Well-defined PEPPSI-themed palladium–NHC complexes: synthesis, and catalytic application in the direct arylation of heteroarenes

Article abstract of DOI:10.1002/aoc.5387

In this study, a series of benzimidazolium salts were synthesized as unsymmetrical N-heterocyclic carbene (NHC) precursors. Benzimidazolium salts were used for synthesis of the PEPPSI (pyridine enhanced precatalyst preparation stabilization and initiation)-themed, six new Pd-complexes with the general formula [PdX₂(NHC)(pyridine)]. The structures of all compounds were characterized by various spectroscopic techniques such as ¹H NMR, ¹³C NMR and FT-IR. The more detailed structural characterization of four of the complexes was determined by single-crystal X-ray diffraction study. The catalytic activities of all Pd-complexes were evaluated in the direct arylation of the 2-acetylfuran and 2-acetylthiophene with aryl bromides in the presence of 1 mol% catalyst loading.

Full text of DOI:10.1002/aoc.5387

Received: 20 September 2019
DOI: 10.1002/aoc.5387
Revised: 28 October 2019
Accepted: 2 November 2019
F U L L P A P E R
Well‐defined PEPPSI‐themed palladium–NHC complexes:
synthesis, and catalytic application in the direct arylation of
heteroarenes
Murat Kaloğlu1,2
İsmail Özdemir1,2
| Nevin Gürbüz1,2
| İlkay Yıldırım3
| Namık Özdemir4
|
1 Faculty of Science and Arts, Department
of Chemistry, İnönü University, 44280
Malatya, Turkey
In this study, a series of benzimidazolium salts were synthesized as unsymmet-
rical N‐heterocyclic carbene (NHC) precursors. Benzimidazolium salts were
used for synthesis of the PEPPSI (pyridine enhanced precatalyst preparation
stabilization and initiation)‐themed, six new Pd‐complexes with the general
formula [PdX2(NHC)(pyridine)]. The structures of all compounds were charac-
terized by various spectroscopic techniques such as 1H NMR, 13C NMR and FT‐
IR. The more detailed structural characterization of four of the complexes was
determined by single‐crystal X‐ray diffraction study. The catalytic activities of
all Pd‐complexes were evaluated in the direct arylation of the 2‐acetylfuran
and 2‐acetylthiophene with aryl bromides in the presence of 1 mol% catalyst
loading.
2 Catalysis Research and Application Center,
İnönü University, 44280 Malatya, Turkey
3 Vocational School of Health Services,
Department of Radiotherapy, Biruni
University, 34010 İstanbul, Turkey
4 Faculty of Education Department of
Mathematics and Science Education,
Ondokuz Mayıs University, 55139
Samsun, Turkey
Correspondence
İsmail Özdemir, İnönü University, Faculty
of Science and Arts, Department of
Chemistry, 44280. Malatya, Turkey.
Email: ismail.ozdemir@inonu.edu.tr
KEYWORDS
direct arylation, heteroarene, N‐heterocyclic carbene, palladium, PEPPSI
Funding information
Technological and Scientific Research
Council of Turkey TÜBİTAK, Grant/
Award Number: 117R010
1 | INTRODUCTION
corrosive chemicals, etc.[9] Therefore, there is a need for
more environmentally attractive and atom economical
Transition metal‐catalyzed cross‐coupling reactions have
become one of the most important synthetic tools for
synthesis of bi(hetero)aryl units in modern organic synthe-
sis.[1–4] Suzuki,[5] Stille,[6] Negishi[7] or Kumada[8] type
reactions are widely used in the metal‐catalyzed synthesis
of bi(hetero)aryls. Despite the large applicability of these
reactions, in most of them, the starting coupling partners
should bear either a metal‐containing functionality or a
halogen atom and related groups. In this case in fact the
preliminary preparation of two independent, expensive,
starting materials is required. These processes also have
some disadvantages such as generating stoichiometric
amounts of waste materials, the use of unstable and
methods in which total synthesis steps and by‐product for-
mation are minimized. The direct arylation method repre-
sents an ideal alternative to conventional cross‐coupling
reactions in the synthesis of bi(hetero)aryls, because of its
step‐economic and eco‐friendly advantages.[10] More
importantly, this method not only minimizes by‐product
formation, but also has a great advantage as it makes
organic synthesis easier (Scheme 1).
Over the last few decades, considerable advances have
been achieved in direct arylation methods. Various transi-
tion metals such as Pd, Ru, Rh, etc., have been shown to be
effective for the direct arylation reactions.[11] However,
among them, the Pd‐complexes are the most powerful
Appl Organometal Chem. 2019;e5387.
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© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5387

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KALOĞLU ET AL.
SCHEME 1 Comparison of conventional cross‐coupling reactions and direct arylation method
and widely used catalysts in direct arylation. In recent
years, Pd‐catalyzed direct arylation of (hetero)arenes with
(pseudo)halides have received significant attention as
eco‐friendly and economic alternatives to conventional
methods. Here, aryl (pseudo)halides are the most com-
monly employed coupling partners for Pd‐catalyzed direct
arylation reactions.[12] Among these reactions, the direct
arylation of heteroarenes is particularly attractive owing
to the fact that these moieties are present in many biologi-
cally active compounds. One of the first examples of Pd‐
catalyzed direct arylation of heteroarenes was reported by
Nakamura, Tajima and Sakai in 1982[13] and by Ohta in
1985.[14] Following these pioneering studies, Pd‐catalyzed
direct arylation of heteroarenes by aryl halides has been
demonstrated to be a convenient and attractive methodol-
ogy for the synthesis of bi(hetero)aryls. To date, Pd‐
catalyzed direct arylation of heteroarenes, especially five‐
membered heterocycles such as thiopene[15] and furan,[16]
has been described by a large number of researchers.
N‐Heterocyclic carbenes (NHCs) are neutral two‐
electron donors and they are known to be strong Lewis
bases and excellent nucleophiles that bind metals better
than phosphines.[17] One of the key features of NHCs com-
pared with other classes of ligands is their stability. The
strong σ‐donating but poor π‐accepting ability of NHC
ligands leads to the formation of many stable metal–NHC
complexes. Another features of NHCs is their tunability
by the attachment of different substituents allowing the
complexes to be obtained with desired electronic and steric
properties.[17] In 1968, the first metal complexes containing
NHC ligand were reported,[18] but these ligands received
little attention in those years. However, after the first isola-
tion and characterization of stable free NHC in 1991,[19]
interest in these ligands increased exponentially. Next, the
first use of the NHCs in Pd‐catalyzed Heck reaction in
1995[20] presented a new class of ligands to the catalysis area.
Nowadays, NHCs have become one of the most widely used
ligand classes in organometallic chemistry and catalysis.
After the discovery of Organ's PEPPSI (pyridine
enhanced precatalyst preparation stabilization and initia-
tion)‐themed Pd‐complexes,[21] this type of complex has
shown remarkable catalytic activities towards various
carbon–carbon and carbon–heteroatom coupling reac-
tions. PEPPSI‐themed Pd–NHC complexes represent a
new class of Pd‐catalysts that are completely different
from other Pd–NHC complexes and easier to synthesize
and use.[22] In recent years, PEPPSI‐themed Pd‐
complexes have been used as effective catalysts in direct
arylation and successful results have been obtained.[23]
In this context, recently we successfully reported the syn-
thesis and structural characterization of PEPPSI‐themed
Pd‐complexes with different NHC ligands, and we have
investigated the atalytic activity of these complexes in
direct arylation reactions.12h, 15h Herein, we now report
the successful synthesis of six new PEPPSI‐themed Pd–
NHC complexes (3a–3f) of the general formula [PdX2
(NHC)(pyridine)], (X = Cl, Br; NHC = 1,3‐disubstituted
benzimidazole‐2‐ylidene), and their full characterization
by various spectroscopic techniques. The solid‐state struc-
tures of the four Pd‐complexes (3a–3c and 3 f) have been
established by single‐crystal X‐ray diffraction study. In
the present study, the catalytic application of all Pd‐
catalysts has been tested in the direct arylation of 2‐
acetylfuran and 2‐acetylthiophene with aryl bromides in
the presence of 1 mol% catalyst loading (Figure 1).
2 | EXPERIMENTAL SECTION
2.1 | Synthesis
The general procedures for the 1‐(2,2‐diethoxyethyl)benz-
imidazole (1), NHC ligand precursors (2a–2f) and their
corresponding PEPPSI‐themed Pd–carbene complexes
(3a–3f) are shown in Scheme 2. Compounds 1, 2b, 2c

KALOĞLU ET AL.
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FIGURE 1 Pd‐catalyzed direct
arylation of C2‐substituted furan and
thiophene with aryl bromides
and 2d were synthesized as previously described in the
under reduced vacuum. 1‐(2,2‐Diethoxyethyl)benzimid-
azole was isolated as a yellowish gel in 85% yields. For
the 1H NMR, 13C NMR and FT‐IR spectra of 1 see
supporting information file, pages S1 and S2.
literature.[24]
2.2 | General procedure for the
preparation of 1‐(2,2‐diethoxyethyl)
benzimidazole (1)
2.2.1 | 1‐(2,2‐Diethoxyethyl)benzimid-
azole, 1[24]
For the preparation of 1‐(2,2‐diethoxyethyl)benzimid-
azole (1), benzimidazole (5.907 g, 50.0 mmol) and potas-
sium hydroxide (2.805 g, 50.0 mmol) were dissolved in
ethyl alcohol (50 ml), the reaction mixture was stirred
at room temperature for 1 h. Then, 2,2‐diethoxyethyl bro-
mide (9.853 g, 50.0 mmol) was slowly added, and the
solution was heated to reflux for 5 h. The mixture cooled
to room temperature and the precipitated potassium bro-
mide was removed by filtration. The solvent was removed
by distillation. The crude product was then distilled
Yield 85%, 9.95 g (yellowish gel); b.p.: 140–150°C-
(under ~50 Torr pressure); FT‐IR (vC(2)‐N): 1494 cm−1
.
1H NMR (400 MHz, CDCl3, 25°C, TMS): δ (ppm) = 1.14
[t, J = 7.0 Hz, 6H, NCH2CH(OCH2CH3)2]; 3.40 and 3.70
[dq, J = 9.2, 7.0 Hz, 4H, NCH2CH(OCH2CH3)2]; 4.25
[d,
J = 5.3 Hz, 2H, NCH2CH(OCH2CH3)2]; 4.67
[t, J = 5.3 Hz, 1H, NCH2CH(OCH2CH3)2]; 7.25–7.32
(m, 2H, NC6H4N); 7.45 and 7.80 (dd, J = 6.8,
1.9 Hz, 2H, NC6H4N); 7.96 (s, 1H, NCHN). 13C NMR
SCHEME 2 General pathway for the preparation of 1, 2a–2f and 3a–3f compounds

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KALOĞLU ET AL.
(101 MHz, CDCl3, 25°C, TMS): δ (ppm) = 15.21
[NCH2CH(OCH2CH3)2]; 48.12 [NCH2CH(OCH2CH3)2];
2.3.2 | 1‐(2,2‐Diethoxyethyl)‐3‐(2,4,6‐
trimethylbenzyl)benzimidazolium
chloride, 2b[24]
63.91
[NCH2CH(OCH2CH3)2];
100.86
[NCH2CH
(OCH2CH3)2]; 109.65, 120.31, 122.13, 122.92, 134.07
and 143.83 (NC6H4N); 143.55 (NCHN).
Yield 76%, 1.53 g (white solid); m.p.: 172–173°C; FT‐IR (vC
(2)‐N): 1560 cm−1. 1H NMR (400 MHz, CDCl3, 25°C, TMS):
δ (ppm) = 1.10 [t, J = 7.0 Hz, 6H, NCH2CH (OCH2CH3)2];
2.30 and 2.33 [s, 9H, NCH2C6H2(CH3)3‐2,4,6]; 3.64 and
3.74 [dq, J = 14.1, 7.0 Hz, 4H, NCH2CH(OCH2CH3)2];
4.84 [d, J = 3.9 Hz, 2H, NCH2CH (OCH2CH3)2]; 4.98 [t,
J = 3.9 Hz, 1H, NH2CH (OCH2CH3)2]; 5.80 [s, 2H,
NCH2C6H2(CH3)3‐2,4,6]; 6.95 [s, 2H, NCH2C6H2(CH3)3‐
2,4,6]; 7.33 (d, J = 8.4 Hz, 1H, NC6H4N); 7.45–7.49 and
7.54–7.58 (m, 2H, NC6H4N); 7.84 (d, J = 8.4 Hz, 1H,
NC6H4N); 10.78 (s, 1H, NCHN). 13C NMR (101 MHz,
2.3 | General procedure for the
preparation of NHC ligand precursors
(2a–2f)
1‐(2,2‐Diethoxyethyl)benzimidazole (1.17 g, 5.0 mmol)
was dissolved in degassed dimethylformamide (3 ml)
and alkyl halide derivative (5.0 mmol) was added at
room temperature. The reaction mixture was stirred at
80°C for 36 h under argon. After completion of the reac-
tion, the solvent was removed by vacuum and Et2O
(15 ml) was added to obtain a white crystalline solid,
which was filtered off. The solid was washed with
Et2O (3 × 10 ml) and dried under vacuum. The crude
product was recrystallized from EtOH/Et2O mixture
(1:5, v/v) at room temperature, and completely dried
under vacuum. All NHC ligand precursors (2a–2f) were
isolated as air‐ and moisture‐stable white solids in high
CDCl3, 25°C, TMS):
δ (ppm) = 15.10 [NCH2CH
(OCH2CH3)2]; 20.18 and 21.07 [NCH2C6H2 (CH3)3‐2,4,6];
46.97 [NCH2CH(OCH2CH3)2]; 49.91 [NCH2C6H2(CH3)3‐
2,4,6]; 64.41 [NCH2CH (OCH2CH3)2]; 99.87 [NCH2CH
(OCH2CH3)2]; 113.09, 114.69, 125.07, 126.66, 126.88,
130.19, 130.97, 132.78, 137.99 and 139.77 [NC6H4N and
NCH2C6H2(CH3)3‐2,4,6]; 143.92 (NCHN).
2.3.3 | 1‐(2,2‐Diethoxyethyl)‐3‐(2,3,5,6‐
tetramethylbenzyl)benzimidazolium
chloride, 2c[24]
1
yields. For the H NMR, 13C NMR and FT‐IR spectrum
of NHC ligand precursors (2a–2f) see supporting infor-
mation file, pages S3–S14.
Yield 90%, 1.87 g (white solid); m.p.: 146–147°C; FT‐IR
1
(vC(2)‐N): 1563 cm−1. H NMR (400 MHz, CDCl3, 25°C,
TMS): δ (ppm) = 1.08 [t, J = 7.0 Hz, 6H, NCH2CH
(OCH2CH3)2]; 2.25 and 2.27 [s, 12H, NCH2C6H(CH3)4‐
2,3,5,6]; 3.63 and 3.72 [dq, J = 14.0, 7.0 Hz, 4H,
2.3.1 | 1‐(2,2‐Diethoxyethyl)‐3‐(4‐
methoxybenzyl)benzimidazolium
chloride, 2a
NCH2CH(OCH2CH3)2]; 4.87 [d,
NCH2CH(OCH2CH3)2]; 4.96 [t,
J
J
=
=
4.0 Hz, 2H,
4.0 Hz, 1H,
NCH2CH(OCH2CH3)2]; 5.82 [s, 2H, NCH2C6H(CH3)4‐
2,3,5,6]; 7.09 [s, 1H, NCH2C6H(CH3)4‐2,3,5,6]; 7.53–7.62
(m, 3H, NC6H4N); 7.89 (d, J = 7.8 Hz, 1H, NC6H4N);
10.06 (s, 1H, NCHN). 13C NMR (101 MHz, CDCl3, 25°
C, TMS): δ (ppm) = 15.05 [NCH2CH(OCH2CH3)2]; 15.99
and 20.55 [NCH2C6H(CH3)4‐2,3,5,6]; 47.28 [NCH2CH
(OCH2CH3)2]; 49.96 [NCH2C6H(CH3)4‐2,3,5,6]; 64.29
[NCH2CH(OCH2CH3)2]; 99.99 [NCH2CH (OCH2CH3)2];
113.12, 114.66, 126.87, 127.02, 127.63, 131.04, 132.79,
133.62, 134.12 and 135.08 [NC6H4N and NCH2C6H(CH3)
4‐2,3,5,6]; 142.96 (NCHN).
Yield 87%, 1.70 g (white solid); m.p.: 130–131°C; FT‐IR (vC
(2)‐N): 1560 cm−1. 1H NMR (400 MHz, CDCl3, 25°C, TMS):
δ (ppm) = 1.12 [t, J = 7.0 Hz, 6H, NCH2CH(OCH2CH3)2];
3.67 and 3.77 [dq, J = 13.9, 7.0 Hz, 4H, NCH2CH
(OCH2CH3)2]; 3.76 [s, 3H, NCH2C6H4(OCH3)‐4); 4.79 (d,
J
=
4.2 Hz, 2H, NCH2CH(OCH2CH3)2]; 5.03 [t,
J = 4.1 Hz, 1H, NCH2CH(OCH2CH3)2]; 5.77 [s, 2H,
NCH2C6H4(OCH3)‐4]; 6.88 [d,
NCH2C6H4(OCH3)‐4]; 7.50 [d,
J
J
=
=
8.3 Hz, 2H,
8.6 Hz, 2H,
NCH2C6H4(OCH3)‐4]; 7.52–7.64 (m, 3H, NC6H4N); 7.82
(d, J = 8.0 Hz, 1H, NC6H4N); 11.80 (s, 1H, NCHN). 13C
NMR (101 MHz, CDCl3, 25°C, TMS): δ (ppm) = 15.16
[NCH2CH(OCH2CH3)2]; 49.96 [NCH2CH(OCH2CH3)2];
51.01 [NCH2C6H4(OCH3)‐4]; 55.33 [NCH2C6H4(OCH3)‐
4]; 64.58 [NCH2CH (OCH2CH3)2]; 100.27 [NCH2CH
(OCH2CH3)2]; 113.18, 114.67, 114.75, 124.77, 126.62,
126.80, 130.05, 130.76, 132.68 and 160.19 [NC6H4N and
NCH2C6H4(OCH3)‐4]; 144.12 (NCHN).
2.3.4 | 1‐(2,2‐Diethoxyethyl)‐3‐(2,3,4,5,6‐
pentamethylbenzyl)benzimidazolium chlo-
ride, 2d[24]
Yield 95%, 2.04 g (white solid); m.p.: 120–121°C; FT‐IR (vC
(2)‐N): 1562 cm−1. 1H NMR (400 MHz, CDCl3, 25°C, TMS): δ

KALOĞLU ET AL.
5 of 16
(ppm) = 1.07 [t, J = 7.0 Hz, 6H, NCH2CH(OCH2 CH3)2];
2.25 and 2.29 [s, 15H, NCH2C6(CH3)5‐2,3,4,5,6]; 3.64 and
3.74 [dq, J = 14.1, 7.0 Hz, 4H, NCH2CH(OCH2 CH3)2];
4.90 [d, J = 3.4 Hz, 2H, NCH2CH(OCH2CH3)2]; 4.97 [t,
J = 3.3 Hz, 1H, NCH2CH(OCH2CH3)2]; 5.77 [s, 2H,
NCH2C6(CH3)5‐2,3,4,5,6]; 7.45 (d, J = 8.3 Hz, 1H,
NC6H4N); 7.51 (t, J = 7.7 Hz, 1H, NC6H4N); 7.58 (t,
J = 7.7 Hz, 1H, NC6H4N); 7.87 (d, J = 8.3 Hz, 1H, NC6H4N);
10.27 (s, 1H, NCHN). 13C NMR (101 MHz, CDCl3, 25°C,
TMS): δ (ppm) = 15.04 [NCH2CH(OCH2CH3)2]; 17.00,
17.05 and 17.34 [NCH2C6(CH3)5‐2,3,4,5,6]; 47.77
[NCH2CH(OCH2CH3)2]; 50.03 [NCH2C6(CH3)5‐2,3,4,5,6];
64.37 [NCH2CH (OCH2CH3)2]; 99.80 [NCH2CH
(OCH2CH3)2]; 112.86, 114.83, 124.83, 126.71, 126.83,
131.02, 132.93, 133.61, 133.98 and 137.42 [NC6H4N and
NCH2C6(CH3)5‐2,3,4,5,6]; 143.38 (NCHN).
7.58 (m, 3H, NC6H4N); 7.84 (d, J = 8.5 Hz, 1H, NC6H4N);
11.30 (s, 1H, NCHN). 13C NMR (101 MHz, CDCl3, 25°C,
TMS): δ (ppm) = 15.18 [NCH2CH(OCH2CH3)2]; 31.36
{NCH2C6H3[C(CH3)3]2‐3,5}; 34.94 {NCH2C6H3[C(CH3)3]
2‐3,5}; 49.97 [NCH2CH (OCH2CH3)2]; 52.09 {NCH2C6H3
[C(CH3)3]2‐3,5}; 64.65 [NCH2CH(OCH2CH3)2]; 100.08
[NCH2CH(OCH2CH3)2]; 113.12, 114.78, 122.42, 123.24,
126.68, 126.78, 130.88, 131.74, 132.74 and 152.18
{NC6H4N and NCH2C6H3 [C(CH3)3]2‐3,5}; 143.61
(NCHN).
2.4 | General procedure for the
preparation of PEPPSI‐themed
palladium‐carbene complexes (3a–3f)
NHC ligand precursors (2a–2f; 1.0 mmol) were converted
with high yields into the PEPPSI‐themed Pd–carbene
complexes (3a–3f) by reaction with PdCl2 (1.0 mmol) in
refluxing pyridine in the presence of K2CO3 (5.0 mmol)
as a base at 80°C for 16 h. Then, all volatiles were removed
under vacuum, and the solid residue was washed with n‐
pentane (2 × 5 ml). The crude product was dissolved in
CH2Cl2 (5 ml), then filtered through a pad of celite and sil-
ica gel (70–230 mesh) to remove the unreacted PdCl2 and
NHC ligand. The crude complex was crystallized from
CH2Cl2–n‐pentane mixture (1:5, v/v) at room temperature
and completely dried under vacuum. All Pd‐complexes
were isolated as air‐ and moisture‐stable bright yellow
2.3.5 | 1‐(2,2‐Diethoxyethyl)‐3‐(4‐tert‐
butylbenzyl)benzimidazolium bromide, 2e
Yield 81%, 1.87 g (white solid); m.p.: 162–163°C; FT‐IR
1
(vC(2)‐N): 1557 cm−1. H NMR (400 MHz, CDCl3, 25°C,
TMS): δ (ppm) = 1.05 [t, J = 7.0 Hz, 6H, NCH2CH
(OCH2CH3)2]; 1.20 {s, 9H, NCH2C6H4 [C(CH3)3]‐4}; 3.61
and 3.71 [dq, J = 13.9, 7.0 Hz, 4H, NCH2CH(OCH2CH3)
2]; 4.73 [d, J = 4.1 Hz, 2H, NCH2CH (OCH2CH3)2]; 4.99
[t, J = 4.1 Hz, 1H, NCH2CH (OCH2CH3)2]; 5.69 {s, 2H,
NCH2C6H4[C(CH3)3]‐4}; 7.32 {d, J = 8.3 Hz, 2H,
NCH2C6H4[C(CH3)3]‐4}; 7.39 {d, J = 8.3 Hz, 2H,
NCH2C6H4[C(CH3)3]‐4}; 7.45–7.58 (m, 3H, NC6H4N);
7.75 (d, J = 7.1 Hz, 1H, NC6H4N); 11.28 (s, 1H, NCHN).
13C NMR (101 MHz, CDCl3, 25°C, TMS): δ (ppm) = 15.17
[NCH2CH(OCH2CH3)2]; 31.18 {NCH2C6H4[C(CH3)3]‐4};
1
solids in moderate to high yields. For the H NMR, 13C
NMR and FT‐IR spectrum of 3a‐3f Pd‐complexes see
supporting information file, pages S15–S26.
2.4.1 | Trans‐dichloro[1‐(2,2‐
diethoxyethyl)‐3‐(4‐methoxybenzyl)benz-
imidazole‐2‐ylidene]‐(pyridine)‐palladium
(II), 3a
34.68
{NCH2C6H4[C(CH3)3]‐4};
50.00
[NCH2CH
(OCH2CH3)2]; 51.07 {NCH2C6H4 [C(CH3)3]‐4}; 64.65
[NCH2CH(OCH2CH3)2]; 99.99 [NCH2CH(OCH2CH3)2];
113.11, 114.71, 126.34, 126.73, 126.90, 128.20, 129.54,
130.82, 132.60 and 152.90 {NC6H4N and NCH2C6H4[C
(CH3)3]‐4}; 143.58 (NCHN).
Yield 71%, 0.216 g (bright yellow solid); m.p.: 149–150°C;
1
FT‐IR (vC(2)‐N): 1405 cm−1. H NMR (400 MHz, CDCl3,
25°C, TMS): δ (ppm) = 1.04 [t, J = 7.0 Hz, 6H,
NCH2CH(OCH2CH3)2]; 3.54 and 3.79 [dq, J = 9.4,
7.0 Hz, 4H, NCH2CH(OCH2CH3)2]; 3.70 [s, 3H,
NCH2C6H4(OCH3)‐4]; 4.94 [d, J = 5.5 Hz, 2H, NCH2CH
(OCH2CH3)2]; 5.49 [t, J = 5.5 Hz, 1H, NCH2CH
(OCH2CH3)2]; 6.09 [s, 2H, NCH2C6H4(OCH3)‐4]; 6.79–
6.83 [m, 2H, NCH2C6H4(OCH3)‐4]; 7.03 (d, J = 3.9 Hz,
2H, NC6H4N); 7.12–7.15 (m, 1H, NC6H4N); 7.29–7.33
[m, 2H, NCH2C6H4(OCH3)‐4]; 7.46 (d, J = 8.7 Hz, 2H,
pyridine); 7.56 (d, J = 8.2 Hz, 1H, NC6H4N); 7.73 (tt,
J = 7.7, 1.6 Hz, 1H, pyridine); 8.93 (dd, J = 6.5, 1.5 Hz,
2H, pyridine). 13C NMR (101 MHz, CDCl3, 25°C, TMS):
2.3.6 | 1‐(2,2‐Diethoxyethyl)‐3‐(3,5‐di‐tert‐
butylbenzyl)benzimidazolium bromide, 2f
Yield 73%, 1.89 g (white solid); m.p.: 166–167°C; FT‐IR
1
(vC(2)‐N): 1562 cm−1. H NMR (400 MHz, CDCl3, 25°C,
TMS): δ (ppm) = 1.11 [t, J = 6.9 Hz, 6H, NCH2CH
(OCH2CH3)2]; 1.28 {s, 18H, NCH2C6H3 [C(CH3)3]2‐3,5};
3.68 and 3.77 [dq, J = 14.1, 7.0 Hz, 4H, NCH2CH
(OCH2CH3)2]; 4.83 [d, J = 3.7 Hz, 2H, NCH2CH
(OCH2CH3)2]; 5.07 [t, J = 3.7 Hz, 1H, NCH2CH
(OCH2CH3)2]; 5.78 {s, 2H, NCH2C6H3 [C(CH3)3]2‐3,5};
7.26 and 7.41 {s, 3H, NCH2C6H3 [C(CH3)3]2‐3,5}; 7.51–
δ
(ppm) = 15.35 [NCH2CH (OCH2CH3)2]; 51.52

6 of 16
KALOĞLU ET AL.
[NCH2CH(OCH2CH3)2]; 52.80 [NCH2C6H4(OCH3)‐4];
1H, NC6H4N); 7.72 (tt, J = 7.7, 1.6 Hz, 1H, pyridine);
8.85 (dd, J = 6.4, 1.5 Hz, 2H, pyridine). 13C NMR
(101 MHz, CDCl3, 25°C, TMS): δ (ppm) = 15.36
[NCH2CH(OCH2CH3)2]; 16.48 and 20.63 [NCH2C6H
(CH3)4‐2,3,5,6]; 50.40 [NCH2CH(OCH2CH3)2]; 51.58
[NCH2C6H(CH3)4‐2,3,5,6]; 64.64 [NCH2CH(OCH2CH3)2];
102.78 [NCH2CH(OCH2CH3)2]; 110.86, 112.34, 122.57,
123.03, 130.50, 132.53, 134.34, 134.41, 135.05 and 136.62
[NC6H4N and NCH2C6H(CH3)4‐2,3,5,6]; 124.56, 138.15
and 151.11 (pyridine); 163.77 [Pd‐C(2)].
55.26
[NCH2C6H4(OCH3)‐4];
64.65
[NCH2CH
(OCH2CH3)2]; 102.66 [NCH2CH (OCH2CH3)2]; 111.10,
112.47, 114.25, 122.99, 123.08, 127.00, 129.41, 133.79,
135.92 and 159.90 [NC6H4N and NCH2C6H4(OCH3)‐4];
124.63, 138.25 and 151.20 (pyridine); 163.82 [Pd‐C(2)].
2.4.2 | Trans‐dichloro[1‐(2,2‐
diethoxyethyl)‐3‐(2,4,6‐trimethylbenzyl)
benzimidazole‐2‐ylidene]‐(pyridine)‐palla-
dium (II), 3b
2.4.4 | Trans‐dichloro[1‐(2,2‐
Yield 64%, 0.199 g (bright yellow solid); m.p.: 192–193°C;
diethoxyethyl)‐3‐(2,3,4,5,6‐
1
FT‐IR (vC(2)‐N): 1400 cm−1. H NMR (400 MHz, CDCl3,
pentamethylbenzyl)benzimidazole‐2‐yli-
dene]‐(pyridine)‐palladium (II), 3d
25°C, TMS): δ (ppm) = 1.03 [t, J = 7.0 Hz, 6H, NCH2CH
(OCH2CH3)2]; 2.25 and 2.26 [s, 9H, NCH2C6H2(CH3)3‐
2,4,6]; 3.53 and 3.78 [dq, J = 9.4, 7.0 Hz, 4H, NCH2CH
(OCH2CH3)2]; 4.93 [d, J = 5.5 Hz, 2H, NCH2CH
(OCH2CH3)2]; 5.49 [t, J = 5.5 Hz, 1H, NCH2CH
(OCH2CH3)2]; 6.11 [s, 2H, NCH2C6H2(CH3)3‐2,4,6]; 6.40
(d, J = 8.3 Hz, 1H, NC6H4N); 6.86 [s, 2H, NCH2C6H2
(CH3)3‐2,4,6]; 6.90 (t, J = 7.4 Hz, 1H, NC6H4N); 7.08 (t,
J = 7.4 Hz, 1H, NC6H4N); 7.31 (ddd, J = 7.6, 5.1, 1.3 Hz,
2H, pyridine); 7.54 (d, J = 8.2 Hz, 1H, NC6H4N); 7.73 (tt,
J = 7.7, 1.6 Hz, 1H, pyridine); 8.89 (dd, J = 6.5, 1.5 Hz,
2H, pyridine). 13C NMR (101 MHz, CDCl3, 25°C, TMS):
δ (ppm) = 15.36 [NCH2CH(OCH2CH3)2]; 20.80 and 21.11
[NCH2C6H2(CH3)3‐2,4,6]; 49.96 [NCH2CH (OCH2CH3)2];
Yield 77%, 0.250 g (bright yellow solid); m.p.: 215–216°C;
1
FT‐IR (vC(2)‐N): 1402 cm−1. H NMR (400 MHz, CDCl3,
25°C, TMS): δ (ppm) = 1.03 (t, J = 7.0 Hz, 6H, NCH2CH
(OCH2CH3)2); 2.15, 2.22 and 2.24 [s, 15H, NCH2C6(CH3)
5‐2,3,4,5,6]; 3.53 and 3.78 [dq, J = 9.4, 7.0 Hz, 4H,
NCH2CH(OCH2CH3)2]; 4.93 [d, J = 5.5 Hz, 2H, NCH2CH
(OCH2CH3)2]; 5.49 [t, J = 5.5 Hz, 1H, NCH2CH
(OCH2CH3)2]; 6.36 [s, 2H, NCH2C6(CH3)5‐2,3,4,5,6]; 6.34
(d, J = 8.3 Hz, 1H, NC6H4N); 6.85 (t, J = 7.4 Hz, 1H,
NC6H4N); 7.07 (t, J = 7.4 Hz, 1H, NC6H4N); 7.30 (ddd,
J = 7.6, 5.2, 1.3 Hz, 2H, pyridine); 7.52 (d, J = 8.2 Hz,
1H, NC6H4N); 7.72 (tt, J = 7.7, 1.6 Hz, 1H, pyridine); 8.86
(dd, J = 6.4, 1.5 Hz, 2H, pyridine). 13C NMR (101 MHz,
51.53
[NCH2C6H2(CH3)3‐2,4,6];
64.66
[NCH2CH
(OCH2CH3)2]; 102.79 [NCH2CH (OCH2CH3)2]; 110.90,
112.36, 122.64, 123.06, 127.51, 129.66, 134.21, 135.70,
138.55 and 138.78 [NC6H4N and NCH2C6H2 (CH3)3‐
2,4,6]; 124.58, 138.15 and 151.18 (pyridine); 163.80 [Pd‐C
(2)].
CDCl3, 25°C, TMS): δ (ppm) = 15.36 [NCH2CH
(OCH2CH3)2]; 16.95, 17.32 and 17.48 [NCH2C6(CH3)5‐
2,3,4,5,6]; 51.11 [NCH2CH(OCH2CH3)2]; 51.58 [NCH2C6
(CH3)5‐2,3,4,5,6]; 64.65 [NCH2CH(OCH2 CH3)2]; 102.80
[NCH2CH(OCH2CH3)2]; 111.04, 112.25, 122.48, 122.98,
127.77, 133.36, 134.47, 134.63, 136.67 and 136.96 [NC6H4N
and NCH2C6(CH3)5‐2,3,4,5,6]; 124.54, 138.13 and 151.11
(pyridine); 163.57 [Pd‐C(2)].
2.4.3 | Trans‐dichloro[1‐(2,2‐
diethoxyethyl)‐3‐(2,3,5,6‐
tetramethylbenzyl)benzimidazole‐2‐yli-
dene]‐(pyridine)‐palladium (II), 3c
2.4.5 | Trans‐dibromo[1‐(2,2‐
diethoxyethyl)‐3‐(4‐tert‐butylbenzyl)benz-
imidazole‐2‐ylidene]‐(pyridine)‐palladium
(II), 3e
Yield 75%, 0.238 g (bright yellow solid); m.p.: 191–192°C;
1
FT‐IR (vC(2)‐N): 1403 cm−1. H NMR (400 MHz, CDCl3,
25°C, TMS): δ (ppm) = 1.03 [t, J = 7.0 Hz, 6H,
NCH2CH(OCH2CH3)2]; 2.17 and 2.18 [s, 12H, NCH2C6H
(CH3)4‐2,3,5,6]; 3.53 and 3.77 [qq, J = 9.4, 7.0 Hz, 4H,
Yield 61%, 0.221 g (bright yellow solid); m.p.: 91–92°C;
1
FT‐IR (vC(2)‐N): 1406 cm−1. H NMR (400 MHz, CDCl3,
NCH2CH(OCH2CH3)2]; 4.93 [d,
NCH2CH(OCH2CH3)2]; 5.49 [t,
J
J
=
=
5.5 Hz, 2H,
5.5 Hz, 1H,
25°C, TMS): δ (ppm) = 1.07 [t, J = 7.0 Hz, 6H,
NCH2CH(OCH2CH3)2]; 1.22 {s, 9H, NCH2C6H4 [C
(CH3)3]‐4}; 3.57 and 3.80 [dq, J = 9.4, 7.0 Hz, 4H,
NCH2CH(OCH2CH3)2]; 6.13 [s, 2H, NCH2C6H(CH3)4‐
2,3,5,6]; 6.37 (d, J = 8.3 Hz, 1H, NC6H4N); 6.88 (t,
J = 7.4 Hz, 1H, NC6H4N); 7.01 [s, 1H, NCH2C6H(CH3)4‐
2,3,5,6]; 7.06 (t, J = 7.4 Hz, 1H, NC6H4N); 7.30 (ddd,
J = 7.5, 5.1, 1.3 Hz, 2H, pyridine); 7.53 (d, J = 8.2 Hz,
NCH2CH(OCH2CH3)2]; 4.91 [d,
J
=
5.5 Hz, 2H,
NCH2CH(OCH2CH3)2]; 5.55 [t,
J
=
5.5 Hz, 1H,
NCH2CH(OCH2CH3)2]; 6.07 {s, 2H, NCH2C6H4 [C
(CH3)3]‐4}; 6.96–7.02 (m, 2H, NC6H4N); 7.13 (dd,

KALOĞLU ET AL.
7 of 16
J = 8.2, 1.3 Hz, 1H, NC6H4N); 7.27–7.41 {m, 4H,
NC6H4N and NCH2C6H4[C(CH3)3]‐4}; 7.45 (d,
J = 8.2 Hz, 2H, pyridine); 7.57 (d, J = 8.2 Hz, 1H,
NC6H4N); 7.75 (tt, J = 7.7, 1.6 Hz, 1H, pyridine); 8.95
using X‐RED32.[25] The structures were solved by direct
methods with SIR2019[26] and refined by means of the
full‐matrix full‐matrix least‐squares calculations on F 2
using SHELXL‐2018.[27] All carbon‐bound H atoms were
located in difference electron‐density map and then
treated as riding atoms in geometrically idealized posi-
tions, with C─H = 0.93 (aromatic CH), 0.97 (CH2), 0.96
(CH3) and 0.98 Å (methine CH), and with Uiso
(H) = kUeq(C), where k = 1.5 for the methyl groups
and 1.2 for all other H atoms. In 3c, there is a disordered
solvent water molecule with very large displacement
parameters which could not be modeled properly. The
diffused electron densities resulting from this were
removed by the SQUEEZE routine in PLATON.[28] There
are two cavities of volume 105 Å3 per unit cell centered
at (0, 0.5, 0) and (0.5, 1, 0.5). Each cavity contains
approximately 23 electrons which were assigned to two
solvent water molecules. Since Z is equal to 4, each Pd
complex has one solvent water equivalent. In the final
refinement, these contributions were removed from the
intensity data to produce better refinement results. The
crystallographic data and refinement parameters are
summarized in Table 1. Molecular graphic was created
by using OLEX2.[29]
(dd,
J =
6.5, 1.5 Hz, 2H, pyridine). 13C NMR
(101 MHz, CDCl3, 25°C, TMS): δ (ppm) = 15.43
[NCH2CH(OCH2CH3)2}; 31.34 {NCH2C6H4[C(CH3)3]‐4};
34.60
{NCH2C6H4[C(CH3)3]‐4};
51.84
[NCH2CH
(OCH2CH3)2]; 53.45 {NCH2C6H4 [C(CH3)3]‐4}; 64.63
[NCH2CH(OCH2CH3)2]; 102.10 [NCH2CH(OCH2CH3)2];
111.20, 112.47, 122.83, 122.94, 125.77, 127.78, 131.77,
134.10, 136.12 and 151.12 [NC6H4N and NCH2C6H2
(CH3)3‐2,4,6]; 124.67, 138.03 and 152.55 (pyridine);
163.43 [Pd‐C(2)].
2.4.6 | Trans‐dibromo[1‐(2,2‐
diethoxyethyl)‐3‐(3,5‐di‐tert‐butylbenzyl)
benzimidazole‐2‐ylidene]‐(pyridine)‐palla-
dium (II), 3f
Yield 69%, 0.269 g (bright yellow solid); m.p.: 151–152°
C; FT‐IR (vC(2)‐N): 1404 cm−1 1H NMR (400 MHz,
.
CDCl3, 25°C, TMS): δ (ppm) = 1.03 [t, J = 7.0 Hz,
6H, NCH2CH(OCH2CH3)2]; 1.20 {s, 18H, NCH2C6H3 [C
(CH3)3]2‐3,5}; 3.54 and 3.79 [dq, J = 9.4, 7.0 Hz, 4H,
NCH2CH(OCH2CH3)2]; 4.91 [d,
NCH2CH(OCH2CH3)2]; 5.59 [t,
J
J
=
=
5.6 Hz, 2H,
5.6 Hz, 1H,
2.6 | General procedure for the
direct arylation of C2‐substituted
heteroarenes
NCH2CH(OCH2CH3)2]; 6.11 {s, 2H, NCH2C6H3 [C
(CH3)3]2‐3,5}; 6.90 (d, J = 8.1 Hz, 1H, NC6H4N); 6.99
(t, J = 7.6 Hz, 1H, NC6H4N); 7.12 (t, J = 7.8 Hz, 1H,
NC6H4N); 7.27 {s, 3H, NCH2C6H3[C(CH3]3}2‐3,5); 7.28
(ddd, J = 7.6, 5.2, 1.3 Hz, 2H, pyridine); 7.57 (d,
J = 8.2 Hz, 1H, NC6H4N); 7.69 (tt, J = 7.7, 1.6 Hz,
1H, pyridine); 8.97 (dd, J = 6.4, 1.4 Hz, 2H, pyridine).
13C NMR (101 MHz, CDCl3, 25°C, TMS): δ (ppm) = 15.40
[NCH2CH(OCH2CH3)2]; 31.47 {NCH2C6H3[C(CH3)3]2‐
3,5}; 34.97 {NCH2C6H3[C(CH3)3]2‐3,5}; 51.71 [NCH2CH
(OCH2CH3)2]; 54.34 {NCH2C6H3[C(CH3)3]2‐3,5}; 64.74
[NCH2CH(OCH2CH3)2]; 102.03 [NCH2CH (OCH2CH3)
2]; 111.28, 112.45, 121.78, 122.36, 122.70, 122.82,
133.86, 134.07, 136.11 and 151.32 [NC6H4N and
NCH2C6H2(CH3)3‐2,4,6]; 124.64, 137.99 and 152.99 (pyr-
idine); 163.38 [Pd‐C(2)].
Typically, C2‐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
argon atmosphere. Subsequently, the Pd–carbene cata-
lyst (3a–3f) (0.01 mmol, 1 mol%) was added to stirred
solution in a Schlenk tube, and the closed Schlenk tube
was stirred at 120°C (oil bath temperature). At the end
of the reaction, the solution was cooled to room temper-
ature and dichloromethane (2 ml) was then added to
the crude mixture. This solution was used for GC anal-
ysis and yields (%) were calculated according to aryl
bromide.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis of 1‐(2,2‐diethoxyethyl)
benzimidazole
2.5 | X‐Ray analysis of the
palladium‐carbene complexes
1‐(2,2‐Diethoxyethyl)benzimidazole (1) was synthesized
as previously described in the literature.[24] Compound 1
was synthesized by the reaction of benzimidazole with
2,2‐diethoxyethyl bromide in ethyl alcohol at 78°C for
5 h. Then it was isolated as a viscous liquid in moderate
X‐Ray diffraction data were recorded with a STOE IPDS
II diffractometer at room temperature using graphite‐
monochromated Mo Kα radiation by applying the ω‐scan
method. Data collection and cell refinement were carried
out using X‐AREA[25] while data reduction was applied

8 of 16
KALOĞLU ET AL.

KALOĞLU ET AL.
9 of 16
to high yield (85%). Compound 1 was characterized by 1H
NMR, 13C NMR and FT‐IR spectroscopies. In the 13C
NMR spectra of 1, the characteristic peak of the imino
carbon (NCHN), resonance was detected as typical singlet
at δ = 14,355 ppm. The signal of the NCHN proton was
also detected as a singlet at δ = 7.96 ppm for 1. FT‐IR
spectra showed a broad band at 1494 cm−1 for the ‐
C=N‐ bond vibration of 1.
3.2 | Synthesis of NHC ligand precursors
The NHC ligand precursors (2a–2f) were synthesized by
the reaction of 1‐(2,2‐diethoxyethyl)‐benzimidazole with
substituted benzyl halides in dimethylformamide at 80°
C for 36 h. The salts 2b–2d were synthesized as previ-
ously described in the literature.[24] All NHC ligand pre-
cursors were characterized by various spectroscopic
1
techniques such as H NMR, 13C NMR and FT‐IR. The
FT‐IR data clearly indicated that the 2a–2f ligand precur-
sors exhibit a characteristic ν(CN) band typically at 1560,
1560, 1563, 1562, 1557 and 1562 cm−1, respectively. In
1
the H NMR spectra, the C(2)–H proton downfield reso-
nances of 2a–2f were observed as sharp singlets at
δ = 11.80, 10.78, 10.06, 10.27, 11.28 and 11.30 ppm,
respectively. In the 13C NMR spectra, the C(2)‐carbon res-
onance of 2a–2f compounds appeared at δ = 144.12,
143.92, 142.96, 143.38, 143.58 and 143.61 ppm, respec-
tively, as a single signal. These spectroscopic values are
in line with those found for other benzimidazolium salts
in the literature.12e–h, 15f, 16h
3.3 | Synthesis of PEPPSI‐themed
palladium–carbene complexes
The PEPPSI‐themed Pd–carbene complexes (3a–3f) were
synthesized according to procedures similar to those
reported previously by Organ for other Pd–NHC–PEPPSI
complexes.[21] The 3a–3f complexes, which are highly
moisture‐ and air‐stable both in solution and in solid state
against air, light and moisture, could be stored at room
temperature for months without an obvious decline in
catalytic efficiency. They are soluble in most organic sol-
vents, such as dichloromethane, chloroform, ethanol
and acetonitrile, with the exception of non‐polar ones,
such as pentane and hexane. Next, Pd‐complexes were
succesfully characterized by various spectroscopic tech-
niques such as NMR, FT‐IR and single‐crystal X‐ray anal-
1
ysis. In the H NMR and 13C NMR spectra of the Pd‐
complexes, the absence of the characteristic signals of
the acidic C(2)‐H proton and C(2)‐carbon of the
benzimidazolium salts suggests the formation of a Pd–
carbene bond. Also, in the 13C NMR spectra, the

10 of 16
KALOĞLU ET AL.
characteristic C(2)‐carbene signals of 3a–3f complexes
appear as singlets at δ = 163.82, 163.80, 163.77, 163.57,
163.43 and 163.38 ppm, respectively. The FT‐IR data
clearly indicate that the PEPPSI‐themed Pd–carbene
complexes exhibit a characteristic ν(CN) band typically
between 1400 and 1406 cm−1. Owing to the flow of elec-
trons from the NHC ligand to the Pd, the C–N bond is
weakened, and as a result, a decrease in the v(CN)
stretching frequency is expected. Also, for 3a–3c and 3 f
complexes appropriate single crystals and structures of
these complexes were elucidated by X‐ray diffraction
studies. These complexes show typical spectroscopic
signatures, which are in line with those recently reported
for other similar types of Pd–carbene complexes.12h, 15h
3.4 | Description of the crystal structures
of the palladium–carbene complexes
The molecular diagrams of 3a–3c and 3f with the adopted
atom‐labeling scheme are shown in Figures 2–5, while
important bond distances and angles are listed in
Table 2. The asymmetric unit of 3f contains the two inde-
pendent molecules, labeled A and B. In the following dis-
cussion, parameters related to molecule B are given in
square brackets.
FIGURE
3
Molecular structure of 3b showing the atom
numbering scheme
The Pd–carbene complexes are structurally similar, in
which Pd is surrounded by the carbon atom of NHC
FIGURE
2
Molecular structure of 3a showing the atom
FIGURE
4
Molecular structure of 3c showing the atom
numbering scheme.
numbering scheme.

KALOĞLU ET AL.
11 of 16
NHC and pyridine ligands, and the anion atoms are trans
to each other for all complexes. The cis angles varying
from 86.0(2) to 92.6(2)° and the trans angles changing
from 175.11(4) to 178.80(12)° are a little off from the ideal
values of 90 and 180°. The two anion atoms are bent
toward the NHC ligand rather than the non‐bulky pyri-
dine ligand, as evident from the X─Pd─C (X is the anion
atom) angles smaller than those for X─Pd─Npyridine. The
four‐coordinate geometry index, τ4,[30] is 0.04 for 3a, 0.05
for 3b, 0.07 for 3c and 0.05 for 3 f, which has to be 0 for
an ideal square‐planar geometry and has to be 1 for a per-
fect tetrahedral geometry. The values of τ4 indicate that
the coordination polyhedron of the Pd atoms is a slightly
distorted square‐planar.
The average Pd─CNHC bond distance [1.946 Å] is
smaller than the sum of the individual covalent radii of
the Pd and carbon atoms (2.12 Å), while the average
Pd─Npyridine bond distance [2.109 Å] is nearly equal to
the sum of the covalent individual radii of the Pd and
nitrogen atoms (2.10 Å).[31] The Pd─Br and Pd─Cl bond
lengths are in the typical range, and the internal
N─C─N ring angle at the carbene centers varies from
105.9(7) to 107.6(2)° in the complexes. These values lie
within the same range as those observed for analogous
PEPPSI‐themed Pd‐complexes.[32] The carbene ring is
nearly perpendicular to the PdCNX2 coordination plane
with a dihedral angle of 75.58(9)° in 3a, 68.82(16)° in
3b, 80.44(12)° in 3c and 73.8(3)° [77.4(3)°] in 3 f, which
is typical for NHC complexes to reduce steric congestion.
FIGURE
5
Molecular structure of 3f showing the atom
numbering scheme. For the sake of clarity, only molecule A is
shown.
and the nitrogen atom of the pyridine ring. The remain-
ing coordination sites are occupied by two chloride atoms
in 3a–3c and by two bromide atoms in 3f. The metal
coordination environments of the complexes feature a
slightly distorted square‐planar geometry, in which the
TABLE 2 Selected geometric parameters for 3a–3c and 3 f
Parameters
3a
3b
3c
3f
Molecule A
Molecule B
Bond lengths (Å)
Pd1─X1
2.3091(8)
2.2993(9)
2.114(2)
1.953(3)
1.351(4)
1.350(4)
2.305(2)
2.3008(11)
2.3124(12)
2.113(3)
1.947(4)
1.351(5)
1.356(5)
2.4273(12)
2.4247(12)
2.091(8)
1.946(8)
1.341(9)
1.362(8)
2.4286(12)
2.4364(12)
2.096(8)
Pd1─X2
2.315(2)
2.129(5)
1.944(5)
1.346(6)
1.365(6)
Pd1─N3
Pd1─C1
1.940(8)
N1─C1
1.357(10)
1.358(8)
N2─C1
Bond angles (deg)
X1─Pd1─X2
X1─Pd1─N3
X2─Pd1─N3
X1─Pd1─C1
X2─Pd1─C1
N3─Pd1─C1
N1─C1─N2
175.23(3)
92.16(8)
92.40(8)
88.10(8)
87.38(8)
178.80(12)
107.6(2)
176.30(8)
91.90(17)
91.77(17)
88.29(18)
88.07(18)
177.2(2)
175.11(4)
91.33(10)
92.43(10)
86.33(12)
90.12(12)
175.62(13)
107.1(3)
175.14(5)
90.8(2)
91.1(2)
90.6(2)
87.7(2)
178.0(3)
106.0(7)
175.91(5)
92.6(2)
91.4(2)
89.9(2)
86.0(2)
177.4(3)
105.9(7)
106.5(4)
X1, Cl1 or Br1; X2, Cl2 or Br2.

12 of 16
KALOĞLU ET AL.
On the other hand, the dihedral angle between the
pyridine ring and the coordination plane is 37.68(14)°
for 3a, 35.8(3)° for 3b, 41.35(17)° for 3c and 56.1(6)°
[48.7(5)°] for 3 f.
bromides. The reaction worked well for a wide variety
of aryl bromides such as 3‐bromoquinoline,
bromobenzene, 4‐bromotoluene, 4‐bromobenzaldehyde
and 4‐bromoacetophenone, affording 2‐acetylfuran for
30 examples, and 2‐acetylthiophene for another 30 exam-
ples, and the results are summarized in Tables 3 and 4,
respectively.
3.5 | The direct arylation of 2‐acetylfuran
and 2‐acetylthiophene with aryl bromides
Firstly, under the optimal conditions, direct arylation of
2‐acetylfuran with aryl bromides was examined and the C
(5)‐arylated furan derivatives were obtained. Full conver-
sion was obtained when 3‐bromoquinoline was used with
2‐acetylfuran with 71–85% GC yields (Table 3, entries 4–9).
C(5)‐arylated 2‐acetylfuran was obtained with 85% GC
yield in the presence of 3f catalyst. Similar results were
obtained when bromobenzene was used. In this case 2‐
acetyl‐5‐phenylfuran was obtained in 68–92% GC yield
(Table 3, entries 10–15). 4‐Bromotoluene, which contained
an electron‐rich aryl bromide, was also obtained at 69–87%
GC yield with high conversion (Table 3, entries 16–21). 2‐
Acetyl‐5‐(4‐methylphenyl)furan was obtained with 87%
yield in the presence of 3d catalyst (Table 3, entry 19).
The coupling of 2‐acetylfuran with electron‐poor aryl bro-
mide such as 4‐bromobenzaldehyde proceeds nicely. 4‐
Bromobenzaldehyde gave the C(5)‐arylated furan with
69–89% GC yields with full conversion (Table 3, entries
22–27). This product was obtained at 80% yield in the pres-
ence of 3f catalyst (Table 3, entry 27). The electron‐poor 4‐
bromoacetophenone was also a good substrate to afford
the corresponding products 2‐acetyl‐5‐(4‐acetylphenyl)
furan at between 76 and 80% CG yields (Table 3, entries
28–33).
In 1990, the first examples of Pd‐catalyzed direct arylation
of furans and thiophenes were reported by Ohta.14b In this
pioneering work, the direct C(2)‐arylation of furan and
thiophene was carried out in medium to good yields with
electron‐rich or electron‐poor aryl bromides using [Pd
(PPh3)4] as the catalyst, potassium acetate (KOAc) as the
base and dimethylacetamide (DMA) as the solvent. In
the last two decades, Pd‐catalyzed direct arylation was
successfully performed using DMA/KOAc combina-
tion.[15,16] Therefore, in this study, we selected DMA as
the solvent, and KOAc as the base. The direct arylation
of furan or thiophene itself to prepare C(2)‐arylated prod-
ucts remains difficult as the formation of C(2)‐ and C(5)‐
diarylated products from C(2)‐arylated products appears
to be faster than the C(2)‐arylation of furan or thiophene.
Therefore, the use of a blocking group at the C(2)‐position
of furans and thiophenes in order to control the selectivity
towards C(5)‐arylation has also been described.12d There-
fore, regioselective arylation on only the C(5)‐position of
C(2)‐blocked furan or thiophene was observed. It is sup-
posed that because the C(2)‐position is blocked, the acidic
C(5)‐position is used for arylation and bonding of
electron‐deficient aryl groups is difficult compared with
electron‐rich aryl groups. For this reason, we selected C
(2)‐blocked 2‐acetylfuran and 2‐acetylthiophene as
heteroaromatic subsrates, and we focused on the direct
arylation at the C(5)‐position of these heteroarenes.
Using the same reaction conditions, we investigated
the reactivity of 2‐acetylthiophene in Pd‐catalyzed direct
C(5)‐arylation. As shown in Table 4, high‐yield C(5)‐
arylated products were obtained. When 2‐acetylthiophene
was arylated with 3‐bromoquinoline, bromobenzene, 4‐
Initially, to optimize the reaction conditions, the
arylation of 2‐acetylfuran with 3‐bromoquinoline was car-
ried out at 120°C for 2 h without the addition of any Pd‐
catalyst in order to examine the effect of the catalyst on
the reaction. No product was formed without the addition
of Pd‐catalyst (Table 3, entry 1). Then we examined the
effect of the reaction time on the reaction. When the reac-
tion time was reduced from 2 to 1 h in the presence of 3f
catalyst, 85% yield was observed (Table 3, entry 2). When
the reaction time was reduced to 0.5 h in the presence of
3f catalyst, the percentage conversion was significantly
reduced to 60% with 81% yield (Table 3, entry 3). After
these preliminary trials, it was concluded that the best
conditions for the direct arlyation were achieved at 120°
C, 2 h. Next, we evaluated the scope and limitations of
the Pd‐complexes 3a–3f for the direct arylation of 2‐
acetylfuran and 2‐acetylthiophene with different aryl
bromotoluene,
4‐bromobenzaldehyde
and
4‐
bromoacetophenone, products were obtained using only
1 mol% Pd‐complexes (3a–3f) as catalyst, and yields at
84–90, 67–88, 65–91, 69–85 and 81–92% were observed,
respectively (Table 4, entries 1–30). When the reaction of
2‐acetylthiophene with 3‐bromoquinoline was investi-
gated, C(5)‐arylated product was obtained at 90% GC yield
in the presence of 3d catalyst (Table 4, entry 4). The reac-
tion of 2‐acetylthiophene with bromobenzene generated
the 5‐phenyl‐2‐acetylthiophene at 87 and 88% yield in
the presence of 3b and 3c catalysts, respectively (Table 4,
entries 8,9). The reaction of 2‐acetylthiophene with 4‐
bromotoluene gave the expected product at 91 and 86%
yield in the presence of 3b and 3d catalysts, respectively
(Table 4, entries 14,16). The reaction of 2‐acetylthiophene
with 4‐bromobenzaldehyde gave the expected product at
85% yield in the presence of 3e catalyst (Table 4, entry 23).

KALOĞLU ET AL.
13 of 16
TABLE 3 Pd‐catalyzed direct C(5)‐arylation of 2‐acetylfuran with aryl bromidesa
Entry
Aryl bromide
[Pd]
Product
Conversion (%)b
Yield (%)c
1d
2e
3f
None
3f
—
85
—
85
81
79
71
81
74
84
85
71
68
92
90
76
90
69
86
84
87
80
85
82
69
76
75
85
89
76
78
73
71
80
80
3f
60
4
3a
3b
3c
3d
3e
3f
100
100
100
100
100
100
90
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
36
27
28
29
30
31
32
33
3a
3b
3c
3d
3e
3f
96
92
100
94
100
85
3a
3b
3c
3d
3e
3f
100
100
100
96
100
100
100
100
100
100
100
95
3a
3b
3c
3d
3e
3f
3a
3b
3c
3d
3e
3f
95
100
100
100
100
aConditions: [Pd] 3a–3f (0.01 mmol, 1 mol%), 2‐acetylfuran (2.0 mmol), aryl bromide (1.0 mmol), KOAc (2.0 mmol), DMA (2 ml), 120°C, 2 h.
bConversions were calculated with respect to aryl bromide from the results of GC spectrometry.
cGC yield.
dWithout any [Pd] catalyst.
eThe reaction was carried out at 1 h.
fThe reaction was carried out at 0.5 h.

14 of 16
KALOĞLU ET AL.
TABLE 4 Pd‐catalyzed direct C(5)‐arylation of 2‐acetylthiophene with aryl bromidesa
Entry
Aryl bromide
[Pd]
Product
Conversion (%)b
Yield (%)c
1
3a
3b
3c
3d
3e
3f
100
97
84
89
87
90
85
88
67
87
88
85
83
83
65
91
83
86
79
84
72
69
71
74
85
74
86
81
86
89
90
92
2
3
100
100
100
100
90
4
5
6
7
3a
3b
3c
3d
3e
3f
8
95
9
92
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
36
27
28
29
30
95
100
94
3a
3b
3c
3d
3e
3f
90
100
100
100
96
100
97
3a
3b
3c
3d
3e
3f
100
100
100
100
100
100
95
3a
3b
3c
3d
3e
3f
100
100
100
100
aConditions: [Pd] 3a–3f (0.01 mmol, 1 mol%), 2‐acetylthiophene (2.0 mmol), aryl bromide (1.0 mmol), KOAc (2.0 mmol), DMA (2 ml), 120°C, 2 h.
bConversions were calculated with respect to aryl bromide from the results of GC spectrometry.
cGC yield.
Moderate to high yields of 5‐(4‐acetylphenyl)‐2‐
acetylthiophene were obtained for the coupling with 4‐
bromoacetophenone (Table 4, entries 25–30).
The Pd‐catalyzed direct arylation of furan and thio-
phene with a variety of electrophilic reagents has been
previously described.[15,16] In 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 arylation of furan and thio-
phene in the presence of Pd‐catalysts. In the present
work, 1 mol% catalyst loading was used, and the reaction
time was shortened to 2 h. Moreover, thiophene and
furan derivatives can be efficiently and selectively
arylated at the C(5)‐position. In this study, satisfactory

KALOĞLU ET AL.
15 of 16
results were obtained as compared with previously
reported similar studies.[15,16]
application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (Fax: +44 1223 336 033, e‐mail: deposit@ccdc.
cam.ac.uk, https://www.ccdc.cam.ac.uk/structures/).
Finally, the catalytic activities of the PEPPSI‐themed
new Pd–carbene complexes were investigated in the direct
C(5)‐arylation of C(2)‐blocked furan and thiophene. In
most cases, high yields were observed with all complexes.
Only a minor effect of the NHC ligand on the Pd‐complex
was observed for the coupling of aryl bromide with
heteroaromatics. Surprisingly, similar conversions were
obtained for the coupling of each aryl bromide. There is
no significant difference between these complexes on the
catalytic activity of direct arylation of heteroaromatics by
aryl bromides. The only significant difference between
3a–3e complexes indicates that electronic and steric
effects also play some role in these processes.
ORCID
Murat Kaloğlu https://orcid.org/0000-0002-2770-5532
Nevin Gürbüz https://orcid.org/0000-0003-3201-3597
İlkay Yıldırım https://orcid.org/0000-0002-7423-5043
Namık Özdemir https://orcid.org/0000-0003-3371-9874
İsmail Özdemir https://orcid.org/0000-0001-6325-0216
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How to cite this article: Kaloğlu M, Gürbüz N,
Yıldırım İ, Özdemir N, Özdemir İ. Well‐defined
PEPPSI‐themed palladium–NHC complexes:
synthesis, and catalytic application in the direct
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