Synthesis, characterization, and microwave-promoted catalytic activity of novel benzimidazole salts bearing silicon-containing substituents in Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions under aerobic conditions

Article abstract of DOI:10.3906/kim-1109-5

A number of benzimidazole derivatives (1-8) were synthesized and the catalytic activity of these compounds in a catalytic system including Pd(OAc)₂ and K₂CO₃ in DMF-H₂O was evaluated in Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions of aryl iodides, bromides, and chlorides with styrene and arylboronic acids under microwave irradiation and aerobic conditions. The yields of both the Heck-Mizoroki and the Suzuki-Miyaura cross coupling reactions with aryl iodides and aryl bromides were nearly quantitative. The synthesized 1-substituted benzimidazole (1) and benzimidazole salts (2-8) were identified by ¹Hand ¹³C-NMR and IR spectroscopic methods, and micro analysis. The molecular structure of 7 was also determined by X-ray crystallography.

Full text of DOI:10.3906/kim-1109-5

Turk J Chem
36 (2012) , 201 – 217.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-1109-5
Synthesis, characterization, and microwave-promoted
catalytic activity of novel benzimidazole salts bearing
silicon-containing substituents in Heck-Mizoroki and
Suzuki-Miyaura cross-coupling reactions under aerobic
conditions∗
1,†
2
1
1
3
¨
¨
˙
˙
¨
3
˙
4
Hasan KUC¸ UKBAY , Nihat S¸IRECI , Ulku¨ YILMAZ , Selma DENIZ , Mehmet AKKURT ,
¨
¨
¨
¨
Zeliha BAKTIR ,Orhan BUYUKGUNGOR
1
˙
Department of Chemistry, Faculty of Science and Arts, Ino¨nu¨ University,
44280 Malatya-TURKEY
e-mail: hkucukbay@inonu.edu.tr
2Faculty of Education, Adıyaman University, 02040 Adıyaman-TURKEY
3Department of Physics, Faculty of Sciences, Erciyes University,
38039 Kayseri-TURKEY
4Department of Physics, Faculty of Science and Arts, Ondokuz Mayıs University,
55139 Samsun-TURKEY
Received: 13.09.2011
A number of benzimidazole derivatives (1-8) were synthesized and the catalytic activity of these com-
pounds in a catalytic system including Pd(OAc)2 and K2 CO3 in DMF-H2 O was evaluated in Heck-Mizoroki
and Suzuki-Miyaura cross-coupling reactions of aryl iodides, bromides, and chlorides with styrene and aryl-
boronic acids under microwave irradiation and aerobic conditions. The yields of both the Heck-Mizoroki and
the Suzuki-Miyaura cross coupling reactions with aryl iodides and aryl bromides were nearly quantitative.
The synthesized 1-substituted benzimidazole (1) and benzimidazole salts (2-8) were identified by 1 H-
and 13 C-NMR and IR spectroscopic methods, and micro analysis. The molecular structure of 7 was also
determined by X-ray crystallography.
Key Words: Benzimidazole salt, N-heterocyclic carbene, palladium catalysis, Heck-Mizoroki coupling
reaction, Suzuki-Miyaura coupling reaction, microwave, crystal structure analysis
∗Dedicated to Prof. Dr. Christian Bruneu for his 60th birthday.
†Corresponding author
201

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Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
Introduction
During the past 50 years, the most important carbon-carbon bond forming methodologies have involved using
transition metals to mediate the reactions in a controlled and selective manner.1 In particular, the discovery of
palladium catalyzed C-C coupling reactions, such as Heck-Mizoroki coupling2,3 in the early 1970s and Suzuki-
Miyaura coupling4 in the early 1980s, has been applied to a diverse array of fields, ranging from natural products’
synthesis to material science, including biologically important molecules using as drugs and agrochemicals.5
Stilbene and biaryl moieties, which are products of Heck-Mizoroki and Suzuki-Miyaura cross coupling reactions,
respectively, are high-value synthetic targets and can be found as substructures in many pharmaceutically and
biologically active compounds.
Despite the fact that they are air-sensitive, degradable at high temperature, and toxic, tertiary phosphines
are employed as ligand for palladium complexes in both Heck-Mizoroki and Suzuki-Miyaura cross-coupling
reactions.6 The major limitation with phosphine ligands in catalytic reactions is the termination of the catalytic
cycle due to oxidation of phosphines to phosphine oxides, formation of stable phosphido-bridged catalytically
inactive dimers, and the cleavage of the P-C bond, causing degradation of the ligand. These problems led
chemists to discover new phosphorous free ligands.7
8,9
¨
Since the first synthesis of N-heterocyclic carbene complexes, by Ofele and Wanzlick
in the 1960s
and the isolation of free carbenes by Arduengo et al.10 in 1991, N-heterocyclic carbenes (NHC) have gained
remarkable attention.11−13 They have become a very important class of ligands due to their stability to air and
moisture, their low toxicity, and their strong σ -donor but π -acceptor abilities. They are considered alternatives
to phosphine ligands in metal complexes. For this reason, the design and synthesis of novel and effective NHCs
have attracted a great deal of attention from both academia and industry.14
However, both NHCs and electron-rich alkenes that can be used as a source of NHCs are highly air
and moisture sensitive and require handling under strict inert conditions.15−17 In contrast to the cumbersome
preparation and isolation of NHCs and electron-rich alkenes, in situ generation of NHCs has the advantages
of using a strong base, diazolium salts, and a common palladium source such as PdCl2 or Pd(OAc)2 under
mild conditions in a number of catalytic syntheses, particularly C-C cross-coupling reactions. In particular,
Pd(OAc)2 /benzimidazole or imidazole ligands could be very effective catalytic system in these reactions. 18−23
Furthermore, microwave-promoted synthesis is an area of increasing interest in both academic and
industrial laboratories. In recent years, heating and driving chemical reactions by microwave energy has been
an increasingly popular theme in the scientific community. The use of metal catalysts in conjunction with
microwaves may have significant advantages over traditional heating methods since the inverted temperature
gradient under microwave conditions may lead to an increased lifetime of catalyst through the elimination of
wall effects.24 There are extensive studies about Heck-Mizoroki and Suzuki-Miyaura type C-C cross-coupling
reactions incorporating microwave irradiation using various ligands other than benzimidazole moiety.25−33
However, to the best of our knowledge, there is only one example about the catalytic properties of
alkylsilyl substituted benzimidazole derivatives in Suzuki-Miyaura C-C cross-coupling reactions under microwave
irradiation published in the literature.22 The nature, size, and electronic properties of the substituents on the
nitrogen atom(s) of the benzimidazole may play a crucial role in tuning the catalytic activity. In order to find
a more efficient palladium catalyst, we synthesized a series of new benzimidazole salts, II, III, 2-8 (Scheme 1),
202

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Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
containing trimethylsilylmethyl or dimethylphenylsilylmethyl moieties, and we aimed to investigate the activity
of in-situ prepared Pd-NHC based catalytic systems for the Heck-Mizoroki and Suzuki-Miyaura cross-coupling
reactions under microwave heating.
CH3
Si
CH3
Si
H
N
L
L
1.
KOH/EtOH
2.(CH3)2L SiCH2Cl
CH3
CH3
N
N
N
+
DMF
-
+
R X
X
N
N
R
I L: -CH3
1 L: -C6H6
II R: -CH2CH=CH2, L: CH3, X: Br
III R: -CH2-C6H5, L: CH3, X: Cl
2 R: -CH2CH2CH2CN, L: CH3, X: Cl
3 R: -CH2C6H4CH3-p, L: CH3, X: Br
4 R: -CH2CH2C6H5, L: CH3, X: Br
5 R: -CH3, L: C6H5, X: I
6 R: -CH2CH3, L: C6H5, X: I
7 R: -CH2CH=CH2, L: C6H5, X: Br
8 R: -CH2C6H5, L: C6H5, X: Cl
Scheme 1. Synthesis pathways of the new silyl substituted benzimidazole derivatives.
Herein, we describe the synthesis of new benzimidazole (1) and benzimidazole salts (1-8) containing
trimethylsilylmethyl or dimethylphenyl moieties. The compounds were fully characterized by elemental anal-
ysis, IR, 13 C-NMR, and 1 H-NMR spectroscopy and the molecular structure of 7 was determined by X-ray
crystallography. We also report the microwave-assisted catalytic activity of Pd(OAc)2 /trimethylsilylmethyl or
dimethylphenyl substituted benzimidazole catalytic system in Heck-Mizoroki and Suzuki-Miyaura cross-coupling
reactions.
Experimental section
General chemical procedure
All preparations were carried out in an atmosphere of purified argon using standard Schlenk techniques.
Starting materials and reagents used in the reactions were supplied commercially by Aldrich or Merck Chemical
Co. Solvents were dried with standard methods and freshly distilled prior to use. All catalytic activity
experiments were carried out in a microwave oven system manufactured by Milestone (Milestone Start S
Microwave Labstation for Synthesis) under the aerobic conditions. 1 H-NMR (300 MHz) and 13 C-NMR (75
MHz) spectra were recorded using a Bruker DPX-300 high performance digital FT NMR spectrometer. Infrared
spectra were recorded as KBr pellets in the range 4000-400 cm−1 on a Perkin-Elmer FT-IR spectrophotometer.
Elemental analyses were performed by LECO CHNS-932 elemental analyzer. Melting points were recorded
using an electrothermal-9200 melting point apparatus, and are uncorrected.
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Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
Benzimidazole derivatives, I,34 II,35 and III,36 used in this work were prepared according to the
literature procedures.
GC-MS analysis
GC-MS spectra were recorded on an Agilient 6890 N GC and 5973 Mass Selective Detector with an HP-
INNOWAX column of 60 m length, 0.25 mm diameter, and 0.25 μm film thicknesses. GC-MS parameters for
both Suzuki and Heck-Mizoroki coupling reactions were as follows: initial temperature 60 ◦ C; initial time, 5
min; temperature ramp 1, 30 ◦ C/min; final temperature, 200 ◦ C; ramp 2, 20 ◦ C/min; final temperature 250
◦ C; run time 30.17 min; injector port temperature 250 ◦ C; detector temperature 250 ◦ C, injection volume, 1.0
μL; carrier gas, helium; mass range between m/z 50 and 550.
Synthesis of benzimidazole derivatives
1-(Dimethylphenylsilylmethy)benzimidazole, 1. To a mixture of benzimidazole (1.18 g; 10 mmol) and KOH
(0.56 g, 10.0 mmol) in EtOH (10 mL) was added (chloromethyl)dimethylphenylsilane (1.85 mL, 10.2 mmol)
and the mixture was heated under reflux for 4 h. The mixture was then cooled, after which potassium chloride
was filtered off and washed with a little EtOH. The solvent was then removed from the filtrate in vacuo. After
distillation of the crude product under reduced pressure, yellow oily compound 1 (1.74 g, 65%) was obtained, bp
89 ◦ C/1 mmHg; υ(C=N) = 1590 cm−1 . Anal. found: C 72.09, H 6.75, N 10.43%. Calculated for C16 H18 N2 Si:
C 72.13, H 6.81, N 10.52%. 1 H-NMR (δ, DMSO-d6): 7.93 ppm (s, 1H, NCHN), 7.70-7.55 (m, 4H, C6 H4),
7.47-7.03 (m, 5H, C6 H5 Si), 4.11 (s, 2H, CH2 Si ) and 0.31 (s, 6H, Si(CH3)2). 13 C-NMR (δ, DMSO-d6): 139.6
(NCHN), 136.3, 134.2, 133.2, 130.1, 129.5 and 128.2 (C6 H4), 124.5, 122.2, 119.7 and 111.1 (C6 H5 Si), 35.0
(CH2 Si) and 1.1 (Si(CH3)2).
3-(3-Cyanopropyl)-1-(trimethylsilylmethyl)benzimidazolium chloride, 2. A mixture of 1-(trimethylsilylmet-
hyl)benzimidazole (1.02 g, 5.0 mmol) and 4-chlorobutyronitrile (0.5 mL, 5.6 mmol) in dimethylformamide (5
mL) was refluxed for 3 h. The mixture was then cooled and the volatiles were removed under vacuum. The
residue was crystallized from dimethylformamide/ethanol (1:1). White crystals of the title compound 2 (1.13
g, 73%) were obtained, mp 133-134 ◦ C: υ(C=N) = 1557 cm−1 , υ(C≡N) = 2247 cm−1 , Anal. found: C 58.37,
H 7.16, N 13.56%. Calculated for C15 H22 N3 ClSi: C 58.51, H 7.20, N 13.65%. 1 H-NMR (δ, DMSO-d6):
9.98 (s, 1H, NCHN), 8.18-8.07 (m, 2H, C6 H4 -C4 and C7), 7.27-7.53 (m, 2H, C6 H4 -C5 and C6), 4.64 (t,
2H, CH2 CH2 CH2 CN, J =6.9 Hz), 4.24 (s, 2H, CH2 Si), 2.70 (t, 2H, CH2 CH2 CH2 CN, J =6.9 Hz), 2.25 (q,
2H, CH2 CH2 CH2 CN, J =6.9 Hz) and 0.11 (s, 9H, (CH3)3 Si). 13 C-NMR (δ, DMSO-d6): 141.9 (NCHN),
132.4, 131.5, 126.9, 126.7, 114.5 and 113.9 (C6 H4), 120.3 (CH2 CH2 CH2 CN), 45.9 (CH2 CH2 CH2 CN), 25.1
(CH2 CH2 CH2 CN), and 14.2 (CH2 CH2 CH2 CN), 38.4 (CH2 Si) and -2.2 ((CH3)3 Si).
General method for the synthesis of compounds 3-8.
Equivalent amounts of the appropriate 1-substituted benzimidazole (I, 1) and the alkyl halide were refluxed in
dimethylformamide (5 mL) for 3 h. Then the mixture was cooled to room temperature and the volatiles were
removed under reduced pressure. The residue was crystallized from dimethylformamide/ethanol (1:1) under an
argon atmosphere to avoid bonding crystal water, as in some of our previous research.37
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Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
1-(Trimethylsilylmethyl)-3-p-xylylbenzimidazolium bromide, 3. Yield 1.66 g (white crystals), 85%, mp
125-127 ◦ C: υ(C=N) = 1558 cm−1 . Anal. found: C 58.58, H 6.46, N 7.15%. Calculated for C19 H25 N2 BrSi:
C 58.60, H 6.47, N 7.19%. 1 H-NMR (δ, DMSO-d6): 9.95 (s, 1H, NCHN), 8.12-8.10 (m, 1H, C6 H4), 8.02-
7.98 (m, 1H, C6 H4), 7.70-7.62 (m, 2H, C6 H4), 7.42 (d, 2H, C6 H4 − p-xylyl, J = 8.0 Hz), 7.22 (d, 2H,
C6 H4 − p-xylyl, J = 8.0 Hz), 5.78 (s, 2H, CH2p-xylyl), 4.29 (s, 2H, CH2 Si), 2.28 (s, 3H, CH3p-xylyl) and
0.10 (s, 9H, (CH3)3 Si). 13 C-NMR (δ, DMSO-d6): 141.6 (NCHN), 138.6, 132.4, 131.8, 131.1, 129.9 and 128.7
(C6 H4), 127.1, 126.8, 114.6 and 114.3 (C6 H5), 50.0 (CH2p-xylyl), 38.5 (CH2 Si), 21.2 (CH3 p-xylyl) and –2.2
((CH3)3 Si).
1-(Trimethylsilylmethyl)-3-phenethylbenzimidazolium bromide, 4. Yield 1.70 g (white crystals), 87%, mp
231-232 ◦ C: υ(C=N) = 1559 cm−1 . Anal. found: C 58.53, H 6.47, N 7.13%. Calculated for C19 H25 N2 BrSi: C
58.60, H 6.47, N 7.19%. 1 H-NMR (δ, DMSO-d6): 9.59 (s, 1H, NCHN), 8.12-8.05 (m, 2H, C6 H4), 7.70-7.64 (m,
2H, C6 H4), 7.27-7.16 (m, 5H, C6 H5), 4.83 (t, 2H, CH2 phenethyl, J = 6.9 Hz), 4.17 (s, 2H, CH2 Si), 3.27 (t,
2H, CH2 phenethyl, J = 6.9 Hz) and 0.01 (s, 9H, (CH3)3 Si). 13 C-NMR (δ, DMSO-d6): 141.3 (NCHN), 137.3,
132.1, 131.2, 129.2, 129.0 and 127.4 (C6 H4), 126.9, 126.8, 114.4 and 114.2 (C6 H5), 48.1 (CH2 phenethyl),
38.2(CH2 Si), 34.9 (CH2 phenethyl) and –2.4 ((CH3)3 Si).
1-(Dimethylphenylsilylmethyl)-3-methylbenzimidazolium iodide, 5. A mixture of 1-(dimethylphenylsilyl-
methyl)benzimidazole (1.34 g, 5.0 mmol) and iodomethane (0.4 mL, 6.4 mmol) in dimethylformamide (5 mL)
was refluxed for 3 h. The mixture was then cooled and the volatiles were removed under vacuum. The residue
was crystallized from dimethylformamide/ethanol (1:1). White crystals of the title compound 5 (1.56 g, 76%)
were obtained, mp 122-123 ◦ C: υ(C=N) = 1559 cm−1 . Anal. found: C 49.73, H 5.11, N 6.72%. Calculated
for C17 H21 N2 ISi: C 50.00, H 5.18, N 6.86%. 1 H-NMR (δ, DMSO-d6): 9.48 (s, 1H, NCHN), 8.05-7.70 (m,
4H, C6 H4), 7.65-7.33 (m, 5H, C6 H5 Si), 4.44 (s, 2H, CH2 Si), 4.08 (s, 3H, CH3) and 0.41(s, 6H, Si(CH3)2).
13 C-NMR (δ, DMSO-d6): 141.8 (NCHN), 134.9, 134.3, 132.1, 132.0, 131.9 and 130.5 (C6 H4), 128.5, 126.8,
114.2 and 113.9 (C6 H5 Si), 37.9 (CH2 Si), 33.8 (CH3) and –3.8 (Si(CH3)2).
1-(Dimethylphenylsilylmethyl)-3-ethylbenzimidazolium iodide, 6. Yield 1.67 g (white crystals), 79%, mp
112-114 ◦ C: υ(C=N) = 1559 cm−1 . Anal. found: C 51.02, H 5.46, N 6.54%. Calculated for C18 H23 N2 ISi: C
51.18, H 5.49, N 6.63%. 1 H-NMR (δ, DMSO-d6): 9.62 (s, 1H, NCHN), 8.08-8.05 (m, 1H, C6 H5), 7.94-7.91
(m, 1H, C6 H4), 7.67-7.61 (m, 2H, C6 H4), 7.52-7.31 (m, 5H, C6 H5 Si), 4.47 (s, 2H, CH2 Si), 4.45 (q, 2H, CH2
ethyl,J = 7.2 Hz), 0.83 (t, 3H, CH3 ethyl, J = 7.2 Hz) and 0.42(s, 6H, Si(CH3)2). 13 C-NMR (δ, DMSO-d6):
141.4 (NCHN), 134.8, 134.3, 132.0, 131.8, 131.3 and 130.5 (C6 H4), 128.5, 126.9, 126.4 and 114.2 (C6 H5 Si),
48.4 (CH2 ethyl), 37.8 (CH2 Si), 22.6 (CH3 ethyl) and –3.7 (Si(CH3)2).
1-(Dimethylphenylsilylmethyl)-3-allylbenzimidazolium bromide monohydrate, 7. Yield 1.52 g (white crys-
tals), 78%, mp 107-108 ◦ C: υ(C=N) = 1558 cm−1 . Anal. found: C 56.99, H 6.94, N 6.63%. Calculated for
C19 H23 N2 BrSi: C 57.00, H 6.94, N 6.65%. 1 H-NMR (δ, DMSO-d6): 9.48 (s, 1H, NCHN), 7.96-7.90 (m,
2H, C6 H4), 7.67-7.56 (m, 2H, C6 H4), 7.51-7.32 (m, 5H, C6 H5 Si), 6.04 (m, 1H, CH allyl), 5.28 (m, 2H, CH2
allyl), 5.14 (d, 2H, CH2 allyl, J = 5.7 Hz), 4.45 (s, 2H, CH2 Si) and 0.40 (s, 6H, Si(CH3)2). 13 C-NMR (δ,
DMSO-d6): 141.5 (NCHN), 134.7, 134.3, 132.2, 131.7, 131.2 and 130.5 (C6 H4), 128.5, 127.0, 126.6 and 120.4
(C6 H5 Si), 114.5 (CH allyl), 114.1 (CH2 allyl), 49.0 (CH2 allyl), 38.0 (CH2 Si) and –3.8 (Si(CH3)2).
1-(Dimethylphenylsilylmethyl)-3-benzylbenzimidazolium chloride, 8. Yield 1.62 g (white crystals), 82%,
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Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
mp 170-171 ◦ C: υ(C=N) = 1557 cm−1 . Anal. found: C 70.23, H 6.39, N 7.12%. Calculated for C23 H25 N2 ClSi:
C 70.29, H 6.41, N 7.13%. 1 H-NMR (δ, DMSO-d6): 9.80 (s, 1H, NCHN), 7.94-7.91 (m, 2H, C6 H4), 7.60-7.56
(m, 2H, C6 H4), 7.51-7.29 (m, 10H, C6 H5 benzyl and 5H C6 H5 Si), 5.76 (s, 2H, CH2 benzyl), 4.49 (s, 2H,
CH2 Si) and 0.42 (s, 6H, Si(CH3)2). 13 C-NMR (δ, DMSO-d6): 141.6 (NCHN), 134.6, 134.5, 134.3, 132.3,
131.0, 130.5, 129.5, 129.1, 128.8, 128.5, 127.1, 126.7, 114.6 and 114.1 (C6 H4 -ArH; C6 H5 -benzyl and C6 H5 Si),
50.1 (CH2 benzyl), 38.0 (CH2 Si) and –3.7 (Si(CH3)2).
General procedure for the Heck-Mizoroki reactions
Pd(OAc)2 (1 mmol%), benzimidazolium chlorides (II, III, 1-8) (2 mmol%), the aryl halide (1 mmol), styrene
(1.2 mmol), K2 CO3 (2 mmol), water (3 mL), and DMF (3 mL) were added to the microwave apparatus and
the mixture was heated at 120 ◦ C (300 W) for 10 min. It was carried out over a ramp time of 3 min to
reach 120 ◦ C. At the end of the reaction, the mixture was cooled and the product was extracted with ethyl
acetate/n-hexane (1:5) and filtered through a pad of silica gel with copious washing. The percent conversion
was determined by GC-MS based on aryl halide using the normalizing peak areas method.
Table 1. The Heck-Mizoroki coupling reactions of aryl halides with styrene.
Pd(OAc)2 (1 mol %)
II, III, 1-8 (2 mol %), mw (300 W)
R
+
X
R
z
DMF/ H2O (1:1), 120 oC, 10 min
K2CO3 (2 equiv)
z
Entry
1
R
Z
X
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
Salt Conversiona (%) TOF (h−1
)
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
II
II
II
II
72b
88c
92d
97e
864
528
1104
291
—
2
3
4
5
no ndf
no 09g
6
27
7
II
II
10h
60
8
97
89i
94i
92i
582
588
594
588
594
588
594
588
588
558
576
534i
564i
552i
9
III 98
10
11
12
13
14
15
16
17
18
2
3
4
5
6
7
8
II
99
98
99
98
99
98
98
93
96i
91i
94i
576i
546i
564i
OCH3 CH
OCH3 CH
87i
90i
522i
540i
III 96
206

¨
¨
Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
Table 1. Continued.
19
20
21
22
23
24
25
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
CH
CH
CH
CH
CH
CH
CH
I
I
I
I
I
I
I
2
3
4
5
6
7
8
99
97
99
97
99
98
99
594
582
95i 594
91i 582
95i 594
588
570i
546i
570i
594
26 COCH3 CH Br II 94
27 COCH3 CH Br III 96
564
576
28 COCH3 CH Br
29 COCH3 CH Br
30 COCH3 CH Br
31 COCH3 CH Br
32 COCH3 CH Br
33 COCH3 CH Br
34 COCH3 CH Br
2
3
4
5
6
7
8
98
97
98
88
98
90
91
588
582
588
528
588
540
546
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
H
H
N
N
N
N
N
N
N
N
N
Br II 63
Br III 64
378
384
H
Br
Br
Br
Br
Br
Br
Br
2
3
4
5
6
7
8
67
64
63
68
75
70
72
402
H
384
H
378
H
408
H
450
H
420
H
432
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
CH Cl II 36
CH Cl III 41
216
246
CH Cl
CH Cl
CH Cl
CH Cl
CH Cl
CH Cl
CH Cl
2
3
4
5
6
7
8
48
40
46
39
56
48
55
288
240
276
234
336
288
330
aConversions were determined by GC-MS based on the aryl halide. Reaction conditions: temperature ramped to 80 ◦C
(3 min) and held for 5b min and 10c min. Temperature ramped to 120 ◦C (3 min) and held for 5d min and 20e min.
Temperature ramped to 120 ◦C (3 min) and held for 10f min and 20g min without salt (II). On preheated oil bath, 10h
min with thermal heating at 120 ◦C. Isolated yields.i n.d., not detected. TOF = TON/time (h); TON = (% Conversion)
× (mol-substrate)/(mol-Catalyst).
207

¨
¨
Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
The Heck-coupling yields between styrene with phenyl iodide or p-iodoanisole were also determined as
an isolated yield for comparison purposes with the GC based conversion (Table 1, entries 8, 9, 12-14, 17, 18,
and 21-23). The isolated yields were determined as follows: at the end of the coupling reaction, the mixture was
cooled to room temperature; the contents of the reaction vessel were poured into a separatory funnel. Water (3
mL) and ethyl acetate (5 mL) were added, and the coupling product was extracted and removed. After further
extraction of the aqueous phase with ethyl acetate (5 mL) and combining the extracts, the ethyl acetate was
removed in vacuo leaving the trans-stilbene or trans-p-methoxystilbene products as a pale white solid, which
was characterized by comparison of NMR data with those in the literature.
General procedure for the Suzuki reactions
In a typical run, a microwave reaction vessel was charged with a mixture of aryl halide (1 mmol), styrene (1.2
mmol), K2 CO3 (2 mmol), Pd(OAc)2 (1 mmol %), and benzimidazolium salt (2 mmol). To the mixture was
then added DMF (3 mL) and water (3 mL). The mixture was reacted over a ramp time of 3 min to 120 ◦ C and
heated at 120 ◦ C for 10 min. After completion of the reaction, work-up procedures similar to those described
above for the Heck-Mizoroki reaction were performed.
Table 2. The Suzuki-Miyaura coupling reactions of aryl halides with phenylboronic acid.
Pd(OAc)2 (1 mol %)
II, III,1-8 (2 mol %), mw (300 W)
X
R
B(OH)2
R
+
DMF/ H2O (1:1), 120 oC, 10 min
K2CO3 (2 equiv)
z
z
Entry
R
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Z
X
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
Salt Conversiona (%) TOF (h−1
)
1
2
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
II
II
II
II
76b
87c
94d
98e
912
522
1128
294
—
3
4
5
no ndf
no 11g
6
33
7
II
II
09h
54
8
97
91i
96i
582
594
594
594
594
594
594
594
594
594
594
546i
576i
9
III 99
10
11
12
13
14
15
16
17
18
2
3
4
5
6
7
8
II
99
99
99
99
99
99
99
99
97i
93i
94i
582i
556i
564i
OCH3 CH
OCH3 CH
90i
93i
540i
558i
III 99
208

¨
¨
Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
Table 2. Continued.
19
20
21
22
23
24
25
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
CH
CH
CH
CH
CH
CH
CH
I
I
I
I
I
I
I
2
3
4
5
6
7
8
99
99
99
99
99
99
99
594
594
95i 594
92i 594
97i 594
594
570i
552i
582i
594
26 COCH3 CH Br II 97
27 COCH3 CH Br III 96
582
576
28 COCH3 CH Br
29 COCH3 CH Br
30 COCH3 CH Br
31 COCH3 CH Br
32 COCH3 CH Br
33 COCH3 CH Br
34 COCH3 CH Br
2
3
4
5
6
7
8
98
98
99
98
99
98
99
588
588
594
588
594
588
594
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
H
H
N
N
N
N
N
N
N
N
N
Br II 69
Br III 67
414
402
H
Br
Br
Br
Br
Br
Br
Br
2
3
4
5
6
7
8
69
71
76
74
80
78
77
414
H
426
H
456
H
444
H
480
H
468
H
462
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
CH Cl II 50
CH Cl III 57
300
342
CH Cl
CH Cl
CH Cl
CH Cl
CH Cl
CH Cl
CH Cl
2
3
4
5
6
7
8
62
58
60
68
65
70
76
372
348
360
408
390
420
456
aConversions were determined by GC-MS based on the aryl halide. Reaction conditions: temperature ramped to 80
◦C (3 min) and held for 5b min and 10c min. Temperature ramped to 120 ◦C (3 min) and held for 5d min and 20e
min. Temperature ramped to 120 ◦C (3 min) and held for 10f min and 20g min without salt (II). On preheated oil
i
bath, 10hmin with thermal heating at 120 ◦C. Isolated yields. n.d., not detected. TOF = TON/time (h); TON = (%
Conversion) × (mol-substrate)/(mol-Catalyst).
209

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Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
The Suzuki-Miyaura coupling yields between phenylboronic acid with phenyl iodide or p-iodoanisole were
also determined as an isolated yield for comparison purposes with the GC based conversion (Table 2, entries 8,
9, 12-14, 17, 18, and 21-23). The isolated yields for this purpose were determined using procedures similar to
those described above for the Heck-Mizoroki reaction.
Crystal structure determination of 7
A good-quality single crystal of 7 was mounted with epoxy glue at the tip of glass capillaries for the X-ray
diffraction experiments. X-ray measurements for 7 were carried out on a STOE IPDS 2 diffractometer with
˚
MoKα (λ = 0.71073 A) radiation at T = 296 K. Lattice parameters were obtained by least-squares fit to
the optimized setting angles of the collected reflections by means of X-AREA. 38 Data were reduced by using
X-RED3238 software applying integration absorption corrections. The molecular structure of 7 was solved
by direct methods using the SIR-97 package39 and refinements by full-matrix least-squares on F2 using the
SHELXL-97 program.40 The water H atoms were located from a difference Fourier map and refined with
˚
˚
distance restraints of O—H = 0.83(2) A and H...H = 1.31 (2) A, and with Uiso (H) = 1.5 Ueq (O). The other H
˚
atoms were positioned geometrically, with C—H = 0.93–0.97 A, and refined as riding with Uiso (H) = 1.2 or 1.5
eq (C). Figures 1 and 2 were drawn by PLATON program.41 The crystal data, data collection, and refinement
U
details of compound 7 are given in Table 3.
Results and discussion
In this work, non-symmetric benzimidazolium salts, which are conventional NHC precursors, bearing silicon-
containing substituents, II, III, and 2-8, were used. These salts were prepared from the treatment of 1-
alkylbenzimidazoles (I and 1) with appropriate alkyl halides in refluxing DMF with good yields of 73%-87%.
The synthesis of the benzimidazoles (I, II, III, and 1-8) is summarized in Scheme 1. All benzimidazolium
salts synthesized in this work are air- and moisture-stable in the solid state as well as in solution. The new
1
compounds 1-8 were identified by spectroscopic methods and elemental analysis. The NC HN δ[13 C{ H}]
signal of the benzimidazolium salts is usually around 142
4 ppm.22,42 For the benzimidazolium salts 2-8 it
was found to be 141.9, 141.6, 141.3, 143.6, 141.8, 141.4, 141.5, and 141.6 ppm, respectively. These values are
in good agreement with the previously reported results.20−22,43 The NCH N protons of the benzimidazolium
salts were observed as singlets in the 1 H-NMR spectrum at 9.98, 9.95, 9.59, 9.48, 9.62, 9.48, and 9.80 ppm,
respectively. These chemical shift values are typical for NCH N protons of benzimidazolium salts because of
their increased acidity.19−22 In comparison, the corresponding values of the starting materials I and 1 are 8.10
and 7.93 ppm, which are significantly smaller than in benzimidazolium salts.
The benzimidazolium salts (2-8) show IR absorption bands at 1557, 1558, 1559, 1559, 1559, 1558, and
1557 cm−1 , respectively, which are assigned to ν (C=N). These IR absorption values are in good agreement
with previously reported values for benzimidazolium salts.19−21 These values are slightly smaller than expected
for a standard ν (C=N) because of the π -electron delocalization on the imidazolium ring.
210

¨
¨
Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
Table 3. The crystal data, data collection, and refinement details of compound 7.
Empirical Formula
Formula weight
C19H23N2Si.H2O.Br
405.40
Temperature (K)
296
Crystal system
Monoclinic
P 21/c
Space group
The number of reflections used to
determine the unit cell
30505
◦
˚
Cell parameters (A, )
a
b
c
8.8485 (4)
24.5969 (14)
11.0667 (5)
120.689 (3)
2071.29 (19)
4
β
3
˚
Volume (A )
Z
Absorption coefficient, μ(Mo Kα) (mm−1
)
2.05
Crystal size mm3
0.17 × 0.21 × 0.33
1.300
Dx (Mg.m−3
)
F(000)
840
θmax (◦)
26.5
Number of reflection measured
22205
Number of uniqe reflection
4288
Number of observed reflection
3300
I > 2σ(I)
h
–11 → 11
–30 → 30
–13 → 13
0.033
k
l
Rint
Number of refined parameters
Goodness of fit on F 2
R[F 2 > 2σ(F 2)] ,wR2
(Δσ)max
224
1.08
0.052, 0.117
0.001
−3
˚
Δρmax (e.A
)
)
0.62
−3
˚
Δρmax (e.A
–0.51
The Heck-Mizoroki coupling reaction
Palladium-catalyzed reactions for the formation of new carbon-carbon bonds are now recognized as essential
in the tool box every synthetic chemist.44 Of the many coupling reactions involving palladium catalysis, the
211

¨
¨
Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
Heck-Mizoroki reaction, the coupling of aryl halides with terminal olefins, emerged as one of the most widely
used transformations from both academic and industrial laboratories.45
The in situ generated catalytic system derived from the benzimidazolium salts and Pd(OAc)2 has
been used successfully in a series of carbon-carbon bond formation reactions. We recently reported the
optimal reaction conditions for the Heck-Mizoroki coupling reactions including some benzimidazolium or bis-
benzimidazolium salts/Pd(OAc)2 and base as a catalyst system under microwave and conventional heating
conditions.20−22 In the present report, a series of aryl iodide, aryl bromide, aryl chloride, and 2-pyridyl bromide
compounds were used for the coupling reaction with styrene. The recently reported optimal parameters for the
base and solvent20 were used to obtain stilbene derivatives, after test reactions to determine optimum reaction
time and temperature using phenyl iodide and styrene (Table 1, entries 1-8). Finally, we found that use of the
catalytic system consisting of 1% Pd(OAc)2 , 2% mol of II-III, 2-8, and 2% mol K2 CO3 in DMF-H2 O (1:1)
at 120 ◦ C/300 W microwave heating led to the best conversion within 10 min.
It has been observed that no increase in catalytic reaction yield is found by prolonging the reaction time
from 10 to 20 min (Table 1, entry 4). Furthermore, no coupling reaction occurred in the absence of II within
5 min (Table 1, entry 5) and very low yield was detected within 10 min (Table 1, entry 6). We also tested the
catalytic yield using a conventional heating system (i.e. preheated oil bath) after 10 min at 120 ◦ C and a low
yield was detected, 10% (Table 1, entry 7). These experiments indicate that benzimidazole salts and microwave
heating may play the crucial role in the catalytic system.
A complete list of the results obtained with optimal conditions is given in Table 1. Altogether 5 different
aryl halides bearing electron-donating, electron-withdrawing groups and 2-bromopyridine, which is an electron-
poor heterocyclic compound, reacted with styrene and gave the coupled products in almost high yield. As
expected, the yields of the Heck-Mizoroki coupling reactions with styrene and aryl chlorides were low.
The benzimidazolium salt bearing dimethylphenyl substituents (4-8) are generally more effective than
the other salts examined. Probably, acting as a Lewis base, the phenyl substituent on the silicon atom of the
benzimidazolium salts affects positively the catalytic activity. It is noteworthy that 2-bromopyridine was also
tolerated in this system, and the cross-coupling products were obtained in satisfactory yields (Table 1, entries
35-43). The reactivity of the aryl halide component on Heck-Mizoroki coupling reactions decreases in the order
I > Br > Cl, which is in good agreement with previous reports.20,21,46 In many studies, it was reported that
the Heck-Mizoroki coupling reaction with aryl chlorides with electron withdrawing substituents did not give
detectable amounts of Heck-Mizoroki coupling products in conventional heating systems.46−48 In contrast, low
or moderate yields were obtained for aryl chloride even bearing electron donating methoxy substituent with
styrene using the microwave heating system (Table 1, entries 44-52). It is important to note that the end of all
these reactions was clearly indicated by black particles in the reaction mixture, which are probably palladium
nanoparticles.
The Suzuki-Miyaura coupling reactions
Nowadays, the Suzuki-Miyaura reaction is one of the key techniques in the construction of molecular skeletons
in organic synthesis. The catalytic yield of the coupling is dependent on a variety of parameters such as temper-
ature, solvent, base, and nature of catalyst loading. We recently reported the optimum reaction conditions for
the Suzuki-Miyaura/Heck-Mizoroki coupling reaction, including some benzimidazolium or bis-benzimidazolium
212

¨
¨
Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
salts/Pd(OAc)2 and base as a catalyst system under microwave and conventional heating conditions.20−22 The
recently reported optimal parameters (except time and temperature)20 were used to obtain biaryl derivatives,
after test reactions for optimization of the reacting time and temperature using phenyl iodide and phenylboronic
acid (Table 2, entries 1-8). Finally, we found that use of the catalytic system consisting of 1% Pd(OAc)2 , 2%
mol of II-III, 2-8, and 2% mol K2 CO3 in DMF-H2 O (1:1) at 120 ◦ C/300 W microwave heating led to the
best conversion within 10 min.
In the present report, a series of aryl iodide, aryl bromide, aryl chloride, and 2-pyrdyl bromide were used
for coupling partner with phenylboronic acid. The Suzuki-Miyaura cross coupling catalyzed by Pd(OAc)2 -
benzimidazole salt-K2 CO3 system is presented in detail (Table 2).
After having established the optimized coupling reaction conditions, the scope of the reaction and
efficiencies of the benzimidazolium salts were evaluated by investigating the coupling of the phenylboronic
acid with some p-substituted aryl halides. Under the optimized conditions, reaction of phenyl iodide, p-
iodoanisole, and p-bromoacetophenone with phenylboronic acid gave almost as high a yield using a catalytic
system consisting of 2 mol % benzimidazole salts (II, III, or 1-8), 1 mol % Pd(OAc)2 and 2 equivs K2 CO3
in DMF-H2 O (1:1) at 120 ◦ C by microwave irradiation (300 W) within 10 min. On the other hand, p-
chloroanisole, which bears a strong electron donating methoxy group, gave a low or moderate yield using the
optimized conditions. It is noteworthy that aryl chlorides are arguably the most useful substrates because
of their lower cost and the wide range of commercially available compounds.6 It is also worth noting that the
satisfactory coupling products of 2-pyridyl bromide with phenylboronic acid were also obtained under optimized
conditions. We also tested the catalytic yields using the conventional heating system in a preheated oil bath
for 10 min at 120 ◦ C, but we obtained only 9% yield using benzimidazole salt, II, and phenyl iodide optimized
conditions (Table 2, entry 7). Control experiments showed that the Suzuki coupling reaction did not occur in
the absence of II in 10 min (Table 2, entry 5) and very low yield was detected within 20 min (Table 2, entry
6) under microwave heating. A complete list of the results obtained with optimal conditions is given in Table
2. Of the 5 different aryl halides used in the Suzuki-Miyaura coupling with phenylboronic acid, those with
electron-withdrawing substituents and aryl iodides were found to give the highest yield (Table 2, entries 1-34).
Contrary to expectations, using 2-pyridyl bromide, which has an electron-deficient ring, as a coupling partner
reduced the catalytic yield of the Suzuki-Miyaura reaction (Table 2, entries 35-43) As accepted, Suzuki-Miyaura
cross-coupling yields of aryl chloride with electron-donating methoxy substituent were lower than those of aryl
iodides and aryl bromides (Table 2, entries 44-52). These yields for the aryl chlorides may be acceptable due to
their low cost and the challenge to activate the Csp2 -Cl bond; it has attracted much attention from both the
industry and scientific research laboratories to couple aromatic chlorides with arylboronic acides.42
The benzimidazolium salt bearing dimethylphenyl substituent on the silicon atoms (4-8) are also found
to be more effective than other salts examined in Suzuki-Miyaura reactions (Table 2) and the tables indicate
that the results for the Suzuki-Miyaura reaction are parallel to the Heck-Mizoroki data in general (Table
1). According to the previously reported results, electron-donating substituent on the nitrogen atoms of the
benzimidazole salts may play a crucial role for the better catalytic activity in the catalytic system. It is
important to note that, at the end of the all these reactions, palladium black was observed on the wall of the
reaction vessel, probably derived from palladium nanoparticles. As can be seen in Table 2, a high C-C coupling
product was obtained from the reaction of aryl iodides and aryl bromides with phenylboronic acid as expected.
213

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Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
X-ray crystallography of 7
The structure of compound 7 was determined by X-ray diffraction analysis. Compound 7 crystallizes in the
monoclinic space group P21 /c. The general view and atom numbering scheme for 7 is shown in Figure 1.
Figure 1. View of the molecule, showing the atom labeling scheme. Displacement ellipsoids for non-H atoms are drawn
at the 30% probability level.
The benzimidazole ring system (N1/N2/C1–C7) of the title compound (I) is almost planar [maximum
◦
˚
deviation = –0.022(3) A] and makes an angle of 72.66(16) with the phenyl ring (C12–C17). The bond
lengths and bond angles in (I) have reasonable values, and they are compatible with those found for similar
compounds.35,36 The angles around the Si atoms with a distorted tetrahedral geometry vary from 103.84(17)◦
to 111.7 (3)◦ .
O—H...Br, C-H...Br, and C—H...O hydrogen bonds link the molecules (Table 4 and Figure 2). Further-
more, π − π stacking interactions are observed between the centroids of the benzene (C1–C6) and imidazole
˚
(N1/N2/C1/C6/C7) rings of the benzimidazole ring system [Cg1...Cg2(1 – x, 1 – y, 1 – z) = 3.4970(17) A and
˚
Cg2...Cg2(1 – x, 1 – y, 1 – z) = 3.6740(18) A, Cg1 and Cg2 are the centroids of the imidazole and benzene
rings of the benzimidazole ring system, respectively].
214

¨
¨
Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
Figure 2. The packing and hydrogen bonding interactions of (I), viewing down the a-axis. All hydrogen atoms not
involved in hydrogen bonding have been omitted for clarity.
Table 4. Hydrogen-bond parameters (A, ◦ ) for 7.
˚
D—H
H...A
D...A
D—H...A
169 (5)
171 (4)
176
O1—HO2...Br1
O1—HO1...Br1i
C8—H8A...Br1ii
C8—H8B...Br1iii
C11—H11A...O1iv
C17—H17...O1iv
0.85 (4) 2.55 (5) 3.385 (4)
0.85 (4) 2.53 (4) 3.366 (5)
0.97
0.97
0.97
0.93
2.92
2.92
2.57
2.54
3.890 (4)
3.832 (4)
3.471 (6)
3.453 (5)
157
155
168
Symmetry codes: (i) 1 – x, 1 – y, 2 – z; (ii) 1 + x, y, z; (iii) 1 – x, 1 – y, 1 – z; (iv) x, y, –1 + z.
Conclusions
We prepared 1 new 1-substituted benzimidazole (1) and 7 non-symmetric 1,3-disubstituted benzimidazolium
salts (2-8) bearing silicon containing alkyl groups on the 1 nitrogen atom of the azolium ring. The catalytic
studies were done using in situ generated systems from Pd(OAc)2 /benzimidazolium salt and K2 CO3 for the
Heck- Mizoroki and Suzuki-Miyaura cross coupling reactions. The catalyst systems in the Heck-Mizoroki and
Suzuki-Miyaura reactions gave better yields under microwave-assisted moderate conditions and very short reac-
tion times compared to those given in the literature.19,46,47 The Heck-Mizoroki and Suzuki-Miyaura reactions
were carried out using 300 W power microwave irradiation at 120 ◦ C in only 10 min. It can be concluded that
Heck-Mizoroki and Suzuki-Miyaura coupling reactions may be accelerated by microwave irradiation even using
aryl chlorides bearing electron-donating methoxy substituent. In addition, coupling of 2-pyridyl bromide with
215

¨
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Synthesis, characterization, and microwave-promoted catalytic activity of..., H. KUC¸UKBAY, et al.
both styrene and phenylboronic acid affords corresponding stilbene and biaryl products in satisfactory yields
over 10 min, respectively.
Supporting Information
Supporting information can be found in the online version of this article. CCDC holds the supplementary
crystallographic data 827530. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; Fax (+44) 1223-336-033; or email deposit@ccdc.cam.ac.uk.
Acknowledgements
˙
¨
This work was financially supported by the Ino¨nu¨ University Research Fund (I.U. B.A.P. 2010/124 and
2010/125). The authors acknowledge the Faculty of Arts and Sciences, Ondokuz Mayıs University, Turkey,
for the use of the Stoe IPDS 2 diffractometer (purchased under grant F.279 of the University Research Fund).
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