Dichlorido(3-chloropyridine-N)[1,3-dialkylbenzimidazol-2-ylidene]palladium(II) complexes: Synthesis, characterization and catalytic activity in the arylation reaction

Article abstract of DOI:10.1016/j.ica.2015.01.019

Six novel Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation (PEPPSI) themed palladium N-heterocyclic carbene (Pd-NHC) complexes (2a-f) were synthesized in good yields from the reaction of 1,3-dialkylbenzimidazolium salts with PdCl

Full text of DOI:10.1016/j.ica.2015.01.019

Accepted Manuscript
Dichlorido(3-chloropyridine-N)[1,3-dialkylbenzimidazol-2-ylidene]palladi-
um(II) complexes: Synthesis, characterization and catalytic activity in the
arylation reaction
Senem Akkoç, Yetkin Gök
PII:
S0020-1693(15)00021-3
http://dx.doi.org/10.1016/j.ica.2015.01.019
ICA 16368
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
15 November 2014
9 January 2015
11 January 2015
Please cite this article as: S. Akkoç, Y. Gök, Dichlorido(3-chloropyridine-N)[1,3-dialkylbenzimidazol-2-
ylidene]palladium(II) complexes: Synthesis, characterization and catalytic activity in the arylation reaction,
Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.01.019
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Dichlorido(3-chloropyridine-N)[1,3-dialkylbenzimidazol-2-ylidene]palladium(II)
complexes: Synthesis, characterization and catalytic activity in the arylation reaction
Senem Akkoça* and Yetkin Gökb
aErciyes University, Faculty of Science, Department of Chemistry, 38039, Kayseri, Turkey
bInönü University, Faculty of Science and Arts, Department of Chemistry, 44280, Malatya,
Turkey
ABSTRACT
Six novel Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation
(PEPPSI) themed palladium N-heterocyclic carbene (Pd-NHC) complexes (2a-f) were
synthesized in good yields from the reaction of 1,3-dialkylbenzimidazolium salts with
PdCl2 in the presence of K2CO3 as base in 3-chloropyridine. These synthesized complexes
were characterized by means of elemental analysis, FT-IR, 1H and 13C NMR spectroscopic
methods. The fully characterized novel complexes (2a-f) were tested as catalysts in the
direct arylation of 2-n-propylthiazole, 2-n-butylthiophene and 2-n-butylfuran with different
aryl bromides at 130 ºC for 1 h. The complexes exhibited fairly high catalytic activities
under the given conditions. The highest conversions were achieved when complexes 2a, 2e
and 2f were used as catalysts in direct arylation.
Keywords: PEPPSI palladium complexes; N-heterocyclic carbene; Benzimidazole;
Catalyst; Direct arylation.
Corresponding author. Tel: +90 352 437 52 62 Fax: +90 352 437 49 33
E-mail address: senemakkoc@erciyes.edu.tr
1

1. Introduction
After the independent discovery of N-heterocyclic carbenes (NHCs) by Wanzlick
[1] and Öfele [2], the interest in NHCs has increased with the studies of Arduengo in 1991
[3]. NHCs have several interesting properties such as high stability, low toxicity, strong σ-
donating, weak π-accepting abilities, and steric and electronic effects in generating strong
bonds to a metal [4-8]. Because of these characteristic properties of metal NHC complexes,
various benzimidazole and imidazole based Pd-NHC and PEPPSI Pd-NHC complexes
have been used to carry out Sonogashira [9, 10], Suzuki-Miyaura [11-13], Kumada-
Tamao-Corriu [12, 14], Negishi [12, 15-17], Hiyama [18] and Mizoroki-Heck [4, 19]
cross-coupling reactions. The use of these complexes has also been reported for arylation
[20-22], amination [23], allylation [24], Buchwald-Hartwig amination [25] and
hydroarylation reactions [26].
Our interest was aroused by a study which describes synthesized Pd-PEPPSI
complexes using an imidazole nucleus by Organ et al. in 2006 [15]. A lot of work has been
done by many groups since 2006 on these types of complexes, which are extremely stable
in air and moisture. In general, an imidazole nucleus has been used in these kinds of
studies [27-31]. Therefore, we used a benzimidazole nucleus for this study. We present the
synthesis of six novel PEPPSI themed [1,3-dialkylbenzimidazol-2-ylidene]PdCl2(3-
chloropyridine) complexes (2a-f) which were prepared from 1,3-dialkylbenzimidazolium
salts with PdCl2 in the presence of K2CO3 as a base in 3-chloropyridine. The structures of
these compounds were identified with spectroscopic and elemental analysis techniques.
The novel generated PEPPSI complexes were tested for catalytic activity in the direct
arylation reaction. As a catalyst, these complexes gave very good results at short reaction
time and low catalyst loading (2a-f) when they were used in this reaction for the formation
of aryl-aryl bonds.
2

2. Experimental
2.1 Materials and methods
All reactions for the synthesis of the PEPPSI Pd-NHC complexes (2a-f) were done
using standard Schlenk type flasks and standard high vacuum line techniques. The solvents
used were of analytical grade. All reagents and solvents were purchased from Sigma-
Aldrich, Merck and Fluka. All 1H NMR and 13C NMR analyses were performed in CDCl3.
1
The H NMR and 13C NMR spectra were recorded by using a Bruker AC300P FT
spectrometer operating at 300.13 MHz (1H) and 75.47 MHz (13C). Chemical shifts (δ)
were given in ppm according to relative tetramethylsilane (TMS). The coupling constants
(J) were given in Hz. The FT-IR spectra of the synthesized novel complexes were recorded
in the 450–4000 cm-1 region with a Shimadzu FT-IR 8400 spectrophotometer. Melting
points were determined in open capillary tubes with an Electrothermal-9200 melting point
device. Gas chromatographic analysis was conducted with an Agilent 6890N Network GC
System using an HP-5 column with a length of 30 m, column diameter of 0.32 mm, a
column filler size of 0.25 µm and temperature range from -60 °C to 325 °C. Elemental
analyses were performed by using CHNS-932 LECO apparatus.
Synthesis
of
1,3-dibenzylbenzimidazol-2-ylidene-N-(3-
chloropyridine)dichloropalladium(II) complex, 2a
1,3-dibenzylbenzimidazolium salt (0.2 g, 0.6 mmol), PdCl2 (0.1 g, 0.6 mmol) and
K2CO3 (0.4 g) in 3-chloropyridine (3 mL) were heated at 80 ºC for 16 h. After completion
of the reaction, the reaction mixture was cooled to room temperature and was added to
dichloromethane (20 mL). The obtained mixture was filtered into a silica gel column and
then the solvent was removed under reduced pressure. The product was washed with
diethyl ether (3×8 mL) and dried under vacuum. Very light yellow colored crystals were
3

obtained; yield: 70 %; m.p.: 215-216 ºC; FT-IRν(CN): 1409.9 cm-1. 1H NMR (300.13 MHz,
13
CDCl3, 298 K, TMS), δ; 6.28 (s, 4 H, NCH2C6H5); 7.12-8.87 (m, 18 H, Ar-H). C NMR
(75.47 MHz, CDCl3), δ; 53.2 (NCH2C6H5); 111.5, 123.4, 124.9, 125.3, 127.7, 128.0,
128.2, 128.3, 129.1, 132.7, 134.9, 138.2, 149.3 and 150.3 (Ar-C); 163.4 (N-CNHC-N).
Elemental analysis calcd. (%) for C26H22Cl3N3Pd (589.25 g/mol): C 53.00, H 3.76, N 7.13;
found: C 52.96, H 3.80, N 7.11.
Synthesis
of
1-naphthalenomethyl-3-benzylbenzimidazol-2-ylidene-N-(3-
chloropyridine)dichloropalladium(II) complex, 2b
According to the same conditions and procedure for the 2a complex, the 2b
complex was prepared from 1-naphthalenomethyl-3-benzylbenzimidazolium salt (0.2 g,
0.5 mmol) and PdCl2 (0.09 g, 0.5 mmol) in 3-chloropyridine (3 mL) at 80 ºC for 16 h. A
cream colored powder was obtained; yield: 68 %; m.p.: 120-121 ºC; FT-IRν(CN): 1413.7
cm-1. 1H NMR (300.13 MHz, CDCl3, 298 K, TMS), δ; 6.31 (s, 2 H, NCH2C6H5); 6.72 (s, 2
H, NCH2C10H7); 7.08-7.92 (m, 20 H, Ar-H). 13C NMR (75.47 MHz, CDCl3), δ; 50.4
(NCH2C6H5); 53.4 (NCH2C10H7); 111.4, 111.5, 122.6, 123.5, 124.8, 125.6, 126.0, 126.8,
128.3, 128.9, 129.1, 130.1, 130.9, 132.6, 133.7, 135.0, 138.1, 147.5, 148.2 and 150.3 (Ar-
C); 164.2 (N-CNHC-N). Elemental analysis calcd. (%) for C30H24Cl3N3Pd (639.31 g/mol): C
56.36, H 3.78, N 6.57; found: C 56.30, H 3.82, N 6.55.
Synthesis
of
1-phenyl-3-naphthalenomethylbenzimidazol-2-ylidene-N-(3-
chloropyridine)dichloropalladium(II) complex, 2c
According to the same conditions and procedure for the 2a complex, the 2c
complex was prepared from 1-phenyl-3-naphthalenomethylbenzimidazolium salt (0.2 g,
4

0.5 mmol) and PdCl2 (0.09 g, 0.5 mmol) in 3-chloropyridine (3 mL). A cream colored
1
powder was obtained; yield: 69 %; m.p.: 149-150 ºC; FT-IRν(CN): 1503.8 cm-1. H NMR
(300.13 MHz, CDCl3, 298 K, TMS), δ; 6.78 (s, 2 H, NCH2C10H7); 6.99-8.89 (m, 20 H, Ar-
H). 13C NMR (75.47 MHz, CDCl3), δ; 50.4 (NCH2C10H7); 111.2, 111.5, 122.8, 123.9,
124.7, 125.7, 126.1, 126.9, 128.0, 128.8, 129.1, 129.7, 130.9, 132.5, 133.8, 134.1, 135.9,
137.0, 138.0, 149.1 and 150.2 (Ar-C); 164.1 (N-CNHC-N). Elemental analysis calcd. (%)
for C29H22Cl3N3Pd (625.28 g/mol): C 55.70, H 3.55, N 6.72; found: C 55.75, H 3.57, N
6.72.
Synthesis
of
1-naphthalenomethyl-3-(4-methylbenzyl)benzimidazol-2-ylidene-N-(3-
chloropyridine)dichloropalladium(II) complex, 2d
According to the same conditions and procedure for the 2a complex, the 2d
complex was prepared from 1-naphthalenomethyl-3-(4-methylbenzyl)benzimidazolium
salt (0.2 g, 0.5 mmol) and PdCl2 (0.09 g, 0.5 mmol) in 3-chloropyridine (3 mL). A cream
1
colored powder was obtained; yield: 71 %; m.p.: 107-108 ºC; FT-IRν(CN): 1507.2 cm-1. H
NMR (300.13 MHz, CDCl3, 298 K, TMS), δ; 2.36 (s, 3 H, NCH2C6H4CH3); 6.27 and 6.71
13
(s, 4 H, NCH2C6H4CH3; NCH2C10H7); 6.98-8.98 (m, 22 H, Ar-H). C NMR (75.47 MHz,
CDCl3), δ; 21.1 (NCH2C6H4CH3); 50.5 and 53.2 (NCH2C10H7; NCH2C6H4CH3); 111.4,
111.5, 122.6, 123.5, 124.8, 125.6, 126.0, 126.8, 128.3, 128.9, 129.1, 130.1, 130.9, 132.6,
133.7, 135.0, 138.0, 148.9, 149.2 and 150.3 (Ar-C); 163.9 (N-CNHC-N). Elemental analysis
calcd. (%) for C31H26Cl3N3Pd (653.34 g/mol): C 56.99, H 4.01, N 6.43; found: C 56.93, H
4.07, N 6.43.
5

Synthesis
of
1-phenyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazol-2-ylidene-N-(3-
chloropyridine)dichloropalladium(II) complex, 2e
According to the same conditions and procedure for the 2a complex, the 2e
complex was prepared from 1-phenyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazolium salt
(0.2 g, 0.5 mmol) and PdCl2 (0.09 g, 0.5 mmol) in 3-chloropyridine (3 mL). A very light
cream colored powder was obtained; yield: 65 %; m.p.: 212-213 ºC; FT-IRν(CN): 1500.5
1
cm-1. H NMR (300.13 MHz, CDCl3, 298 K, TMS), δ; 2.31 and 2.34 (s, 12 H, CH3); 6.36
13
[s, 2 H, NCH2CH(CH3)4]; 6.99-8.86 (m, 20 H, Ar-H). C NMR (75.47 MHz, CDCl3), δ;
16.5 and 20.6 (CH3); 51.0 [NCH2CH(CH3)4]; 110.9, 111.6, 123.2, 123.7, 124.8, 128.2,
129.5, 130.1, 132.7, 134.4, 135.3, 135.9, 137.0, 137.9, 147.6, 149.1 and 150.2 (Ar-C);
162.9 (N-CNHC-N). Elemental analysis calcd. (%) for C29H28Cl3N3Pd (631.33 g/mol): C
55.17, H 4.47, N 6.66; found: C 55.11, H 4.49, N 6.65.
Synthesis
of
1-phenyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazol-2-ylidene-N-(3-
chloropyridine)dichloropalladium(II) complex, 2f
According to the same conditions and procedure for the 2a complex, the 2f
complex was prepared from 1-phenyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazolium
salt (0.2 g, 0.5 mmol) and PdCl2 (0.09 g, 0.5 mmol) in 3-chloropyridine (3 mL). A cream
1
colored powder was obtained; yield: 63 %; m.p.: 241-242 ºC; FT-IRν(CN): 1508.2 cm-1. H
NMR (300.13 MHz, CDCl3, 298 K, TMS), δ; 2.09 and 2.45 (s, 15 H, CH3); 6.38 [s, 2 H,
13
NCH2C(CH3)5]; 6.75-8.87 (m, 20 H, Ar-H). C NMR (75.47 MHz, CDCl3), δ; 16.9, 17.4
and 17.5 (CH3); 51.7 [NCH2C(CH3)5]; 110.9, 111.7, 123.1, 123.6, 124.8, 127.4, 128.2,
129.5, 132.5, 133.2, 134.6, 134.8, 135.9, 136.2, 137.0, 137.9, 149.1 and 150.2 (Ar-C);
6

162.8 (N-CNHC-N). Elemental analysis calcd. (%) for C30H30Cl3N3Pd (645.36 g/mol): C
55.83, H 4.69, N 6.51; found: C 55.88, H 4.75, N 6.53.
2.2 General method for direct arylation
Heteroaryl derivative (2-n-butylfuran, 2-n-butylthiophene and 2-n-propylthiazole)
(2 mmol), aryl bromide (p-bromoacetophenone, p-bromoanisole, p-bromotoluene and
bromobenzene) (1 mmol), PEPPSI Pd-NHC complexes 2a-f (0.003 mmol), KOAc (1
mmol) and N,N-dimethylacetamide (DMAc) (2 mL) were added to a small Schlenk tube
equipped with a magnetic stirring bar as described in the literature [20-22]. The Schlenk
tube was heated in an oil bath at 130 ºC for 1 h. When the reaction was completed, the
solvent in the reaction media was removed by heating the reaction vessel under vacuum.
Then, the pentane and diethyl ether mixture (3:1) was added to the reaction ambient. As a
result of this process, the obtained product was purified by using flash chromatography on
silica gel. The purity of the compound was checked by GC and NMR. Conversions were
based on aryl bromides.
3. Results and Discussion
3.1 Synthesis of PEPPSI palladium N-heterocyclic carbene complexes
1,3-dialkylbenzimidazolium salts as carbene precursors, which were synthesized in
our previous studies [32, 33], were obtained by the reaction of N-alkylbenzimidazole with
alkyl halides in DMF at 80 ºC for 24 hrs. The PEPPSI Pd-NHC complexes were obtained
by the treatment of 1,3-dialkylbenzimidazolium salts with PdCl2 in the presence of K2CO3
in 3-chloropyridine at 80 oC for 16 h in 70 %, 68 %, 69 %, 71 %, 65 % and 63 % yields for
2a-f, respectively (Scheme 1). These compounds were soluble in different solvents such as
dimethyl sulfoxide, chloroform and dichloromethane. The formation of the six novel
7

1
symmetrical and nonsymmetrical substituted complexes was confirmed by FT-IR, H
NMR and 13C NMR spectroscopic methods and elemental analysis techniques. These
spectroscopic methods, which are seen to be spectroscopically pure, are consistent with the
recommended formulae. In the 1H NMR spectra, the benzimidazolium salts as compared to
the PEPPSI Pd-NHC complexes were identified by a characteristic proton peak at the 2-
position (NCHN) of the benzimidazole ring which appeared highly downfield shifted as
sharp singlets at 11.70, 9.59 and 11.38 ppm for 1c, 1e and 1f, respectively [31]. It was
known that the PEPPSI Pd-NHC complexes had successfully synthesized because the
signal of the acidic benzimidazolium C2-H proton between 9 and 12 ppm in the 1H NMR
spectra was not observed. The Pd-Ccarbene resonances of these novel PEPPSI Pd-NHC
complexes in the 13C NMR spectra appeared highly downfield shifted at δ ca. 163.4, 164.2,
164.1, 163.9, 162.9 and 162.8 ppm for 2a-f, respectively. When the results of the
elemental analysis, which is one of the analytical techniques used to prove the synthesis of
compounds, were evaluated, it was observed that the calculated values were very close to
the found values. The FT-IR data clearly indicated the presence of ν(CN) at 1409.9, 1413.7,
1503.8, 1507.2, 1500.5 and 1508.2 cm-1 for the PEPPSI Pd-NHC complexes (2a-f),
respectively. Unfortunately, we were unable to obtain an appropriate single crystal from
these new complexes for X-ray diffraction.
8

Cl
Pd
Cl
Cl
Pd
N
N
N
N
N
N
Cl
Cl
Cl
2a
2b
N
Cl
R
+
Cl
Pd
Cl
Cl
Pd
Cl
N
N
N
N
N
Cl- + PdCl2
+
5K2CO3
N
N
N
Cl
Cl
R'
1
2c
2f
Cl
Pd
Cl
Cl
Pd
Cl
N
N
N
N
N
N
Cl
Cl
2e
2d
Scheme 1. Synthesis of dichlorido(3-chloropyridine-N)[1,3-dialkylbenzimidazol-2-
ylidene]palladium(II) complexes (2a-f).
3.2 Catalytic practice of PEPPSI Pd-NHC complexes (2a-f) in direct arylation reactions
The catalytic activities in the direct arylation reactions of the six novel PEPPSI Pd-
NHC complexes (2a-f) as catalysts for forming aryl-aryl bonds, which are highly crucial in
9

organic chemistry, were investigated. Conversions of intended products changed
depending on the conditions of the reaction. When the catalytic activity results of the
PEPPSI Pd-NHC complexes were analyzed, it was found that the catalytic activity values
of these compounds were very similar to each other and they resulted in very high
conversions. Small differences can be attributed to the substituents present on the ligands.
Electron factors determined the reactivity of the substrates. Aryl bromides as substrates
can tolerate different functional groups like OCH3 and CH3. The best conversions were
acquired when aryl bromides containing a moderate level of both electron-donating and
electron-withdrawing groups were employed. However when aryl bromide containing the
electron-neutral group was used, lower conversions were obtained according to other
groups. The obtained results were given in Table 1-3.
3.2.1 Using PEPPSI Pd-NHC catalysts, the direct arylation of various heteroaromatic
groups with arylbromides
In our previous study, we saw that Pd-NHC complexes were very effective in the
direct arylation of different heteroaromatic groups with various aryl halides [21]. Thus, we
began by applying these reaction conditions in the direct arylation of novel PEPPSI Pd-
NHC complexes with different aryl bromides (Table 1-3). Initially, we commenced our
works by attempting to make the direct arylation at the 5-position of 2-n-propylthiazole
using PEPPSI Pd-NHC complexes as catalyst. In this screening process, the reaction of 2-
n-propylthiazole and p-bromoacetophenone was used as a model reaction. The following
reaction may be given as an example: treatment of 2-n-propylthiazole (2 mmol) with p-
bromoacetophenone (1 mmol) in the presence of PEPPSI Pd-NHC complex 2c (0.003
mmol) and potassium acetate (KOAc) (1 mmol) as a base in N,N-dimethylacetamide
o
(DMAc) (2 mL) by heating in an oil bath at 130 C for 1 h afforded product in 78 %
10

conversion. After that, we investigated the PEPPSI Pd-NHC catalyzed direct arylation at
the 5-position of 2-n-propyl thiazole with p-bromoanisole and p-bromotoluene in excellent
conversions; such as 99.9 % and 100 % (Table 1; entries: 4-8). Then, we conducted
research on the catalytic activities in the direct arylation of 2-n-butylthiophene and 2-n-
butyl furan rings with identical aryl bromides as used in the 2-n-propylthiazole in the
presence of PEPPSI Pd-NHC complexes (2a-f) as catalysts (Table 2, 3).
Table 1. Pd-NHC catalysed direct arylation of 2-n-propylthiazole by using aryl
bromides.[a]
N
N
DMAc, KOAc, 130 oC, 1h
2
+
R
Br
R
Pr
Pr
S
S
2a-f
Entry
Ar-Br
PEPPSI Pd-NHC
2c
Conv. (%)
78
1
2
3
4
5
6
7
8
O
97
2e
H3C C
Br
98
2f
100
2a
2c
100
H3CO
H3C
Br
100
2e
2a
2e
100
Br
99.9
[a]Reaction conditions: 2-n-propylthiazole (2 mmol), aryl bromide (1 mmol), PEPPSI Pd-
o
NHC (0.003 mmol), KOAc (1 mmol), DMAc (2 mL), 130 C, 1 h. Product purity was
checked by GC and NMR, conversions were calculated according to aryl bromide.
Table 2. Pd-NHC catalysed direct arylation of 2-n-butylfuran by using aryl bromides.[a]
DMAc, KOAc, 130 oC, 1h
2
R
+
R
Br
Bu
Bu
O
O
2a-f
11

Entry
1
Ar-Br
PEPPSI Pd-NHC
% Conv.
99
2a
2b
2e
2f
O
2
99.9
99.6
99
H3C C
Br
3
4
5
84
2a
2d
2e
2a
2b
2c
2d
2a
2b
2c
2d
2e
6
90
H3CO
H3C
Br
7
88
8
94
9
99
Br
10
11
12
13
14
15
16
76
90
87
75
Br
70
83
95
[a]Reaction conditions: 2-n-butylfuran (2 mmol), aryl bromide (1 mmol), PEPPSI Pd-NHC
o
(0.003 mmol), KOAc (1 mmol), DMAc (2 mL), 130 C, 1 h. Product purity was checked
by GC and NMR, conversions were calculated according to aryl bromide.
Use of p-bromoanisole with 2-n-propylthiazole gave product in 100 % conversion
(Table 1, entries: 4-6), but when p-bromoanisole was used with 2-n-butylfuran it resulted
in a lower conversion, such as 84 %, 88 % and 90 % (Table 2, entries: 5-7). When p-
bromoanisole was put into the reaction ambient instead of p-bromoacetophenone as a
coupling partner with 2-n-propylthiazole, the conversion of product increased (Table 1,
entries: 4-6). The use of p-bromoanisole with 2-n-butylthiophene gave the desired
coupling product for different complexes (2a, 2c and 2f) as catalysts in good conversion,
such as 96 %, 100 % and 98 %, respectively (Table 3, entries: 5-7). Aryl bromide bearing
12

an electron-donating group like p-bromotoluene smoothly coupled with 2-n-
propylthiazole, 2-n-butylfuran and 2-n-butylthiophene and provided the corresponding
products in moderate conversions (Table 1-3).
Table 3. Pd-NHC catalysed direct arylation of 2-n-butylthiophene by using aryl
bromides.[a]
DMAc, KOAc, 130 oC, 1h
2
R
+
R
Br
Bu
Bu
S
S
2a-f
Entry
Ar-Br
PEPPSI Pd-NHC
% Conv.
100
100
98
1
2
2a
2c
2e
2f
2a
2c
2f
2c
2f
2f
O
H3C C
Br
3
4
99
5
96
6
100
98
H3CO
H3C
Br
7
8
100
99
Br
9
10
89
Br
[a]Reaction conditions: 2-n-butylthiophene (2 mmol), aryl bromide (1 mmol), PEPPSI Pd-
o
NHC (0.003 mmol), KOAc (1 mmol), DMAc (2 mL), 130 C, 1 h. Product purity was
checked by GC and NMR, conversions were calculated according to aryl bromide.
In this study, as previously stated, we investigated the catalytic activities of six
novel PEPPSI Pd-NHC complexes in the direct arylation reaction using electron-
withdrawing, electron-donating and electron-neutral groups by applying short time periods
such as 1 h. The best results for p-bromoacetophenone and p-bromotoluene were obtained
13

with catalysts 2a, 2e and 2f. The best conversions for p-bromoanisole were gained with
catalysts 2c and 2f. Generally, the synthesized novel complexes were found to be effective
catalysts in the direct arylation reaction.
4. Conclusions
In summary, six novel Pd-PEPPSI-type NHC complexes (2a-f), which were
1
confirmed by FT-IR, H NMR, 13C NMR and elemental analysis, were synthesized from
different 1,3-dialkylbenzimidazolium salts with PdCl2 in the presence of K2CO3 as base in
3-chloropyridine in good yields. These novel PEPPSI Pd-NHC complexes, which were
obtained from the successful deprotonation of benzimidazolium salts, are practical
catalysts for the direct regioselective C5 arylation of heteroaromatic groups using both
electron-deficient and electron-rich aryl bromides as coupling partners. The best results in
terms of conversions were obtained with low catalysts loading.
Acknowledgments
We would like to thank to Erciyes University in Kayseri and İnönü University in
Malatya for their financial support.
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16

Synopsis Absract:
• PEPPSI themed six novel palladium N-heterocyclic carbene complexes were
synthesized.
• Their structures were characterized by elemental analysis and spectroscopic
methods.
• These complexes were tested as catalysts in the direct arylation and they exhibited
high catalytic activities.
17

Synopsis Absract:
Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation (PEPPSI)
themed six novel palladium N-heterocyclic carbene (Pd-NHC) complexes (2a-f) were
synthesized and characterized by elemental analysis and spectroscopic methods. These
novel complexes (2a-f) were tested as catalysts in the direct arylation of 2-n-
propylthiazole, 2-n-butylthiophene and 2-n-butylfuran with different aryl bromides at 130
ºC for 1 h. These complexes exhibited high catalytic activities.
Graphical abstract:
N
Cl
R
R
+
Cl
Pd
Cl
N
N
Cl- + PdCl2
+
5K2CO3
N
N
R'
N
Cl
R'
2a-f
1
In Pd-NHC catalysts (2a-f), direct arylation of 2-n-propyl thiazole, 2-n-butyl
thiophene and 2-n-butyl furan by using various aryl bromides.
E
A
Z
+
E
PEPPSI Pd-NHC (0.005 mmol)
R
A
Z
DMAc (2 mL), KOAc (1 mmol), 1 h
R
Br
E=S
E=S
E=O
Z=N
Z=H
Z=H
A=CH2CH2CH3
A=CH2CH2CH2CH3
A=CH2CH2CH2CH3
18