Synthesis of [PdBr2(benzimidazole-2-ylidene)(pyridine)] complexes and their catalytic activity in the direct C[sbnd]H bond activation of 2-substituted heterocycles

Article abstract of DOI:10.1016/j.poly.2021.115091

A series of unsymmetrical 1,3-disubstituted benzimidazolium chlorides, 2a-f, having two nitrogen atoms substituted by various alkyl groups were synthesized as N-heterocyclic carbene (NHC) precursors in high yields. The benzimidazolium salts are readily co

Full text of DOI:10.1016/j.poly.2021.115091

Polyhedron 199 (2021) 115091
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Synthesis of [PdBr2(benzimidazole-2-ylidene)(pyridine)] complexes and
their catalytic activity in the direct CAH bond activation of 2-substituted
heterocycles
e
a,c,
Sarra Lasmari a,b, Nevin Gürbüz a,c, Raouf Boulcina b,d, Namık Özdemir , Ismail Özdemir
⇑
_
a Catalysis Research and Application Center, Inönü University, 44280 Malatya, Turkey
b Laboratory of Synthesis of Molecules with Biological Interest, Faculty of Exact Sciences, Mentouri -Constantine 1 University, 25000 Constantine, Algeria
c Department of Chemistry, Faculty of Science and Art, Inönü University, 44280 Malatya, Turkey
d Department of Sciences and Technology, Faculty of Technology, Mostefa Benboulaïd-Batna 2 University, 5000 Batna, Algeria
e Ondokuz Mayıs University, Faculty of Education, Department of Mathematics and Science Education, 9055139 Samsun, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of unsymmetrical 1,3-disubstituted benzimidazolium chlorides, 2a-f, having two nitrogen atoms
substituted by various alkyl groups were synthesized as N-heterocyclic carbene (NHC) precursors in high
yields. The benzimidazolium salts are readily converted into the corresponding PEPPSI-type palladium–
NHC complexes 3a-f (PEPPSI = pyridine-enhanced precatalyst preparation, stabilization and initiation).
The structures of all the compounds have been characterized by 1H NMR, 13C NMR and IR spectroscopy,
as well as the X-ray diffraction technique (3a, 3d and 3e), which support the proposed structures. Next,
the palladium-NHC-PEPPSI complexes were used as catalysts in the direct C(5)-arylation of 2-acetyl furan
and 2-acetylthiophene with various aryl bromides. These complexes exhibited moderate to high catalytic
activities and gave CAH activation selectively at the C(5)-position of 2-acetylfuran and 2-
acetylthiophene.
Received 3 December 2020
Accepted 26 January 2021
Available online 14 February 2021
Dedicated to the memory of Professor
Dahmane Tebbani.
Keywords:
N-Heterocyclic carbene
Benzimidazole
Palladium
CAH bond activation
Arylation
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
membered heterocycles such as furan and thiophene, has been
widely described by a large number of researchers [15].
Robust synthetic routes for the synthesis of bi(hetero)aryls are
still a challenging area of interest due to the lower reactivity of
heterocycles in some of the recent booming arenas of synthetic
chemistry. Cross-coupling reactions, such as Heck [1], Suzuki-
Miyaura [2], Stille [3], Sonogashira [4] and arylation reactions [5],
generally required the use of palladium catalysts to form CAC
[6], and carbon-heteroatom bonds [7], are one of the most impor-
tant subjects of organic chemistry and have attracted great atten-
tion [8–11]. In the literature, many practical methods have been
developed for the transition metal-catalyzed arylation of hetero-
cyclic compounds, such as pyrrole, furan and thiophene at the C
(2), C(4) and C(5) positions [12]. Arylation reactions by the cleav-
age of CAH bonds are considered to be a more attractive strategy
[13]; in this process, a variety of arylating reagents are used,
including aryl halides, aryl organometallic reagents [14] and unac-
tivated arenes. Recently, the direct arylation of (hetero) arenes by
catalyzed palladium-NHC-PEPPSI complexes, in particular five-
Organ et al. [16] are the pioneers of ‘‘Pyridine Enhanced Precat-
alyst Preparation Stabilization and Initiation” (PEPPSI); this
method has been applied in several fields. In particular, nucle-
ophilic N-heterocyclic carbenes (NHCs) have become one of the
best ligands for transition metal catalysis [17]. Generally, C5-aryla-
tion is achieved with the use of PEPPSI-type palladium-NHC com-
plexes [18]; the excellent performance of these complexes on
organic transformations is due to both the strong d-donor and
weak
p-acceptor ability of the NHC ligand [19]. There are many
advantages to palladium-NHC-PEPPSI complexes [20]. First, it has
been affirmed that the use of the pyridine group facilitates the
binding of substrates to the complex in the oxidative addition step.
Secondly, PEPPSI-type Pd-NHCs have high stability against oxygen
and water [21]. These properties of PEPPSI-type complexes are
highly important for the efficiency of the catalyst.
The direct arylation of heteroarenes with aryl halides has
become a most valuable method for the formation of C(sp2)-C
(sp2) bonds in contemporary organic synthesis, because of the
numerous applications of heteroaromatic compounds, such as
pharmaceuticals, cosmetics, natural products, biologically active
⇑
Corresponding author.
E-mail address: deposit@ccdc.cam.ac.uk (I. Özdemir).
_
https://doi.org/10.1016/j.poly.2021.115091
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
compounds and functional materials [22–25]. Thiophene, furan
and thiazole derivatives show valuable biological activity and pre-
sent considerable interest in pharmaceutical chemistry. For exam-
ple, Canagliflozin [26] is a drug for the treatment of type-2
diabetes, Nitrofurantoin [27] is a nitrofuran antibiotic used to treat
uncomplicated urinary tract infections, Febuxostat [28] is pre-
scribed to patients with gout suffering from hyperuricemia and it
is used for chronic management, Vaborbactam [29] has been used
in trials investigating the treatment of bacterial infections in sub-
jects with varying degrees of renal insufficiency and Motapizone
[30–35] is used against platelet aggregation (Scheme1). Because
of these properties, the discovery of simple and direct routes to
access heteroarene derivatives using a simple catalytic system
remains an important challenge for organic chemists.
in parts per million (ppm) relative to tetramethylsilane (TMS) as an
internal standard. Coupling constants (J values) are given in hertz
(Hz). NMR multiplicities were abbreviated as follows: s = singlet,
d = doublet, t = triplet, p = pentet, dd = doublet of doublets, tt = tri-
plet of triplets, ddd = doublet of doublet of doublets, m = multiplet.
1H NMR spectra were referenced to residual protiated solvents
(d = 7.28 ppm for CDCl3) and 13C NMR chemical shifts were
reported relative to deuterated solvents (d = 77.16 ppm for CDCl3).
The catalytic solutions were analyzed with a Shimadzu GC 2025
equipped with a GC-FID sensor and an RX-5 ms column of 30 m
length, 0.25 mm diameter and 0.25 lm film thickness. The C, H
and N elemental analyses were determined by an Elementar Vario
MICRO CUBE.
The palladium-catalyzed direct arylation of several heteroaro-
matics [36–48], via CAH bond activation using aryl halides has
led to successes in recent years. For this reason, we synthesized
some novel 1,3-disubstituted benzimidazolium salts (2a-f) and
their palladium-NHC-PEPPSI complexes (3a-f). All the new com-
pounds were characterized by different techniques, such as 1H
and 13C NMR and IR. Also, the solid-state structures of the palla-
dium complexes 3a, 3d and 3e have been established by a sin-
gle-crystal X-ray diffraction study. Next, the palladium-NHC-
PEPPSI complexes were used as catalysts in the direct C(5)-aryla-
tion of 2-substituted heteroaryl derivatives (thiophene and furan)
with various aryl bromides. The reactive C(2)-position of the
heteroarenes was blocked to maximize the yields of the monoary-
lated products. The C(5)-arylated heteroaryl derivatives were
selectively obtained in moderate to high yields.
2.2. Synthesis of 1-(1, 3-dioxolane-2-yl)methyl)benzimidazole (1)
A mixture of benzimidazole (7 g, 0.06 mol) and potassium
hydroxide (8.5 g, 0.15 mol) was dissolved in DMSO (10 mL) at
50 °C for 2 h, after which 2-chloromethyl 1,3-dioxalane (7.26 g,
0.06 mol) was added and the temperature was increased to 80 °C
for 72 h. The mixture was cooled to room temperature and
extracted with CH2Cl2 (3 x15 mL) , then the solvent was removed
by vacuum to give the title compound 1 (6.6 g, 94%) ,1H NMR
(400 MHz, CDCl3) d, ppm: 7.92 (s, 1H, NCHN), 7.77 (d, J = 7.4 Hz,
1H, arom of benzimidazole), 7.46 (d, J = 7.9 Hz, 1H, arom of benz-
imidazole), 7.32–7.19 (m, 2H, arom of benzimidazole), 5.26–5.15
(m, 1H, CHCH2), 4.31 (s, 2H, CHCH2), 3.77 (m, 2H, OCH2CH2O),
3.68–3.58 (m, 2H, OCH2CH2O). 13C NMR (100 MHz, CDCl3) d,
ppm: 144.28 (C), 143.49 (CH), 134.70 (C), 123.11 (CH), 122.16
(CH), 120.21 (CH), 110.27 (CH), 101.66 (CH2), 65.59 (2CH2), 47.43
(CH2).
Over the past few years, our research group has previously used
palladium-NHC-PEPPSI complexes to catalyze organometallic reac-
tions under different conditions [49].
2. Materials and methods
2.3. Synthesis of the benzimidazolium salts (2a-2f)
2.1. General remarks
1-(1,3-Dioxalan-2-yl)methyl)benzimidazole (1.2 g, 1.0 mmol)
was dissolved in degassed dimethylformamide (3 mL) and the
alkyl halide derivative (1.0 mmol). The reaction mixture was stir-
red at 70 °C for 48 h under argon. After completion of the reaction,
the solvent was removed by vacuum and Et2O (15 mL) was added
to the obtained solid, which was filtered off. The solid was washed
with Et2O (3 Â 10 mL) and dried under a vacuum. The crude pro-
duct was recrystallized from a DCM/Et2O mixture (1:5, v/v) at
room temperature, then completely dried under vacuum. All the
NHC ligand precursors (2a–2f) were isolated as air and moisture
stable in high yields.
All manipulations were performed in Schlenk-type flasks under
an argon atmosphere. The melting point measurements were
determined in open capillary tubes with an Electrothermal-9200
melting points apparatus. The IR spectra were recorded on a Gladi
ATR unit (Attenuated Total Reflection) in the range 450–4000 cmÀ1
with a Perkin Elmer Spectrum 100 Fourier-transform infrared spec-
trometer. Routine 1H and 13C NMR spectra were recorded with a
Bruker AscendTM 400 Avance III HD NMR spectrometer with sample
solutions prepared in CDCl3. The chemical shifts (d) were reported
Scheme 1. Examples of bioactive furan thiophene and thiazole derivatives.
2

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
2.3.1. 1-(1,3-Dioxalane-2-yl-methyl)-3-(4-methylbenzyl)
benzimidazolium chloride (2a)
2.3.5. 1-(1,3-Dioxalane-2-yl)methyl)-3-(2,3,4,5,6-pentamethylbenzyl)
benzimidazolium chloride (2e)
Yield 72%, white solid, m.p.: 142–143 °C. FT-IR (cmÀ1): m(CN)
1557. 1H NMR (400 MHz, CDCl3) d, ppm:11.62 (s, 1H, NCHN),
7.79 (d, J = 8.0 Hz, 1H, C6H4), 7.63–7.48 (m, 3H, C6H4), 7.38 (d,
J = 8.0 Hz, 2H, CH2C6H4CH3), 7.18 (d, J = 8.0 Hz, 2H, CH2C6H4CH3),
5.85 (s, 2H, CH2C6H4CH3), 5.40 (t, J = 3.1 Hz, 1H, CHCH2), 4.93 (d,
J = 3.2 Hz, 2H, CHCH2), 3.88–3.86 (m, 2H, OCH2CH2O), 3.85–3.83
(m, 2H, OCH2CH2O), 2.32 (s, 3H, CH2C6H4CH3). 13C NMR
(100 MHz, CDCl3) d, ppm:144.86 (NCH), 139.24 (C), 132.30 (C),
130.80 (C), 130.01 (C), 129.74 (C), 128.21 (C), 127.01 (CH),
126.90 (CH), 113.89 (CH), 113.46 (CH), 99.98 (CH), 65.51 (CH2),
51.39 (CH2), 48.74 (CH2), 21.19 (CH3). Elemental analysis, calcd
for C19H21N2O2ClÁH2O: C 62.89, H 6.39, N 7.72; found: C 61.81, H
6.29, N 7.89%.
Yield 67%, white solid, m.p. 175–176 °C. FT-IR (cmÀ1): m(CN)
1558. 1H NMR (400 MHz, CDCl3) d, ppm: 10.52 (s, 1H, NCHN),
7.80 (d, J = 8.4 Hz, 1H, C6H4), 7.59 (t, J = 7.8 Hz, 1H, C6H4), 7.50 (t,
J = 7.8 Hz, 1H, C6H4), 7.38 (d, J = 8.4 Hz, 1H, C6H4), 5.83 (s, 2H, CH2-
C6C(CH3)5), 5.34 (t, J = 2.8 Hz, 1H, CHCH2), 5.03 (d, J = 2.7 Hz, 2H,
CHCH2), 3.83 (t, J = 7.0 Hz, 2H, OCH2CH2O), 3.76 (t, J = 7.3 Hz, 2H,
OCH2CH2O), 2.29 (s, 9H, CH2C6(CH3)3(CH3)2), 2.25 (s, 6H, CH2C6(-
CH3)3(CH3)2). 13C NMR (100 MHz, CDCl3) d, ppm: 144.30 (CNH),
137.42 (C), 133.98 (C), 132.53 (C), 131.03 (C), 126.97 (C), 126.89
(CH), 124.78 (CH), 114.00 (CH), 113.22 (CH), 99.99 (CH), 65.39
(CH2), 48.64 (CH2), 48.15 (CH2), 17.36 (CH3), 17.05 (CH3), 17.01
(CH3). Elemental analysis, calcd for C23H29N2O2Clx0.5CH2Cl2: C
63.66, H 6.82, N 6.32; found: C 63.44, H 6.96, N 6.74%.
2.3.6. 1-(1,3-Dioxalane-2-yl)methyl)-3-(anthracen-9-ylmethyl)
benzimidazolium chloride (2f)
2.3.2. 1-(1,3-Dioxalane-2-yl)methyl)-3-(4-tet-butylbenzyl)
benzimidazolium bromide (2b)
Yield 58%, yellow solid, m.p. 197–198 °C. FT-IR (cmÀ1): m(CN)
1564. 1H NMR (400 MHz, CDCl3) d, ppm: 11.51 (s, 1H, NCHN),
8.61 (s, 1H, CH2C6H4C6HC6H4), 8.53 (d, J = 8.9 Hz, 2H, CH2C6H4C6-
HC6H4), 8.09 (d, J = 8.5 Hz, 2H, CH2C6H4C6HC6H4 and C6H4), 7.73–
7.65 (m, 3H, C6H4), 7.57–7.51 (m, 2H, CH2C6H4C6HC6H4), 7.42 (t,
J = 7.9 Hz, 1H, CH2C6H4C6HC6H4), 7.18 (t, J = 7.9 Hz, 1H, CH2C6H4C6-
HC6H4), 7.04 (d, J = 8.5 Hz, 1H, CH2C6H4C6HC6H4), 6.96 (s, 2H, CH2-
C6H4C6HC6H4), 5.33 (t, J = 2.8 Hz, 1H, CHCH2), 4.87 (d, J = 2.8 Hz,
2H, CHCH2), 3.79 (t, J = 7.0 Hz, 2H, OCH2CH2O), 3.62 (t, J = 7.1 Hz,
2H, OCH2CH2O). 13C NMR (100 MHz, DMSO) d, ppm: 143.49
(NHC), 133.52 (C), 133.42 (C), 133.24 (C), 131.79 (C), 130.17 (C),
129.11 (CH), 128.15 (CH), 127.88 (CH), 126.94 (CH), 124.57 (CH),
123.21 (CH), 115.82 (CH), 115.25 (CH), 101.02 (CH), 65.94 (CH2),
48.78 (CH2), 44.78 (CH2). Elemental analysis, calcd for C26H23N2O2-
ClxH2O: C 69.56, H 5.61, N 6.24; found: C 67.66, H 5.62, N 6.11%.
Yield 94%, white solid, m.p.: 140–141 °C. FT-IR (cmÀ1): m(CN)
1551. 1H NMR (400 MHz, CDCl3) d, ppm: 11.32 (s, 1H, NCHN),
7.81 (m, J = 13.7 Hz, 1H, C6H4), 7.66–7.53 (m, 3H, C6H4), 7.44–
7.38 (m, J = 8.3 Hz, 4H, CH2C6H4(C(CH3)3), 5.84 (s, 2H, CH2C6H4(C
(CH3)3)), 5.39 (t, J = 3.0 Hz, 1H, CHCH2), 4.91 (d, J = 3.0 Hz, 2H,
CHCH2), 3.86 (d, J = 4.9 Hz, 4H, OCH2CH2O), 1.27 (s, 9H, CH2C6H4(-
C(CH3)3). 13C NMR (100 MHz, CDCl3) d, ppm: 152.60 (C), 144.25
(NCH), 132.40 (C), 130.96 (C), 129.71 (CH), 128.11 (CH), 127.19
(CH), 126.46 (CH), 113.97 (CH), 113.59 (CH), 100.00 (CH), 65.69
(CH2), 53.57 (CH2), 51.36 (CH2), 48.90 (C), 34.81 (CH3), 31.31
(CH3). Elemental analysis, calcd for C22H27N2O2BrxH2Ox0.5CH2Cl2:
C 57.83, H 6.30, N 6.06; found: C 58.37, H 6.60, N 6.64%.
2.3.3. 1-(1,3-Dioxalane-2-yl)methyl-3-(2, 4,6-trimethylbenzyl)
benzimidzolium chloride (2c)
Yield 71%, white solid, m.p.: 188–189 °C. FT-IR (cmÀ1): m(CN)
1552. 1H NMR (400 MHz, CDCl3) d, ppm: 11.08 (s, 1H, NCHN),
7.78 (d, J = 8.4 Hz, 1H, C6H4), 7.56 (t, J = 7.8 Hz, 1H, C6H4), 7.45 (t,
J = 7.8 Hz, 1H, C6H4), 7.22 (d, J = 8.4 Hz, 1H, C6H4), 6.94 (s, 2H, CH2-
C6H2(CH3)3), 5.86 (s, 2H, CH2C6H2(CH3)3), 5.35 (t, J = 2.9 Hz, 1H,
CHCH2), 4.96 (d, J = 2.9 Hz, 2H, CHCH2), 3.89–3.83 (m, 2H, OCH2-
CH2O), 3.81–3.77 (m, 2H, OCH2CH2O), 2.33 (s, 6H, CH2C4H2(CH3)2
CH3), 2.31 (s, 3H, CH2C4H2(CH3)2 CH3). 13C NMR (100 MHz, CDCl3)
d, ppm: 145.53 (CNH), 140.26 (C), 138.43 (C), 132.87 (C), 131.46
(C), 130.71 (C), 127.39 (CH), 125.50 (CH), 114.35 (CH), 113.85
(CH), 100.36 (CH), 65.91 (CH2), 49.02 (CH2), 47.83 (CH2), 21.57
(CH3), 20.65 (CH3). Elemental analysis, calcd for C21H25N2O2ClxH2-
O: C 64.52, H 6.96, N 7.17; found: C 65.67, H 6.68, N 7.34%.
2.4. General procedure for the synthesis of the PEPPSI-type palladium-
NHC complexes (3a-3f)
The benzimidazolium salts (2a–2f, 1.0 mmol) with PdCl2
(1.0 eq) and pyridine (2.0 eq), in the presence of K2CO3 (5.0 eq)
and KBr (10.0 eq), were dissolved in acetonitrile at 80 °C for 10 h.
Next, all volatiles were removed under vacuum and the solid resi-
due was washed with hexane (2 Â 5 mL). The crude product was
purified by column chromatography using CH2Cl2 to afford the cor-
responding Pd-PEPPSI-NHC complex. The palladium complex was
crystallized from a CH2Cl2/hexane solvent mixture (1:6, v/v) at
room temperature, and completely dried under vacuum. The ben-
zimidazole-2-ylidene based Pd-PEPPSI-NHC complexes were iso-
lated as air- and moisture-stable bright yellow solids.
2.3.4. 1-(1,3-Dioxolane-2- methyl)-3-(2,3,5,6-tetramethylbenzyl)
benzimidazolium chloride (2d)
2.4.1. Dibromo[1-((1,3-dioxalane-2-yl)-3-(4-methylbenzyl)
benzimidazol-2-ylidene](pyridine)palladium (II) (3a)
Yield 78%, white solid; m.p.: 167–168 °C. FT-IR (cmÀ1): m(CN)
1555. 1H NMR (400 MHz, CDCl3) d, ppm: 10.68 (s, 1H, NCHN),
7.81 (d, J = 8.4 Hz, 1H, C6H4), 7.59 (t, J = 7.8 Hz, 1H, C6H4), 7.49 (t,
J = 7.8 Hz, 1H, C6H4), 7.34 (d, J = 8.4 Hz, 1H, C6H4), 7.09 (s, 1H,
C6H4), 5.87 (s, 2H, CH2C6H(CH3)4), 5.34 (t, J = 2.9 Hz, 1H, CHCH2),
5.00 (d, J = 2.9 Hz, 2H, CHCH2), 3.86–3.83 (m, 2H, OCH2CH2O),
3.79–3.75 (m, 2H, OCH2CH2O), 2.27 (s, 6H, CH2C6CH(CH3)2CH3)2),
2.25 (s, 6H, CH2C6CH(CH3)2CH3)2). 13C NMR (100 MHz, CDCl3) d,
ppm: 144.24 (NCH), 134.91 (C), 133.88 (C), 133.45 (C), 132.24
(C), 130.76 (C), 127.83 (CH), 126.73 (CH), 113.78 (CH), 113.07
(CH), 99.74 (CH), 65.13 (CH2), 53.19 (CH2), 48.41 (CH2), 47.53
Yield 90%, yellow crystal; m.p.: 217–218 °C. FT-IR (cmÀ1): m(CN)
1407. 1H NMR (400 MHz, CDCl3) d, ppm: 9.04 (dd, J = 6.5, 1.6 Hz,
2H, NC5H5), 7.76 (tt, J = 7.6, 1.6 Hz, 1H, NC5H5), 7.59 (d, J = 8.2 Hz,
1H, C6H4), 7.46 (d, J = 8.0 Hz, 2H, CH2C6H4CH3), 7.34 (ddd, J = 7.6,
5.0, 1.4 Hz, 1H, NC5H5), 7.25–7.21 (m, 1H, C6H4), 7.15 (d,
J = 7.9 Hz, 2H, CH2C6H4CH3), 7.12–7.07 (m, 1H, C6H4), 7.03 (d,
J = 8.1 Hz, 1H,C6H4), 6.14 (s, 2H, CH2C6H4CH3), 5.82 (t, J = 4.5 Hz,
1H, CHCH2), 5.10 (d, J = 4.5 Hz, 2H, CHCH2), 4.12–4.08 (m, 2H,
OCH2CH2O), 3.98–3.94 (m, 2H, OCH2CH2O), 2.33 (s, 3H, CH2C6H4-
CH3). 13C NMR (100 MHz, CDCl3) d, ppm: 164.54 (NCH), 152.64
(Cpyridine), 137.94 (C), 137.87 (C), 135.83 (C),134.24 (C), 131.72
(C), 129.48 (CH), 128.00 (CH), 124.56 (CH), 123.16 (CH), 123.09
(CH), 111.76 (CH), 111.38 (CH), 102.49 (CH), 65.32 (CH2), 53.58
(CH2), 20.39 (CH3), 15.83 (CH3). Elemental analysis, calcd for C21
25N2O2ClxH2O: C 64.52, H 6.96, N 7.17; found: C 65.67, H 6.68,
N 7.34%.
-
H
(CH2), 51.72 (CH2), 21.19 (CH3). Elemental analysis, calcd for C24
-
3

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
H25N3O2Br2Pd: C 44.10, H 3.85, N 6.43; found: C 43.74, H 3.81, N
6.62%.
1H, C6H4), 6.93–6.84 (m, 1H, C6H4), 6.24 (d, J = 8.4 Hz, 1H, C6H4),
6.20 (s, 2H, CH2C6(CH3)5), 5.83 (t, J = 4.6 Hz, 1H, CHCH2), 5.09 (d,
J = 4.6 Hz, 2H, CHCH2), 4.14–4.08 (m, 2H, OCH2CH2O), 3.98–3.90
(m, 2H, OCH2CH2O), 2.33 (s, 3H, CH2C6(CH3)5), 2.29 (s, 6H, CH2C6(-
CH3)5), 2.25 (s, 6H, CH2C6(CH3)5). 13C NMR (100 MHz, CDCl3) d,
ppm: 164.79 (NCH), 153.42 (Cpyridine), 138.67 (C), 136.91 (C),
136.39 (C), 135.77 (C), 135.77 (C), 135.63 (C), 133.92 (CH),
128.42 (CH), 125.33 (CH), 123.89 (CH), 123.30 (CH), 112.26 (CH),
112.13 (CH), 103.42 (CH), 66.09 (CH2), 53.11 (CH2), 52.56 (CH2),
18.45 (CH3), 18.10 (CH3), 17.73 (CH3). Elemental analysis, calcd
for C28H33N3O2Br2Pd: C 47.38, H 4.69, N 5.92; found: C 46.53, H
4.52, N 5.81%.
2.4.2. Dibromo[1-((1,3-dioxalane-2-yl)-3-(4-(tert-butyl)benzyl)
benzimidazol-2-ylidene)](pyridine)palladium (II) (3b)
Yield 68%, yellow crystal; m.p.: 219–220 °C. FT-IR (cmÀ1): m(CN)
1404. 1H NMR (400 MHz, CDCl3) d, ppm: 9.03 (dd, J = 6.5, 1.6 Hz,
2H, NC5H5), 7.75 (tt, J = 7.7, 1.6 Hz, 1H, NC5H5), 7.59 (d, J = 8.2 Hz,
1H,C6H4), 7.52 (d, J = 8.4 Hz, 2H, NC5H5), 7.38–7.30 (m, 4H,CH2C6-
H4C(CH3)3), 7.25–7.20 (m, 1H, C6H4), 7.14–6.98 (m, 2H, C6H4),
6.13 (s, 2H, CH2C6H4C(CH3)3)), 5.81 (t, J = 4.5 Hz, 1H, CHCH2),
5.09 (d, J = 4.5 Hz, 2H,CHCH2), 4.10 (t, J = 7.0 Hz, 2H, OCH2CH2O),
3.96 (t, J = 6.9 Hz, 1H, OCH2CH2O), 1.28 (s, 9H, CH2C6H4C(CH3)3).
13C NMR (100 MHz, CDCl3) d, ppm: 164.51 (NCH), 152.67 (C),
151.10 (Cpyridine), 137.92 (C), 135.83 (C), 134.29 (C), 131.74 (CH),
127.82 (CH), 125.73 (CH), 124.56 (CH), 123.14 (CH), 123.07 (CH),
111.76 (CH), 111.42 (CH), 102.51 (CH), 65.32 (CH2), 53.50 (CH2),
51.73 (CH2), 34.58 (CH3), 31.31 (CH3). Elemental analysis, calcd
for C27H31N3O2Br2Pd: C 46.61, H 4.49, N 6.04; found: C 46.72, H
4.56, N 6.20%.
2.4.6. Dibromo[1-((1,3-dioxalane-2-yl)-3-(anthracen-9-ylmethyl)
benzimidazol-2-ylidene](pyridine)palladium (II) (3f)
Yield 92%, yellow crystal; m.p.: 291–292 °C. FT-IR (cmÀ1): m(CN)
1405. 1H NMR (400 MHz, CDCl3) d, ppm: 9.00 (dd, J = 6.5, 1.6 Hz,
2H, NC5H5), 8.65 (s, 1H, CH2C6H4C6HC6H4), 8.62 (d, J = 4.7 Hz, 2H,
CH2C6H4C6HC6H4 and C6H4), 8.07 (d, J = 8.2 Hz, 2H, CH2C6H4C6HC6-
H4), 7.75 (tt, J = 7.7, 1.6 Hz, 1H, NC5H5), 7.57–7.44 (m, 5H, CH2C6H4-
C6HC6H4 andC6H4), 7.34 (ddd, J = 7.6, 5.1, 1.4 Hz, 2H, NC5H5), 7.11
(s, 2H, CH2C6H4C6HC6H4), 7.07–6.97 (m, 1H, C6H4), 6.67–6.51 (m,
1H, C6H4), 5.93 (d, J = 8.4 Hz, 1H, C6H4), 5.87 (t, J = 4.5 Hz, 1H,
CHCH2), 5.14 (d, J = 4.5 Hz, 2H, CHCH2), 4.16–4.04 (m, 2H, OCH2-
CH2O), 4.02–3.89 (m, 2H, OCH2CH2O). 13C NMR (100 MHz, CDCl3)
d, ppm: 164.52 (NCH), 152.62 (Cpyridine), 137.90 (C), 135.71 (C),
134.59 (C), 131.67 (C), 131.31 (C), 130.25 (C), 129.74 (CH),
129.20 (CH), 127.34 (CH), 125.38 (CH), 124.81 (CH), 124.56 (CH),
124.30 (CH), 123.08 (CH), 122.61 (CH), 111.59 (CH), 102.58 (CH),
65.30 CH2), 51.79 (CH2), 49.34 (CH2). Elemental analysis, calcd
for C31H27N3O2Br2Pd: C 50.33, H 3.68, N 5.68; found: C 50.03, H
3.50, N 5.71%.
2.4.3. Dibromo[1-((1,3-dioxalane-2-yl)-3-(2,4,6-trimethylbbenzyl)
benzimidazol-2-ylidene](pyridine)palladium (II) (3c)
Yield 72%, yellow crystal; m.p.: 262–263 °C. FT-IR (cmÀ1): m(CN)
1403. 1H NMR (400 MHz, CDCl3) d, ppm: 9.04 (dd, J = 6.5, 1.6 Hz,
2H, NC5H5), 7.78 (tt, J = 7.7, 1.6 Hz, 1H, NC5H5), 7.56 (d, J = 8.2 Hz,
1H, C6H4), 7.36 (ddd, J = 7.6, 5.1, 1.4 Hz, 2H, NC5H5), 7.18–7.14
(m, 1H, C6H4), 6.95 (s, 2H, CH2C6H2(CH3)3), 6.94–6.89 (m, 1H,
C6H4), 6.28 (d, J = 8.4 Hz, 1H, C6H4), 6.14 (s, 2H, CH2C6H2(CH3)3),
5.82 (t, J = 4.6 Hz, 1H, CHCH2), 5.09 (d, J = 4.6 Hz, 2H, CHCH2),
4.12–4.04 (m, 2H, OCH2CH2O), 3.97–3.93 (m, 2H, OCH2CH2O),
2.35 (s, 3H, CH2C6H2(CH3)3), 2.33 (s, 6H, CH2C6H2(CH3)3). 13C
NMR (101 MHz, CDCl3) d, ppm: 164.25 (NCH), 152.64 (Cpyridine),
139.04 (C), 138.74 (C), 137.91 (C), 135.61 (C), 134.69 (C), 129.54
(CH), 127.36 (CH), 124.56 (CH), 123.17 (CH), 122.66 (CH), 111.57
(CH), 111.14 (CH), 102.59 (CH), 65.29 (CH2), 51.73 (CH2), 51.10
(CH2), 22.66 (CH3),21.14 (CH3), 20.94 (CH3). Elemental analysis,
calcd for C26H29N3O2Br2Pd: C 45.81, H 4.29, N 6.16; found: C
47.13, H 4.56, N 6.15%.
2.5. General procedure for the Pd-NHC catalyzed direct arylation of
C2-substituted heteroarenes
Typically, the C(2)-substituted heteroarene (2.0 mmol), aryl
bromide (1.0 mmol), KOAc (2.0 mmol) and DMA (2 mL) were
added to an oven-dried Schlenk tube under an argon atmosphere.
Subsequently, the Pd–carbene catalyst (3a–3f) (0.01 mmol, 1 mol
%) was added to the stirred solution in the Schlenk tube, then the
closed Schlenk tube was stirred at 130 °C for 1 h. (oil bath temper-
ature). At the end of the reaction, the solution was cooled to room
temperature and dichloromethane (2 mL) was added to the crude
mixture. The solution was filtered through a pad of celite to
remove the solid particles and then used for GC analysis. The yields
were calculated according to the (hetero)aryl halide by GC analysis.
2.4.4. Dibromo-[1-((1,3-dioxalane-2-yl)methyl)-3-(2,3,5,6-
tetramethylbenzyl)benzimdazol-2-ylidene](pyridine)palladium(II)
(3d)
Yield 70%, yellow crystal; m.p.: 226–227 °C. FT-IR (cmÀ1): m(CN)
1376. 1H NMR (400 MHz, CDCl3) d, ppm: 9.01 (dd, J = 6.4, 1.5 Hz,
2H, NC5H5), 7.77 (tt, J = 7.7, 1.6 Hz, 1H, NC5H5), 7.56 (d, J = 8.2 Hz,
1H, NC5H5), 7.38–7.33 (m, 2H,C6H4), 7.26 (s, 1H, CH2C6H(CH3)4),
7.16 (ddd, J = 8.2, 3.2, 1.6 Hz, 1H,C6H4), 7.10 (d, J = 4.4 Hz, 1H,
C6H4), 6.97–6.90 (m, 1H, C6H4), 6.17 (s, 2H, CH2C6H(CH3)4), 5.83
(t, J = 4.6 Hz, 1H, CHCH2), 5.09 (d, J = 4.6 Hz, 2H, CHCH2), 4.12–
4.09 (m, 2H, OCH2CH2O), 3.97–3.93 (m, 2H, OCH2CH2O), 2.27 (s,
6H, CH2C6H(CH3)4), 2.25 (s, 6H, CH2C6H(CH3)4). 13C NMR
(100 MHz, CDCl3) d, ppm: 164.01 (NCH), 152.42 (Cpyridine), 151.51
(Cpyridine), 137.69 (C), 135.63 (C), 135.08 (C), 134.99 (C), 134.71
(C), 134.16 (CH), 132.44 (CH), 130.17 (CH), 124.37 (CH), 122.97
(CH), 122.42 (CH), 111.38 (CH), 110.95 (CH), 102.43 (CH), 65.11
(CH2), 51.61 (CH2), 51.36 (CH2), 29.52 (CH3), 20.42 (CH3), 16.50
(CH3),16.40 (CH3).
2.6. Mercury poisoning experiment
2-Acetylfuran (2.0 mmol), 4-bromobenazldeyde (1.0 mmol),
KOAc (2.0 mmol) and DMA (2 mL) were added to an oven-dried
Schlenk tube under an argon atmosphere. Subsequently, the Pd–
carbene catalyst 3e (0.01 mmol, 1 mol%) was added to the stirred
solution in the Schlenk tube and one drop of Hg was added with
a syringe to the reaction mixture. The closed Schlenk tube was stir-
red at 130 °C for 1 h (oil bath temperature). At the end of the reac-
tion, the solution was cooled to room temperature and
dichloromethane (2 mL) was added to the crude mixture. The solu-
tion was filtered through a pad of celite to remove the solid parti-
cles and then used for GC analysis. The yields were calculated
according to the (hetero)aryl halide by GC analysis.
2.4.5. Dibromo[1-((1,3-dioxalane-2-yl)-3-(2,3,4,5,6-
pentamethylbenzyl)benzimidazol-2-ylidene](pyridine)palladium (II)
(3e)
Yield 77%, yellow crystal; m.p.: 268–269 °C. FT-IR (cmÀ1): m(CN)
1394. 1H NMR (400 MHz, CDCl3) d, ppm: 9.02 (dd, J = 6.5, 1.6 Hz,
2H, NC5H5), 7.77 (tt, J = 7.6, 1.6 Hz, 1H, NC5H5), 7.55 (d, J = 8.2 Hz,
1H,C6H4), 7.35 (ddd, J = 7.6, 5.1, 1.4 Hz, 2H, NC5H5), 7.17–7.12 (m,
4

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
2.7. X-ray analysis
zole with various alkyl chlorides in DMF at 70 °C for 48 h, Scheme 2.
The salts were soluble in polar solvents like methanol, ethanol,
DMSO and DMF, but insoluble in non-polar solvents like diethyl
ether, dichloromethane and chloroform. The benzimidazolium
salts 2a–f are air- and moisture-stable, both in the solid-state
and in solution. The benzimidazolium salts were characterized
using 1H NMR, 13C NMR , elemental analysis and FT-IR spec-
troscopy, which confirmed the proposed structures. The NMR spec-
tra of all the compounds were analyzed in d-CDCl3. In the 1H NMR
spectra, the acidic protons (NCHN) for 2a-f were seen at d 11.62,
11.32, 11.08, 10.68, 10.52 and 11.51 ppm, respectively, as a charac-
teristic sharp singlet. In the 13C NMR spectra of 2a-f, the NCHN car-
bon atom was detected as a typical singlet at d 144.86, 152.60,
145.53, 144.24, 144.30 and 143.49 ppm, respectively. These NMR
values are in line with those found for other benzimidazolium salts
in the literature [54]. The formation of the benzimidazolium salts
was also evidenced by their IR spectra, which showed an absorp-
tion at 1557, 1551, 1552, 1555, 1558 and 1564 cmÀ1 for the respec-
tive CN bond vibrations of 2a–f.
X-ray data of the compounds were collected with an STOE IPDS
II diffractometer at room temperature using graphite-monochro-
mated Mo Ka radiation by applying the x-scan method. Data col-
lection and cell refinement were carried out using X-AREA [50],
while data reduction was applied using X-RED32 [50]. The struc-
tures were solved using the charge-flipping algorithm by SUPER-
FLIP [51] and refined using full-matrix least-squares calculations
on F2 using SHELXL-2018 [52]. All H atoms were placed geometri-
cally and treated using a riding model, fixing the bond lengths at
0.93, 0.98, 0.97 and 0.96 Å for aromatic CH, methine CH, CH2 and
CH3 atoms, respectively. The displacement parameters of the H
atoms were fixed at Uiso(H) = 1.2Ueq (1.5Ueq CH3) of their parent
atoms. In 3d, the dioxolane ring was disordered over two positions
with occupancy factors of 0.597(9)/0.403(9) %. Crystal data, data
collection and structure refinement details are collected in Table 1.
The molecular graphics were generated using OLEX2 [53].
The general procedure for the preparation of the PEPPSI-type
palladium-NHC complexes 3a–f is shown in Scheme 3. The reac-
tions were carried out in the presence of pyridine as an N-donor
ligand in acetonitrile (MeCN) at 80 °C for 10 h, and the target com-
plexes were obtained. The benzimidazolium salts 2a–f were incor-
porated into the PEPPSI-type palladium–NHC complexes 3a–f by
their reaction with PdCl2 in pyridine, by heating at 80 °C for 10 h
in the presence of K2CO3 as a base. The air and moisture-stable
PEPPSI-type palladium-NHC complexes are yellow in color and sol-
uble in common organic solvents such as acetone, dichloro-
methane, chloroform, DMF, ethyl acetate and acetonitrile
(Scheme 3).
3. Results and discussion
3.1. Synthesis and spectral characterization of the benzimidazoles salts
and their palladium complexes
The synthesis of the target 1,3-disubstituted benzimidazolium
salts 2a–f has been achieved via a two-step N-alkylation process,
as depicted in Scheme 2. The step first involves the alkylation of
benzimidazole with 2-chloromethyl-1,3-dioxalane in the presence
of KOH in DMSO at 80 °C for 72 h to the corresponding benzimida-
zole, enhancing the reactivity of the other nitrogen atom. The addi-
tion of another alkylating agent to the remaining nitrogen atom
gives the desired 1,3-disubstituted benzimidazolium salts 2a–f.
The new 1,3-disubstituted benzimidazolium salts 2a–f were
prepared by reacting 1-(1,3-dioxolane-2-yl) methyl)-benzimida-
The complexes 3a-3f were characterized by 1H NMR, 13C NMR,
IR and X-ray diffraction techniques. In the 1H NMR spectra of the
palladium-carbene complexes complexes 3a-3f, the characteristic
Table 1
Crystal data and structure refinement parameters for 3a, 3d and 3e.
Parameters
3a
3d
3e
CCDC depository
Color/shape
2042615
2042616
2,042,617
Yellow/prism
[PdBr2(C19H20N2O2)(C5H5N)]
653.69
296(2)
0.71073 Mo K
Yellow/prism
[PdBr2(C22H26N2O2)(C5H5N)]
695.77
296(2)
0.71073 Mo K
Yellow/prism
[PdBr2(C23H28N2O2)(C5H5N)]
709.79
296(2)
0.71073 Mo K
Monoclinic
Chemical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a
a
a
Orthorhombic
P212121 (No. 19)
Monoclinic
C2/c (No. 15)
P21/c (No. 14)
Unit cell parameters
a, b, c (Å)
11.5282(7), 13.3631(8), 16.5657(9)
22.6751(10), 8.4460(3), 28.5415(11)
13.8789(9), 13.6413(11), 15.3642(10)
a
, b,
c
(°)
90, 90, 90
2552.0(3)
4
1.701
3.883
90, 96.922(3), 90
5426.3(4)
8
1.703
3.658
90, 102.057(5), 90
2844.7(4)
4
1.657
3.491
Volume (Å3)
Z
Dcalc. (g/cm3)
l
(mmÀ1
)
Absorption correction
Tmin., Tmax.
Integration
0.2587, 0.6374
1288
Integration
0.4302, 0.8061
2768
Integration
0.2679, 0.7310
1416
F000
Crystal size (mm3)
Diffractometer/measurement
method
0.63 Â 0.20 Â 0.15
0.41 Â 0.14 Â 0.07
0.48 Â 0.44 Â 0.07
STOE IPDS II/
x
scans
STOE IPDS II/
x
scans
STOE IPDS II/x scans
Index ranges
À13 ꢀ h ꢀ 13, À15 ꢀ k ꢀ 15,
À19 ꢀ l ꢀ 19
À29 ꢀ h ꢀ 29, À10 ꢀ k ꢀ 10,
À36 ꢀ l ꢀ 36
À14 ꢀ h ꢀ 16, À16 ꢀ k ꢀ 16,
À18 ꢀ l ꢀ 18
h range for data collection (°)
Reflections collected
Independent/observed reflections
Rint.
1.958 ꢀ h ꢀ 25.049
1.437 ꢀ h ꢀ 27.443
2.016 ꢀ h ꢀ 25.047
23,213
4528/3354
0.2157
23,339
6179/3230
0.0776
26,921
5035/4172
0.1479
Refinement method
Full-matrix least-squares on F2
4528/0/290
Full-matrix least-squares on F2
6179/223/366
Full-matrix least-squares on F2
5035/0/325
Data/restraints/parameters
Goodness-of-fit on F2
1.058
0.970
1.106
Final R indices [I greater than 2
r
(I)]
R1 = 0.0798, wR2 = 0.1665
R1 = 0.1067, wR2 = 0.1810
2.16, À0.70
R1 = 0.0695, wR2 = 0.1697
R1 = 0.1380, wR2 = 0.2011
1.30, À1.08
R1 = 0.0462, wR2 = 0.1092
R1 = 0.0585, wR2 = 0.1148
1.27, À0.70
R indices (all data)
D
qmax, D
qmin. (e/Å3)
5

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
Scheme 2. Synthesis and structure of the benzimidazolium salts 2a-f.
down-field signals for the acidic C(2)-H protons of the benzimida-
zolium salts 2a-2f disappeared. This suggests the formation of the
palladium-NHC-PEPPSI complexes. Also, the formation of carbenes
is correlated by a shift of the (CN) vibration from 1551 to
1564 cmÀ1 in the benzimidazolium salts to1376-1407 cmÀ1 in
ues show that the distortion in the coordination polyhedron of 3d
is a little more than for the other complexes.
The average Pd─CNHC bond distance [1.961 Å] is smaller than
the sum of the individual covalent radii of the palladium and car-
bon atoms (2.12 Å), while the average Pd─Npyridine bond distance
[2.116 Å] is close to the sum of the individual covalent radii of
the palladium and nitrogen atoms (2.10 Å) [57]. The Pd─Br bond
lengths are in the typical range and interestingly bent toward the
NHC ligand rather than toward the non-bulky pyridine ligand.
These values are in good agreement with those found in other
Pd-NHC-pyridine-Br2 complexes [58–64]. In the NHC ligands, the
dioxolane ring adopts an envelope conformation and the internal
NACAN ring angle at the carbene centers vary from 106.9(7) to
107.2(4)°. The carbene ring is nearly perpendicular to the PdCNBr2
coordination plane, with a dihedral angle of 76.8(5)° in 3a, 76.3(3)°
in 3d and 75.53(14)° in 3e, which is typical for NHC complexes,
reducing steric congestion. Furthermore, the dihedral angle
between the pyridine ring and the coordination plane is found to
be 68.9(8)° in 3a, 53.7(6)° in 3d and 58.3(2)° in 3e.
the coordinated carbenes [55]; the FT-IR data indicated m(CN) at
1407, 1404, 1403, 1376, 1394 and 1405 cmÀ1 for the PEPPSI Pd-
NHC complexes 3a-f, respectively. The Pd–carbene resonances of
these novel PEPPSI Pd-NHC complexes in the 13C NMR spectra
appeared highly downfield shifted at d 164.54 164.51, 164.25,
164.01, 164.79 and 164.52 ppm for 3a-f, respectively. The analyti-
cal data are in good agreement with the compositions proposed for
all the newly prepared compounds.
3.2. Description of the crystal structures of the palladium-carbene
complexes 3a, 3d and 3e.
The molecular diagrams of 3a, 3d and 3e, with the adopted
atom-labelling scheme, are shown in Figs. 1-3, while important
bond distances and angles are listed in Table 2.
The three complexes show slightly distorted square-planar
geometries around the palladium center, which are encompassed
by the carbonic carbon atom of NHC, the nitrogen atom of the pyr-
idine ring and two bromo ligands in a trans configuration. The cis
angles, varying from 87.3(6) to 93.12(11)°, and the trans angles,
changing from 173.04(5) to 179.6(7)°, deviate from their expected
3.3. Catalytic studies
Over the last two decades, the transition-metal-catalyzed direct
CAH arylation of arenes with aryl halides has appeared as an effi-
cient method for the preparation of heteroarene derivatives, such
as arylated thiophenes, furans and thiazole [65]. In 1990, Ohta
et al. reported the arylation of thiophenes, furans and thiazoles
with aryl halides, via a CAH bond activation, in moderate to good
yields using 5 mol% Pd(PPh3)4 as the catalyst [66]. Since these
values of 90 and 180°. The four-coordinate geometry index
an ideal square-planar geometry and 1 for a perfect tetrahedral
geometry) [56] is 0.03 for 3a, 0.07 for 3d and 0.04 for 3e. The 4 val-
s4 (0 for
s
6

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
Scheme 3. Synthesis of the Pd-PEPPSI complexes 3a-f.
Fig. 2. Molecular structure of 3d, showing the atom-labelling scheme. Displace-
ment ellipsoids are drawn at the 20% probability level and H atoms are shown as
small spheres of arbitrary radii. For clarity, only the major part of the disordered
dioxolane ring is shown.
Fig. 1. Molecular structure of 3a, showing the atom-labelling scheme. Displace-
ment ellipsoids are drawn at the 20% probability level and H atoms are shown as
small spheres of arbitrary radii.
7

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
Fig. 3. Molecular structure of 3e, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 20% probability level and H atoms are shown as small spheres
of arbitrary radii.
Table 2
low catalyst loading and short-time reaction conditions (less than
2 mol% and 15 h) would be a considerable advantage for industrial
applications and sustainable development. Thus, to find an effec-
tive and selective procedure allowing high yields of arylation prod-
ucts and using low catalyst loading and short reaction times is still
subject to significant improvement.
Selected geometric parameters for 3a, 3d and 3e.
Parameters
3a
3d
3e
Bond lengths (Å)
Pd1─Br1
Pd1─Br2
Pd1─N3
Pd1─C1
N1─C1
N1─C2
N2─C1
N2─C7
Bond angles (°)
Br1─Pd1─Br2
Br1─Pd1─N3
Br2─Pd1─N3
Br1─Pd1─C1
Br2─Pd1─C1
N3─Pd1─C1
N1─C1─N2
2.450(2)
2.435(2)
2.132(12)
1.963(16)
1.37(2)
1.416(18)
1.348(19)
1.386(19)
2.4289(11)
2.4186(12)
2.097(8)
2.4461(6)
2.4298(6)
2.120(4)
1.964(4)
1.354(5)
1.396(6)
1.358(6)
1.404(5)
1.957(9)
Here, we used the optimized conditions for the catalytic reac-
tion which were determined in our previous works [15,47,58]. In
a standard experiment the C2-substituted heteroarene (2.0 mmol),
aryl bromide (1.0 mmol), KOAc (2.0 mmol), DMA (2 mL) and the
Pd-NHC complex 3a-f (0.01 mmol) were added to a Schlenk tube
under an inert atmosphere. The sealed Schlenk tube was stirred
at 130 °C for 1 h. The reaction mixture was cooled to room temper-
ature at the end of the reaction, then CH2Cl2 (2 mL) was added and
the resulting solution was filtered through a short SiO2 pad. The fil-
trate was analyzed by GC. The yields were based on the corre-
sponding (hetero)aryl halide. Initially, under the optimal
conditions, the direct arylation of 2-acetylfuran with 4-bromoben-
zaldehyde and bromobenzene was examined and the C(5)-arylated
furan derivatives were obtained. We observed good yields of the
target product when 4-bromobenzaldehyde was used with 2-
acetylfuran, with 78–90% GC yields (Table 3, entries 1–6). Similar
results were obtained when bromobenzene was used. In this case,
2-acetyl-5-phenylfuran was obtained in 72–89% GC yield (Table 3,
entries 7–12).
1.363(10)
1.392(12)
1.336(10)
1.385(10)
175.87(9)
93.0(4)
90.6(4)
87.3(6)
89.1(6)
173.04(5)
92.4(2)
90.6(2)
88.8(2)
88.4(2)
177.6(3)
106.9(7)
176.32(2)
93.12(11)
90.01(11)
87.88(12)
89.03(12)
178.47(18)
107.2(4)
179.6(7)
107.1(13)
exciting results, the palladium-catalyzed direct arylation of hetero-
aryl derivatives with aryl halides or triflates has proved to be a
powerful method for the synthesis of arylated heterocycles [67–
77]. So far, to our knowledge, all the procedures reported for aryla-
tion via CAH bond activation of heteroarenes using ligand-free cat-
alysts required 5–10 mol% catalyst, [76,77] except one which
employs only 1 mol% [78]. Such couplings under a low catalyst
concentration employ palladium associated with sophisticated
ligands. Therefore, the discovery of more effective conditions for
the direct coupling of furan derivatives with aryl halides under
Using the same reaction conditions, we investigated the reactiv-
ity of 2-acetylthiophene for the Pd-catalyzed direct C(5)-arylation.
As shown in Table 4, high-yield C(5)-arylated products were
8

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
Table 3
Palladium(II)–NHC-catalyzed direct C5-arylation of 2-acetylfuran with aryl bromides.
Entry
Catalyst
Aryl bromide
Product
Conv (%)
Yield (%)
1
2
3
3a
3b
3c
100
100
100
86
90
82
4
5
6
3d
3e
3f
88
100
98
78
86
85
7
8
9
3a
3b
3c
100
100
96
85
89
72
10
11
12
3d
3e
3f
100
95
98
88
86
82
a Conditions: [Pd] 3a-3f (0.01 equiv., 1 mol%), 2-acetylfuran or 2-acetylthiophene (2 equiv.), (hetero)aryl halide (1 equiv.), KOAc (2 equiv.), DMA (2 mL), 130 °C. b Yields were
calculated with respect to the (hetero)aryl halide from the results of GC spectrometry.
obtained. When 2-acetylthiophene was arylated with 4-bro-
mobenzotrifluoride, 4-bromobenzaldehyde, bromobenzene, 4-
bromoanisole and 3-bromoquinoline, products were obtained
using only1 mol% of the Pd-complexes 3a–3f as catalysts, and
yields of 82–87, 68–92, 80–92, 62–70 and 77–94% were observed,
respectively (Table 4, entries 1–30).
when 4-bromoanisole was used. In this case, 2-aldehyde-5-(4-
methoxyphenyl) was obtained in 73–79% GC yield (Table 5, entries
19–24). When the reaction of 2-aldehydefuran with 3-
bromoquinoline was investigated, the C(5)-arylated product was
obtained in 90% GC yield in the presence of the catalyst 3a (Table 5,
entry 25).
When the reaction of 2-acetylthiophene with 4-bromobenzotri-
fluoride was investigated, the C(5) arylated product was obtained
in 87% GC yield in the presence of the catalyst 3b (Table 2, entry
2). The reaction of 2-acetylthiophene with 4-bromobenzaldehyde
gave the expected product in 92% yield in the presence of the cat-
alyst 3a (Table 4, entry 7). The reaction of 2-acetylthiophene with
bromobenzene generated the 5-phenyl-2-acetylthiophene in 92
and 94% yields, and with 4-bromoanisole, the C(5)-arylated pro-
duct was obtained in 70% yield (Table 4, entries 13, 17 and 22).
We examined the reactivities of electron-deficient heterocycles,
such as 3-bromoquinoline, as heteroaryl bromides. A selected reac-
tion was observed using 3-bromoquinoline. With this substrate,
the target product 5-(quinoline-3-yl)-2acetythiophene was
obtained in 94% yield in presence of the 3c and 3f catalysts after
1 h (Table 4, entries 27, 30).
Also, we tried to evaluate the scope and limitations of the pal-
ladium-carbene catalysts 3a-3f for the direct C(5)-arylation of 2-
aldehydefuran with (hetero)aryl halides and the results are sum-
marized in Table 5. When electron-withdrawing para-substituents
such as aldehyde and trifluoromethyl on the aryl bromide were
investigated with 2-aldehydefuran, the target products were
obtained in moderate to high yields in presence of 1 mol% of the
3a-3f catalysts after 1 h.
The Pd-catalyzed direct arylation of furan and thiophene with a
variety of electrophilic reagents has been previously described
[79,80]. In the previous studies, similar or close substrates have
been employed with higher catalyst loading (1–20 mol%), and a
higher reaction time (1–48 h) has been chosen for the direct aryla-
tion of furan and thiophene in the presence of Pd-catalysts. In the
present work, 1 mol% catalyst loading was used, and the reaction
time was shortened to 1 h. Moreover, thiophene and furan deriva-
tives can be efficiently and selectively arylated at the C(5)-position.
Finally, the palladium-catalyzed direct arylation of acetyl furan
with aryl bromides has been as been compared to the previously
published complexes. For example, 2-acetylfuran and 4-bro-
mobenzaldeyde were chosen for comparison. Based on previous
literature reports, similar substrates have been employed with
higher reaction times, such as 2 h, for the direct arylation of 2-
acetylfuran [81]). In the present work, the reaction time was short-
ened to 1 h for aryl bromides (Table 6). Moreover, in the present
study satisfactory results were obtained as compared to previous
results.
The ability of Hg(0) to poison metal-particle heterogeneous cat-
alysts, by amalgamating the metal or adsorbing on the metal sur-
face, has been known for more than 90 years and is a widely
used test [82]. This experiment was performed by adding Hg(0)
to the reaction solution. The suppression of the catalysis by Hg
(0) is evidence for a heterogeneous catalyst; if Hg(0) does not sup-
press the catalysis that is evidence for a homogeneous catalyst.
Hence we carried out the mercury poisoning experiment to assess
whether the reaction system was homogeneous or heterogeneous.
The Hg(0)-poisoning experiments were performed with the cata-
lyst 3e in the presence of excess Hg. The results showed no signif-
icant inhibition of conversion to products when complex 3e was
When 4-bromobenzotrifluoride and 2-aldehydefuran were
reacted in the presence of the Pd-NHC catalysts (3a-3f), 74–88%
yields were obtained (Table 5, entries 1–6). When 4-
bromobenzaldehyde was used as the aryl halide, after 1 h, moder-
ate to high yields of 72–87% were obtained in the presence of 3a-3f
(Table 5, entries 7–12). When 4-bromobenzaldehyde was used as
the aryl halide, moderate yields (83%) were obtained in presence
of the catalyst 3a (Table 5, entry 13). Similar results were obtained
9

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
Table 4
Palladium(II)–NHC-catalyzed direct C5-arylation of 2-acetythiophene with aryl bromides.
Entry
Catalyst
Arylbromide
Product
Conv (%)
Yield (%)
1
2
3
3a
3b
3c
92
95
90
86
87
85
4
5
6
7
8
3d
3e
3f
3a
3b
100
91
90
99
75
84
86
82
92
76
9
3c
3d
3e
3f
3a
3b
3c
76
86
78
80
95
95
96
68
83
77
83
92
84
85
10
11
12
13
14
15
16
17
18
19
20
21
3d
3e
3f
3a
3b
3c
93
98
92
94
92
92
80
94
80
64
62
62
22
23
24
25
26
3d
3e
3f
3a
3b
96
91
92
100
90
70
60
62
77
86
27
28
29
30
3c
3d
3e
3f
98
92
100
100
94
92
92
94
a Conditions: [Pd] 3a-3f (0.01 equiv., 1 mol%), 2-acetylfuran or 2-acetylthiophene (2 equiv.), (hetero)aryl halide (1 equiv.), KOAc (2 equiv.), DMA (2 mL), 130 °C. b Yields were
calculated with respect to the (hetero)aryl halide from the results of GC spectrometry.
used as a catalyst. Thus, the present catalysis appears to be
homogeneous.
ether side chains, on the metal may radically increase the catalytic
performance of the catalyst. The chelating nature of these ligands
promotes the production of highly stable complexes. The hemil-
abile part of such ligands is capable of reversible dissociation to
produce vacant coordination sites, allowing complexation of sub-
strates during the catalytic cycle. In this direct CAH bond aryla-
tion, we believe that the NHC ligands bearing the 1,3-dioxalane
group in complexes 3a-f provide the synergetic steric and elec-
tronic effects to confer the metal center the appropriate properties
We attributed the performance differences to the well-accor-
dance electronic and steric properties of the NHC ligand. It is
known that oxidative additions of electron-withdrawing sub-
strates to electron-rich palladium-complexes and reductive elim-
ination of the product from large, sterically hindered palladium-
complexes proceed more readily. Therefore, the presence of an
NHC ligand bearing a different second donating group, such as
10

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
Table 5
Palladium(II)–NHC-catalyzed direct C5-arylation of 2-aldehydefuran with aryl bromides.
Entry
Catalyst
Arylbromide
Product
Conv (%)
Yield (%)
1
2
3
3a
3b
3c
84
95
88
74
80
81
4
5
6
7
8
9
3d
3e
3f
3a
3b
3c
96
82
85
99
100
100
88
75
79
72
87
83
10
11
12
13
14
15
3d
3e
3f
3a
3b
3c
100
100
99
90
79
80
70
81
83
79
67
73
16
17
18
19
20
21
3d
3e
3f
3a
3b
3c
79
84
81
83
98
90
64
78
71
73
79
75
22
23
24
25
26
27
3d
3e
3f
3a
3b
3c
92
91
94
98
98
96
79
77
77
95
90
71
28
29
30
3d
3e
3f
94
96
98
60
78
85
Note: a Conditions: [Pd] 3a-3f (0.01 equiv., 1 mol%), 2-acetylfuran or 2-acetylthiophene (2 equiv.), (hetero)aryl halide (1 equiv.), KOAc (2 equiv.), DMA (2 mL), 130 °C. b Yields
were calculated with respect to the (hetero)aryl halide from the results of GC spectrometry.
to make the key steps of the catalytic cycles optimum. The pro-
posed catalytic pathway according to the above explanations is
shown in Scheme 4.
When the proposed catalytic cycle is examined, initially, oxida-
tive addition of the aryl halides to the pre-activated [(NHC)Py-Pd0]
species affords the [(NHC)PyX-PdII-Aryl] intermediate. In this step,
the NHC ligands increase the electron density around the metal
center and the oxidative addition step more readily takes place.
Next, the exchange of the X ligand with KOAc gives [(NHC)Py
(AcO)-PdII-Aryl]. The nature of the base used in this step is very
important. Then, the intermediate [(NHC)Py(AcO)-PdII-Aryl] reacts
with furan to give [(NHC)Py(furan)-PdII-Aryl] by C-H activation. In
this step, the chelating nature of the 1,3-dioxalane substituent pro-
motes the production of highly stable complexes. Finally, reductive
11

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
Table 6
Palladium-catalyzed direct C5-arylation of 2-acetylfuran.
Entry
Catalyst
Conditions
Conv. [%]
Yield [%]
Ref.
1
2
3
4
5
3e
A
B
C
D
130 °C, 1h
150 °C, 2h
120 °C, 2h
120 °C, 2h
120 °C, 2h
100
93
100
100
100
86
79
75
74
75
this work
[81a]
[81b]
[81c]
[81d]
elimination of the intermediate [(NHC)(Py)(furan)-PdII-Aryl] pro-
duces the desired C5-arylated furan products.
complexes. This study is of environmental and economic interest
owing to the low catalyst loading and shorter reaction time. In this
study, only AcOH and HBr were formed as a by-products by the use
of the direct arylation method and thus by-product formation was
minimized compared with the multistep traditional transition
metal-catalyzed reactions. Further studies focusing on the synthe-
sis of novel benzimidazole-2-ylidene linked palladium-PEPPSI
complexes and their catalytic application for the CAH bond aryla-
tion of heteroarenes are currently underway by our research group.
4. Conclusion
We have prepared a series of benzimidazolium salts as N-hete-
rocyclic carbene precursors and their new Pd-PEPPSI-NHC com-
plexes have been prepared in good yields. The benzimidazole
ligands are easy to prepare and handle, and these ligands are most
suitable to prepared PEPPSI catalysts. The catalytic activities of all
the palladium complexes were investigated in the direct C(5)-ary-
lation of 2-acetylfuran, 2-acetylthiophene and 2-aldehydefuran
with (hetero)aryl halides. It was found that all the palladium com-
plexes were effective catalysts for the CAH activation process. Also,
no catalyst poisoning was observed in the presence of mercury,
indicating homogeneous catalysis.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
It was found that the new Pd-complexes were effective cata-
lysts for this direct arylation. Overall, except in a few cases, satis-
factory results were obtained. Since the newly prepared NHC
ligands in this work are similar to each other, no significant differ-
ences were observed between the catalytic activities of the Pd-
Acknowledgments
This study was supported by the Technological and Scientific
Research Council of Turkey TÜBITAK (Project No: 117R010).
_
12

S. Lasmari, N. Gürbüz, R. Boulcina et al.
Polyhedron 199 (2021) 115091
Scheme 4. Proposed catalytic pathway for the CAH bond arylation of furans.
[12] S.I. Gorelsky, Organometallics 31 (2012) 794–797.
Appendix A. Supplementary data
_
[13] N. Sßahin, G. Serdarog˘lu, S. Demir Düsßünceli, M.N. Tahir, C. Arıcı, I. Özdemir, J.
Coord. Chem. 72 (2019) 3258–3284.
CCDC 2042615-2042617 contain the supplementary crystallo-
graphic data for the compounds reported in this article. These data
can be obtained free of charge on application to 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.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2021.115091.
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