Carbon-carbon bond formation catalyzed by PEPPSI Pd-NHC

Article abstract of DOI:10.1016/j.ica.2017.01.025

Five new palladium complexes were efficiently synthesized from the reaction of benzimidazolium salts, potassium carbonate (K₂CO₃) and palladium chloride (PdCl₂) in pyridine (for 3–5) or 3-chloropyridine (for 6 and 7). The synthesized complexes were characterized and tested in Suzuki-Miyaura cross-coupling reaction as catalysts. In the presence of catalysts 3–7, biaryl products were obtained in moderate yields when phenylboronic acid was used as boronic acid derivative. However, the coupling of thianaphthene-2-boronic acid with 1-chloro-4-nitrobenzene generated low yields although a longer period of time was used in comparison to the coupling of phenylboronic acid with aryl chlorides.

Full text of DOI:10.1016/j.ica.2017.01.025

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Research paper
Carbon-carbon bond formation catalyzed by PEPPSI Pd-NHC
Senem Akkoç, İlhanÖzer İlhan, Yetkin Gök, Veysel Kayser
PII:
S0020-1693(16)31029-5
http://dx.doi.org/10.1016/j.ica.2017.01.025
ICA 17418
DOI:
Reference:
Inorganica Chimica Acta
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26 January 2017
29 January 2017
Please cite this article as: S. Akkoç, I. İlhan, Y. Gök, V. Kayser, Carbon-carbon bond formation catalyzed by PEPPSI
Pd-NHC, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.01.025
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Carbon-carbon bond formation catalyzed by PEPPSI Pd-NHC
Senem Akkoça,b*, İlhan Özer İlhana, Yetkin Gökc and Veysel Kayserb*
aDepartment of Chemistry, Faculty of Sciences, Erciyes University, Talas Street, 38039,
Kayseri, Turkey
bFaculty of Pharmacy, The University of Sydney, NSW, 2006, Sydney, Australia
cDepartment of Chemistry, Faculty of Arts and Sciences, Inönü University, 44280, Malatya,
Turkey
ABSTRACT
Five new palladium complexes were efficiently synthesized from the reaction of
benzimidazolium salts, potassium carbonate (K2CO3) and palladium chloride (PdCl2) in
pyridine (for 3-5) or 3-chloropyridine (for 6 and 7). The synthesized complexes were
characterized and tested in Suzuki-Miyaura cross-coupling reaction as catalysts. In the
presence of catalysts 3-7, biaryl products were obtained in moderate yields when
phenylboronic acid was used as boronic acid derivative. However, the coupling of
thianaphthene-2-boronic acid with 1-chloro-4-nitrobenzene generated low yields although a
longer period of time was used in comparison to the coupling of phenylboronic acid with aryl
chlorides.
Keywords: N-Heterocyclic carbene; PEPPSI complex; Palladium; Suzuki-Miyaura cross-
coupling reaction; Catalyst.
1

Corresponding author*: E-mail: senemakkoc44@gmail.com; senemakkoc@erciyes.edu.tr;
Tel: +90 352 437 52 62; Fax: +90 352 437 49 33.
Corresponding author*: E-mail: veysel.kayser@sydney.edu.au; T: +61 2 9351 3391; F: +61
2 9351 4391
1. Introduction
N-heterocyclic carbene (NHC) metal complexes have been known for the last fifty
years [1] and they have been tested in a range of fields [2-8]. The application of metal
complexes containing NHC has attracted much attention recently, in particular as catalysts in
carbon-carbon (C-C) and carbon-nitrogen (C-N) bond forming reactions [2, 3, 9-11]. For
example, the uploading of low amounts of these compounds as catalysts has been found to be
very effective for Suzuki-Miyaura cross-coupling [9].
Suzuki-Miyaura cross-coupling reaction is mostly used to synthesize substituted
biphenyls and poly-olefins and can be described as a coupling of boronic acid derivatives (R1-
BY2) with mesylate, halide or triflate (R2-X) using a palladium catalyst in the presence of a
base (Scheme 1) [12, 13]. One of the important aspects of this reaction is that green solvents,
especially water, can be used which are inexpensive, non-flammable, non-toxic and can be
easily separated from organic products. This additionally increases the value and importance
of this reaction.
R1-BY2
R2-X
R1-R2
Pd catalyst
+
Base, solvent
Scheme 1. General definition of Suzuki-Miyaura cross-coupling [12].
2

Herein, we tested the catalytic properties of five newly synthesized benzimidazole-
based Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation (PEPPSI) Pd-
NHC complexes in Suzuki-Miyaura cross-coupling reaction for the formation of biaryl
products. Two different boronic acid derivatives (thianaphthene-2-boronic acid and
phenylboronic acid) and various aryl chlorides (4-chloroacetophenone, 4-methoxy-1-
chlorobenzene, 4-chlorotoluene and 1-chloro-4-nitrobenzene) were used for this coupling
reaction. Catalysts 3-7 were mucher effective in the all coupling of phenylboronic acid with
aryl chlorides than the coupling of thianaphthene-2-boronic acid with 1-chloro-4-nitrobenzene
at 80 oC.
2. Experimental
2.1. General considerations
All reactions and purifications for the preparation of 1-7 were carried out under
ambient conditions in 100 mL Schlenk tubes. All reagents and solvents were purchased from
Sigma-Aldrich (Interlab A.S., USA), Scharlau (Barcelona, Spain) and Merck (Darmstadt,
1
Germany), and were used without further purification. H and 13C NMR spectra were
performed in CDCl3 solvent and were recorded using a Bruker-300 or 400 MHz Ultra Shield
instrument. Chemical shifts (δ) were given in ppm relative to tetramethylsilane (TMS).
1
Coupling constants (J) were given in hertz (Hz). H NMR signals are labeled as singlet (s),
doublet (d), pentet (p) and multiplet (m). An Agilent Technologies 6890N Network GC
system was used for the catalytic experiment measurements. An Electrothermal-9200 melting
point apparatus was used for measuring melting points. A Shimadzu FT-IR 8400
spectrophotometer was used for recording the FT-IR spectra of compounds.
3

2.2. New palladium complexes
2.2.1. General preparation of compounds, 3-7
The benzimidazolium salts used in this study were synthesized as described previously
[14, 15]. Compounds 3-7 were prepared from the reaction of 2 (1 mmol), PdCl2 (1 mmol) and
o
K2CO3 (5 mmol) in pyridine or 3-chloropyridine (3 mL) for 16 h at 80 C. The solvent was
removed under vacuum. The obtained complexes were washed with diethyl ether (3 x 5 mL)
and dried under vacuum and had different colors. These benzimidazole-based complexes were
stable against atmospheric moisture and air. Compounds 3-7 were further characterized by 1H
NMR, 13C NMR, FT-IR and were soluble in organic solvents such as chloroform and
dichloromethane.
2.2.2. Data for the complexes
2.2.2.1.
For
[1-phenyl-3-(3,4,5-trimethoxybenzyl)benzimidazol-2-
ylidene](pyridyl)palladium(II) dichloride, 3
1-phenyl-3-(3,4,5-trimethoxybenzyl)benzimidazolium chloride (0.2 g, 1 mmol),
K2CO3 (0.34 g, 5 mmol) and PdCl2 (0.086 g, 1 mmol) in pyridine (3 mL) were stirred at 80 oC
for 16 h. After the reaction was finished, 10 mL of dichloromethane was added to the reaction
medium. The resulting solution mixture was filtered on silica gel and through a pad of celite
in order to remove unreacted PdCl2. The solvent in the reaction medium was removed and
dried under vacuum. The reaction yielded as a cream colored solid (C28H27N3O3Cl2Pd: 630.86
g/mol), which was washed thrice with 5 mL of diethyl ether. Yield: 150 mg, 48.9%; m.p.:
198-199 ºC; color: cream; IR: 1234.3 (C-O); 1409.9 (CN); 2831.3, 2912.3 and 2993.3
(aliphatic C-H); 3058.9 cm-1 (aromatic C-H). 1H NMR (300 MHz, 298 K, CDCl3), δ: 3.85 and
3.89 [m, 9 H, NCH2C6H2(OCH3)3-3,4,5]; 6.25 [s, 2 H, NCH2C6H2(OCH3)3-3,4,5]; 7.03-8.87
(m, 16 H, Ar-H). 13C NMR (75 MHz, 298 K, CDCl3), δ: 53.68, 56.65 and 60.85
4

[NCH2C6H2(OCH3)3-3,4,5]; 65.87 [NCH2C6H2(OCH3)3-3,4,5]; 105.47, 111.16, 111.58,
123.76, 123.80, 124.49, 125.01, 127.96, 129.38, 129.63, 130.53, 133.97, 136.01, 137.01,
137.82, 138.07, 138.64, 151.16, 153.36 and 153.69 (Ar-C); 164.71 (NCN).
2.2.2.2.
For
[1-phenyl-3-(2,3,5,6-tetramethlbenzyl)benzimidazol-2-ylidene]
(pyridyl)palladium(II) dichloride, 4
Complex 4 (C29H29N3Cl2Pd: 596.89 g/mol) was synthesized under the same conditions
and procedure as for 3, from 1-phenyl-3-(2,3,5,6-tetramethylbenzyl)benzimidazolium chloride
(0.2 g, 1 mmol), K2CO3 (0.37 g, 5 mmol) and PdCl2 (0.094 g, 1 mmol) in pyridine (3 mL).
Yield: 210 mg, 66.3%; m.p.: 275-276 ºC; color: cream; IR: 1446.5 (CN); 2964.4 (aliphatic C-
H); 3022.2 and 3080.1 cm-1 (aromatic C-H). 1H NMR (300 MHz, 298 K, CDCl3), δ: 2.25 and
2.38 [m, 12 H, NCH2C6H(CH3)4-2,3,5,6]; 6.42 [s, 2 H, NCH2C6H(CH3)4-2,3,5,6]; 6.99-8.83
13
(m, 14 H, Ar-H). C NMR (75 MHz, 298 K, CDCl3), δ: 16.53 and 20.63 [NCH2C6H(CH3)4-
2,3,5,6]; 51.19 [NCH2C6H(CH3)4-2,3,5,6]; 110.89, 111.60, 123.12, 123.58, 124.41, 125.01,
128.18, 129.30, 129.50, 130.20, 132.71, 134.36, 134.49, 135.25, 135.90, 137.12, 137.96,
149.35 and 151.14 (Ar-C); 164.41 (NCN).
2.2.2.3.
For
[1-phenyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazol-2-
ylidene](pyridyl)palladium(II) dichloride, 5
Complex 5 (C30H31N3Cl2Pd: 610.91 g/mol) was synthesized under the same conditions
and procedure as for 3, from 1-phenyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazolium
chloride (0.2 g, 1 mmol), K2CO3 (0.35 g, 5 mmol) and PdCl2 (0.09 g, 1 mmol) in pyridine (3
mL). Yield: 160 mg, 51.1%; m.p.: 202-204 ºC; color: cream; IR: 1479.3 (CN); 2912.3
5

1
(aliphatic C-H); 3006.8 cm-1 (aromatic C-H). H NMR (400 MHz, 298 K, CDCl3), δ: 2.22-
2.43 [m, 15 H, NCH2C6(CH3)5-2,3,4,5,6]; 5.29 [s, 2 H, NCH2C6(CH3)5-2,3,4,5,6]; 6.35-8.83
(m, 14 H, Ar-H). 13C NMR (100 MHz, 298 K, CDCl3), δ: 16.45, 17.08, 17.49, 18.02 and
42.33 [NCH2C6(CH3)5-2,3,4,5,6]; 51.88 [NCH2C6(CH3)5-2,3,4,5,6]; 108.39, 109.18, 110.80,
111.77, 120.85, 121.85, 123.70, 124.35, 126.11, 127.55, 128.63, 129.60, 130.23, 133.17,
134.59, 135.19, 136.18, 137.16, 137.89, 149.86, 151.15, 152.31 and 153.36 (Ar-C); 164.30
(NCN).
2.2.2.4.
For
[1-phenyl-3-((2,3-dihydrobenzo[b][1,4]dioxin-2-yl)methyl)benzimidazol-2-
yliden](3-chloropyridyl)palladium(II) dichloride, 6
Complex 6 (C27H22N3Cl3O2Pd: 633.26 g/mol) was synthesized under the same
conditions and procedure as for 3, from 1-phenyl-3-((2,3-dihydrobenzo[b][1,4]dioxin-2-
yl)methyl) benzimidazolium bromide (0.2 g, 1 mmol), K2CO3 (0.33 g, 5 mmol) and PdCl2
(0.08 g, 1 mmol) in 3-chloropyridine (3 mL). Yield: 150 mg, 50.2%; m.p.: 191-193 ºC; color:
light brown; IR: 1255.6 and 1259.4 (C-O); 1488.9 (CN); 2964.4 (aliphatic C-H); 3030.0 cm-1
1
(aromatic C-H). H NMR (400 MHz, 298 K, CDCl3), δ: 4.54 [d, J: 8.0 Hz, 2 H,
NCH2CHCH2O2C6H4]; 5.05 [p, J: 8.0 Hz, 1 H, NCH2CHCH2O2C6H4]; 5.39 [d, J: 8.0 Hz, 2 H,
NCH2CHCH2O2C6H4]; 6.88-8.90 (m, 17 H, Ar-H). 13C NMR (100 MHz, 298 K, CDCl3), δ:
48.87 [NCH2CHCH2O]; 65.25 [NCH2CHCH2O]; 72.13 [NCH2CHCH2O]; 111.29, 117.56,
121.68, 122.16, 123.85, 124.96, 125.94, 127.92, 128.28, 129.65, 132.67, 135.09, 135.75,
136.89, 138.12, 142.34, 143.10, 149.86, 150.88 and 151.47 (Ar-C); 163.80 (NCN).
6

2.2.2.5.
For
[1-phenyl-3-naphthalen-1-ylmethylbenzimidazol-2-ylidene](3-
chloropyridyl)palladium(II) dichloride, 7
Complex 7 (C29H22N3Cl3Pd: 625.28 g/mol) was synthesized under the same conditions
and procedure as for 3, from 1-phenyl-3-naphthalen-1-ylmethylbenzimidazolium chloride (0.2
g, 1 mmol), K2CO3 (0.37 g, 5 mmol) and PdCl2 (0.09 g, 1 mmol) in 3-chloropyridine (3 mL).
Yield: 70 mg, 20.8%; m.p.: 197-198 ºC; color: dark brown; IR: 1402.2 (CN); 2912.3 and
1
2970.2 (aliphatic C-H); 3058.9 cm-1 (aromatic C-H). H NMR (400 MHz, 298 K, CDCl3), δ:
13
5.66 [s, 2 H, NCH2C10H7]; 6.78-8.89 (m, 20 H, Ar-H). C NMR (100 MHz, 298 K, CDCl3),
δ: 65.88 [NCH2C10H7]; 108.75, 109.11, 111.51, 121.56, 122.02, 123.32, 124.72, 125.23,
125.76, 126.06, 126.89, 127.70, 128.00, 128.86, 129.10, 129.73, 131.21, 132.86, 133.89,
134.74, 137.99 and 138.84 (Ar-C); 151.41 (NCN).
2.3. General procedure for the Suzuki-Miyaura reaction
A typical Suzuki-Miyaura cross-coupling reaction was performed using 1 mmol aryl
chloride, 1.5 mmol boronic acid derivative, 2 mmol base, 0.008 mmol compounds 3-7 as
catalyst and N,N-dimethylformamide (DMF)-water (H2O) (2-2 mL) as solvent in a 25 mL
Schlenk flask . This mixture was heated in an oil bath for the duration of the desired reaction
time. After the 5 mL of hexane and 1 mL of ethyl acetate were added to the reaction medium,
the solution was filtered using a mini-column for purification. The purity and yield of the
compounds was checked by GC. The yields were related to the unreacted aryl chlorides.
3. Results and Discussion
3.1. Synthesis of PEPPSI Pd-NHC complexes, 3-7
7

Compounds 3-7 were prepared from benzimidazolium salts, K2CO3 and PdCl2 in
pyridine (for 3-5) or 3-chloropyridine (for 6 and 7) (Scheme 1) as described previously [3, 4].
The Pd-NHC complexes (3-7) were characterized by FT-IR, 1H and 13C NMR spectroscopic
methods. In the 1H NMR spectra, benzylic proton (NCH2) was resonated at low fields δ 6.25,
6.42, 5.29, 4.54 and 5.66 cm-1 for 3-7, respectively. The absence of NCHN signal proton in a
downfield for 3-7 indicated the successful formation of NHC complexes. The structure of
expected complexes was further verified with the 13C NMR: the carbene carbon resonance
(for metal bound carbon atom in 2-position, NCN-Pd) shifted significantly downfield at δ
164.71, 164.41, 164.30, 163.80 and 151.41 ppm for 3-7, respectively. Specifically, this signal
was observed in the upfield area for benzimidazolium salts compared to that of the metal
complexes [14, 15]. In addition, benzylic carbon (NCH2) was resonated at δ 65.87, 51.19,
51.88, 48.87 and 65.88 cm-1 for 3-7, respectively. Lastly, the FT-IR data indicated the
presence of ν(CN) at 1409.9, 1446.5, 1479.3, 1488.9 and 1402.2 cm-1 for 3-7, respectively. The
same band appeared in a different region for benzimidazolium salts; in between 1500 and
1595 cm-1 [14, 15]. The structures of the complexes 3-7 derived benzimidazolium salts were
verified by the appearance of this band in different regions.
8

Cl
Pd
Cl
Pd
Cl
N
N
N
N
N
N
Cl
O
O
3
O
4
Cl
Pd
Cl
N
N
N
5
X
NH
N
N
N
+
N
(CH3O)2CHNMe2
R
X-
NH2
DMF
1
R
2
Cl
Pd
Cl
N
N
N
+
+
Cl
N
O
Y
PdCl2
+
O
6
K2CO3
Cl
Pd
Cl
N
N
N
Cl
7
Scheme 1. Synthesis of new PEPPSI Pd-NHC complexes.
3.3. Catalytic applications of synthesized compounds in Suzuki-Miyaura coupling reaction
The air stable compound 3 was tested for its catalytic activity in order to determine a
suitable reaction time for the formation of biaryl product from phenylboronic acid (1.5 mmol)
with 4-chloroacetophenone (1.0 mmol) under very mild reaction conditions using K2CO3 (2.0
mmol) as an inorganic base and a mixture of DMF/H2O (2/2 mL) as solvent.
9

Fig. 1. The effect of time on Suzuki-Miyaura cross-coupling of phenylboronic acid with 4-
chloroacetophenone as catalyzed by 3.
O
3 (0.008 mmol)
K2CO3, DMF/H2O, 80 oC
Cl
COCH3
B(OH)2
+
CH3
Yield changes depending on time
91.4
100
90
80
70
60
50
40
30
20
10
0
90.3
90.7
66
30
60
90
120
Time (minute)
Time-screening experiments showed that the reaction resulted in only a 66% yield in
30 minutes, but the yield was 91% when the reaction time was extended until 2 h (Fig. 1).
Nevertheless, the yield remained almost the same between 1 h to 1.5 h and it was mostly
completed in an hour with a high yield (Fig. 1).
Table 1
Suzuki-Miyaura cross-coupling of phenylboronic acid with three different aryl chlorides. a, b
3-7 (0.008 mmol)
B(OH)2
R
+
R
Cl
K2CO3, DMF/H2O
80 oC, 1.5 h
Entry
R
Pd-NHC Yield
1
2
CH3
CH3
4
5
67
30
10

3
4
CH3
6
7
3
4
5
6
7
3
4
5
6
7
21
99.9
43
CH3
5
OCH3
OCH3
OCH3
OCH3
OCH3
COCH3
COCH3
COCH3
COCH3
COCH3
6
99.9
47
7
8
54
95
9
10
11
12
13
14
91
98
95
99
90
aReaction condition: phenylboronic acid (1.5 mmol), aryl chlorides (1.0 mmol), K2CO3 (2.0 mmol), 3-7 (0.008
mmol), DMF-H2O (2-2 mL). bYields were determined by GC.
After determining a suitable reaction time for the formation of the C-C bond, the
synthesized complexes were tested for their catalytic qualities in the coupling reaction of
phenylboronic acid with different aryl chlorides for 1.5 h at 80 °C. When 4-chlorotoluene as
aryl halide was used, a better result was obtained from the catalyst 7 compared to that of the
catalysts 4-6 under the same experimental conditions (Table 1, entry 4). The yield was 99%
(Table 1, entry 4). Among the five complexes 3-7, 2,3,5,6-tetramethylbenzyl substituted with
compound 4 displayed the top catalytic activity for the coupling of 4-chloroanisole with
phenylboronic acid (Table 1, entry 6). Naphthalen-1-ylmethyl substituted with compound 7
was also found to be very effective in the coupling of 4-methoxy-1-chlorobenzene with
phenylboronic acid (Table 1, entry 9). However, 2,3,4,5,6-pentamethylbenzyl substituted with
compound 5 and ((2,3-dihydrobenzo[b][1,4]dioxin-2-yl)methyl) substituted with compound 6
showed lower catalytic activities for the same coupling reaction (Table 1, entries 2, 3, 7 and
8). Finally, catalyst properties of these complexes were tested in the carbon-carbon bond
formation of 4-chloroacetophenone with phenylboronic acid. All catalyst candidates gave
very high activity in this coupling reaction. Surprisingly, even compounds 5 and 6 exhibited
potential catalyst properties for this reaction as opposed to earlier two couplings (Table 1,
entries 2 and 3).
11

Fig. 2. Suzuki-Miyaura cross-coupling of thianaphthene-2-boronic acid with 1-chloro-4-
nitrobenzene. a, b
S
S
3-7 (0.008 mmol)
DMF-H2O
B(OH)2
O2N
Cl
+
+
NO2
K2CO3
3h, 80 oC
Yield changes depending on catalysts
14.35
16
14
12
10
8
9.4
5.8
6
4
1.05
2
3.7
0
3
4
5
6
7
Catalysts
In the cross-coupling of 1-chloro-4-nitrobenzene (1.0 mmol) with thianaphthene-2-
boronic acid (1.5 mmol), complexes 3-7 showed a wide range of catalytic properties with
o
varying yields from 1.05 to 14.05% after 3 h at 80 C in DMF-H2O (2-2 mL) mixture with
compound loadings of 0.8 mmol % and 2 mmol of K2CO3 (Fig. 2). Compounds 6 and 7
containing the 3-chloropyridine group showed better catalytic activities than those of
compounds 3-5 containing pyridine groups. Higher reaction yield for the product 2-(4-
nitrophenyl)benzo[b]thiophene was obtained when compound 6 containing the ((2,3-
dihydrobenzo[b][1,4]dioxin-2-yl)methyl) group was used compared to the rest of the
compounds (Fig. 2).
4. Conclusion
12

Several new PEPPSI Pd-NHC complexes were synthesized, characterized and their
catalytic activities were determined. Overall, compounds 3-7 demonstrated good catalytic
activities for the coupling of phenylboronic acid with 4-chloroacetophenone in a Suzuki-
Miyaura reaction. However, lower catalytic activities were observed for the coupling
reactions of 1-chloro-4-nitrobenzene with thianaphthene-2-boronic acid although the reaction
was maintained for a longer period of time. Finally, the compound 7 displayed superior
catalytic properties compared to the other complexes and is a good candidate to use as a
catalyst in C-C bond forming reactions.
Acknowledgments
This work was financially supported by Erciyes University Research Fund (FBA-
2013-4307).
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−) menthyl and (+)
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14

GRAPHICAL ABSTRACT
Cl
Pd
Cl
X
N
N
NH
N
N
N
3-chloropyridine
PdCl2, K2CO3
(CH3O)2CHNMe2
R
X-
N
+
N
NH2
DMF
Y
1
R
R
2
3-7
Tested in Suzuki-Miyaura
cross-coupling reaction
as catalysts

Highlights
•
New PEPPSI Pd-NHC complexes (3-7) were efficiently synthesized and
characterized.
•
•
These complexes were tested in Suzuki-Miyaura cross-coupling reaction as catalysts.
Biaryl products were obtained in moderate yields in the presence of catalysts 3-7.
15