The first use of [PdBr2(imidazolidin-2-ylidene)(pyridine)] catalysts in the direct C-H bond arylation of C2-substituted furan and thiophene

Article abstract of DOI:10.1007/s11164-021-04444-4

Abstract: N-Heterocyclic carbene (NHC)-linked PEPPSI-type palladium complexes have recently been used in the direct C-H bond arylation of heteroarenes. However, in most of the published works, NHC ligands containing benzimidazole and imidazole ring have b

Full text of DOI:10.1007/s11164-021-04444-4

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04444-4
The frst use of [PdBr2(imidazolidin‑2‑ylidene)(pyridine)]
catalysts in the direct C‑H bond arylation of C2‑substituted
furan and thiophene
Murat Kaloğlu1,2 · Nazan Kaloğlu1,2 · Namık Özdemir3 · İsmail Özdemir1,2
Received: 13 January 2021 / Accepted: 25 March 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
N-Heterocyclic carbene (NHC)-linked PEPPSI-type palladium complexes have
recently been used in the direct C-H bond arylation of heteroarenes. However, in
most of the published works, NHC ligands containing benzimidazole and imidazole
ring have been used, but using NHC ligands containing saturated imidazoline ring is
quite rare. Therefore, in this study, four new 1,3-disubstituted imidazolinium salts as
NHC ligand precursors, and their four new PEPPSI-type palladium complexes were
synthesized. The structures of all new compounds were fully characterized by dif-
ferent spectroscopic and analytical techniques. The more detailed structural charac-
terization of one of the palladium complexes was determined by single-crystal X-ray
difraction study. The catalytic activities of all palladium complexes were evalu-
ated in the direct C-H bond arylation of the 2-acetylfuran and 2-acetylthiophene
with (hetero) aryl bromides and readily available and inexpensive aryl chlorides in
presence of 1 mol% catalyst loading at 120 °C. Under the given conditions, (het-
ero)aryl halides were successfully applied as the arylating reagents to achieve the
C5-arylated furans and thiophenes in acceptable to high yields.
*
İsmail Özdemir
ismail.ozdemir@inonu.edu.tr
https://www.inonu.edu.tr/
1
Faculty of Science and Arts, Department of Chemistry, İnönü University, 44280 Malatya,
Turkey
2
3
Catalysis Research and Application Center, İnönü University, 44280 Malatya, Turkey
Faculty of Education, Department of Mathematics and Science Education, Ondokuz Mayıs
University, 55139 Samsun, Turkey
Vol.:(0123456789)
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M. Kaloğlu et al.
Graphic abstract
Keywords N-Heterocyclic carbene · Imidazolidin-2-ylidene · Palladium-PEPPSI
complex · Direct C-H bond arylation · Furan · Thiophene
Introduction
Since the frst isolation and characterization of stable free carbene by Arduengo in
1991, N-heterocyclic carbenes (NHCs) have played a central role as ligands in coor-
dination chemistry [1–7]. In 1995, the frst use of the NHCs in palladium-catalyzed
Heck reaction by Herrmann revealed that NHCs are efective ligands, and they have
emerged as a new ligand class in area of organometallic chemistry and catalysis
[8]. The structural variability of NHCs makes them excellent scafolds that allow to
adapt the electronic and steric properties of a particular metal complex. Moreover,
the strong σ-donating but poor π-accepting ability of NHCs leads to the formation of
many stable metal complexes. Therefore, NHCs ofer many advantages over phos-
phine ligands such as superior activity, stronger σ-donating properties, better stabil-
ity and structural versatility [9, 10]. As a result of these, the importance of NHCs as
ligands for metal complexes used in catalysis, material science and medical applica-
tions is unquestionable.
Bi(hetero)aryl substructures, which are important building units, are pre-
sent in many natural products, biologically active molecules, functional materi-
als and pharmaceuticals [11–13]. The robust synthetic routes for the synthesis
of bi(hetero)aryls are still a challenging area of interest due to lower reactivity
of heterocycles toward some of the recent booming arena of synthetic chemistry.
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The frst use of…
Today, developing efcient synthetic approaches to construct new C(sp2)–C(sp2)
bonds of bi(hetero)aryl units by transition-metal-catalyzed reactions is an impor-
tant research area of modern organic synthesis [14]. Among these reactions, the
Suzuki–Miyaura, Stille and Negishi couplings represent some of the important
procedures [15–18]. However, they require the preliminary preparation of an
organometallic derivative of an aryl derivative and provide an organometallic
salt as the by-product. In this regard, the direct arylation of heteroarenes via C-H
bond activation has emerged in recent years as an attractive alternative to tra-
ditional methods [19–25]. A chief advantage of this protocol is that the major
by-products are HX along with a base instead of metallic salts obtained by tra-
ditional procedures. Moreover, no prior preparation of an organometallic deriva-
tive is required, reducing the number of steps to prepare these compounds [26].
Today, palladium-catalyzed direct arylation of heteroarenes with aryl (pseudo)
halides ofers a more economical and environmental-friendly approach for the
construction of bi(hetero)aryl units because it obviates the use of organometallic
reagents, which are generally difcult to synthesize and are, in some cases, unsta-
ble [27–32].
The palladium-catalyzed direct arylation of some fve-membered heteroaromat-
ics was frstly reported by Nakamura, Tajima and Sakai in 1982 [33]. In 1990, the
frst examples of palladium-catalyzed direct arylation of furans and thiophenes were
reported by Ohta et al. [34]. Since then, the palladium-catalyzed direct arylation of
fve-membered heteroaromatics with aryl (pseudo)halides has emerged as one of the
most powerful methods for the preparation of bi(hetero)aryls. To date, palladium-
catalyzed direct arylation of a wide variety of heteroaromatics such as (benzo)
furans, (benzo)thiophenes, pyrroles, indoles, thiazoles, oxazoles, imidazoles, pyra-
zoles or triazoles has been largely described by a large number of researchers [26,
35].
After the discovery of palladium-PEPPSI complexes by Organ et al. in 2006
[36], (PEPPSI = Pyridine Enhanced Precatalyst Preparation Stabilization and Ini-
tiation), PEPPSI complexes have shown remarkable catalytic activities toward
many organic reactions. Pd-NHC-PEPPSI complexes represent a class of pal-
ladium catalysts completely diferent from other palladium-NHC complexes.
Unlike other types of palladium complexes, Pd-NHC-PEPPSI complexes are eas-
ier to synthesize and use [37]. The high activity of Pd-NHC-PEPPSI complexes
in catalysis has been based on to the presence of a loosely bound throw-away pyr-
idine ligand that makes way for the incoming substrate [38]. Despite the fact that
palladium-catalyzed direct arylation has become invaluable for catalysis chem-
istry, and signifcant advances have been reported, Pd-NHC-PEPPSI complexes
have been weakly applied as catalysts in the direct arylation reactions to date
[39]. However, in a limited number of studies, several examples about synthe-
sis of Pd-NHC-PEPPSI complexes bearing (benz)imidazol-2-ylidene ligands and
their catalytic application in the direct arylation reactions were reported to date
by various research groups [40, 41]. In some cases, saturated ring imidazolidin-
2-ylidene-based palladium complexes for the direct C-H bond arylation of heter-
oaromatic compounds are known to exhibit higher efciency than their structur-
ally analog unsaturated ring imidazol-2-ylidene-based palladium complexes [42,
1 3

M. Kaloğlu et al.
43]. But, to the best of our knowledge, only very few reports are available on the
direct arylation of heteroarenes with (hetero)aryl halides over Pd-NHC-PEPPSI
complexes exploiting imidazolidin-2-ylidene ligand [44–46].
In the light of the information mentioned above, recently, we have reported the
frst examples of the direct C4-arylation of 3,5-dimethylisoxazole [47], and the direct
C5-arylation of N-methylpyrrole-2-carboxaldehyde [48], with (hetero)aryl halides
catalyzed by Pd-NHC-PEPPSI complexes bearing imidazolidin-2-ylidene ligand.
These exciting results thus prompted us to further investigate its application on the
direct arylation of other heteroarenes catalyzed by imidazolidin-2-ylidene-based
palladium-PEPPSI catalysts. In this regard, herein, we have synthesized a series
of new imidazolinium salts (3a–3d) as carbene precursors and their corresponding
palladium-PEPPSI complexes (4a–4d) (Fig. 1). All new compounds were charac-
1
terized by diferent techniques such as H NMR, 13C NMR, FT-IR and elemental
analysis. Also, the solid-state structure of the palladium complex 4d has been estab-
lished by single-crystal X-ray difraction study. Then, the catalytic properties of all
palladium complexes were evaluated in the direct C5-arylation of the 2-acetylfuran
and 2-acetylthiophene with (hetero)aryl halides in presence of 1 mol% palladium
catalyst loading. The C5-arylated furans and thiophenes were selectively obtained
in moderate to high yields. To the best of our knowledge, this work is the frst report
of the direct C5-arylation of 2-substituted furan and thiophene with (hetero)aryl hal-
ides in presence of imidazolidin-2-ylidene-based palladium-PEPPSI complexes.
Results and discussion
General procedure
The general synthesis pathway for the N-(4-phenoxybutyl)ethylenediamine
(1), 1-(4-phenoxybutyl)imidazoline (2), 1,3-disubstituted imidazolinium salts
(3a–3d), and their corresponding Pd-NHC-PEPPSI complexes (4a–4d) is shown
in the Scheme 1. Some physical and spectroscopic data of the compounds 1–4 are
summarized in Table 1.
Fig. 1 Imidazolidin-2-ylidene based palladium-PEPPSI complexes used in this study
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The frst use of…
Scheme 1 The general synthesis pathway of compounds 1–4
Table 1 Physical and spectroscopic properties of the compounds 1–4
Compound Formula
Isolated
M.p. (°C) IR (vC(2)-N
)
1H NMR
(C(2)-H)
(ppm)
13C NMR
(C(2))
yield (%)
(cm−1
)
(ppm)
1
2
C
12H20N2O
75
91
80
72
69
78
46
42
35
110-115a
125-130a
79–80
89–90
134–135
79–80
124–125
69–70
192–193
─
─
─
C13H18N2O
C22H29BrN2O
1598
1655
1661
1650
1665
1602
1600
1600
1597
6.87
9.98
10.00
9.29
9.99
─
─
─
─
158.86
158.14
158.13
157.21
158.37
180.70
180.77
181.40
181.38
3a
3b
3c
3d
4a
4b
4c
4d
C
C
C
C
C
C
23H31BrN2O
24H33BrN2O
20H24BrClN2O
27H33Br2N3OPd
28H35Br2N3OPd
29H37Br2N3OPd
C25H28Br2ClN3OPd 50
120–121
a Boiling point under~50 Torr pressure
Preparation and characterization of N‑(4‑phenoxybutyl)ethylenediamine (1)
N-Alkylation of the ethylenediamine with 4-phenoxybutyl bromide gave the
N-(4-phenoxybutyl)ethylenediamine (1) as colorless viscous liquid in 75% yield.
1
This compound was characterized by H NMR and 13C NMR and IR spectros-
1
copy, and elemental analysis studies. In the H NMR spectra of the 1, the sig-
nal of the N–H protons of ethylenediamine was detected as multiplet between
δ=1.54–1.60 ppm, and aliphatic CH2 protons of ethylenediamine were detected
as multiplet between δ=2.57 and 2.61 ppm in accordance with the expected inte-
grals. Aliphatic CH2 protons of 4-phenoxybutyl substituent were detected as pentet
at δ=1.58 and 1.74 ppm and were detected as triplet at δ=2.71 and 3.88 ppm.
Aromatic ring protons of 4-phenoxybutyl substituent were detected as multiplet
between δ=6.79 and 7.20 ppm with 5H intensity. In 13C NMR spectra, aliphatic
carbon resonances of ethylenediamine appeared at δ=41.76 and 49.52 ppm as sin-
gle signal. Aliphatic carbon resonances of 4-phenoxybutyl substituent were detected
at δ=26.79, 27.12, 52.47 and 67.60 ppm. Aromatic ring carbon resonances of
1 3

M. Kaloğlu et al.
4-phenoxybutyl substituent appeared at δ=114.48, 120.53, 129.41 and 159.01 ppm.
IR data clearly indicated that N-(4-phenoxybutyl)ethylenediamine exhibits a charac-
teristic ν(C-N) and ν(N–H) band. In the IR spectra, C-N bond vibrations of the 1 were
observed as sharp band at 1032 and 1241 cm−1. N–H bond vibration also appeared
as a broadband at 3290 cm−1. Elemental analysis data of the 1 were also consistent
with the expected structure.
Preparation and characterization of 1‑(4‑phenoxybutyl)imidazoline (2)
As can be seen in Scheme 1, the 1-(4-phenoxybutyl)imidazoline (2) was obtained
by cyclization of the N-(4-phenoxybutyl)ethylenediamine (1) with N,N-dimethylfor-
mamide dimethyl acetal, as yellowish viscous liquid in 96% yield. The formation of
the 2 was confrmed by the characteristic signals in 1H NMR and 13C NMR spectra,
and elemental analysis data. In 1H NMR spectra, characteristic C(2)-H proton down-
feld resonance of the 2 was observed as a sharp singlet at δ=6.87 ppm. Aliphatic
C(4)-H and C(5)-H protons of imidazoline ring were detected as triplet at δ=3.24
and 3.84 ppm in accordance with the expected integrals, respectively. In 13C NMR
spectra, C(2) carbon resonance of imidazoline ring appeared at δ=158.86 ppm as
a single signal, while C(4) and C(5) carbon resonances were observed at δ=47.40
and 48.39 ppm. The IR spectrum of the 2 displays the characteristic vibration band
of C=N bond at 1598 cm−1. Also, C-N bond vibrations of the 2 were observed as a
sharp band at 1022 and 1240 cm−1. These data suggest the formation of imidazoline
ring. Elemental analysis data were also consistent with the expected structure.
Preparation and characterization of imidazolinium salts as carbene precursors
(3a–3d)
The new imidazolinium salts 3a-3d were synthesized as carbene precursors by inter-
action of the 1-(4-phenoxybutyl)imidazoline (2) with substituted benzyl bromides.
The reactions were carried out in anhydrous dimethylformamide (DMF) at 80 °C
for 16 h, and the target salts were obtained as white solids between 69 and 80%
yields. The imidazolinium salts were fully characterized by the combination of
1H NMR, 13C NMR, and IR spectroscopic techniques and elemental analyses. In
1
the H NMR spectra, the signal of the acidic C(2)-H proton down-feld resonance
of imidazolinium ring for 3a–3d salts was observed as sharp singlets at δ=9.98,
10.00, 9.29 and 9.99 ppm, respectively. Aliphatic CH2 protons of benzyl substitu-
ents for imidazolinium salts 3a–3d were detected as singlet at δ=4.75, 4.81, 4.87
and 4.88 ppm, respectively. In the 13C NMR spectra, C(2)-carbon resonances of the
3a–3d salts appeared at δ=158.14, 158.13, 157.21 and 158.37 ppm, respectively,
as single signal. These downfeld signals indicate the formation of imidazolinium
salts. Also, aliphatic carbon resonances of benzyl substituents for the 3a–3d salts
were detected as single signal at δ=52.25, 51.94, 48.15 and 51.41 ppm, respec-
tively. As shown in Table 1, the IR data clearly indicated that the imidazolinium
salts 3a–3d exhibit a characteristic vC(2)-N vibration band typically at 1655, 1661,
1 3

The frst use of…
1650 and 1665 cm−1, respectively. Elemental analysis data were also consistent with
the expected structures.
Preparation and characterization of palladium‑carbene complexes (4a–4d)
As can be seen in Scheme 1, the new palladium-carbene complexes 4a–4d were pre-
pared by metallation of the corresponding imidazolinium salts (3a–3d) with PdCl2.
The reactions were carried out in presence of pyridine as N-donor ligand in ace-
tonitrile (MeCN) at 80 °C for 16 h, and the target complexes were obtained as yel-
low solids between 35 and 50% yields. These PEPPSI-type palladium-carbene com-
plexes were very stable against air and moisture in the solid state. They are soluble in
most organic solvents, such as CH2Cl2, CHCl3, EtOAc and DMSO, with the excep-
tion of non-polar ones, such as pentane, hexane and Et2O. Formation of palladium-
carbene complexes is supported by NMR, IR spectroscopies and elemental analysis
1
techniques. In the H NMR spectra of complexes 4a–4d, the characteristic down-
feld signals for the acidic C(2)–H protons of the imidazolinium salts 3a–3d disap-
peared in the 1H NMR spectra of the palladium-carbene complexes. In addition, the
down-feld signals of pyridine ligand between δ=7.21 and 8.97 ppm indicate the
formation of PEPPSI-type palladium-carbene complexes. 13C NMR chemical shifts
provide a useful diagnostic tool for metal-carbene complexes. In the 13C NMR spec-
tra of complexes 4a–4d, characteristic signal of C(2)-carbon of imidazolinium salts
3a–3d between δ=157.21 and 158.37 ppm were completely disappeared, and the
characteristic Pd–C(2) carbene bond signals of the complexes 4a–4d were observed
as singlet. In the 13C NMR spectra, the carbene signals of the palladium-complexes
4a–4d were observed at δ=180.70, 180.77, 181.40 and 181.38 ppm, respectively.
Also, characteristic down-feld signals of the aromatic carbons of the pyridine
ligand support the formation of PEPPSI-type palladium-carbene complexes. The
IR data clearly indicated that palladium-carbene complexes exhibit a characteristic
v(CN) stretching frequency peaks typically between 1597 and 1602 cm−1. Due to the
fow of electrons from the imidazolidin-2-ylidene ligand to the palladium, the C-N
bond is weakened, and as a result, a decreasing in the v(CN) stretching frequency is
expected. Also, the microanalysis data of the palladium-carbene complexes agree
closely with the theoretical requirements of their structures.
Description of the crystal structure of the palladium‑carbene complex 4d
The molecular structure of complex 4d with the adopted atom-labeling scheme is
shown in Fig. 2, while important bond distances and angles are listed in Table 2.
There are two crystallographically independent molecules in the asymmetric unit of
the complex. In the following discussion, parameters related to the second molecule
are given in square brackets.
The complex has a slightly distorted square-planar geometry, the metal
center being coordinated to the carbenic carbon atom of NHC ligand, the nitro-
gen atom of the pyridine ring, and two bromo ligands in a trans confguration.
In the square-planar coordination, atoms Pd1, Br1, Br2, N3 and C1 deviate by
1 3

M. Kaloğlu et al.
Fig. 2 Molecular structure of complex 4d, showing the atom-labeling scheme. Displacement ellipsoids
are drawn at the 10% probability level and H atoms are shown as small spheres of arbitrary radii. For the
sake of clarity, only one of two molecules in the asymmetric unit is shown
0.0218(4) [0.0102(4)], 0.0529(6) [0.0957(6)], 0.0528(7) [0.0961(6)], − 0.060(4)
[− 0.094(4)] and − 0.068(5) [− 0.108(4)] Å, respectively, from the mean plane
through these fve atoms. The cis angles varying from 88.08(13) to 92.03(10)°
1 3

The frst use of…
Table 2 Selected geometric
parameters for complex 4d
Parameters
Molecule I
Molecule II
Bond lengths (Å)
Pd1─Br1
Pd1─Br2
Pd1─N3
Pd1─C1
N1─C1
N1─C2
N2─C1
N2─C3
2.4247(6)
2.4263(7)
2.107(3)
1.962(5)
1.321(6)
1.466(6)
1.318(6)
1.461(6)
2.4234(6)
2.4262(6)
2.110(4)
1.959(5)
1.322(6)
1.462(6)
1.319(6)
1.459(7)
Bond angles (°)
Br1─Pd1─Br2
Br1─Pd1─N3
Br2─Pd1─N3
Br1─Pd1─C1
Br2─Pd1─C1
N3─Pd1─C1
N1─C1─N2
176.96(3)
90.69(10)
92.03(10)
89.32(13)
88.08(13)
175.14(19)
110.3(4)
174.98(3)
91.17(9)
91.98(9)
89.32(12)
87.97(12)
173.72(19)
110.0(4)
[from 87.97(12) to 91.98(9)°] and the trans angles changing from 175.14(19) to
176.96(3)° [from 173.72(19) to 174.98(3)°] deviate from their expected values
of 90 and 180°. For quantitative evaluation of the extent of distortion around the
metal center, the structural indexes ꢀ4 [49] and ꢀꢀ [50] were employed; where ꢀ
4
and ꢀ (ꢀ > ꢁ) are the two greatest valence angles [ꢀ = 176.96° and ꢀ = 175.14°
for molecule I and ꢀ = 174.98° and ꢀ = 173.72° for molecule II] and ꢀ is the
ideal tetrahedral angle (ꢀ = 109.5°).
360◦−(ꢁ+ꢂ)
360◦−2ꢃ
ꢂ−ꢁ
180◦−ꢂ
+
360◦−ꢃ 180◦−ꢃ
ꢀ4 =
ꢀ� =
4
The calculated ꢀ4 and ꢀꢀ geometry indices are 0.06 [0.08] and 0.05 [0.08],
4
respectively, pointing out a slightly distorted square-planar geometry.
Although the Pd─CNHC bond distance of 1.962(5) [1.959(5) Å] is smaller
than the sum of the individual covalent radii of the palladium and carbon atoms
(2.12 Å), the Pd─Npyridine bond distance of 2.107(3) [2.110(4) Å] is equal to
the sum of the individual covalent radii of the palladium and nitrogen atoms
(2.10 Å) [51]. The Pd─Br bond lengths are found in the typical range, and all
the aforementioned values are comparable with those found in other Pd-NHC-
Pyridine-Br2 complexes [23, 52–57]. In the NHC ligands, the internal N─C─N
ring angle at the carbene center is 110.3(4)° [110.0(4)]. The carbene ring is
almost perpendicular to the PdCNBr2 coordination plane with a dihedral angle
of 88.4(2)° [83.9(2)°], which is typical for NHC complexes to reduce steric con-
gestion. Furthermore, the dihedral angle between the pyridine ring and the coor-
dination plane is found to be 49.8(2)° [48.1(2)°].
1 3

M. Kaloğlu et al.
Infuence of the reaction conditions for the palladium‑carbene catalyzed direct
arylation
In order to determine the most suitable conditions in the palladium-carbene cata-
lyzed direct arylation of C2-substituted furan and thiophene, as can be seen in Eq. 1,
the reaction of 2-acetylfuran with 3-bromoquinoline was examined as a model reac-
tion. Last two decades, the palladium-catalyzed direct arylation of fve-membered
heterocycles was successfully performed using DMA/KOAc combination. Based
on previously reported conditions for the direct arylation of furan and thiophene
[58–71], we employed KOAc (2 equiv.) as the base and DMA as the solvent. The
efect of the catalysts, temperature, reaction time and catalyst loading was examined
on the reaction. Yields were calculated with respect to 3-bromoquinoline from the
GC results. Selected results from our preliminary studies are summarized in Table 3.
The arylation of 2-acetylfuran with 3-bromoquinoline was carried out at 150 °C
for 4 h without the addition of any palladium catalyst in order to examine the efect
of the catalyst on the reaction. However, under these reaction conditions, no forma-
tion of the desired product was obtained (Table 3, entry 1). In order to identify the
most active catalyst among complexes 4a–4d, reactions were carried out at 150 °C
in DMA with a palladium-loading of 1 mol% (Table 3, entries 2–5). In the presence
of these catalysts, yields ranged from 90 to 97%. As a result of these preliminary
studies, it was observed that the most active catalyst was complex 4c with 97% yield
Table 3 The direct C5-arylation of 2-acetylfuran with 3-bromoquinolinea
Entry
[Pd] (mol-%)
Temperature (°C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
─
150
150
150
150
150
150
120
90
120
120
120
120
4
4
4
4
4
4
4
4
3
2
1
2
─
4a (1)
4b (1)
4c (1)
4d (1)
PdCl2 (1)
4c (1)
4c (1)
4c (1)
4c (1)
4c (1)
4c (0.5)
94
91
97
90
78c
95
67
95
92
72
55
9
10
11
12
aConditions: 2-acetylfuran (2 equiv.), 3-bromoquinoline (1 equiv.), KOAc (2 equiv.), DMA (2 mL)
bYields were calculated with respect to 3-bromoquinoline from the results of GC spectrometry
cIn situ generated catalytic system with 3c salt as carbene precursor (2 equiv.) and PdCl2 (1 equiv.) was
used
1 3

The frst use of…
(Table 3, entry 4). It was observed that the catalytic activity of complexes 4a–4d
was enhanced by having sterically hindered and bulky groups on the NHC ligand.
When in situ generated palladium/ligand catalytic system with 2 mol% of 3c
NHC precursor and 1 mol% of PdCl2 was used at 150 °C for 4 h, only 78% yield was
observed (Table 3, entry 6). Thus, it was understood that isolated palladium com-
plex 4c was more active than in situ generated palladium/ligand system. Later, the
efect of temperature on the yield was examined. When the temperature is reduced
from 150 °C to 120 °C in presence of 1 mol% catalyst 4c, the yield decreased to
95% (Table 3, entry 7). However, this decrease in the yield is within acceptable lim-
its. When the reaction temperature was decreased from 120 °C to 90 °C in pres-
ence of 1 mol% catalyst 4c, it was observed that the yield decreased up to 67%
(Table 3, entry 8). At this temperature, the reaction did not take place in satisfactory
yield. Therefore, it was decided that the optimum temperature for the model reac-
tion was 120 °C. Next, the efect of the reaction time on the yield was examined.
When the reaction time was regularly reduced from 4 to 2 h, no signifcant difer-
ence was observed on the yield (Table 3, entries 9,10). But, when the reaction time
was reduced from 2 to 1 h, the yield dropped to 72% (Table 3, entry 11). Therefore,
the optimum reaction time for the model reaction was decided to be 2 h. Finally, the
efect of catalyst-loading on the yield was also investigated. When the catalyst-load-
ing was decreased from 1 mol% to 0.5 mol% at 120 °C, only 55% yield was achieved
after 2 h (Table 3, entry 12).
After these preliminary studies summarized in Table 3, we tried to evaluate
the scope and limitations of the palladium-carbene catalysts 4a–4d for the direct
C5-arylation of 2-acetylfuran and 2-acetylthiophene with (hetero)aryl halides
(Eq. 2). The reaction worked well for a wide variety of (hetero)aryl bromides, and
even some aryl chlorides. The results of the direct C5-arylation of 2-acetylfuran and
2-acetylthiophene with (hetero)aryl halides are summarized in Table 4.
When the reaction of 2-acetylfuran with a neutral aryl bromide such as bro-
mobenzene was investigated, yields at between 65 and 90%. When bromobenzene
and 2-acetylthiophene were interacted, 71–88% yields were obtained (Table 4,
entries 1–4). When chlorobenzene was used with both heterocycles in the presence
of 4c catalyst, which is the most active catalyst, high yields could not be obtained
despite the 18 h reaction (Table 4, entry 3). When the reaction of both heterocycles
with an electron-rich aryl bromide such as 4-bromotoluene was investigated, yields
were obtained at between 60 and 88% for 2-acetylfuran and 71–91% for 2-acetylth-
iophene (Table 4, entries 5–8). When 4-chlorotoluene was used as aryl halide, after
18 h, moderate yields were obtained in presence of 4c catalyst for both heterocy-
cles (Table 4, entry 7). Similar yields were obtained for 2-acetylfuran (58–90%) and
2-acetylthiophene (55–87%) using 4-bromoanisole for 4 h by using only 1 mol%
4a–4d complexes (Table 4, entries 9–12). But, when 4-chloroanisole was used in
presence of 4c catalyst, 55% and 61% yields were observed for the 2-acetylfuran and
2-acetylthiophene after 18 h, respectively (Table 4, entry 11). When the electron-
withdrawing para-substituents such as aldehyde, acetyl, fuoro and trifuoromethyl
on the aryl bromide were investigated with 2-acetylfuran, the target products were
obtained in moderate to high yields in presence of 1 mol% 4a–4d catalysts after 2 h.
When electron-withdrawing para-substituents were used with 2-acetylthiophene,
1 3

M. Kaloğlu et al.
Table 4 The Pd-carbene catalyzed direct C5-arylation of 2-acetylfuran and 2-acetylthiophene with (het-
ero)aryl halidesa
(Eq. 2)
Yield (%)b,c
Entry
(Hetero)aryl halide
[Pd]
Product
Time (h)
Y = O
82
Y = S
84
1
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
2
2
2
2
2
2
2
2
4
4
4
4
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
8
8
8
8
4
4
4
4
2
2
2
2
4
4
4
4
2
3
4
5
6
7
8
9
71
90 (66)
65
85
76
88 (70)
60
75
58
90 (55)
74
95 (82)
81
93
76
94
88
97 (84)
78
86
81
91
73
80
62
89 (75)
70
77
58
84
45
78
64
83
55
88
81
92
79
88 (57)
71
80
71
91 (73)
75
71
55
87 (61)
60
88
75
96 (87)
79
86
80
97 (83)
79
78
69
88
76
84
80
92 (81)
67
71
52
74
39
85
62
91
67
83
70
86
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
83
77
61
80
67
63
60
77
48
49
a
Conditions: [Pd] 4a–4d (0.01 equiv., 1 mol%), 2-acetylfuran or 2-acetylthiophene (2 equiv.), (hetero)
aryl halide (1 equiv.), KOAc (2 equiv.), DMA (2 mL), 120 °C
b Yields were calculated with respect to (hetero)aryl halide from the results of GC spectrometry
c Yields of the aryl chlorides obtained at 18 h were shown in parentheses
similar results were also obtained (Table 4, entries 13–28). However, the para-sub-
stituted 4-chlorobenzaldehyde, 4-chloroacetophenone and 4-chlorobenzotrifuoride
led to the formation of the target products in moderate yields after 18 h. When
1 3

The frst use of…
sterically hindered electron-donating 2-bromotoluene was used as aryl halide with
2-acetylfuran, 84% yield was obtained in the presence of 4c catalyst after 8 h. When
2-acetylthiophene was used under the same conditions, the yield dropped to 74%
(Table 4, entry 31). In the presence of 2-bromobenzonitrile, yields varying between
55 and 83% with 2-acetylfuran and 62–91% with 2-acetylthiophene were obtained
after 4 h (Table 4, entries 33–36). Then, we examined the reactivities of electron-
defcient heterocycles such as 3-bromoquinoline and 2-bromothiophene as heter-
oaryl bromides. When 3-bromoquinoline was used, high yields were obtained for
both 2-acetylfuran and 2-acetylthiophene in the presence of 4e-4 h catalysts after 2 h
(Table 4, entries 37–40). When 2-bromothiophene was used as the coupling partner,
a yield of 80% was obtained at the end of 4 h for 2-acetylfuran in the presence of 4c
of catalyst, while a yield of 77% was obtained for 2-acetylthiophene (Table 4, entry
43).
As a result, we investigated that the catalytic activities of imidazolidin-2-ylidene-
linked palladium-PEPPSI complexes in the direct C5-arylation 2-acetylfuran were
similar to 2-acetylthiophene with (hetero)aryl bromides and aryl chlorides by apply-
ing short time periods. The catalytic system was tolerable to a variety of functional
groups such as aldehyde, acetyl, fuoro, trifuoromethyl and nitrile on the aryl hal-
ides, producing the corresponding products in moderate to high yields. Generally,
the reactivity of 2-acetylfuran was similar to 2-acetylthiophene, and the yields for
substrates containing electron-withdrawing groups were higher than those for sub-
stituents containing electron-donating group. Although small diferences in reactivi-
ties were observed for 4a-4d catalysts due to similar nature of the NHC moieties, it
can be said that the most efective catalyst in the direct arylation of heteroaromatics
is complex 4c containing bulky 2,3,5,6-tetramethylbenzyl substituent.
Conclusion
In summary, we prepared a series of four new imidazolinium salts as imidazolidin-
2-ylidene ligand precursors, and their four new PEPPSI-type palladium complexes.
All new compounds were characterized using diferent spectroscopic and analitical
techniques. The catalytic activities of the all palladium complexes were investigated
in the direct C5-arylation of 2-acetylfuran and 2-acetylthiophene with (hetero)aryl
halides. It was found that all the palladium complexes were efective catalysts for
these reactions. Overall, except in a few cases, satisfactory results were obtained in
all trials. This study was performed by using 1 mol% of palladium catalyst. There-
fore, this low catalyst loading procedure was economically attractive. In this study,
only AcOH and HBr were formed as a by-product by the use of direct arylation
method and thus the by-product formation was minimized compared to the multi-
step traditional transition metal-catalyzed reactions. To our knowledge, this study is
the frst report of the direct C5-arylation of 2-acetylfuran and 2-acetylthiophene with
(hetero)aryl halides catalyzed by imidazolidin-2-ylidene-linked palladium-PEPPSI
complexes. Moreover, further studies focused on the synthesis of new imidazolidin-
2-ylidene-linked palladium-PEPPSI complexes and their catalytic application for the
C-H bond arylation of heteroarenes are currently underway by our research group.
1 3

M. Kaloğlu et al.
Experimental
General remarks
All manipulations were performed in Schlenk-type fasks under argon atmosphere.
The melting point measurements were determined in open capillary tubes with an
Electrothermal-9200 melting points apparatus. The C, H and N elemental analy-
sis measurements were determined by LECO CHNS-932 elemental analyzer. The
FT-IR spectra were recorded on GladiATR unit (Attenuated Total Refection) in
the range of 450–4000 cm−1 with a Perkin Elmer Spectrum 100 Fourier-transform
1
infrared spectrometer. Routine H NMR and 13C NMR spectra were recorded with
a Bruker Ascend™ 400 Avance III HD NMR spectrometer with sample solutions
prepared in CDCl3. The chemical shifts (δ) were reported in parts per million (ppm)
relative to tetramethylsilane (TMS) as internal standard. Coupling constants (J val-
ues) were given in hertz. NMR multiplicities were abbreviated as follows: s=sin-
glet, d=doublet, t=triplet, p=pentet, dd=doublet of doublets, tt=triplet of triplets,
1
ddd=doublet of doublet of doublets, m=multiplet. H NMR spectra were refer-
enced to residual protiated solvents (δ=7.28 ppm for CDCl3), 13C NMR chemical
shifts were reported relative to deuterated solvents (δ=77.16 ppm for CDCl3). The
catalytic solutions were analyzed with a Shimadzu GC 2025 equipped with GC-FID
sensor and RX-5 ms column of 30 m length, 0.25 mm diameter and 0.25 μm flm
thickness.
Preparation of N‑(4‑phenoxybutyl)ethylenediamine (1)
The N-(4-phenoxybutyl)ethylenediamine (1) was prepared by the N-alkylation of the
ethylenediamine with 4-phenoxybutyl bromide. Lithium (0.35 g; 50 mmol) was little
by little added in freshly distilled and dried ethylenediamine (30 mL) under argon
atmosphere at 110 °C. The solution, which was stirred for 1 h, was then cooled to
room temperature, and 4-phenoxybutyl bromide (11.5 g; 50.2 mmol) was added to
this solution. Then, anhydrous toluene (30 mL) was added to the solution. After
1 min, lithium bromide precipitate started to form. The mixture was stirred for a
further 1 h at 110 °C, and it was then cooled to room temperature. The precipitated
lithium bromide was removed by fltered of and was washed with 20 mL of tolu-
ene. Then, all volatiles were removed under vacuum. The crude product was dis-
tilled under reduced vacuum. The N-(4-phenoxybutyl)ethylenediamine was isolated
as colorless gel in 75% yield.
Yield 75%, 7.880 g (colorless gel); bp: 110–115 °C (under~50 Torr pres-
1
sure); FT-IR (vC-N): 1032 and 1241 cm−1; (vN-H): 3290 cm−1. H NMR (400 MHz,
CDCl3, 25 °C, TMS): δ (ppm)=1.54–1.60 (m, 3H, N-Hs of ethylenediamine);
1.58 (p, J=6.7 Hz, 2H, NCH2CH2CH2CH2OC6H5); 1.74 (p, J=6.7 Hz, 2H,
NCH2CH2CH2CH2OC6H5); 2.57–2.61 (m, 4H, NCH2CH2N); 2.71 (t, J=5.9 Hz,
2H, NCH2CH2CH2CH2OC6H5); 3.88 (t, J=6.4 Hz, 2H, NCH2CH2CH2CH2OC6H5);
6.79–6.85 and 7.16–7.20 (m, 5H, arom. Hs of NCH2CH2CH2CH2OC6H5). 13C NMR
(101 MHz, CDCl3, 25 °C, TMS): δ (ppm)=26.79 (NCH2CH2CH2CH2OC6H5);
1 3

The frst use of…
27.12 (NCH2CH2CH2CH2OC6H5); 41.76 and 49.52 (NCH2CH2N); 52.47
(NCH2CH2CH2CH2OC6H5); 67.60 (NCH2CH2CH2CH2OC6H5); 114.48, 120.53,
129.41 and 159.01 (arom. Cs of NCH2CH2CH2CH2OC6H5). Elemental analysis
calcd. (%) for C12H20N2O: C 69.19, H 9.68, N 13.45; found (%): C 69.56, H 9.37,
1
N 13.86. (For the H NMR, 13C NMR and IR spectrum of the 1, see SI fle, pages
S1-S2).
Preparation of 1‑(4‑phenoxybutyl)imidazoline (2)
The 1-(4-phenoxybutyl)imidazoline (2) was prepared by the cyclization of the
N-(4-phenoxybutyl)ethylenediamine (1) with N,N-dimethylformamide dimethyl
acetal. For the preparation of the 2, the N-(4-phenoxybutyl)ethylenediamine (1)
(7,88 g; 37,8 mmol) was reacted with N,N-dimethylformamide dimethyl acetal
(4,96 g; 41,6 mmol) at 90–110 °C for 3 h. End of the reaction, unreacted excess
acetal were removed under vacuum. Then, the crude product was distilled under
reduced vacuum. The 1-(4-phenoxybutyl)imidazoline was isolated as yellowish gel
in 91% yield.
Yield 91%, 7.480 g (yellowish gel); bp: 125–130 °C (under~50 Torr pres-
1
sure); FT-IR (vC-N): 1022 and 1240 cm−1; (vC=N): 1598 cm−1. H NMR (400 MHz,
CDCl3, 25 °C, TMS): δ (ppm)=1.71–1.79 (m, 2H, NCH2CH2CH2CH2OC6H5);
1.81–1.87 (m, 2H, NCH2CH2CH2CH2OC6H5); 3.19 (t, J=6.9 Hz, 2H,
NCH2CH2CH2CH2OC6H5); 3.24 (t, J=9.9 Hz, 2H, NCH2CH2N); 3.84 (td,
J=9.9, 1.6 Hz, 2H, NCH2CH2N); 4.01 (t, J=6.0 Hz, 2H, NCH2CH2CH-
2CH2OC6H5); 6.87 (s, 1H, NCHN); 6.90–6.98 and 7.28–7.32 (m, 5H, arom. Hs
of NCH2CH2CH2CH2OC6H5). 13C NMR (101 MHz, CDCl3, 25 °C, TMS): δ
(ppm)=25.43 (NCH2CH2CH2CH2OC6H5); 26.54 (NCH2CH2CH2CH2OC6H5);
47.40 and 48.39 (NCH2CH2N); 54.74 (NCH2CH2CH2CH2OC6H5); 67.22
(NCH2CH2CH2CH2OC6H5); 114.45, 120.74, 129.49 and 157.78 (arom. Cs of
NCH2CH2CH2CH2OC6H5); 158.86 (NCHN). Elemental analysis calcd. (%) for
C13H18N2O: C 71.53, H 8.31, N 12.83; found (%): C 71.51, H 8.41, N 12.77. (For
the 1H NMR, 13C NMR and IR spectrum of the 2, see SI fle, pages S3-S4).
General procedure for the preparation of imidazolinium salts as carbene
precursors (3a–3d)
The new imidazolinium salts (3a–3d) were prepared by interaction of the 1-(4-phe-
noxybutyl)imidazoline (2) with substituted benzyl bromides. The 1-(4-phenoxybu-
tyl)imidazoline (1,09 g; 5,0 mmol) and substituted benzyl bromide (5.0 mmol) were
dissolved in degassed DMF (5 mL), and the solution was stirred at 80 °C for 16 h.
After completion of the reaction, the mixture was allowed to cool to room tempera-
ture, and diethyl ether (15 mL) was added to solution. Then, obtained white solid
was fltered of, was washed with diethyl ether (3×10 mL), and dried under vacuum.
The crude product was recrystallized from EtOH/Et2O solvent system (1:5, v/v) at
room temperature and completely dried under vacuum. The new imidazolinium salts
(3a–3d) were isolated as air- and moisture-stable white-brown crystalline solids.
1 3

M. Kaloğlu et al.
1‑(4‑Phenoxybutyl)‑3‑(3,5‑dimethylbenzyl)imidazolinium bromide (3a)
Yield 80%, 1.675 g (white solid); mp: 79–80 °C; FT-IR (vC-N): 1038 and
1
1243 cm−1; (vC(2)-N): 1655 cm−1. H NMR (400 MHz, CDCl3, 25 °C, TMS):
δ (ppm)=1.85–1.89 (m, 2H, NCH2CH2CH2CH2OC6H5); 1.91–1.95 (m, 2H,
NCH2CH2CH2CH2OC6H5); 2.31 (s, 6H, NCH2C6H3(CH3)2–3,5); 3.75 (t, J=6.8 Hz,
2H, NCH2CH2CH2CH2OC6H5); 3.79–3.82 and 3.92–3.98 (m, 4H, NCH2CH2N);
4.00 (t, J=5.6 Hz, 2H, NCH2CH2CH2CH2OC6H5); 4.75 (s, 2H, NCH2C6H3
(CH3)2–3,5); 6.87 (d, J=7.8 Hz, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5);
6.94 (t, J=7.4 Hz, 1H, arom. H of NCH2CH2CH2CH2OC6H5); 6.97 (m,
3H, arom. Hs of NCH2C6H3(CH3)2–3,5); 7.27 (dd, J=8.7, 7.6 Hz, 2H,
arom. Hs of NCH2CH2CH2CH2OC6H5); 9.98 (s, 1H, NCHN). 13C NMR
(101 MHz, CDCl3, 25 °C, TMS): δ (ppm)=21.23 (NCH2C6H3(CH3)2–3,5);
24.24 (NCH2CH2CH2CH2OC6H5); 26.13 (NCH2CH2CH2CH2OC6H5); 47.73
(NCH2CH2CH2CH2OC6H5); 48.25 and 48.36 (NCH2CH2N); 52.25 (NCH2C6H3
(CH3)2–3,5); 66.80 (NCH2CH2CH2CH2OC6H5); 114.43, 120.86, 129.55 and 158.61
(arom. Cs of NCH2CH2CH2CH2OC6H5); 126.58, 130.73, 132.26 and 139.00 (arom.
Cs of NCH2C6H3(CH3)2–3,5); 158.14 (NCHN). Elemental analysis calcd. (%) for
C22H29BrN2O: C 63.31, H 7.00, N 6.71; found (%): C 63.94, H 6.80, N 7.13. (For
the 1H NMR, 13C NMR and IR spectrum of the 3a, see SI fle, pages S5-S6).
1‑(4‑Phenoxybutyl)‑3‑(4‑isopropylbenzyl)imidazolinium bromide (3b)
Yield 72%, 1.561 g (white solid); mp: 89–90 °C; FT-IR (vC-N): 1035 and
1
1237 cm−1; (vC(2)-N): 1661 cm−1. H NMR (400 MHz, CDCl3, 25 °C, TMS): δ
(ppm)=1.22 (d, J=6.9 Hz, 6H, NCH2C6H4(CH(CH3)2)-4); 1.83–1.89 (m, 2H,
NCH2CH2CH2CH2OC6H5); 1.90–1.94 (m, 2H, NCH2CH2CH2CH2OC6H5);
2.89 (hept, J=6.9 Hz, 1H, NCH2C6H4(CH(CH3)2)-4); 3.74 (t, J=7.1 Hz, 2H,
NCH2CH2CH2CH2OC6H5); 3.77–3.82 and 3.92–3.97 (m, 4H, NCH2CH2N);
4.00 (t, J=5.7 Hz, 2H, NCH2CH2CH2CH2OC6H5); 4.81 (s, 2H, NCH2C6H4(C
H(CH3)2)-4); 6.87 (d, J=7.9 Hz, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5);
6.93 (t, J=7.3 Hz, 1H, arom. H of NCH2CH2CH2CH2OC6H5); 7.22 (d,
J=8.1 Hz, 2H, arom. Hs of NCH2C6H4(CH(CH3)2)-4); 7.26 (dd, J=8.4,
7.5 Hz, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5); 7.32 (d, J=8.1 Hz, 2H,
arom. Hs of NCH2C6H4(CH(CH3)2)-4); 10.00 (s, 1H, NCHN). 13C NMR
(101 MHz, CDCl3, 25 °C, TMS): δ (ppm)=23.88 (NCH2C6H4(CH(CH3)2)-4);
24.20 (NCH2CH2CH2CH2OC6H5); 26.11 (NCH2CH2CH2CH2OC6H5); 33.85
(NCH2C6H4(CH(CH3)2)-4); 47.68 (NCH2CH2CH2CH2OC6H5); 48.24 and 48.37
(NCH2CH2N); 51.94 (NCH2C6H4(CH(CH3)2)-4); 66.77 (NCH2CH2CH2CH2OC6H5);
114.42, 120.83, 129.53 and 158.59 (arom. Cs of NCH2CH2CH2CH2OC6H5);
127.31, 128.95, 129.77 and 149.91 (arom. Cs of NCH2C6H4(CH(CH3)2)-4); 158.13
(NCHN). Elemental analysis calcd. (%) for C213H31BrN2O: C 64.03, H 7.24, N 6.49;
found (%): C 64.22, H 7.40, N 6.89. (For the H NMR, 13C NMR and IR spectrum
of the 3b, see SI fle, pages S7-S8).
1 3

The frst use of…
1‑(4‑Phenoxybutyl)‑3‑(2,3,5,6‑tetramethylbenzyl)imidazolinium bromide (3c)
Yield 69%, 1.532 g (white solid); mp: 134–135 °C; FT-IR (vC-N): 1037 and
1
1257 cm−1; (vC(2)-N): 1650 cm−1. H NMR (400 MHz, CDCl3, 25 °C, TMS):
δ (ppm)=1.79–1.82 (m, 2H, NCH2CH2CH2CH2OC6H5); 1.84–1.89 (m, 2H,
NCH2CH2CH2CH2OC6H5); 2.18 and 2.22 (s, 12H, NCH2C6H(CH3)4–2,3,5,6);
3.69 (t, J=6.8 Hz, 2H, NCH2CH2CH2CH2OC6H5); 3.78–3.84 and 3.94–3.97
(m, 4H, NCH2CH2N); 4.01 (t, J=6.7 Hz, 2H, NCH2CH2CH2CH2OC6H5);
4.87 (s, 2H, NCH2C6H(CH3)4–2,3,5,6); 6.79 (d, J=7.9 Hz, 2H, arom.
Hs of NCH2CH2CH2CH2OC6H5); 6.89 (t, J=7.3 Hz, 1H, arom. H of
NCH2CH2CH2CH2OC6H5); 6.96 (s, 1H, arom. H of NCH2C6H(CH3)4–2,3,5,6);
7.21 (t, J=8.0 Hz, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5); 9.29 (s, 1H,
NCHN). 13C NMR (101 MHz, CDCl3, 25 °C, TMS): δ (ppm)=16.11 and
20.51 (NCH2C6H(CH3)4–2,3,5,6); 24.24 (NCH2CH2CH2CH2OC6H5); 25.97
(NCH2CH2CH2CH2OC6H5); 46.85 (NCH2CH2CH2CH2OC6H5); 48.15 (NCH2C6H
(CH3)4–2,3,5,6); 48.18 and 48.48 (NCH2CH2N); 66.90 (NCH2CH2CH2CH2OC6H5);
114.37, 120.82, 129.50 and 158.55 (arom. Cs of NCH2CH2CH2CH2OC6H5);
128.09, 132.89, 133.82 and 134.76 (arom. Cs of NCH2C6H(CH3)4–2,3,5,6); 157.21
(NCHN). Elemental analysis calcd. (%) for C214H33BrN2O: C 64.71, H 7.47, N 6.29;
found (%): C 64.89, H 7.53, N 6.70. (For the H NMR, 13C NMR and IR spectrum
of the 3c, see SI fle, pages S9-S10).
1‑(4‑Phenoxybutyl)‑3‑(4‑chlorobenzyl)imidazolinium bromide (3d)
Yield 78%, 1.642 g (white solid); mp: 79–80 °C; FT-IR (vC-N): 1038 and 1241 cm−1;
(vC(2)-N): 1665 cm−1. 1H NMR (400 MHz, CDCl3, 25 °C, TMS): δ (ppm)=1.83–1.86
(m, 2H, NCH2CH2CH2CH2OC6H5); 1.91–1.94 (m, 2H, NCH2CH2CH2CH2OC6H5);
3.71 (t, J=6.9 Hz, 2H, NCH2CH2CH2CH2OC6H5); 3.77–3.82 and 3.92–3.97 (m,
4H, NCH2CH2N); 4.00 (t, J=5.5 Hz, 2H, NCH2CH2CH2CH2OC6H5); 4.88 (s, 2H,
NCH2C6H4(Cl)-4); 6.87 (d, J=8.0 Hz, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5);
6.94 (t, J=7.3 Hz, 1H, arom. H of NCH2CH2CH2CH2OC6H5); 7.27 (t, J=7.9 Hz,
2H, arom. Hs of NCH2CH2CH2CH2OC6H5); 7.34 (d, J=8.0 Hz, 2H, arom. Hs
of NCH2C6H4(Cl)-4); 7.41 (d, J=8.2 Hz, 2H, arom. Hs of NCH2C6H4(Cl)-4);
9.99 (s, 1H, NCHN). 13C NMR (101 MHz, CDCl3, 25 °C, TMS): δ
(ppm)=24.18 (NCH2CH2CH2CH2OC6H5); 26.15 (NCH2CH2CH2CH2OC6H5);
47.73 (NCH2CH2CH2CH2OC6H5); 48.39 and 48.46 (NCH2CH2N); 51.41
(NCH2C6H4(Cl)-4); 66.77 (NCH2CH2CH2CH2OC6H5); 114.42, 120.89, 129.55 and
158.55 (arom. Cs of NCH2CH2CH2CH2OC6H5); 129.43, 130.45, 131.15 and 136.06
(arom. Cs of NCH2C6H4(Cl)-4); 158.37 (NCHN). Elemental analysis calcd. (%) for
C20H24BrClN2O: C 56.69, H 5.71, N 6.61; found (%): C 56.84, H 5.68, N 6.93. (For
the 1H NMR, 13C NMR and IR spectrum of the 3d, see SI fle, pages S11-S12).
1 3

M. Kaloğlu et al.
General procedure for the preparation of palladium‑carbene complexes (4a–4d)
An acetonitrile (10 mL) solution of imidazolinium salts 3a-3d (1 mmol), PdCl2
(0.177 g, 1 mmol), K2CO3 (0.691 g, 5 mmol), KBr (1.190 g, 10 mmol) and pyridine
(0.119 g, 1.5 mmol) was stirred at 80 °C for 16 h. The reaction mixture was then fl-
tered through Celite, the fltrate was evaporated under vacuum, and the solid residue
was purifed by column chromatography (CH2Cl2) to aford the corresponding pal-
ladium-carbene complex. Then, palladium-carbene complexes were recrystallized
from CH2Cl2/n-pentane solvent system (1:5, v/v). The palladium-carbene complexes
4a–4d were isolated as air- and moisture-stable yellow solids in 35–50% yields.
Dibromo‑[1‑(4‑phenoxybutyl)‑3‑(3,5‑dimethylbenzyl)imidazolidin‑2‑ylidene]
(pyridine)palladium(II) (4a)
Yield 46%, 0.312 g (yellow solid); mp: 124–125 °C; FT-IR (vC-N): 1071 and
1
1251 cm−1; (vC(2)-N): 1602 cm−1. H NMR (400 MHz, CDCl3, 25 °C, TMS):
δ (ppm)=1.87–1.94 (m, 2H, NCH2CH2CH2CH2OC6H5); 1.96–2.02 (m, 2H,
NCH2CH2CH2CH2OC6H5); 2.25 (s, 6H, NCH2C6H3(CH3)2–3,5); 3.34–3.40 and
3.56–3.61 (m, 4H, NCH2CH2N); 4.03 (t, J=5.9 Hz, 2H, NCH2CH2CH2CH2OC6H5);
4.14 (t, J=7.3 Hz, 2H, NCH2CH2CH2CH2OC6H5); 5.17 (s, 2H, NCH2C6
H3(CH3)2–3,5); 6.84 (s, 1H, arom. H of NCH2C6H3(CH3)2–3,5); 6.86 (d,
J=6.5 Hz, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5); 7.11 (s, 2H, arom. Hs of
NCH2C6H3(CH3)2–3,5); 7.17–7.19 (m, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5);
7.21 (ddd, J=7.9, 5.2, 1.4 Hz, 2H, arom. Hs of pyridine); 7.64 (tt, J=7.4, 1.5 Hz,
1H, arom. H of pyridine); 8.87 (dd, J=6.3, 1.4 Hz, 2H, arom. Hs of pyridine). 13C
NMR (101 MHz, CDCl3, 25 °C, TMS): δ (ppm)=21.30 (NCH2C6H3(CH3)2–3,5);
24.32 (NCH2CH2CH2CH2OC6H5); 26.51 (NCH2CH2CH2CH2OC6H5); 47.72
(NCH2CH2CH2CH2OC6H5); 48.41 and 50.34 (NCH2CH2N); 54.90 (NCH2C6H3
(CH3)2–3,5); 67.26 (NCH2CH2CH2CH2OC6H5); 114.56, 120.60, 129.45 and 158.98
(arom. Cs of NCH2CH2CH2CH2OC6H5); 126.67, 129.70, 135.10 and 137.79 (arom.
Cs of NCH2C6H3(CH3)2–3,5); 124.49, 138.31 and 152.45 (arom. Cs of pyridine);
180.70 (Pd-Ccarbene). Elemental analysis calcd. (%) for C27H33Br2N3OPd: C 47.56, H
4.88, N 6.16; found (%): C 47.59, H 4.81, N 6.13. (For the 1H NMR, 13C NMR and
IR spectrum of the 4a, see SI fle, pages S13-S14).
Dibromo‑[1‑(4‑phenoxybutyl)‑3‑(4‑isopropylbenzyl)imidazolidin‑2‑ylidene]
(pyridine)palladium(II) (4b)
Yield 42%, 0.289 g (yellow solid); mp: 69–70 °C; FT-IR (vC-N): 1069 and
1
1238 cm−1; (vC(2)-N): 1600 cm−1. H NMR (400 MHz, CDCl3, 25 °C, TMS): δ
(ppm)=1.27 (d, J=6.8 Hz, 6H, NCH2C6H4(CH(CH3)2)-4); 2.02–2.04 (m, 2H,
NCH2CH2CH2CH2OC6H5); 2.05–2.08 (m, 2H, NCH2CH2CH2CH2OC6H5); 2.93
(hept, J=6.8 Hz, 1H, NCH2C6H4(CH(CH3)2)-4); 3.46 and 3.68 (t, J=9.8 Hz, 4H,
NCH2CH2N); 4.13 (t, J=5.3 Hz, 2H, NCH2CH2CH2CH2OC6H5); 4.24 (t, J=6.8 Hz,
2H, NCH2CH2CH2CH2OC6H5); 5.34 (s, 2H, NCH2C6H4(CH(CH3)2)-4); 6.94–6.96
1 3

The frst use of…
(m, 3H, arom. Hs of NCH2CH2CH2CH2OC6H5); 7.25–7.30 (m, 6H, arom. Hs of
NCH2CH2CH2CH2OC6H5 and NCH2C6H4(CH(CH3)2)-4); 7.51 (ddd, J=7.6, 5.0,
1.3 Hz, 2H, arom. Hs of pyridine); 7.74 (tt, J=7.3, 1.4 Hz, 1H, arom. H of pyridine);
8.97 (dd, J=6.5, 1.5 Hz, 2H, arom. Hs of pyridine). 13C NMR (101 MHz, CDCl3, 25 °C,
TMS): δ (ppm)=24.03 (NCH2C6H4(CH(CH3)2)-4); 24.30 (NCH2CH2CH2CH2OC6H5);
26.51 (NCH2CH2CH2CH2OC6H5); 33.88 (NCH2C6H4(CH(CH3)2)-4); 47.65
(NCH2CH2CH2CH2OC6H5); 48.43 and 50.34 (NCH2CH2N); 54.70 (NCH2C6H4(C
H(CH3)2)-4); 67.28 (NCH2CH2CH2CH2OC6H5); 114.58, 120.60, 129.45 and 158.99
(arom. Cs of NCH2CH2CH2CH2OC6H5); 126.80, 128.85, 132.56 and 148.76 (arom.
Cs of NCH2C6H4(CH(CH3)2)-4); 124.49, 137.78 and 152.44 (arom. Cs of pyridine);
180.77 (Pd-Ccarbene). Elemental analysis calcd. (%) for C28H35Br2N3OPd: C 48.33, H
5.07, N 6.04; found (%): C 48.00, H 5.01, N 6.06. (For the 1H NMR, 13C NMR and IR
spectrum of the 4b, see SI fle, pages S15-S16).
Dibromo‑[1‑(4‑phenoxybutyl)‑3‑(2,3,5,6‑tetramethylbenzyl)
imidazolidin‑2‑ylidene](pyridine)palladium(II) (4c)
Yield 35%, 0.247 g (yellow solid); mp: 192–193 °C; FT-IR (vC-N): 1071 and 1250 cm−1;
(vC(2)-N): 1600 cm−1. 1H NMR (400 MHz, CDCl3, 25 °C, TMS): δ (ppm)=1.89–1.94
(m, 2H, NCH2CH2CH2CH2OC6H5); 1.96–1.99 (m, 2H, NCH2CH2CH2CH2OC6H5);
2.17 and 2.26 (s, 12H, NCH2C6H(CH3)4–2,3,5,6); 3.08 and 3.47 (t, J=9.2 Hz,
4H, NCH2CH2N); 4.03 (t, J=5.7 Hz, 2H, NCH2CH2CH2CH2OC6H5); 4.13 (t,
J=7.1 Hz, 2H, NCH2CH2CH2CH2OC6H5); 5.30 (s, 2H, NCH2C6H(CH3)4–2,3,5,6);
6.83–6.87 (m, 3H, arom. Hs of NCH2CH2CH2CH2OC6H5); 6.90 (s, 1H, arom. H of
NCH2C6H(CH3)4–2,3,5,6); 7.16–7.19 (m, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5);
7.23 (ddd, J=7.5, 5.0, 1.3 Hz, 2H, arom. Hs of pyridine); 7.66 (tt, J=7.8, 1.6 Hz, 1H,
arom. H of pyridine); 8.91 (dd, J=6.6, 1.5 Hz, 2H, arom. Hs of pyridine). 13C NMR
(101 MHz, CDCl3, 25 °C, TMS): δ (ppm)=16.54 and 20.53 (NCH2C6H(CH3)4–2,3,5,6);
24.38 (NCH2CH2CH2CH2OC6H5); 26.50 (NCH2CH2CH2CH2OC6H5); 47.79 and 47.87
(NCH2CH2N); 49.30 (NCH2CH2CH2CH2OC6H5); 50.59 (NCH2C6H(CH3)4–2,3,5,6);
67.28 (NCH2CH2CH2CH2OC6H5); 114.56, 120.57, 129.43 and 158.98 (arom. Cs
of NCH2CH2CH2CH2OC6H5); 131.05, 131.84, 134.03 and 134.52 (arom. Cs of
NCH2C6H(CH3)4–2,3,5,6); 124.49, 137.76 and 152.44 (arom. Cs of pyridine); 181.40
(Pd-Ccarbene). Elemental analysis calcd. (%) for C29H37Br2N3OPd: C 49.07, H 5.25, N
5.92; found (%): C 48.42, H 5.32, N 5.87. (For the 1H NMR, 13C NMR and IR spec-
trum of the 4c, see SI fle, pages S17-S18).
Dibromo‑[1‑(4‑phenoxybutyl)‑3‑(4‑chlorobenzyl)imidazolidin‑2‑ylidene]
(pyridine)palladium(II) (4d)
Yield 50%, 0.345 g (yellow solid); mp: 120–121 °C; FT-IR (vC-N): 1071 and
1
1235 cm−1; (vC(2)-N): 1597 cm−1. H NMR (400 MHz, CDCl3, 25 °C, TMS):
δ (ppm)=1.89–1.95 (m, 2H, NCH2CH2CH2CH2OC6H5); 1.97–2.02 (m, 2H,
NCH2CH2CH2CH2OC6H5); 3.35 and 3.61 (dd, J=11.5, 8.4 Hz, 4H, NCH2CH2N);
4.03 (t, J=5.8 Hz, 2H, NCH2CH2CH2CH2OC6H5); 4.14 (t, J=7.3 Hz, 2H,
1 3

M. Kaloğlu et al.
NCH2CH2CH2CH2OC6H5); 5.24 (s, 2H, NCH2C6H4(Cl)-4); 6.84–6.88 (m, 3H,
arom. Hs of NCH2CH2CH2CH2OC6H5); 7.20 (dd, J=15.9, 8.2 Hz, 4H, arom. Hs of
NCH2C6H4(Cl)-4); 7.27 (d, J=8.4 Hz, 2H, arom. Hs of NCH2CH2CH2CH2OC6H5);
7.45 (ddd, J=8.3, 5.1, 1.4 Hz, 2H, arom. Hs of pyridine); 7.66 (tt, J=7.6, 1.5 Hz,
1H, arom. H of pyridine); 8.86 (dd, J=6.3, 1.4 Hz, 2H, arom. Hs of pyridine). 13C
NMR (101 MHz, CDCl3, 25 °C, TMS): δ (ppm)=24.28 (NCH2CH2CH2CH2OC6H5);
26.49 (NCH2CH2CH2CH2OC6H5); 47.61 (NCH2CH2CH2CH2OC6H5); 48.52 and
50.37 (NCH2CH2N); 54.29 (NCH2C6H4(Cl)-4); 67.22 (NCH2CH2CH2CH2OC6H5);
114.56, 120.63, 129.46 and 158.95 (arom. Cs of NCH2CH2CH2CH2OC6H5); 128.96,
130.25, 133.80 and 133.98 (arom. Cs of NCH2C6H4(Cl)-4); 124.54, 137.86 and
152.43 (arom. Cs of pyridine); 181.38 (Pd-Ccarbene). Elemental analysis calcd. (%)
for C25H28Br2ClN3OPd: C 43.63, H 4.10, N 6.11; found (%): C 43.27, H 4.16, N
1
6.12. (For the H NMR, 13C NMR and IR spectrum of the 4d, see SI fle, pages
S19-S20).
X‑Ray analysis of the complex 4d
X-ray data were collected with a STOE IPDS II difractometer at room temperature
using graphite-monochromated Mo Kα radiation by applying the ω-scan method.
Data collection and cell refnement were carried out using X-AREA [72] while data
reduction was applied using X-RED32 [72]. The structure was solved using the
charge-fipping algorithm by SUPERFLIP [73] and refned by means of the full-
matrix least-squares calculations on F2 using SHELXL-2018 [74]. All H atoms were
placed geometrically and treated using a riding model, fxing the bond lengths at
0.93 and 0.97 Å for CH and CH2 atoms, respectively. The displacement parameters
of the H atoms were fxed at Uiso (H)=1.2Ueq of their parent atoms. The phenoxy-
butyl moiety in one of two molecules in the asymmetric unit of the compound was
disordered over two positions with the occupancy factors of 0.520(5)/0.480(5)%.
Crystal data, data collection and structure refnement details are collected in Table 5.
Molecular diagram was drawn by using OLEX2 [75].
General procedure for the direct C5‑arylation of furan and thiophene
with (hetero)aryl halides
An oven dried Schlenk fask was charged with palladium catalyst (0.01 equiv.,
1 mol%), 2-acetylfuran or 2-acetylthiophene (0.5 mmol, 2 equiv.), (hetero)aryl hal-
ide derivative (0.25 mmol, 1 equiv.), KOAc (0.5 mmol, 2 equiv.) and DMA (2 mL)
under argon atmosphere. Then, the reaction mixture was stirred at 120 °C for difer-
ent durations, as given in Table 4. Completion of the reaction, the solution cooled to
room temperature, and CH2Cl2 (1 mL) was added to Schlenk tube to dilute the solu-
tion. The solution fltered through a pad of celite to remove the solid particles, then,
was used for GC analysis. The yields (%) were calculated according to (hetero)aryl
halide by GC analysis.
1 3

The frst use of…
Table 5 Crystal data and structure refnement parameters for complex 4d
CCDC depository
2,050,747
Color/shape
Dark yellow/block
[PdBr2(C20H23ClN2O)(C5H5N)]
688.17
296(2)
0.71073 Mo Kα
Triclinic
Chemical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
P 1 (No. 2)
Unit cell parameters
a, b, c (Å)
α, β, γ (°)
9.0803(4), 13.8072(6), 22.2565(10)
73.515(4), 88.742(4), 81.980(4)
2649.0(2)
4
1.726
3.841
Volume (Å3)
Z
Dcalc. (g/cm3)
μ (mm−1
)
Absorption correction
Tmin., Tmax
Integration
0.1897, 0.5027
1360
0.75×0.29×0.20
STOE IPDS II
ω scan
−11≤h≤11,−18≤k≤18,−28≤l≤29
1.909≤θ≤27.754
59,552
F000
Crystal size (mm3)
Difractometer
Measurement method
Index ranges
θ range for data collection (°)
Refections collected
Independent/observed refections
Rint
Refnement method
Data/restraints/parameters
Goodness-of-ft on F2
Final R indices [I>2σ(I)]
R indices (all data)
Δρmax., Δρmin. (e/Å3)
12,375/7782
0.0840
Full-matrix least-squares on F2
12,375/409/686
0.978
R1 =0.0495, wR2 =0.0811
R1 =0.0962, wR2 =0.0923
0.83,−0.58
CRediT authorship contribution statement
Murat Kaloğlu contributed to conceptualization, methodology, investigation, writ-
ing-original draft, writing-review and editing, funding acquisition. Nazan Kaloğlu
contributed to conceptualization, writing-original draft, methodology, investiga-
tion. Namık Özdemir contributed to software, writing-original draft, investigation.
İsmail Özdemir contributed to conceptualization, resources, writing-original draft,
supervision.
1 3

M. Kaloğlu et al.
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04444-4.
Acknowledgements This study was supported by the Technological and Scientifc Research Coun-
cil of Turkey TÜBİTAK (Project No: 119R003) and Ondokuz Mayıs University (Project No: PYO.
FEN.1906.19.001).
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