Synthesis of novel benzimidazole salts and microwave-assisted catalytic activity of in situ generated Pd nanoparticles from a catalyst system consisting of benzimidazol salt, Pd(OAc)2 , and base in a Suzuki-Miyaura reaction

Article abstract of DOI:10.3906/kim-1207-18

Novel benzimidazolium salts having N-benzyl or N-(4-substitutedbenzyl) groups were synthesized and their microwave-promoted catalytic activity for the Suzuki-Miyaura cross-coupling reaction were determined using in situ formed palladium(0) nanoparticles (PdNPs) from a catalytic system consisting of Pd(OAc)₂/K₂CO₃ in DMF/H₂ O. PdNPs were characterized by X-ray diffraction (XRD) pattern and particle size of in situ generated PdNPs from the Pd(111) plane was determined to be of diameter 19.6 nm by the Debye-Scherrer equation. Moreover, the yield of the Suzuki-Miyaura reactions with aryl iodides and aryl bromides was found to be nearly quantitative. The synthesized benzimidazole salts (1{5) were identified by ¹H and ¹³C NMR and IR spectroscopic methods, and micro analysis. The molecular structure of 5 was also determined by X-ray crystallography. TUeBITAK.

Full text of DOI:10.3906/kim-1207-18

Turk J Chem
Turkish Journal of Chemistry
(2013) 37: 721 – 733
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1207-18
Research Article
Synthesis of novel benzimidazole salts and microwave-assisted catalytic activity of
in situ generated Pd nanoparticles from a catalyst system consisting of
benzimidazol salt, Pd(OAc)2 , and base in a Suzuki-Miyaura reaction
1
Ulku¨ YILMAZ, Hasan KUC¸ UKBAY,
1,∗
2
¨
¨
¨
¨ ˙ ˙ ˙
Sevim TURKTEKIN C¸ ELIKESIR,
Mehmet AKKURT,2 Orhan BUYUKGUNGOR
3
¨
¨
¨
¨
1
˙
Department of Chemistry, Faculty of Science and Arts, In¨onu¨ University, Malatya, Turkey
2Department of Physics, Faculty of Science, Erciyes University, Kayseri, Turkey
3Department of Physics, Faculty of Arts and Science, Ondokuz Mayıs University, Kurupelit, Samsun, Turkey
Received: 06.07.2012
•
Accepted: 17.03.2013
•
Published Online: 16.09.2013
•
Printed: 21.10.2013
Abstract: Novel benzimidazolium salts having N-benzyl or N-(4-substitutedbenzyl) groups were synthesized and their
microwave-promoted catalytic activity for the Suzuki–Miyaura cross-coupling reaction were determined using in situ
formed palladium(0) nanoparticles (PdNPs) from a catalytic system consisting of Pd(OAc)2 /K2CO3 in DMF/H2 O.
PdNPs were characterized by X-ray diffraction (XRD) pattern and particle size of in situ generated PdNPs from the
Pd(111) plane was determined to be of diameter 19.6 nm by the Debye–Scherrer equation. Moreover, the yield of the
Suzuki–Miyaura reactions with aryl iodides and aryl bromides was found to be nearly quantitative. The synthesized
benzimidazole salts (1–5) were identified by 1 H and 13 C NMR and IR spectroscopic methods, and micro analysis. The
molecular structure of 5 was also determined by X-ray crystallography.
Key words: Benzimidazole salt, N-heterocyclic carbenes, palladium nanoparticles, cross-coupling reaction, Suzuki–
Miyaura coupling, microwave
1. Introduction
The Suzuki–Miyaura reaction is one of the most versatile and utilized reactions for the selective construction of
carbon–carbon bonds, in particular for the formation of biaryls.1−8 Because of the excellent physical and chem-
ical properties of biaryls, they can be used in several organic compound syntheses, such as of monomers for con-
structing polymers, supramolecular compounds, and natural, pharmaceutical, and agrochemical products.9,10
Nowadays, the Suzuki–Miyaura reaction plays an important role in organic synthetic chemistry to obtain new
generation organic materials with many important properties such as electronic, optical, or mechanical.9 There-
fore, in recent years, much effort has been devoted to develop and improve the reaction conditions. For these
purposes, various catalysts or catalytic systems including tertiary phosphines and N-heterocyclic carbenes,
solvent, base, and reaction conditions such as temperature and time, and conventional or microwave heating
systems have been investigated.11−20
Among the catalysts, those having N-heterocyclic carbene ligands have gained enormous popularity
due to their potential advantages over tertiary phosphines such as better σ-donor ability, low toxicity, and
∗Correspondence: hkucukbay@inonu.edu.tr
In memory of Prof Dr Ayhan S Demir
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YILMAZ et al./Turk J Chem
thermal stability.8,21−24 In particular, Pd(II)-NHC complexes are more attractive as pre-catalysts because
of their stability to air, moisture, and heating and they also have excellent long-term storage profiles.8
Pd(OAc)2 /benzimidazole or imidazole ligands could be very effective catalytic systems in these reactions.17,25
In recent years, microwave-assisted organic synthesis has been considered a green technology owing to
its high reaction rates, purity of products, increased yield, decreased electricity cost, and simplified course of
reactions.26−34 The use of metal catalysis in conjunction with microwave heating may also have significant ad-
vantages over traditional heating methods, since the inverted temperature gradients under microwave conditions
may provide an increased lifetime of the catalyst through elimination of wall effects.35
There are extensive studies about the Suzuki-type C–C cross-coupling reaction incorporating microwave
irradiation with high yield in a short time using various ligands other than benzimidazole moiety.27,29−32,34−36
Recently, we have also investigated the catalytic activity of some in situ prepared N-heterocyclic carbene-Pd
complexes for Suzuki–Miyaura cross coupling reactions under microwave heating.37,38 Since the nature, size,
and electronic properties of the substituent on the nitrogen atom(s) of the benzimidazole may play a crucial role
in tuning the catalytic activity, to find more efficient palladium catalysts we have synthesized a series of new
benzimidazolium halides, 1–5 (Scheme), containing benzyl, substituted benzyl, and 3-phenylpropyl moieties,
and we aimed to investigate the activity of in situ Pd-carbene based catalytic systems for Suzuki cross-coupling
reactions.
Reetz and co-workers were the first to report the use of Pd and Pd/Ni nanoparticles for the Suzuki
coupling of aryl bromides and chlorides with phenylboronic acid.39,40 PdNPs are effective catalysts for chemical
transformations due to their large surface area and many research groups have used them as an active catalyst
for Suzuki–Miyaura cross-coupling reactions.16,41−49
Herein, we report on the microwave-assisted catalytic activity of Pd(OAc)2 /benzyl and 3-phenylpropyl
substituted benzimidazolium salts and base catalytic system through in situ formed PdNPs in Suzuki cross-
coupling reactions. The X-ray structural analysis of compound 5 was also determined to clarify whether there
is crystal water in the benzimidazolium compounds, as in our previous work.50
2. Experimental
All preparations were carried out in an atmosphere of purified argon using standard Schlenk techniques. The
starting materials and reagents used in the reactions were supplied commercially by Aldrich or Merck. The
solvents were dried by 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 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 PerkinElmer FT-IR spectrophotometer. The structural characterization of
the samples fabricated was investigated by X-ray diffraction (XRD). An automated Rigaku RadB Dmax X-ray
diffractometer having CuKα radiation was used. Scan speed was selected as 2◦ min−1 in the range of 2θ =
3–80◦ .
Elemental analyses were performed by LECO CHNS-932 elemental analyzer. Melting points were recorded
using an Electrothermal-9200 melting point apparatus, and are uncorrected.
1-(3-Phenylpropyl)benzimidazole (I), used in this work as a starting compound, was prepared by treating
benzimidazole and 3-bromopropylbenzene similar to the literature procedure.51
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YILMAZ et al./Turk J Chem
1 R= H, X= Cl
2 R= CH3, X= Br
X-CH2-C6H4-R
DMF
N
N
N
-
3 R= NO2, X= Cl
+
X
4 R= Cl, X= Cl
5 R= Br, X= Br
N
I
R
Scheme. Synthesis pathways of the benzimidazole derivatives.
2.1. GC-MS analysis
GC-MS spectra were recorded on an Agilient 6890 N GC and 5973 Mass Selective Detector using 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 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.
2.2. Synthesis of benzimidazole salts
Synthesis of 1-benzyl-3-(3-phenylpropyl)benzimidazolium chloride, 1
A mixture of 1-(3-phenylpropyl)benzimidazole (I) (1.00 g, 4.23 mmol) and benzyl chloride (0.50 mL,
4.34 mmol) in dimethylformamide (5 mL) was refluxed for 4 h. The mixture was then cooled and the volatiles
were removed under vacuum. The solid was crystallized from ethanol/diethyl ether (1:1). White crystals of
the title compound 1 (1.16 g, 75%) were obtained, mp 96–98 ◦ C; υmax /cm−1 = 1564 (CN). Anal. found: C
75.37, H 6.30, N 7.20. Calculated for C23 H23 N2 Cl (362.90): C 76.12, H 6.39, N 7.72. 1 H NMR (δ, DMSO-
d6): 10.26 (s, 1H, NCHN), 8.13–7.20 (m, 14H, C6 H4 , C6 H5 , CH2 C6 H5), 5.81 (s, 2H, CH2 C6 H5), 4.59
(t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.2 Hz), 2.73 (t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.8 Hz), 2.29 (quint, 2H,
CH2 CH2 CH2 C6 H5 , J = 7.5 Hz). 13 C NMR (δ, DMSO-d6): 143.1 (NCHN), 141.0, 134.6, 131.8, 131.3, 129.4,
129.2, 129.1, 128.8, 128.7, 127.1, 127.0, 126.5, 114.4, and 114.3 (C6 H4 , C6 H5 , CH2 C6 H5), 50.3 (CH2 C6 H5),
47.1 (CH2 CH2 CH2 C6 H5), 32.4 (CH2 CH2 CH2 C6 H5), 30.4 (CH2 CH2 CH2 C6 H5).
2.3. General method for the synthesis of compounds 2–5
Equivalent amount of the 1-(3-phenylpropyl)benzimidazole and the appropriate alkyl halide were refluxed in
dimethylformamide (5 mL) for 4 h. Then the mixture was cooled to room temperature and the volatiles were
removed under reduced pressure. The residue was crystallized from ethanol/diethyl ether (1:1).
1-(4-Methylbenzyl)-3-(3-phenylpropyl)benzimidazolium bromide, 2.
Yield, 1.64 g (white crystals), 92%; mp 207–208 ◦ C; υmax /cm−1 = 1565 (CN). Anal. found: C 68.27,
H 6.09, N 6.49. Calculated for C24 H25 N2 Br (421.37): C 68.41, H 5.98, N 6.65. 1 H NMR (δ, DMSO-d6):
723

YILMAZ et al./Turk J Chem
10.06 (s, 1H, NCHN), 8.14–7.16 (m, 13H, C6 H4 , C6 H5 , CH2 C6 H4 CH3), 5.73 (s, 2H, CH2 C6 H4 CH3),
4.58 (t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.2 Hz), 2.73 (t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.8 Hz), 2.31
(quint, 2H, CH2 CH2 CH2 C6 H5 , J = 7.5 Hz), 2.29 (s, 3H, CH2 C6 H4 CH3). 13 C NMR (δ, DMSO-d6):
142.8 (NCHN), 141.0, 138.6, 131.8, 131.5, 131.3, 129.9, 128.8, 128.7, 127.1, 127.0, 126.5, and 114.4 (C6 H4 ,
C6 H5 , CH2 C6 H4 CH3), 50.2 (CH2 C6 H4 CH3), 47.1 (CH2 CH2 CH2 C6 H5), 32.4 (CH2 CH2 CH2 C6 H5), 30.5
(CH2 CH2 CH2 C6 H5), 21.2 (CH2 C6 H4 CH3).
1-(4-Nitrobenzyl)-3-(3-phenylpropyl)benzimidazolium chloride, 3.
Yield, 1.53 g (yellow crystals), 88%; mp 154–156 ◦ C; υmax /cm−1 = 1557 (CN). Anal. found: C 67.12,
H 5.57, N 10.01. Calculated for C23 H22 N3 O2 Cl (407.89): C 67.73, H 5.44, N 10.30. 1 H NMR (δ, CDCl3):
12.20 (s, 1H, NCHN), 8.25–7.19 (m, 13H, C6 H4 , C6 H5 , CH2 C6 H4 NO2), 6.16 (s, 2H, CH2 C6 H4 NO2),
4.59 (t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.5 Hz), 2.86 (t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.2 Hz), 2.51 (quint,
2H, CH2 CH2 CH2 C6 H5 , J = 7.5 Hz). 13 C NMR (δ, CDCl3): 148.3 (NCHN), 144.2, 139.8, 139.3, 131.3
131.0, 129.5, 128.7, 128.4, 127.5, 127.4, 126.6, 124.5, 113.3, and 113.2 (C6 H4 , C6 H5 , CH2 C6 H4 NO2), 50.2
(CH2 C6 H4 NO2), 47.2 (CH2 CH2 CH2 C6 H5), 32.6 (CH2 CH2 CH2 C6 H5), 30.2 (CH2 CH2 CH2 C6 H5).
1-(4-Chlorobenzyl-3-(3-phenylpropyl)benzimidazolium chloride, 4.
Yield, 1.41 g (white crystals), 84%; mp 142–143 ◦ C; υmax /cm−1 = 1559 (CN). Anal. found: C 68.93,
H 5.73, N 6.80. Calculated for C23 H22 N2 Cl2 (397.34): C 69.52, H 5.58, N 7.05. 1 H NMR (δ, CDCl3):
12.00 (s, 1H, NCHN), 7.54–7.16 (m, 13H, C6 H4 , C6 H5 , CH2 C6 H4 Cl), 5.91 (s, 2H, CH2 C6 H4 Cl), 4.59
(t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.5 Hz), 2.82 (t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.2 Hz), 2.46 (quint,
2H, CH2 CH2 CH2 C6 H5 , J = 7.5 Hz). 13 C NMR (δ, CDCl3): 143.8 (NCHN), 139.5, 135.2, 131.5, 131.3,
131.0, 129.9, 129.5, 128.6, 128.4, 127.2, 127.1, 126.5, 113.7, and 113.0 (C6 H4 , C6 H5 , CH2 C6 H4 Cl), 50.6
(CH2 C6 H4 Cl), 47.0 (CH2 CH2 CH2 C6 H5), 32.5 (CH2 CH2 CH2 C6 H5), 30.3 (CH2 CH2 CH2 C6 H5).
1-(4-Bromobenzyl)-3-(3-phenylpropyl)benzimidazolium bromide, 5.
Yield, 1.42 g (white crystals), 69%; mp 196–197 ◦ C; υmax /cm−1 = 1563 (CN). Anal. found: C 56.27,
H 4.40, N 5.67. Calculated for C23 H22 N2 Br2 (486.24): C 56.81, H 4.56, N 5.76. 1 H NMR (δ, DMSO-d6):
10.08 (s, 1H, NCHN), 8.14–7.16 (m, 13H, C6 H4 , C6 H5 , CH2 C6 H4 Br), 5.78 (s, 2H, CH2 C6 H4 Br), 4.58
(t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.2 Hz), 2.74 (t, 2H, CH2 CH2 CH2 C6 H5 , J = 7.8 Hz), 2.29 (quint, 2H,
CH2 CH2 CH2 C6 H5 , J = 7.5 Hz). 13 C NMR (δ, DMSO-d6): 143.1 (NCHN), 141.0, 133.9, 132.3, 131.8,
131.2, 131.1, 128.8, 128.7, 127.2, 127.1, 126.5, 122.5, 114.4, and 114.3 (C6 H4 , C6 H5 , CH2 C6 H4 Br), 49.6
(CH2 C6 H4 Br), 47.1 (CH2 CH2 CH2 C6 H5), 32.4 (CH2 CH2 CH2 C6 H5), 30.4 (CH2 CH2 CH2 C6 H5).
2.4. Single-crystal X-ray diffraction analysis of 1-(4-bromobenzyl)-3-(3-phenylpropyl)benzimidazo-
lium bromide (5)
The X-ray data were collected at 296(2) K on a STOE IPDS II diffractometer with MoKα radiation. Data
collection, cell refinement, and data reduction were performed with X-AREA and XRED32.52 Crystal structures
were solved by direct methods using the SIR97 structure solution program and refined on F2 by full matrix
least-square methods on F2 using the SHELXL97 program.53,54
˚
All H atoms were positioned geometrically with C—H = 0.93-0.97 A, and refined using a riding model
with Uiso (H) = 1.2Ueq (C). A summary of the crystal data, experimental details, and refinement results for 5
724

YILMAZ et al./Turk J Chem
is given in Table 1. The molecular structure of 5 in Figure 1 was drawn with ORTEP-3.55 The relevant bond
lengths and bond angles are listed in Table 2.
Table 1. The crystal data, data collection, and refinement values of compound 5.
Crystal data
C23H22BrN2.Br
Mr = 486.23
Triclinic, P-1
Z = 2
Dx = 1.542 Mg m−3
α = 103.098 (5)◦
β = 99.602 (5)◦
γ = 105.739 (5)◦
˚
a = 8.6978 (5) A
˚
b = 9.0916 (5) A
3
˚
˚
c = 14.5678 (9) A
V = 1047.43 (12) A
Mo Kα radiation
T = 296 (2) K
µ = 3.88 mm−1
Crystal shape and color: block, colorless
Data collection
STOE IPDS 2 diffractometer
ω scans
Rint = 0.118
θmax = 26.5◦
h = –10→ 10
k = –11 → 11
l = –18 →18
Absorption correction:integration
Tmin = 0.106, Tmax = 0.196
14,941 measured reflections
4337 independent reflections
3690 reflection with I > 2σ(I)
Refinement
Refinement on F2
Calculated weights
R[F2 > 2σ(F2)] = 0.048
wR(F2) = 0.094
S = 1.09
w = 1/[σ2(Fo2)+ (0.0414P)2+ 0.5155P]
P = (Fo2+ 2Fc2)/3
(∆/σ)max <0.0001−1
˚
∆ρmax = 0.80 e A
−1
˚
∆ρmin = –0.41 e A
4337 reflections
244 parameters
H atoms constrained to parent site Extinction correction: none
◦
˚
Table 2. Selected bond lengths (A), bond angles ( ).
Br1—C12
N1—C1
N1—C7
N1—C8
1.903 (3)
1.389 (4)
1.336 (4)
1.469 (4)
N2—C6
N2—C7
N2—C15
1.396 (4)
1.321 (4)
1.464 (4)
C1—N1—C7 108.4 (2)
C1—N1—C8 126.4 (2)
C7—N1—C8 125.0 (3)
C6—N2—C7 108.0 (2)
N2—C6—C1
N2—C6—C5
N1—C7—N2
N1—C8—C9
107.1 (2)
130.9 (2)
110.4 (2)
113.1 (2)
C6—N2—C15 124.3 (3) Br1—C12—C11 119.0 (2)
C7—N2—C15 127.8 (3) Br1—C12—C13 119.5 (3)
N1—C1—C2 132.2 (3) N2—C15—C16 114.4 (3)
N1—C1—C6 106.2 (2)
2.5. General procedure for the Suzuki–Miyaura reactions
Pd(OAc)2 (1 mmol%), benzimidazolium halides (1–5) (2 mmol %), aryl halide (1 mmol), phenylboronic acid
(1.2 mmol), K2 CO3 (2 mmol), water (3 mL), and DMF (3 mL) were added to the microwave apparatus and
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YILMAZ et al./Turk J Chem
the mixture was heated at 120 ◦ C (300 W) for 10 min. Temperature was ramped up to reach 120 ◦ C in 3
min. At the end of the reaction, the mixture was cooled, and the product was extracted with ethyl acetate/n-
hexane (1:5) and chromatographed on a silica gel column. The purity of coupling products was checked by
NMR and GC-MS, and yields are based on aryl halide. The coupling products were confirmed by increasing
the peaks on gas chromatograms and mass values from MS spectra. All coupling products were also isolated
and characterized by 1 H NMR or MS before the serial catalytic work up each time.
The Suzuki–Miyaura coupling yields between phenylboronic acid and 4-iodotoluene or 4–methylanisole
were also determined as an isolated yield for comparison purposes with the GC-based yields. The isolated yields
were determined as follows. At the end of the Suzuki–Miyaura coupling reaction, the mixture was cooled to
room temperature and 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 4-methylbiphenyl or 4-methoxybiphenyl product as a pale white solid, which
was characterized by comparison of NMR data with those in the literature. The palladium nanoparticles
were obtained as follows. After separating the Suzuki–Miyaura coupling product at the end of the catalytic
reaction, the residue including black palladium nanoparticles was washed 3 times with water and then ethanol
to obtain pure palladium nanoparticles. The PdNPs were tested for the Suzuki–Miyaura coupling reaction at
the optimized conditions after drying.
3. Results and discussion
1-(3-Phenylpropyl)benzimidazole (I) was synthesized from benzimidazole, 3-bromopropylbenzene, and KOH in
refluxing EtOH in good yield of 86%. The molecular structure of compound 5 was confirmed by single crystal
X-ray diffraction to clarify whether there is crystal water in the benzimidazolium compounds. Its molecular
structure is depicted in Figure 1.
Benzimidazolium salts containing aryl alkyl moieties, 1–5, were prepared by treatments of 1-(3-phenylp-
ropyl)benzimidazole with appropriate benzyl halides in refluxing DMF with good yields of 69%–92%. The
synthesis of the benzimidazolium salts 1–5 is summarized in the Scheme. The benzimidazolium salts are air-
and moisture-stable both in the solid state and in solution. The new benzimidazole derivatives (1–5) were
characterized by 1 H NMR, 13 C NMR, IR, and elemental analysis techniques, which support the proposed
structures.
1
The value of δ[13 C{ H}], NC HN in benzimidazolium salts is usually around 142 ꢀ 4.37 For benzimida-
zolium salts 1–5 it was found to be 143.1, 142.8, 148.3, 143.8, and 143.1 ppm, respectively. These values are in
good agreement with the previously reported results.17,38 The NCH N proton signals for the benzimidazolium
salts were observed as singlets at 10.26, 10.06, 12.20, 12.00, and 10.08 ppm, respectively. As expected, the
highest shifts to downfield of the NCH N proton signals were observed where bearing strong electron withdraw-
ing nitro substituent on the phenyl ring. These chemical shift values are also typical for NCH N protons of
benzimidazolium salts for increasing the acidity of the NCH N proton.37,38
The carbon–nitrogen band frequencies, ν(C=N) for benzimidazole salts 1–5 were observed at 1564, 1565,
1557, 1559, and 1563 cm−1 , respectively. Similar to the 13 C NMR and 1 H NMR results, the highest red shift
was observed for compound 3 due to its having a strong electron withdrawing nitro substituent on the phenyl
ring.
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YILMAZ et al./Turk J Chem
Figure 1. View of the title molecule (5), showing the atom labeling scheme. Displacement ellipsoids for non-H atoms
are drawn at the 30% probability level.
In order to find the optimum reaction conditions for the Suzuki coupling reaction, a series of experiments
was performed with catalysis by p-iodotoluene and phenylboronic acid as model compounds. The test reactions
were performed using different bases such as Cs2 CO3 , K2 CO3 , and DBU (1,8-diazabicyclo[5.4.0]undec-7-en)
and different solvents such as DMF/H2 O, EtOH/H2 O, H2 O, C2 H4 (OH)2 /H2 O, and glycerine/H2 O for 5, 10,
60, and 90 min at 60 ◦ C, 80 ◦ C, 100 ◦ C, and 120 ◦ C. It was found that the Suzuki coupling reaction catalyzed
by 2, Pb(OAc)2 , and the base catalyst system gave the highest yield when using DMF/H2 O mixture as a
solvent and Cs2 CO3 or K2 CO3 as a base at 120 ◦ C microwave heating for 10 min. A considerable increase in
the catalytic reactions’ yield was not observed when prolonging the time from 5 to 30 min. After these results,
we chose K2 CO3 as a base as it is cheaper than Cs2 CO3 , and water/DMF as a solvent. We also tested the
catalytic yields using a conventional heating system in a preheated oil bath over 5, 10, 30, 60, and 90 min at
different temperatures. The test experiment results for optimization of the Suzuki–Miyaura coupling reaction
are given in Table 3.
After having established the optimized coupling reaction conditions (Table 3) the scope of the reaction
and efficiencies of the benzimidazolium salts were evaluated by investigating the coupling of the phenylboronic
acid with various p-substituted aryl halides. Under the optimized conditions, reaction of p-bromoacetophenone,
methyl p-bromobenzoate, p-iodoanisole, and p-iodotoluene with phenylboronic acid gave almost as high a yield
as using a catalytic system consisting of 2 mol % benzimidazole salts (1–5), 1 mol % Pd(OAc)2 , and 2 equivs
K2 CO3 in DMF-H2 O (1:1) at 120 ◦ C by microwave irradiation (300 W) over 10 min. On the other hand,
bearing strong electron donating group on the aryl chlorides such as methoxy, weak electron donating methyl,
and medium electron withdrawing formyl group gave a moderate or good 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 We also tested the catalytic yields using a conventional
heating system in a preheated oil bath for 5, 10, and 30 min at 120 ◦ C, but we obtained only 8%, 11%, and 61%
yields, respectively, using benzimidazole salt, 2, and p-iodotoluene in optimized conditions (Table 3, entries
6–8). C¸etinkaya et al. also reported that a similar catalytic system containing some benzimidazolium salts
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YILMAZ et al./Turk J Chem
needed a longer reaction time (3–6 h) for the Suzuki coupling reaction under thermal conditions.17,50 Control
experiments showed that the yield of the Suzuki coupling reaction was decreased in the absence of 2 over 10
min under microwave heating (Table 4, entry 1). The results obtained from optimum conditions for the Suzuki
reactions are given in Table 4. Of the 5 different aryl halides used in the Suzuki coupling with phenylboronic
acid, those with electron-withdrawing substituents were found to give the highest yield (Table 4, entries 11–20).
Table 3. Test experiments for optimization of the Suzuki–Miyaura coupling reactions.
Pd(OAc)2, (1mol %)
2 (2 mol %), heat
B(OH)2
Me
I
+
Me
Solvent, Base (2 mol)
Thermal heating
Yield,%; TOF(h− 1
Microwave heating
a
Entry
Base
Solvent
Time (min)
Conv. ,%; TOF(h− 1
)
°
°
C
)
C (300W)
60
1
2
3
4
5
6
7
8
9
10
11
12
13
14
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Cs2CO3
K2CO3
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
EtOH/H2O
H2O
5
5
60
80
80
100
100
120
120
120
00
03
15
68
87
08
11
61
00
18
90
408
522
48
66
366
67
73
82
402
438
492
80
80
10
60
90
5
10
30
10
10
10
10
10
10
10
n.t.
n.t.
120
120
120
120
120
120
120
120
120
120
-
-
90 88* 540 528*
99 96* 594 576*
99 96* 594 576*
99
76
42
78
91
89
65
594
556
252
468
546
534
390
K2CO3 C2H4(OH)2/H2O
DBU
DBU
K2CO3
DMF/H2O
EtOH/H2O
Glycerine/H2O
15
a
Conversions were determined by GC-MS based on the aryl halide. n.t.: not tested. *Isolated yield.
Benzimidazole salt bearing a strong electron-withdrawing nitro substituent at the benzyl group, 3, is
found the least effective of the salts examined in Suzuki coupling reactions (Table 4, entries 3, 8, 13, 18, 23, 28,
and 32). On the other hand, benzimidazole salt 2, which bears an electron-donating methyl group at the para-
position of the N-benzyl group, is the most effective for the catalytic activity in the Suzuki coupling reactions
among them. Similar catalytic results to ours for the Suzuki cross-coupling reactions have also been reported in
the literature using the catalytic system consisting of palladium compound, base, and various benzimidazolium
or imidazolium salts.12,13,17,56,57
Similar to our previous results, the endpoint of all these reactions was clearly observed black particles
in the reaction mixture, which probably derived from palladium nanoparticles. These nanoparticles may act
as a catalyst themselves or as a reservoir of Pd(0) molecular species, which would be the active catalysts.
These nanoparticles generated from in situ formed Pd-NHC are probably more active than Pd(0) complexes.58
With the aim of proving the catalytic role of the Pd nanoparticles, we also tested in situ formed palladium(0)
nanoparticles at the optimized conditions for Suzuki cross-coupling reactions. As can be seen in Table 2 [entries
2 (TOF = 582 h−1) and 22 (TOF = 462 h−1)], PdNPs were an efficient catalyst at optimized conditions under
microwave heating. The comparison of our results with the previous related studies16,42−44,48,49 showed that
the present study has some advantages, in particular short reaction times, better TOF values, and moderate
reaction conditions.
728

YILMAZ et al./Turk J Chem
Table 4. The Suzuki–Miyaura cross-coupling reactions of aryl halides with phenylboronic acid.
Pd(OAc)2 (1 mol %)
1-5 (2 mol %), mw(300 W)
B(OH)2
X
R
R
+
DMF/ H2O (1:1),120 oC, 10min
K2CO3 (2 equiv)
Entry
R
X
Salt
Yield (%) TOF (h−1
)
1
2b
3
4
5
6
7
8
9
CH3
CH3
CH3
CH3
CH3
CH3
I
I
I
I
I
I
I
I
I
No
48
288
582
PdNPs 97
99 95*
1
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
594 570*
576 534*
588 498*
594 534*
594 522*
594 576*
576 552*
582 540*
594 576*
594
594
588
588
594
594
594
582
588
594
462
456
504
426
432
492
462
474
408
420
444
486
510
456
438
96 89*
98 83*
99 89*
99 87*
99 96*
96 92*
97 90*
99 96*
99
99
98
98
99
OCH3
OCH3
OCH3
OCH3
OCH3
COCH3
COCH3
COCH3
COCH3
COCH3
COOCH3 Br
COOCH3 Br
COOCH3 Br
COOCH3 Br
COOCH3 Br
10
11
12
13
14
15
16
17
18
19
20
21
22c
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
I
I
Br
Br
Br
Br
Br
99
99
97
98
99
CH3
CH3
CH3
CH3
CH3
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
PNPs 77
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
76
84
71
72
82
77
79
68
70
74
81
85
76
73
79
CH3
OCH3
OCH3
OCH3
OCH3
OCH3
CHO
CHO
CHO
CHO
CHO
474
Yields are based on the aryl halide. Reactions were monitored by GC-MS. Conditions: temperature ramped to 120 ◦ C
(3 min) and held for 10 min). * Isolated yields.
b,c Palladium(0) nanoparticles were used as catalyst. TOF = TON/time (h); TON = (Yield %) × (mol-substrate)/(mol-
catalyst).
729

YILMAZ et al./Turk J Chem
Figure 2 shows powder XRD diffraction patterns obtained for the in situ formed palladium(0) nanopar-
ticles. According to XRD diffraction, the Pd nanoparticles have Fm-3m face centered cubic structure, and the
crystal parameters using the Jade program according to the Rietveld-refinement method were calculated as a
◦
◦
◦
˚
= b = c = 3.889 A. The strong diffraction peaks at the Bragg angles of 40.2 , 46.7 , and 68.2 correspond
to the 111, 200, and 220 facets of elemental palladium.16,59 The particle size of the corresponding facets 111,
200, and 220 of elemental palladium were determined as 19.6, 15.5, and 19.6 nm (18.2 ꢀ 2.4 nm) by using
the Debye–Scherrer equation [d = (0.94. λCuKα)/(FWHM.Cosθ)], respectively. These values were also found
experimentally as 19.3, 15.1, and 19.1 nm (17.8 ꢀ 2.4 nm) from the XRD report, respectively.
Figure 2. Powder XRD pattern of in situ formed palladium (0) nanoparticules showing the facets of the palladium.
3.1. Molecular structure of 5
The title compound, C23 H22 BrN2 .Br, crystallizes in the triclinic P-1 space group. All geometric parameters
are comparable with results obtained from previous studies on related benzimidazole derivatives.60,61 The
Figure 3. Packing view of 5 in the unit cell. Hydrogen bonds are indicated as dashed lines. H atoms not involved in
hydrogen bonding have been omitted for clarity.
730

YILMAZ et al./Turk J Chem
benzimidazole ring (N1/N2/C1-C7) is planar with maximum deviation from planarity of 0.026(3) and 0.028(3)
˚
A for atoms N1 and C4, respectively. The dihedral angles between the rings A(N1/N2/C1-C7), B(C9-C14), and
C(C18-C23) are A/B = 86.08(13)◦ , A/C = 80.63(15)◦ , and B/C = 71.49(17)◦ . The crystal structure of 5 is
stabilized by intermolecular C — H ... Br interactions (Table 4; Figure 3). Furthermore, a C—H...π interaction
was found in the crystal structure (Table 5).
Table 5. Hydrogen-bond parameters (A, ◦).
˚
D—H H...A
D...A
D—H...A
171
145
C5—H5...Br2i
C7—H7...Br2ii
C17—17B...Cg3ii
0.93
0.93
0.97
2.79 3.711 (3)
2.68 3.488 (3)
2.75 3.691 (4)
164
Symmetry codes: (i) 1 – x, 1 – y, 1 – z. (ii) 1 – x, 1 – y, 1 – z. Cg3 is the centroid of the C9-C14 benzene ring.
4. Conclusions
We prepared some new benzimidazole salts containing benzyl, p-substituted benzyl, and 3-phenylpropyl moieties
(1–5). The use of the palladium catalyst system including the benzimidazolium salts in the Suzuki coupling
reaction gave better yield under microwave-assisted conditions in short reaction times than thermal heating
conditions and the yields of the reactions were increased even using aryl chlorides. In situ formed PdNPs
(18.2 ꢀ 2.4 nm) also showed good catalytic activity in the Suzuki–Miyaura cross coupling reactions under the
optimized conditions.
5. Supporting information
CCDC holds the supplementary crystallographic data CCDC 838509. 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 e-mail deposit@ccdc.cam.ac.uk.
Acknowledgement
˙
We wish to thank In¨onu¨ University Research Fund (BAPB-2010/124 and 2010/125) for its financial support of
this study. The authors acknowledge the Faculty of Arts and Sciences, Ondokuz Mayıs University, Turkey, for
the use of the Stoe IPDS2 diffractometer (purchased under grant F.279 of the University Research Fund).
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