chemicals decreases or they
are eliminated altogether in the development of environmentally
friendly processes. Therefore, new catalytic systems are needed
which are more active and selective for organic reactions. For this
reason, in order to develop successful catalysts for the formation
of aryl–aryl bonds, in the study reported here, eight novel pure
Pd-NHC complexes (2a–2h) bearing two chlorine atoms were pre-
pared by the interaction of benzimidazol-2-ylidene silver NHC com-
plexes with bis(benzonitrile)palladium(II) chloride (PdCl2(PhCN)2) in
and elemental analysis techniques. These metal carbene complexes
were tested for their catalytic activities in direct arylation, being an
effective method for forming sp2–sp2 C–C bonds. It was found that
these complexes act as catalysts.
Experimental
Materials and Methods
The reactions for the synthesis of Pd-NHC complexes 2a–2h were car-
ried out under argon in flame-dried glassware using standard
Schlenk-type flasks and standard high vacuum line techniques.
Solvents dichloromethane and diethyl ether were of analytical grade.
Prior to their use, they were dried under P4O10 and Na in an inert at-
mosphere, respectively. Some of the necessary reagents were
synthesized in our laboratory and some others were purchased
commercially. Reagents and solvents 2-methylbenzyl chloride, 3-
methylbenzyl chloride, 2,4,6-trimethylbenzyl chloride, 2,3,5,6-
tetramethylbenzyl chloride, 2,3,4,5,6-pentamethylbenzyl chloride,
3,4,5-trimethoxybenzyl chloride, bis(benzonitrile)palladium chloride,
N,N-dimethylformamidedimethylacetal, hexane, dimethylformamide,
dichloromethane and diethyl ether were purchased from commercial
*
Correspondence to: Senem Akkoç, Erciyes University, Faculty of Science, Department
of Chemistry, 38039 Kayseri, Turkey. E-mail: senemakkoc@erciyes.edu.tr
a Erciyes University Faculty of Science, Department of Chemistry, 38039, Kayseri,
Turkey
b Inönü University Faculty of Science and Arts, Department of Chemistry, 44280,
Malatya, Turkey
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

S. Akkoç and Y. Gök
suppliers (Merck, Aldrich, Alfa-Aesar and Fluka). CDCl3 and DMSO-d6
were used as solvents in all 1H NMR and 13C NMR analyses.
(CH3)4-3,5); 50.8 (NCH2C6H(CH3)4-2,3,5,6); 110.7, 128.6, 130.7, 132.3,
134.3, 135.0 (C6H4); 123.1, 128.3, 134.0, 135.6 (NCH2C6H(CH3)4);
111.9, 122.7, 128.9, 137.9 (NC6H5); 182.8 (C–Pd). Elemental analysis.
Calcd for C48H48N4PdCl2 (858.25 g/mol) (%): C, 67.17; H, 5.64; N, 6.53.
Found (%): C, 67.24; H, 5.54; N, 6.54.
The 1H NMR and 13C NMR spectra were obtained using a Bruker
AC300P FT spectrometer operating at 300.13 MHz (1H) and 75.47
MHz (13C). Chemical shifts were measured relative to tetra-
methylsilane. Coupling constants (J) were measured in Hz. The FT-
IR spectra were recorded with a Mattson 1000 spectrophotometer
and wavenumbers were recorded in cmꢀ1. Melting points were
determined in open capillary tubes with an Electrothermal-9200
melting point device. GC analysis was conducted with an Agilent
6890N Network GC system using an HP-5 column (column length
30 m, column diameter 0.32 mm, column filler size 0.25 μm) and
a temperature range from ꢀ60 to 325°C. Elemental analyses were
performed using CHNS-932 LECO apparatus.
Bis[1-phenyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazol-2-ylidene]dichloro-
palladium(II) complex (2c)
Complex 2c, using the same conditions and procedure as for 2a, was
prepared from chloro [1-phenyl-3-(2,3,4,5,6-pentamethylbenzyl)
benzimidazol-2-ylidene]silver(I) complex (1c) (0.21 g, 0.42 mmol)
and PdCl2(PhCN)2 (0.08 g, 0.21 mmol) in dichloromethane (4 ml).
Cream-coloured powder; yield 87%; m.p. 294–295°C; ν(CN)
=
1473.48 cmꢀ1. 1H NMR (300.13 MHz, CDCl3, δ, ppm): 2.29 (s, 12 H,
NCH2C6(CH3)5-2,6); 2.36 (s, 6 H, NCH2C6(CH3)5-4); 2.45 (s, 12 H,
NCH2C6(CH3)5-3,5); 6.27 (s, 4 H, NCH2C6(CH3)5-2,3,4,5,6); 6.79–7.62
(m, 18 H, NC6H5 and C6H4). 13C NMR (75.47 MHz, CDCl3, δ, ppm):
16.9 (NCH2C6(CH3)5-2,6); 17.3 (NCH2C6(CH3)5-4); 17.5 (NCH2C6(CH3)
5-3,5); 53.3 (NCH2C6(CH3)5-2,3,4,5,6); 125.3, 126.1, 128.3, 137.3
(NC6H5); 122.4, 130.6, 131.8, 132.8, 134.8, 135.9 (C6H4); 127.7,
129.1, 133.9, 136.7 (NCH2C6(CH3)5); 166.2 (C–Pd). Elemental analysis.
Calcd for C50H52N4PdCl2 (886.3 g/mol) (%): C, 67.76; H, 5.91; N, 6.32.
Found (%): C, 67.70; H, 5.91; N, 6.32.
Syntheses
Complexes 1a–1h
1,3-Dialkyl-substituted silver NHC complexes 1a–1h containing
both electron-donating and electron-withdrawing moieties were
synthesized from the deprotonation of benzimidazolium salts, as
in our previous study.[28–30]
Bis[1-phenyl-3-(2,4,6-trimethylbenzyl)benzimidazol-2-ylidene]dichloropalladium
(II) complex (2a)
Bis[1-phenyl-3-(3,4,5-trimethoxybenzyl)benzimidazol-2-ylidene]dichloropal-
Chloro [1-phenyl-3-(2,4,6-trimethylbenzyl)benzimidazol-2-ylidene]
silver(I) complex (1a) (0.195 g, 0.42 mmol) and PdCl2(PhCN)2
(0.08g, 0.21 mmol) were dissolved in dried dichloromethane (4 ml)
and then were stirred at room temperature for 18 h in a Schlenk-
type flask. The flask was covered with aluminium foil to avoid light
exposure. The resulting cream-brown solution was filtered over
celite, silica gel and cotton. Then, the solvent was removed under
reduced pressure. The resulting product was crystallized from
dichloromethane–hexane mixture (2:1) at 25°C. Finally, the obtained
crystals were washed twice with 10 ml of diethyl ether and dried
under vacuum.
ladium(II) complex (2d)
Complex 2d, using the same conditions and procedure as for 2a,
was prepared from chloro [1-phenyl-3-(3,4,5-trimethoxybenzyl)
benzimidazol-2-ylidene]silver(I) complex (1d) (0.22 g, 0.42 mmol)
and PdCl2(PhCN)2 (0.08 g, 0.21 mmol) in dichloromethane (4 ml).
Cream-coloured powder; yield 83%; m.p. 305–306°C; ν(CN)
=
1
1590.51 cmꢀ1. H NMR (300.13 MHz, CDCl3, δ, ppm): 3.81, 3.91
(m, 18 H, NCH2C6H2(OCH3)3-3,4,5); 5.86 (s, 4 H, NCH2C6H2(OCH3)
3-3,4,5); 6.65 (s, 4 H, NCH2C6H2(OCH3)3-3,4,5); 7.06–8.04 (m, 18 H,
NC6H5 and C6H4). 13C NMR (75.47 MHz, CDCl3, δ, ppm): 52.9
(NCH2C6H2(OCH3)3-4);
56.8
(NCH2C6H2(OCH3)3-3,5);
60.8
Light cream-coloured powder; yield 85%; m.p. 312–313°C; ν(CN)
=
(NCH2C6H2(OCH3)3-3,4,5); 105.3, 131.2, 134.0, 153.6 (NCH2C6H2
(OCH3)3-3,4,5); 111.4, 123.6, 127.2, 128.4, 129.1, 135.3, 137.7
(NC6H5 and C6H4); 182.1 (C–Pd). Elemental analysis. Calcd for
C46H44N4O6PdCl2 (926.19 g/mol) (%): C, 59.65; H, 4.79; N, 6.05.
Found: C, 59.71; H, 4.78; N, 6.02.
1596.47 cmꢀ1. 1H NMR (300.13 MHz, CDCl3, δ, ppm): 2.31 (s, 12 H,
NCH2C6H2(CH3)3-2,6); 2.40 (s, 6 H, NCH2C6H2(CH3)3-4); 5.89 (s, 4 H,
NCH2C6H2(CH3)3-2,4,6); 6.42 (s, 4 H, NCH2C6H2(CH3)3-2,4,6); 6.95–
7.28 (m, 8 H, C6H4); 7.32–8.09 (m, 10 H, NC6H5). 13C NMR (75.47
MHz, CDCl3, δ, ppm): 20.9 (NCH2C6H2(CH3)3-2,6); 21.2 (NCH2C6H2
(CH3)3-4); 49.9 (NCH2C6H2(CH3)3-2,4,6); 111.8 (NCCHCHCHCHCH-
4); 122.8 (NCCHCHCHCHCH-2,6); 128.3 (NCCHCHCHCHCH-3,5);
138.7 (NCCHCHCHCHCH-1); 110.8, 123.2, 127.8, 128.9 (NCCH-
CHCHCHCN); 134.2, 138.2 (NCCHCHCHCHCN); 128.7, 129.5, 135.6,
137.9 (NCH2C6H2(CH3)3); 182.8 (C–Pd). Elemental analysis. Calcd for
C46H44N4PdCl2 (830.19 g/mol) (%): C, 66.55; H, 5.34; N, 6.75. Found
(%): C, 66.59; H, 5.27; N, 6.74.
Bis[1-(2-diethylaminoethyl)-3-(3-methylbenzyl)benzimidazol-2-ylidene]dichloro-
palladium(II) complex (2e)
Complex 2e, using the same conditions and procedure as for 2a,
was prepared from chloro [1-(2-diethylaminoethyl)-3-(3-methyl-
benzyl)benzimidazol-2-ylidene]silver(I) complex (1e) (0.19 g, 0.41
mmol) and PdCl2(PhCN)2 (0.08 g, 0.21 mmol) in dichloromethane (4
ml).
Bis[1-phenyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazol-2-ylidene]dichloropalla-
dium(II) complex (2b)
Cream-coloured powder; yield 79%; ν(CN) = 1523.17 cmꢀ1 1H
.
NMR (300.13 MHz, DMSO, δ, ppm): 1.06 (t, 12 H, J= 7.6 Hz,
NCH2CH2N(CH2CH3)2); 2.28 (s, 6 H, NCH2C6H4CH3-3); 3.66 (m, 8 H,
NCH2CH2N(CH2CH3)2); 3.85 (m, 4 H, NCH2CH2N(CH2CH3)2); 5.37 v
(m, 4 H, NCH2CH2N(CH2CH3)2); 6.22 (s, 4 H, NCH2C6H4CH3-3); 7.01–
8.02 (m, 16 H, NCH2C6H4CH3-3 and C6H4). 13C NMR (75.47 MHz,
DMSO, δ, ppm): 12.9 (NCH2CH2N(CH2CH3)2); 19.9 (NCH2C6H4CH3-
3); 21.3 (NCH2CH2N(CH2CH3)2); 46.5 (NCH2CH2N(CH2CH3)2);
47.6 (NCH2CH2N(CH2CH3)2); 50.6 (NCH2C6H4CH3-3); 124.2, 125.3,
128.0, 128.9, 133.9, 138.0, 138.4 (NCH2C6H4CH3-3 and C6H4); 183.2
(C–Pd). Elemental analysis. Calcd for C42H54N6PdCl2 (820.24 g/mol) (%):
C, 61.50; H, 6.64; N, 10.25. Found (%): C, 61.59; H, 6.54; N, 10.25.
Complex 2b, using the same conditions and procedure as for 2a,
was prepared from chloro [1-phenyl-3-(2,3,5,6-tetramethylbenzyl)
benzimidazol-2-ylidene]silver(I) complex (1b) (0.201 g, 0.42 mmol)
and PdCl2(PhCN)2 (0.08 g, 0.21 mmol) in dichloromethane (4 ml).
Light yellow-coloured powder; yield 89%; m.p. 317–318°C; ν(CN)
=
1596.68 cmꢀ1. 1H NMR (300.13 MHz, CDCl3, δ, ppm): 2.39 (s, 12 H,
NCH2C6H(CH3)4-2,6); 2.56 (s, 12 H, NCH2C6H(CH3)4-3,5); 5.99 (s, 4 H,
NCH2C6H(CH3)4-2,3,5,6); 6.26 (s, 2 H, NCH2C6H(CH3)4-2,3,5,6); 6.98–
7.40 (m, 8 H, C6H4); 7.50–8.09 (m, 10 H, NC6H5). 13C NMR (75.47
MHz, CDCl3, δ, ppm): 16.5 (NCH2C6H(CH3)4-2,6); 20.6 (NCH2C6H
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Appl. Organometal. Chem. (2014)

Catalytic activities in direct arylation of novel Pd-NHC complexes
Bis[1-(2-diethylaminoethyl)-3-(2,3,5,6-tetramethylbenzyl)benzimidazol-2-ylidene]
Bis[1-(2-morpholinoethyl)-3-naphthalenomethylbenzimidazol-2-ylidene]dichlo-
dichloropalladium(II) complex (2f)
ropalladium(II) complex (2g)
Complex 2f, using the same conditions and procedure as for 2a,
was prepared from chloro [1-(2-diethylaminoethyl)-3-(2,3,5,6-tetra-
methylbenzyl)benzimidazol-2-ylidene]silver(I) complex (1f) (0.21 g, 0.42
mmol) and PdCl2(PhCN)2 (0.08 g, 0.21 mmol) in dichloromethane (4 ml).
Complex 2g, using the same conditions and procedure as for
2a, was prepared from chloro [1-(2-morpholinoethyl)-3-
naphthalenomethyl)benzimidazol-2-ylidene]silver(I) complex (1g)
(0.21 g, 0.41 mmol) and PdCl2(PhCN)2 (0.08 g, 0.21 mmol) in dichlo-
romethane (4 ml).
Cream-coloured powder; yield 89%; m.p. 217–218°C; ν(CN)
=
1
1504.35 cmꢀ1. H NMR (300.13 MHz, CDCl3, δ, ppm): 0.96 (t, 12 H,
J=7.2 Hz, NCH2CH2N(CH2CH3)2); 2.30 (s, 12 H, NCH2C6H(CH3)4-2,6);
2.37 (s, 12 H, NCH2C6H(CH3)4-3,5); 2.68 (q, 8 H, J=7.2 Hz, NCH2CH2N
(CH2CH3)2); 2.82 (t, 4 H, J=7.5 Hz, NCH2CH2N(CH2CH3)2); 3.33 (t, 4 H,
J=7.5 Hz, NCH2CH2N(CH2CH3)2); 5.13 (s, 4 H, NCH2C6H(CH3)4-2,3,5,6);
6.31 (s, 2 H, NCH2C6H(CH3)4-2,3,5,6); 6.34–7.64 (m, 8 H, C6H4). 13C
NMR (75.47 MHz, CDCl3, δ, ppm): 11.8 (NCH2CH2N(CH2CH3)2); 15.9
(NCH2C6H(CH3)4-2,6); 16.6 (NCH2C6H(CH3)4-3,5); 20.5 (NCH2CH2N
(CH2CH3)2); 20.6 (NCH2CH2N(CH2CH3)2); 46.4 (NCH2CH2N(CH2CH3)2);
65.9 (NCH2C6H(CH3)4); 110.8, 111.9, 122.5, 122.9, 134.8, 135.3 (C6H4);
130.0, 130.9, 132.4, 134.2, (NCH2C6H(CH3)4-2,3,5,6); 182.0 (C–Pd).
Elemental analysis. Calcd for C48H66N6PdCl2 (904.4 g/mol) (%): C,
63.75; H, 7.36; N, 9.29. Found (%): C, 63.84; H, 7.23; N, 9.31.
Light cream-coloured powder; yield: 88%; m.p. 226–227°C; ν(CN)
=
1599.32 cmꢀ1. 1H NMR (300.13 MHz, CDCl3, δ, ppm): 2.17, 2.85 (m,
16 H, NCH2CH2N(CH2CH2)2O); 2.91, 4.78 (m, 8 H, NCH2CH2N
(CH2CH2)2O); 5.31 (s, 4 H, NCH2C10H7); 6.39–7.51 (m, 8 H, C6H4);
7.55–7.81 (m, 14 H, NCH2C10H7). 13C NMR (75.47 MHz, CDCl3, δ,
ppm): 41.0 (NCH2CH2N(CH2CH2)2O); 49.2 (NCH2CH2N(CH2CH2)2O);
53.3 (NCH2CH2N(CH2CH2)2O); 56.0 (NCH2CH2N(CH2CH2)2O); 66.6
(NCH2C10H7); 111.3, 112.9, 113.7, 122.5, 125.4, 126.6, 127.0, 127.8,
128.0, 128.5, 129.3, 130.6 131.4, 133.2, 133.9, 134.2, 144.5
(NCH2C10H7 and C6H4); 182.3 (C–Pd). Elemental analysis. Calcd for
C48H50N6O2PdCl2 (920.28 g/mol) (%): C, 62.65; H, 5.48; N, 9.13.
Found (%): C, 62.59; H, 5.48; N, 9.13.
Bis[1-(2-morpholinoethyl)-3-(2-methylbenzyl)
benzimidazol-2-ylidene]dichloropalladium(II)
complex (2h)
Complex 2h, using the same conditions
and procedure as for 2a, was prepared
from chloro [1-(2-morpholinoethyl)-3-(2-
methylbenzyl)benzimidazol-2-ylidene]sil-
ver(I) complex (1h) (0.19 g, 0.40 mmol)
and PdCl2(PhCN)2 (0.08 g, 0.21 mmol) in
N
N
N
N
PdCl2
PdCl2
2
PdCl2
N
2
N
2
O
dichloromethane (4 ml).
Dark cream-coloured powder; yield 83%;
1
m.p. 200–201°C; ν(CN) = 1413.16 cmꢀ1. H
O
O
2d
2c
NMR (300.13 MHz, CDCl3, δ, ppm): 2.25 (m,
8 H, NCH2CH2N(CH2CH2)2O); 2.42 (s, 6 H,
NCH2C6H4CH3-2); 2.87 (t, 8 H, J=8.4 Hz,
2b
NCH2CH2N(CH2CH2)2O); 3.07 (m,
4 H,
N
NCH2CH2N(CH2CH2)2O); 4.46 (t, 4 H, J=9
Hz, NCH2CH2N(CH2CH2)2O); 5.31 (s, 4 H,
NCH2C6H4CH3-2); 7.01–7.87 (m, 16 H,
NCH2C6H4CH3-2 and C6H4). 13C NMR (75.47
MHz, CDCl3, δ, ppm): 19.8 (NCH2C6H4CH3-2);
43.4 (NCH2CH2N(CH2CH2)2O); 49.9 (NCH2CH2N
(CH2CH2)2O); 53.2 (NCH2CH2N(CH2CH2)2O);
53.5 (NCH2CH2N(CH2CH2)2O); 66.2 (NCH2C6
H4CH3-2); 111.2, 113.2, 113.7, 123.7,
126.9, 128.5, 129.5, 130.6, 131.4, 133.9,
134.9, 136.6, 144.6 (NCH2C6H4CH3 and
C6H4); 183.1 (C–Pd). Elemental analysis.
Calcd for C42H50N6O2PdCl2 (848.21 g/mol)
(%): C, 59.47; H, 5.84; N, 9.91. Found (%): C,
59.41; H, 5.96; N, 9.90.
R
N
N
PdCl2
N
N
N
2
PdCl2
+ PdCl2(PhCN)2
AgX
R'
2
N
1a-h
2a
2e
O
O
N
N
N
N
N
PdCl2
N
N
N
N
2
PdCl2
PdCl2
2
2
2-n-Butyl-5-acetophenylfuran (3)
1H NMR (300.13 MHz, CDCl3, δ, ppm): 0.98
(t, 3 H, J = 7.5 Hz, CCH2CH2CH2CH3); 1.43
(m, 2 H, CCH2CH2CH2CH3); 1.71 (tt, 2 H,
J = 7.5 Hz, J= 7.5 Hz, CCH2CH2CH2CH3);
2.73 (t, 2 H, J=7.5 Hz, CCH2CH2CH2CH3);
2.62 (s, 3 H, O¼CCH3); 6.13–7.99 (m, 6 H,
Ar–H).
2f
2h
2g
Scheme 1. Synthesis of [1,3-dialkylbenzimidazol-2-ylidene]dichloridopalladium(II) complexes (2a–2h).
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
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S. Akkoç and Y. Gök
General Procedure for Direct Arylation
according to the method reported in the literature.[8,32,33] Silver
NHC complexes 1a–1h were obtained by the reaction of
benzimidazolium salts with silver oxide in CH2Cl2 at room tempera-
ture for 24 h.[28–30] Eight novel Pd-NHC complexes 2a–2h were syn-
thesized using complexes 1a–1h as the carbene transfer reagent
with PdCl2(PhCN)2 in the presence of CH2Cl2 at 25°C for 24 h in
79–89% yields (Scheme 1). The obtained Pd-NHC complexes are
stable against air and moisture.
Heteroaryl derivative (2-n-butylfuran, 2-n-butylthiophene and 2-
isopropylthiazole) (2 mmol), aryl bromide (4-bromoacetophenone,
4-bromoanisole, 4-bromotoluene and bromobenzene) (1 mmol),
Pd-NHC complexes 2a–2h (0.005 mmol), KOAc (1 mmol) and N,N-
dimethylacetamide (DMAc; 2 ml) were added into a Schlenk tube
equipped with a magnetic stirring bar as indicated in the
literature.[20,31] The Schlenk tube was heated in an oil bath at 130°C
for 1 h. The ambient solvent was removed by heating the reaction
vessel under vacuum. The product was eluted using a pentane–
diethyl ether mixture (3:1). The reaction mixture was purified by flash
chromatography on silica gel. The purity of compounds was checked
using GC and NMR. Conversions were based on aryl bromides.
The Pd-NHC complexes were characterized using both spectro-
1
scopic methods and elemental analysis techniques. H NMR and
13C NMR spectra which appear to be spectroscopically pure are con-
sistent with the proposed formulae. In each 13C NMR spectrum, there
is one signal for the carbenic carbon (NCN) at ca 182.8, 182.8, 166.2,
182.1, 183.2, 182.0, 182.3 and 183.1 ppm for 2a–2h, respectively. The
FT-IR spectrum of each Pd-NHC complex exhibits a characteristic
Results and Discussion
ν
(NCN) band at 1596.47, 1596.68, 1473.48, 1590.51, 1523.17, 1504.35,
1599.32 and 1413.16 cmꢀ1 for 2a–2h, respectively. When results of
the elemental analysis, which is one of the analytical techniques that
verifies synthesis of the compounds, are evaluated, it is observed that
calculated values are very close to found values. Unfortunately, we
Synthesis of Pd-NHC Complexes
Various 1,3-dialkylbenzimidazolium salts were synthesized by the
quaternization of N-alkylbenzimidazole with alkyl halides in DMF
Table 1. Arylation of 2-n-butylfuran with various aryl bromidesa
Entry
R
Pd-NHC
Product
Conversion (%)b
1
2
3
4
5
6
7
8
p-COCH3
2a
2b
2c
2d
2e
2f
75
65
98
3
66
88
100
100
99
2g
2h
9
p-OCH3
p-CH3
H
2a
2b
2c
2d
2f
66
83
93
69
81
66
97
10
11
12
14
15
16
4
5
6
2g
2h
17
18
19
20
22
23
24
2a
2b
2c
2d
2f
93
85
84
100
97
2g
2h
95
96
25
26
27
28
30
31
32
2a
2b
2c
2d
2f
83
88
89
63
89
75
97
2g
2h
aReaction conditions: 2-n-butylfuran (2 mmol), aryl bromide (1 mmol), Pd-NHC (2a–2h) (0.005 mmol), KOAc (1 mmol), DMAc (2
ml), 130°C, 1 h. Product purity was checked using GC and NMR.
bConversions were calculated according to aryl bromide.
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Appl. Organometal. Chem. (2014)

Catalytic activities in direct arylation of novel Pd-NHC complexes
were unable to obtain an appropriate single crystal from these new
complexes for X-ray diffraction.
conversions by using just 0.005 mmol% catalyst (Tables 1–3).
Conversions of products are between 45 and 100% for the 3–14
matching patterns in the most appropriate conditions of solvent,
base and temperature. The best results for the examination of the
temperature are acquired at approximately 130°C.
Direct Arylation of 2-n-Butylfuran/2-n-Butylthiophene/2-Iso-
propylthiazole with Various Aryl Bromides
Generally, Pd(II)-catalysed direct arylation of furan, thiazole or thiophene
gives arylated products at the 2- or 5-positions in high conversions.[34–37]
For example, Pd(II)-catalysed direct arylation of unsubstituted
heteroaromatic groups such as thiazole and furan together with aryl bro-
mides is developed at the C2 position and it generally takes shape at the
C5 position in the presence of 2-substituted heteroaromatic
compound.[20,38] Direct arylation at the C2 or C5 position of aryl halides
and heteroaromatic compounds provides an important and easy way
for the formation of aryl–heteroaryl bonds, that is, the synthesis of biaryls.
Our aim is to obtain the best conversions under the most appro-
priate conditions by decreasing the quantities of synthesized novel
complexes (2a–2f) as catalysts. To achieve this, the 3–14 bound
patterns from couplings of 2-n-butylfuran/2-n-butylthiophene/2-
isopropylthiazole with various aryl bromides are obtained in high
Initially, we studied the coupling of 2-n-butylfuran with 4-
bromoacetophenone in the presence of KOAc as base and complex 2a
as catalyst (Table 1, entry 1). Product 3 is obtained in 75% conversion. Dif-
ferent tests for the obtained coupling product 3 were also done using
other Pd-NHC complexes. Lower conversion for product 3 is obtained
when 2b is used. When the effects of 2c–2h in the formation of product
3 are analysed, conversions of 66–100% are seen. The best results are ob-
tained when complexes 2c and 2f–2h are used. Then, the effects of 2-n-
butylfuran with 4-bromoanisole, 4-bromotoluene and bromobenzene
were investigated in the presence of 2a–2h, as a result of which products
4–6 are obtained in 63–100% conversions (Table 1). Very good conver-
sions are obtained from the direct arylation at the 5-position of
4-bromoacetophenone containing electron-attracting group and
4-bromotoluene containing electron-donating group with 2-n-butylfuran.
Table 2. Arylation of 2-n-butylthiophene with various aryl bromidesa
Entry
R
Pd-NHC
Product
Conversion (%)b
1
2
3
4
5
6
7
8
p-COCH3
2a
2b
2c
2d
2e
2f
100
97
100
100
96
7
97
2g
2h
100
100
9
p-OCH3
p-CH3
H
2a
2b
2c
2d
2f
88
96
10
11
12
14
15
16
96
8
79
100
97
2g
2h
100
17
18
19
20
21
23
24
2a
2b
2c
89
72
93
89
86
97
82
2d
2e
2g
2h
9
25
26
27
28
30
31
32
2a
2b
2c
2d
2f
79
45
83
71
75
93
87
10
2g
2h
aReaction conditions: 2-n-butylthiophene (2 mmol), aryl bromide (1 mmol), Pd-NHC (2a–2h) (0.005 mmol), KOAc (1 mmol),
DMAc (2 ml), 130°C, 1 h. Product purity was checked using GC and NMR.
bConversions were calculated according to aryl bromide.
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
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S. Akkoç and Y. Gök
Table 3. Arylation of 2-isopropylthiazole with various aryl bromidesa
Entry
R
Pd-NHC
Product
Conversion (%)b
1
2
3
4
5
6
7
8
p-COCH3
2a
2b
2c
2d
2e
2f
97
98
98
88
87
72
89
88
11
2g
2h
9
p-OCH3
p-CH3
H
2a
2b
2c
2d
2f
96
91
10
11
12
14
15
16
90
12
13
14
97
100
98
2g
2h
100
17
18
19
20
22
23
24
2a
2b
2c
2d
2f
93
95
79
99
100
97
2g
2h
100
25
26
27
28
30
31
32
2a
2b
2c
2d
2f
91
100
83
94
88
2g
2h
90
89
aReaction conditions: 2-isopropylthiazole (2 mmol), aryl bromide (1 mmol), Pd-NHC (2a–2h) (0.005 mmol), KOAc (1 mmol),
DMAc (2 ml), 130°C, 1 h. Product purity was checked using GC and NMR.
bConversions were calculated according to aryl bromide.
Finally, 2-n-butylthiophene was coupled with 4-bromoace-
tophenone and 4-bromoanisole to give the arylated products 7
and 8 in excellent conversions. When 4-bromotoluene and
bromobenzene with 2-n-butylthiophene are utilized in the direct
arylation reaction, conversions of the obtained products 9 and 10
are seen to be lower than those of products 7 and 8 (Table 2). With
four different aryl bromides, 4-bromoacetophenone containing
electron-attracting group gives the best conversions in direct
arylation of 2-n-butylthiophene. It is found that the most effective
catalyst in the direct arylation of thiophene is 2g containing both
morpholinoethyl and naphthalenomethyl groups. Also, 2-isopro-
pylthiazole reacts with 4-bromoanisole leading to the desired prod-
uct 12 in 90–100% conversions (Table 3). Generally, good results are
obtained when aryl bromides together with heteroaromatic groups
are used in reaction. Almost all catalysts (2a, 2b, 2d, 2f, 2g and 2h)
are highly effective in the direct arylation of 2-isopropylthiazole. As
this study also illustrates, catalytic activities in direct arylation at the
5-position of different homogeneous Pd-NHC complexes were
investigated by our research group previously.[20] Similar catalytic
activity results were obtained in this study, too.
Conclusions
To summarize the research findings of this study, benzimidazole-
based Pd-NHC complexes 2a–2h, which are stable against air and
moisture, were synthesized using silver NHC complexes. Character-
ization of these complexes was done using NMR and FT-IR spectro-
scopic and elemental analysis techniques. The catalytic activities of
the eight novel Pd-NHC complexes were investigated in direct
arylation, which is an efficient method for sp2–sp2 C–C bond forma-
tion, of 2-n-butylfuran/2-n-butylthiophene/2-isopropylthiazole with
4-bromoacetophenone/4-bromoanisole/4-bromotoluene/bromo-
benzene at the 5-position. Desired CAr–CAr bond formation was
achieved efficiently with high regioselectivity. The 5-arylated com-
pounds (3–14) were obtained in high conversions in the presence
of KOAc as base, DMAc as solvent and complexes 2a–2h as catalysts.
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Appl. Organometal. Chem. (2014)

Catalytic activities in direct arylation of novel Pd-NHC complexes
[18] Z. Jina, X.-P. Gua, L.-L. Qiua, G.-P. Wua, H.-B. Songa, J.-X. Fang,
J. Organometal. Chem. 2011, 696, 859.
Acknowledgements
[19] E. Tyrrell, L. Whiteman, N. Williams, J. Organometal. Chem. 2011, 696, 3465.
[20] S. Akkoç, Y. Gök, M. Akkurt, M. N. Tahir, Inorg. Chim. Acta 2014,
413, 221.
This work was financially supported by İnönü University Research
Fund (İUBAP 2011/130).
[21] G. Roymahapatra, S. Giri, A. A. Danopoulos, P. K. Chattaraj,
A. Mahapatra, V. Bertolasi, J. Dinda, Inorg. Chim. Acta 2012, 383, 83.
[22] F. E. Hahn, M. C. Jahnke, Angew. Chem. 2008, 120, 3166.
[23] N. Marion, S. Díez-González, S. P. Nolan, Angew. Chem. Int. Ed. 2007,
46, 2988.
[24] T. Weskamp, F. J. Kohl, W. Hieringer, D. Gleigh, W. A. Herrmann, Angew.
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