Synthesis of N-heterocyclic carbene-palladium-PEPPSI complexes and their catalytic activity in the direct C-H bond activation

Article abstract of DOI:10.1016/j.jorganchem.2017.10.019

A series of benzimidazolium salts having their two nitrogen atoms substituted by bulky groups have been synthesized. The benzimidazolium salts were readily converted into the corresponding palladium-NHC-PEPPSI complexes with general formula [PdBr₂(NHC) (Py)], (NHC = N-heterocyclic carbene; PEPPSI = pyridine-enhanced precatalyst preparation, stabilisation, and initiation). The structures of all new compounds were characterized by NMR, IR spectroscopy and microanalysis techniques, which support the proposed structures. The molecular structure of complex 2g was determined by single-crystal X-ray diffraction study. Next, the palladium-NHC-PEPPSI complexes were used as catalysts in the direct C5-arylation of 1-methylpyrrole-2-carboxaldehyde by aryl halides. These complexes exhibited moderate to high catalytic activities and gave C-H activation selectively at the C5-position of 1-methylpyrrole-2-carboxaldehyde. Both electron-donating and electron-withdrawing substituents were well tolerated with catalytic systems based on these complexes, even non-activated aryl chlorides such as chlorobenzene or 4-chlorotoluene were coupled with pyrrole in moderate yields.

Full text of DOI:10.1016/j.jorganchem.2017.10.019

Accepted Manuscript
Synthesis of N-Heterocyclic carbene-palladium-PEPPSI complexes and their catalytic
activity in the direct C-H bond activation
Nazan Kaloğlu, Murat Kaloğlu, Muhammad Nawaz Tahir, Cengiz Arıcı, Christian
Bruneau, Henri Doucet, Pierre H. Dixneuf, Bekir Çetinkaya, İsmail Özdemir
PII:
S0022-328X(17)30604-6
10.1016/j.jorganchem.2017.10.019
JOM 20139
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 27 August 2017
Revised Date: 6 October 2017
Accepted Date: 14 October 2017
Please cite this article as: N. Kaloğlu, M. Kaloğlu, M.N. Tahir, C. Arıcı, C. Bruneau, H. Doucet, P.H.
Dixneuf, B. Çetinkaya, İ. Özdemir, Synthesis of N-Heterocyclic carbene-palladium-PEPPSI complexes
and their catalytic activity in the direct C-H bond activation, Journal of Organometallic Chemistry (2017),
doi: 10.1016/j.jorganchem.2017.10.019.
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ACCEPTED MANUSCRIPT
Synthesis of N-Heterocyclic Carbene-Palladium-PEPPSI Complexes
and Their Catalytic Activity in The Direct C-H Bond Activation
[
a,b]
[a,b]
[e]
[c]
[d]
[e]
Nazan Kaloğlu*
Murat Kaloğlu,
Muhammad Nawaz Tahir, Cengiz Arıcı, Christian Bruneau,
[
e]
[f] [a,b]
Henri Doucet, Pierre H. Dixneuf, Bekir Çetinkaya and İsmail Özdemir*
a
İnönü University, Faculty of Science and Arts, Department of Chemistry, 44280 Malatya, Turkey
b
İnönü University, Catalysis Research and Application Center, 44280 Malatya, Turkey
c
University of Sargodha, Department of Physics, Sargodha, Pakistan
d
The Scientific and Technological, Research and Council of Turkey, 06100 Ankara, Turkey
e
Université de Rennes 1, Institut des Sciences Chimiques de Rennes, 35042 Rennes, France
f
Ege University, Faculty of Science, Department of Chemistry, 35100 İzmir, Turkey
ABSTRACT: A series of benzimidazolium salts having their two nitrogen atoms substituted by bulky groups have been
synthesized. The benzimidazolium salts were readily converted into the corresponding palladium-NHC-PEPPSI complexes with
2
general formula [PdBr (NHC)(Py)], (NHC = N-heterocyclic carbene; PEPPSI = pyridine-enhanced precatalyst preparation,
stabilisation, and initiation). The structures of all new compounds were characterised by NMR, IR spectroscopy and
microanalysis techniques, which support the proposed structures. The molecular structure of complex 2g was determined by
single-crystal X-ray diffraction study. Next, the palladium-NHC-PEPPSI complexes were used as catalysts in the direct C5-
arylation of 1-methylpyrrole-2-carboxaldehyde by aryl halides. These complexes exhibited moderate to high catalytic activities
and gave C-H activation selectively at the C5-position of 1-methylpyrrole-2-carboxaldehyde. Both electron-donating and
electron-withdrawing substituents were well tolerated with catalytic systems based on these complexes, even non-activated aryl
chlorides such as chlorobenzene or 4-chlorotoluene were coupled with pyrrole in moderate yields.
Keywords: N-heterocyclic carbene, benzimidazolium salts, PEPPSI-type palladium-NHC complexes, pyrroles, direct arylation.
*Corresponding authors
Tel and Fax: +904223410212
E-mail address: ismail.ozdemir@inonu.edu.tr (İ. Özdemir)
nzntemelli@hotmail.com (N. Kaloğlu)
https://www.inonu.edu.tr/

ACCEPTED MANUSCRIPT
1. Introduction
In recent years, NHCs have become a very important class of ligands in organometallic chemistry and catalysis
1-3]. The strong σ-donating but poor π-accepting ability of these NHCs lead to the formation of many stable metal-
[
carbene complexes. For this reason, the metal-NHC complexes have been widely used as highly reactive and rather
selective catalysts for numerous chemical transformations [4-13]. Metal-NHCs have also been utilized extensively in
medicinal applications [14-19]. Nowadays, many research groups around the world are currently focusing on metal-NHC
complexes for various purposes.
Palladium-catalyzed bond-forming reactions are one of the most common tools synthetic chemists employ, in
industry as well as in academia [20-23]. The strong palladium-NHC bonds contribute to the high stability of the active
species, even at low catalyst loading and high temperatures. With a number of commercially available, stable, user-
friendly, and powerful palladium-NHC catalysts, the goal of a universal cross-coupling catalyst is within reach. In
particular, palladium-catalyzed cross-coupling reactions are now widely used to form C-C and C-N bond formation. Such
processes include the Suzuki-Miyaura [24], Mizoroki-Heck [25], Negishi [26], Hiyama [27], Sonogashira [28], amination
[
29], and arylation of arenes reactions [30,31], often involving coupling of an aryl halide with a nucleophilic parthner.
Pyrrole is a very important backbone that is often present in the pharmaceuticals and biologically active
compounds. As selected examples, Lipitor is the leading cholesterol lowering drug [32], Norbinaltorphimine is an opioid
antagonist used in scientific research [33], Tanaproget is a progesterone-receptor agonist [34], Dopamine antagonist is a
type of drug which blocks dopamine receptors by receptor antagonism [35], (Figure 1). Considering the importance of
pyrroles in the pharmaceutical industry, straightforward and eco-friendly preparation of these compounds still represents
a challenging task for chemists.
Figure 1. Examples of biologically active pyrrole derivatives.
The palladium-catalyzed direct arylation of pyrroles by a C-H bond activation using aryl halides has met great
success in recent years [36]. Such arylations are known to occur preferentially at the α-positions to the nitrogen atom
following the typical reactivity profiles of the pyrrole ring. Thus, under direct arylation conditions unsubstituted pyrroles
react at C2- and(or) C5-position.
Ohta and co-workers reported that the direct C2- or C5-arylation of several heteroaromatics with aryl halides
3 4
through C-H bond activation proceeds in moderate to good yields using [Pd(PPh ) ] as catalyst [37]. Since these exciting
results, the palladium-catalyzed direct arylation of various heteroaromatics with aryl halides has proved to be a powerful
method for the synthesis of a wide variety of arylated heterocycles [38]. In this connection, recently we have prepared
novel palladium-bis(NHC) complexes and have used they as effective catalysts in the direct arylation of pyrrole

ACCEPTED MANUSCRIPT
derivatives with aryl chlorides [39]. The general structure of palladyum-bis(NHC) complexes we used is shown in
Scheme 1.
Scheme 1. General structure of recently reported palladium-bis(NHC) complexes.
Up to now, examples of palladium-catalyzed direct arylations of pyrroles by using aryl halides have been
reported [36], but no reactions were found using palladium-NHC-PEPPSI catalysts in the literature. For this reason, we
2
synthesized a series of new palladium-NHC-PEPPSI complexes with the general formula [PdBr (NHC)(Py)], (2a-h). All
1
13
new palladium-NHC-PEPPSI complexes were characterized by H NMR, C NMR, IR spectroscopy and elemental
analysis techniques. Next, palladium-NHC-PEPPSI complexes were tested for direct C5-arylation of 1-methylpyrrole-2-
carboxaldehyde with aryl bromides and aryl chlorides. The reactive C2-position of pyrrole was blocked in order to
maximize the yields of the monoarylated products.
2. Results and discussion
2.1. Synthesis of benzimidazolium salts
The benzimidazolium salts (1a-h) have already been reported in the literature [40]. These salts were prepared
according to general reaction pathway depicted in Scheme 2. The structures of the salts were determined by their
characteristic spectroscopic data and elemental analyses. The results are consistent with the literature [40].
Scheme 2. Synthesis of the benzimidazolium salts (1a-h).

ACCEPTED MANUSCRIPT
2.2. Synthesis of palladium-NHC-PEPPSI complexes
The general procedure for the preparation of PEPPSI-type palladium-NHC complexes (2a-h) according to the
method reported by Organ is shown in Scheme 3 [41]. Benzimidazolium salts (1a-h) were incorporated into the PEPPSI-
2 2 3
type palladium-NHC complexes (2a-h) by reaction with PdCl in refluxing pyridine in the presence of K CO as a base
1
and a large excess of KBr for 16 h. For the H NMR spectra of the palladium-NHC-PEPPSI complexes (2a-h), sharp
peaks in the lower field region belonging to the acidic imino proton of benzimidazolium salts (NCHN) were not observed
1
3
between δ = 10.0-12.0 ppm. Similarly, in the C NMR spectra, imino carbon of benzimidazolium salts (NCHN) were not
1
13
observed between δ = 150-155 ppm. In the H NMR and C NMR spectra of the complexes 2a-h, loss of the the
characteristic peak of the acidic imino proton (NCHN) and imino carbon (NCHN), signal suggests the formation of the
palladium-NHC-PEPPSI complexes. In addition, the characteristic carbenic carbons in compounds 2a-h appeared in the
1
3
C NMR spectra as deshielded singlets at 163.1, 162.7, 164.2, 164.1, 163.8, 162.8, 164.1 and 163.1 ppm, respectively.
The IR data also clearly indicated the presence of (CN) vibration at 1412, 1416, 1411, 1410, 1410, 1416, 1414 and 1411
-1
-1
cm for 2a-h. The formation of carbenes is correlated by a shift of the (CN) vibration from 1551-1564 cm in the
-1
benzimidazolium salts to 1410-1416 cm in the coordinated carbenes [40].
Scheme 3. Synthesis of the palladium-NHC-PEPPSI complexes (2a-h).

ACCEPTED MANUSCRIPT
2.3. Structural characterization of complex 2g
Single crystals of complex 2g suitable for diffraction study were obtained by slow diffusion of n-pentane into a
dichloromethane solution of complex 2g. The molecular structure of complex 2g was confirmed by single-crystal X-ray
diffraction analysis. This complex crystallizes in a centrosymmetric monoclinic P2 /n system and adopts a square-planar
1
geometry. The carbene and the pyridine ligands are in the trans-position, with respective distances to the palladium
3
center of 1.952(7) and 2.087(6) Å. Cell volume and density of the single crystal was calculated as 3853.1(6) Å and
-3
1
.400 g.cm . The molecular structure of complex 2g is shown in Figure 2, and selected bond lengths and angles are
summarized in Table 1.
Figure 2. Perspective view of the molecular structure of 2g.
Table 1. Selected bond lengths [Å] and angles [°] for complex 2g.
Bond lengths
Bond angles
177.4(3)
Pd1-C6
Pd1-N1
Pd1-Br1
Pd1-Br2
N1-C1
N1-C5
N2-C6
N2-C13
N3-C6
N3-C28
1.952(7)
2.087(6)
2.426(1)
2.420(1)
1.320(1)
1.330(1)
1.341(8)
1.451(9)
1.364(9)
1.470(8)
C6-Pd1-N1
C6-Pd1-Br1
C6-Pd1-Br2
N1-Pd1-Br1
N1-Pd1-Br2
Br1-Pd1-Br2
C5-N1-Pd1
C1-N1-Pd1
N2-C6-Pd1
N3-C6-Pd1
89.1(2)
88.8(2)
91.9(2)
90.4(2)
174.37(4)
121.9(5)
121.4(6)
127.6(5)
125.7(5)

ACCEPTED MANUSCRIPT
2.4. Direct C5-arylation of 1-methylpyrrole-2-carboxaldehyde
In order to screen the experimental conditions, we selected the complex 2h as the catalyst. We also selected
the 4-bromoacetophenone and 4-chloroacetophenone as the model coupling partner. The results of the reaction
parameters including solvent, base, temperature and catalyst loading are gathered in Table 2. The chemical
characterizations of the products were made by GC and GC-MS. The conversions were based on the aryl halide by GC.
Table 2. Influence of the reaction conditions for palladium-NHC-PEPPSI catalyzed direct C5-arylation of 1-methylpyrrole-2-
[
a]
carboxaldehyde with 4-bromoacetophenone and 4-chloroacetophenone.
Pd-NHC-PEPPSI 2h
o
[b]
Entry
X
Solvent
Base
Time [h]
Temp. [ C]
Yield [%]
[mol-%]
1
2
3
4
5
6
7
8
9
5
5
5
5
5
5
5
5
5
5
5
5
5
1
0.5
1
1
1
1
1
1
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
DMAc
NMP
Toluene
DMAc
NMP
Toluene
DMAc
NMP
Toluene
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
Cs
Cs
Cs
2
2
2
CO
CO
CO
3
2
2
2
2
2
2
2
2
2
2
2
1
0.5
1
1
1
2
150
150
120
150
150
120
150
150
120
120
90
120
120
120
120
120
120
120
120
120
120
57
41
34
49
37
21
93
70
52
93
71
92
53
90
69
-
3
3
KOPiv.
KOPiv.
KOPiv.
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
10
11
12
13
14
15
16
17
18
19
20
21
-
5
16
42
74
76
10
15
20
[a] Conditions: 1-methylpyrrole-2-carboxaldehyde (2.0 mmol), aryl halide (1.0 mmol), base (1.5 mmol), solvent (2 mL). [b] Yields were calculated
according to aryl halide by GC and GC-MS.
2 3
When NMP or toluene were used as solvent, the reaction gave low yield of only 21-70% with Cs CO , KOPiv.
o
or KOAc as base after 2 h at 120-150 C (Table 2, entries 2, 3, 5, 6, 8, 9). DMAc proved to be the best tested solvent
o
with all bases after 2 h at 150 C (Table 2, entries 1, 4, 7). In this solvent decreasing the reaction temperature from
1
50 °C to 120 °C had no effect on the yield (Table 2, entry 10), but decreasing the reaction temperature from 120 °C to
0 °C had detrimental effect on the yield (Table 2, entry 11). When the reaction time was reduced from 2 h to 1 h no
9
noticeable effect on the yield was observed (Table 2, entry 12), but when the reaction time was reduced to 0.5 h, the
yield dropped to 53% (Table 2, entry 13). In the presence of 1 mol% of 2h as the catalyst, KOAc as the base, DMAc as
the solvent and 4-bromoacetophenone as the coupling partner at 120 °C for 1 h, the C5-arylated product was obtained
in 90% isolated yield (Table 2, entry 14), but when the catalyst loading was reduced from 1 mol% to 0.5 mol%, the
yield decreased to 69% (Table 2, entry 15). Finally, the best conditions leading to 90% yield of 4-bromoacetophenone
with high selectivity in favor of the C5-arylated product were obtained when the reaction was carried out in DMAc in the
o
presence of 1.5 equiv. of KOAc at 120 C for 1 h (Table 2, entry 14). The DMAC / KOAc combination is commonly
used for C-H arylation of 5 membered heterocycles. For this reason, the conditions are also consistent with the
b-d
literature [36 ].
When the less reactive 4-chloroacetophenone was used as substrate in the presence of 1 mol% of catalyst 2h
o
and KOAc as the base at 120 C, almost no conversion was observed after 2 h (Table 2, entry 16 and 17). When the
reaction time was further increased to 20 h, the conversion was improved indicating that the catalysts were still active
reaching a yield of 76% after 20 h.

ACCEPTED MANUSCRIPT
Finally, the scope of the direct C5-arylation of 1-methylpyrrole-2-carboxaldehyde was investigated with various
aryl halides, including five aryl bromides and five aryl chlorides (Table 3) applying our best experimental conditions. Only
a minor effect of the nature of the carbene ligand on the palladium-NHC-PEPPSI complex was observed for the coupling
of aryl bromides with 1-methylpyrrole-2-carboxaldehyde. At elevated temperature, the oxidative addition of the aryl
bromide to palladium is generally easy and does not require the use of very specific ligands. A more important effect of
the nature of the ligand was expected for the coupling of aryl chlorides with the 1-methylpyrrole-2-carboxaldehyde.
Indeed, again in all cases the coupling products were produced in good yields. Surprisingly, similar yields were obtained
for the coupling of electron-deficient aryl chlorides (Table 3, entries 9-24) as with chlorobenzene (Table 3, entries 1-8). In
this case, the presence of the NHC-ligands on palladium likely favors the oxidative addition step.
[
a,b]
Table 3. Direct C5-arylation of 1-methylpyrrole-2-carboxaldehyde by using aryl halides.
[
c]
Yield [%]
Entry
Aryl halide
Catalyst
Product
X = Br
X = Cl
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
2g
2h
2a
2b
2c
2d
2e
2f
62
54
43
75
70
68
72
77
47
59
47
67
56
61
67
88
38
47
41
79
59
68
79
90
50
41
34
66
56
59
64
69
40
43
29
64
50
57
60
68
70
65
51
67
63
61
70
73
59
57
43
65
70
59
63
72
59
67
61
71
61
70
72
74
57
62
42
61
54
63
60
67
51
48
32
53
48
54
62
63
10
11
12
13
14
15
16
2g
2h
17
18
19
20
21
22
23
24
2a
2b
2c
2d
2e
2f
2g
2h
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
2a
2b
2c
2d
2e
2f
2g
2h
2a
2b
2c
2d
2e
2f
2g
2h
[
a] Conditions: Pd-NHC-PEPPSI 2a-h (0.01 mmol), 1-methylpyrrole-2-carboxaldehyde (2.0 mmol), aryl halide (1.0 mmol), KOAc (1.5 mmol), DMAc
2 mL), 120 °C. [b] Reaction time: 1 h for aryl bromides, 15 h for aryl chlorides. [c] Yields were calculated according to aryl halide by GC and GC-
MS.
(

ACCEPTED MANUSCRIPT
3. Conclusion
In conclusion, we have synthesized and characterized eight benzimidazolium salts according to literature [40],
and their corresponding new Pd-NHC-PEPPSI complexes. The catalytic activity of the complexes was investigated in the
direct C5-arylation of 1-methylpyrrole-2-carboxaldehyde by aryl halides. The Pd-NHC-PEPPSI complexes were all found
to be suitable catalysts for the direct C5-arylation of 1-methylpyrrole-2-carboxaldehyde by aryl bromides. With Pd-NHC-
PEPPSI complexes, even non-activated aryl chlorides (such as chlorobenzene or 4-chlorotoluene) were coupled with
1-methylpyrrole-2-carboxaldehyde in moderate to good yields. This good performance possibly arises from strong steric
interactions between the bulky 3,5-di-tert-butylbenzyl substituted NHC ligand and the catalytic centre, which favours
reductive elimination. We can say that there is no significant difference between these complexes on the catalytic activity
of C5-arylation of 1-methylpyrrole-2-carboxaldehyde by aryl halides. The significant difference between 2d and 2c indicates
that electronic effects are also playing some role in these processes. Finally, as the major by-products are KOAc
associated to HBr instead of metallic salts, this procedure is environmentally more attractive than the classical coupling
procedures.
4. Experimental
4.1. General methods
All manipulations were carried out under argon using standard Schlenk line techniques. Chemicals and
solvents were purchased from Sigma-Aldrich Co. (Poole, Dorset, UK). The solvents used were purified by distillation
over the drying agents indicated and were transferred under argon. Pd-NHC-PEPPSI complexes were prepared
according to known methods in the literature [41]. DMAc analytical grade (99%) was not distilled before use. KOAc
(
99%) was employed. Elemental analyses were performed by İnönü University Scientific and Technological Research
Center (Malatya, Turkey). Melting points were measures in open capillary tubes with an Electrothermal-9200 melting
-1
points apparatus. IR spectra were recorded on ATR unit in the range of 400-4000 cm with Perkin Elmer Spectrum 100
1
13
Spectrofotometer. H NMR and C NMR spectra were recorded using a Bruker Avance AMX spectrometer operating at
1
13
4
00 MHz ( H NMR) and at 100 MHz ( C NMR) in CDCl
3
. The NMR studies were carried out in high-quality 5 mm NMR
tubes. The chemical shifts (δ) are reported in ppm relative to CDCl
3
. Coupling constants (J values) are given in hertz.
1
NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, m = multiplet. H NMR spectra are
13
referenced to residual protiated solvents (δ = 7.26 ppm for CDCl
3
), C chemical shifts are reported relative to
deuteriated solvents (δ = 77.16 ppm for CDCl ). All catalytic reactions were monitored on an Agilent 6890N GC and
3
Schimadzu 2010 Plus GC-MS system by GC-FID with an HP-5 column of 30 m length, 0.32 mm diameter, and 0.25 µm
film thickness.
4.2. General procedure for the preparation of benzimidazolium salts (1a-h)
For the preparation of benzimidazolium salts (1a-h), N-(3,5-di-tert-butylbenzyl)benzimidazole, (1.0 mmol) was
dissolved in anhydrous dimethylformamide (DMF), (3 mL) and alkyl bromide (1.0 mmol) was added at room temperature.
The reaction mixture was stirred at 80 °C for 24 h under argon. After completion of the reaction, the DMF was removed
by vacuum and diethyl ether (15 mL) was added to obtain a white crystalline solid, which was filtered off. The solid was
washed with diethyl ether (3×10 mL) and dried under vacuum. The crude product was recrystallized from ethanol/diethyl
ether mixture (1:2, v/v) and completely dried under vacuum. These known compounds were synthesized and
1
13
characterized by H NMR, C NMR, IR and elemental analysis techniques. The results we found, are consistent with the
literature [40].

ACCEPTED MANUSCRIPT
4.3. General procedure for the preparation of palladium-NHC-PEPPSI complexes (2a-h)
Benzimidazolium salts (1a-h) (1.0 mmol) were converted, with moderated yields, into the palladium-NHC-
PEPPSI complexes (2a-h) by reaction with PdCl
2
2 3
(1.0 mmol) in refluxing pyridine in the presence of K CO (5.0 mmol)
o
as a base and a large excess of KBr and the mixture heated with vigorous stirring at 80 C for 16 h. The reaction mixture
2 2 2
was diluted with CH Cl then filtered through a pad of celite and silica gel to remove the unreacted PdCl and
benzimidazolium salt. The solvent was removed under vacuum. The pure complexes were washed with n-pentane (3×5
mL) and completely dried under vacuum. All complexes were isolated as air-stable yellow solids and were isolated in 58-
87% yields.
Dibromo-[1-(3,5-di-tert-butylbenzyl)-3-(2-methoxyethyl)benzimidazole-2-ylidene](pyridine)palladium(II), (2a)
o
1
Yield: 0.27 g, 77%; mp: 206-207 C. H NMR (400 MHz, CDCl
3
): δ 1.21 (s, 18H, NCH
); 5.01 (t, J = 6.7 Hz, 2H, NCH
)-3,5); 6.93-7.71 and 8.98-9.00 (m, 12H, NC N, NCH (C(CH -3,5 and NC
): δ 31.5 (NCH (C(CH )-3,5); 35.0 (NCH (C(CH )-3,5); 48.8 (NCH CH
-3,5); 59.2 (NCH CH OCH ); 71.4 (NCH CH OCH ); 111.4, 111.5, 121.9, 122.5, 122.9, 123.0,
24.6,125.0, 133.9, 134.3, 136.0, 138.0, 138.4, 151.3, 152.7, 154.4 (NC N, NCH (C(CH )-3,5 and NC );
OPd: C, 49.78; H, 5.43; N, 5.80. Found: C, 49.81;
2
C
6
H
3
(C(CH
3
)
3
)
2
)-3,5);
3
.30 (s, 3H, NCH
2
CH
2
OCH
3
); 4.17 (t, J = 6.7 Hz, 2H, NCH
2
CH
2
OCH
3
2
CH OCH
2
3
); 6.09 (s,
1
3
2
H, NCH (C(CH
2
C
6
H
3
3
)
3
)
2
6
H
4
2
C
6
H
3
3
)
3
)
2
5
H
5
).
C
NMR (100 MHz, CDCl
3
2
C
6
H
3
3
)
3
)
2
2
C
6
H
3
3
)
3
)
2
2
2
OCH
3
);
5
1
1
4.4 (NCH
2
C
6
H
3
(C(CH
3
)
3
)
2
2
2
3
2
2
3
6
H
4
2
C
6
H
3
3
)
3
)
2
5 5
H
-1
2 3
63.1 (Pd-Ccarbene). IR (cm ) v(CN): 1412; Anal. Calcd. for C30H39Br N
H, 5.50; N, 5.85.
Dibromo-[1-(3,5-di-tert-butylbenzyl)-3-(2,2-dimethoxyethyl)benzimidazole-2-ylidene](pyridine)palladium(II), (2b)
o
1
Yield: 0.21 g, 69%; mp: 119-120 C. H NMR (400 MHz, CDCl
.44 (s, 6H, NCH CH(OCH ); 4.90 (d, J = 5.5 Hz, 2H, NCH CH(OCH
.10 (s, 2H, NCH (C(CH -3,5); 6.94-7.70 and 8.96-9.00 (m, 12H, NC
C NMR (100 MHz, CDCl ): δ 30.4 (NCH (C(CH -3,5); 33.9 (NCH
3.4 (NCH (C(CH -3,5); 55.3 (NCH CH(OCH ); 103.5 NCH CH(OCH
23.6, 132.8, 133.1, 135.0, 137.0, 150.3, 151.6 (NC N, NCH (C(CH
Pd: C, 49.39; H, 5.48; N, 5.57. Found: C, 49.41; H, 5.50; N, 5.59.
3
): δ 1.21 (s, 18H, NCH
); 5.48 (t, J = 5.5 Hz, 1H, NCH
N, NCH (C(CH -3,5 and NC
(C(CH -3,5); 50.0 (NCH CH(OCH
2
C
6
H
3
(C(CH
3
)
3
)
2
-3,5);
3
6
2
3
)
2
2
3
)
2
2
CH(OCH
3
)
2
);
2
C
6
H
3
3
)
3
)
2
6
H
4
2
C
6
H
3
3
)
3
)
2
5
H
5
).
);
1
3
3
2
C
6
H
3
3
)
3
)
2
2
C
6
H
3
3
)
3
)
2
2
3
)
2
5
1
2
C
6
H
3
3
)
3
)
2
2
3
)
2
2
3
)
2
); 110.3, 110.9, 120.8, 121.8, 121.9,
6
H
4
2
C
6
H
3
3
)
3
)
2
-3,5 and NC ); 162.7 (Pd-Ccarbene). IR
5 5
H
-1
(
cm ) v(CN): 1416; Anal. Calcd. for C31
H
41Br
2
N
3
O
2
Dibromo-[1-(3,5-di-tert-butylbenzyl)-3-(3-methylbenzyl)benzimidazole-2-ylidene](pyridine)palladium(II), (2c):
o
1
Yield: 0.19 g, 58%; mp: 208-209 C. H NMR (400 MHz, CDCl
.25 (s, 3H, NCH (CH )-3); 6.12 (s, 2H, NCH (C(CH -3,5); 6.15 (s, 2H, NCH
.96-8.97 (m, 16H, (NC N, NCH (CH )-3, NCH (C(CH
)-3); 31.5 (NCH (C(CH -3,5); 35.0 (NCH
3
): δ 1.22 (s, 18H, NCH
(CH
). C NMR (100 MHz, CDCl
3 3 2
-3,5); 53.5 (NCH (C(CH ) ) -
2
C
6
H
3
(C(CH
)-3); 6.95-7.68 and
): δ
3 3 2
) ) -3,5);
2
8
2
3
2
C
6
H
4
3
2
C
6
H
3
)
3 3
)
2
2
C
6
H
4
3
13
6
H
4
2
C
6
H
4
3
2
C
6
H
3
3
)
3
)
2
-3,5 and NC
5
H
5
3
1.4 (NCH
2
C
6
H
4
(CH
3
2
C
6
H
3
3
)
3
)
2
2
C
6
H
3
(C(CH
3
)
3
)
2
2
C
6
H
3
,5); 54.4 (NCH
2
C
6
H
4
(CH
3
)-3); 111.4, 111.7, 122.4, 122.8, 122.9, 123.0, 124.5, 125.1, 128.6, 128.8, 134.0, 134.7, 134.8,
-
135.0, 137.9, 138.6, 151.4, 152.7 (NCH
2
C
6
H
4
3 2 6 3 3 3 2 5 5
(CH )-3, NCH C H (C(CH ) ) -3,5 and NC H ); 164.2 (Pd-Ccarbene). IR (cm
1
)
v(CN): 1411; Anal. Calcd. for C35
H41Br
2
N
3
Pd: C, 54.60; H, 5.37; N, 5.46. Found: C, 54.63; H, 5.40; N, 5.49.
Dibromo-[1-(3,5-di-tert-butylbenzyl)-3-(3-methoxybenzyl)benzimidazole-2-ylidene](pyridine)palladium(II), (2d)
o
1
Yield: 0.26 g, 78%; mp: 136-137 C. H NMR (400 MHz, CDCl
.68 (s, 3H, NCH (OCH )-3); 6.14 (s, 2H, NCH (C(CH -3,5); 6.15 (s, 2H, NCH
and 8.96-8.99 (m, 16H, NC N, NCH (OCH )-3, NCH (C(CH -3,5 and NC
CDCl ): δ 31.4 (NCH (C(CH -3,5); 35.0 (NCH (C(CH -3,5); 53.7 (NCH
(OCH )-3); 55.7 (NCH (OCH )-3); 111.4, 111.7, 112.8, 115.0, 120.3, 122.4, 123.0, 124.6, 129.7,133.9,
(OCH )-3, NCH (C(CH -3,5 and NC ); 164.1 (Pd-
OPd: C, 53.49; H, 5.26; N, 5.35. Found: C, 53.53; H, 5.32; N,
3
): δ 1.22 (s, 18H, NCH
2
C
6
H
3
(C(CH
3 3 2
) ) -3,5);
3
2
C
6
H
4
3
2
C
6
H
3
3
)
3
)
2
2
C
6
H
4
(OCH
3
3
)-3); 6.95-7.67
1
6
H
4
2
C
6
H
4
3
2
C
6
H
3
3
)
3
)
2
5
H
5
). C NMR (100 MHz,
3
2
C
6
H
3
3
)
3
)
2
2
C
6
H
3
3
)
3
)
2
2 6 3 3 3 2
C H (C(CH ) ) )-3,5); 54.4
(
NCH
2
C
6
H
4
3
2
C
6
H
4
3
1
34.7, 134.8, 136.6, 138.0, 151.4, 152.7, 160.2 (NCH
2
C
6
H
4
3
2
C
6
H
3
3
)
3
)
2
5 5
H
-1
C
2 3
carbene). IR (cm ) v(CN): 1410; Anal. Calcd. for C35H41Br N
5.43.

ACCEPTED MANUSCRIPT
Dibromo-[1-(3,5-di-tert-butylbenzyl)-3-(2,4,6-trimethylbenzyl)benzimidazole-2-ylidene](pyridine)palladium(II), (2e)
o
1
Yield: 0.26 g, 74%; mp: 259-260 C. H NMR (400 MHz, CDCl
.28 and 2.30 (s, 9H, NCH (CH -2,4,6); 6.13 (s, 2H, NCH (C(CH
,4,6); 6.88-7.71 and 8.96-8.98 (m, 14H, NC N, NCH (CH -2,4,6, NCH
): 21.0 and 21.1 (NCH (CH -2,4,6); 31.5 (NCH
-3,5); 50.9 (NCH (CH (C(CH )-3,5); 111.2, 111.3, 111.5, 121.7,
22.4, 122.5, 122.9, 124.5, 127.6, 129.6, 134.0, 134.6, 135.1, 137.9, 138.3, 138.6, 138.9, 151.2, 152.7 (NC N,
); 163.8 (Pd-Ccarbene). IR (cm ) v(CN): 1410; Anal. Calcd. for
Pd: C, 55.69; H, 5.68; N, 5.27. Found: C, 55.74; H, 5.74; N, 5.32.
3
): δ 1.21 (s, 18H, NCH
-3,5); 6.15 (s, 2H, NCH
(C(CH -3,5 and NC
(C(CH
2 6 3 3 3 2
C H (C(CH ) ) -3,5);
2
2
2
C
6
H
2
3
)
3
2
C
6
H
3
3
)
3
)
2
2
C
6
H
2
3
(CH )
3
-
1
3
6
H
4
2
C
6
H
2
3
)
3
2
C
6
H
3
3
)
3
)
2
5
H
5
).
C
NMR (100 MHz, CDCl
NCH (C(CH
3
δ
2
C
6
H
2
3
)
3
2
C
6
H
3
3 3 2
) ) -3,5); 35.0
(
2
C
6
H
3
3
)
3
)
2
2
C
6
H
2
3
)
3
-2,4,6); 54.4 (NCH
2
C
6
H
3
3 3
)
1
6 4
H
-1
NCH
2
C
6
H
2
(CH
3
)
3
-2,4,6, NCH
2
C
6
H
3
(C(CH
3
)
3
)
2
-3,5 and NC
5
H
5
C
37
H
45Br
2
N
3
Dibromo-[1-(3,5-di-tert-butylbenzyl)-3-(2,3,5,6-tetramethylbenzyl)benzimidazole-2-ylidene](pyridine)palladium(II), (2f)
o
1
Yield: 0.29 g, 77%; mp: 267-268 C. H NMR (400 MHz, CDCl
.20 and 2.21 (s, 12H, NCH H(CH -2,3,5,6); 6.14 (s, 2H, NCH (C(CH
,3,5,6); 6.90-7.32, 7.63-7.69 and 8.91-8.94 (m, 13H, NC N, NCH H(CH
H(CH -2,3,5,6); 30.5 (NCH
-2,3,5,6); 53.4 (NCH (C(CH )-3,5); 110.1,110.5, 120.7, 121.4,
21.8, 123.4, 129.6, 131.5, 133.0, 133.3, 133.5, 134.2, 134.3, 136.8, 150.2, 151.6 (NC N, NCH H(CH -2,3,5,6,
); 162.8 (Pd-Ccarbene). IR (cm ) v(CN): 1416; Anal. Calcd. for C38 Pd: C, 56.21;
3
): δ 1.21 (s, 18H, NCH
-3,5); 6.16 (s, 2H, NCH
-2,3,5,6, NCH (C(CH
(C(CH
2
C
6
H
3
(C(CH
3
)
3
)
2
-3,5);
H(CH
-3,5 and
-3,5); 33.9
2
2
C
6
3
)
4
2
C
6
H
3
3
)
3
)
2
2
C
6
3
)
4
-
2
6
H
4
2
C
6
3
)
4
2
C
6
H
3
3 3 2
) )
1
3
NC
5
H
5
). C NMR (100 MHz, CDCl
3
): δ 15.6 and 19.6 (NCH
2
C
6
3
)
4
2
C
6
H
3
3 3 2
) )
(
NCH
2
C
6
H
3
(C(CH -3,5); 50.3 (NCH
3
)
3
)
2
2
C
6
H(CH
3
)
4
2
C
6
H
3
3 3
)
1
6
H
4
2
C
6
3 4
)
-
1
NCH
2
C
6
H
3
(C(CH
3
)
3
)
2
-3,5 and NC
5
H
5
2 3
H47Br N
H, 5.83; N, 5.17. Found: C, 56.24; H, 5.86; N, 5.20.
Dibromo-[1-(3,5-di-tert-butylbenzyl)-3-(4-tert-butylbenzyl)benzimidazole-2-ylidene](pyridine)palladium(II), (2g)
o
1
Yield: 0.32 g, 87%; mp: 264-265 C. H NMR (400 MHz, CDCl
(C(CH -4); 6.14 (s, 4H, NCH (C(CH -3,5); 6.15 (s, 4H, NCH
.98 (m, 16H, NC N, NCH (C(CH -3,5, NCH (C(CH -4) and NC
NCH (C(CH -4); 31.5 (NCH (C(CH -3,5); 34.6 (NCH (C(CH
(C(CH -4); 54.4 (NCH (C(CH -3,5). C NMR (75 MHz, CDCl
22.9, 124.5, 125.8, 127.8, 132.0, 134.0, 134.8, 134.9, 137.9, 151.1, 151.4, 152.7 (NC
3
): δ 1.23 (s, 27H, NCH
(C(CH -4); 6.95-7.66 and 8.96-
). C NMR (100 MHz, CDCl ): δ 31.4
-4); 35.0 (NCH (C(CH -3,5);
): δ 111.5, 111.6, 121.9, 122.5,
N, NCH (C(CH -3,5,
); 164.1 (Pd-Ccarbene). IR (cm ) v(CN): 1414; Anal. Calcd. for C38 Pd: C, 56.21; H,
2 6 3 3 3 2
C H (C(CH ) ) -3,5 and
NCH
2
C
6
H
4
3
)
3
2
C
6
H
3
3
)
3
)
2
2
C
6
H
4
3 3
)
13
8
6
H
4
2
C
6
H
3
3
)
3
)
2
2
C
6
H
4
3
)
3
5
H
5
3
(
2
C
6
H
4
3
)
3
2
C
6
H
3
3
)
3
)
2
2
C
6
H
4
3
)
3
2
C
6
H
3
3 3 2
) )
1
3
53.4 (NCH
2
C
6
H
4
3
)
3
2
C
6
H
3
3
)
3
)
2
3
1
6
H
4
2
C
6
H
3
3 3 2
) )
-1
NCH
2
C
6
H
4
(C(CH
3
)
3
-4) and NC
5
H
5
2 3
H47Br N
5.83; N, 5.17. Found: C, 56.28; H, 5.89; N, 5.25.
Dibromo-[1,3-bis(3,5-di-tert-butylbenzyl)benzimidazole-2-ylidene](pyridine)palladium(II), (2h)
o
1
Yield: 0.23 g, 67%; Yield: 0.21 g, 72%; mp: 206-207 C. H NMR (400 MHz, CDCl
3
): δ 1.22 (s, 36H,
-3,5); 6.92-7.64 and 8.98-9.00 (m, 15H, NC N,
NMR (100 MHz, CDCl ): 30.5 (NCH (C(CH -3,5); 33.9
(C(CH -3,5); 110.6, 120.7, 121.4, 121.7, 123.5, 133.0, 133.7, 136.8,
-3,5 and NC
Pd: C, 58.11; H, 6.39; N, 4.84. Found: C, 58.16; H, 6.42; N, 4.86.
NCH
2
2
C
6
6
H
3
(C(CH
3
)
3
3
)
)
2
-3,5); 6.17 (s, 4H, NCH
2
C
6
H
3
(C(CH
3
)
3
)
2
6 4
H
13
NCH
C
H
3
(C(CH
3
)
2
-3,5 and NC
-3,5); 53.3 (NCH
N, NCH (C(CH
5
H
5
).
C
3
δ
2
C H
6 3
3 3 2
) )
(
NCH
2
C
6
H
3
(C(CH
3
)
3
)
2
2
C
6
H
3
3 3 2
) )
-1
1
50.3, 151.7 (NC
6
H
4
2
C
6
H
3
3
)
3
)
2
5 5
H ); 163.1 (Pd-Ccarbene). IR (cm ) v(CN): 1411; Anal. Calcd. for
C
42
H
55Br
2
N
3

ACCEPTED MANUSCRIPT
4.4. General procedure for the direct arylation of 1-methylpyrrole-2-carboxaldehyde
An oven dried Schlenk flask was charged with aryl halide (1.0 mmol), 1-methylpyrrole-2-carboxaldehyde (2.0
mmol), KOAc (1.5 mmol), palladium-NHC-PEPPSI complex (0.01 mmol) and DMAc (2 mL). The Schlenk tube was
o
purged several times with argon and it was placed in a preheated oil bath at 120 C, and the reaction mixture was stirred
for different durations given in the specific tables. At the end of the reaction, the solvent was removed under vacuum,
and the residue was charged directly onto a silica gel column. The products were eluted by using n-hexane/diethyl ether
mixture (5:1, v/v). The yields were established by GC and GC-MS based on aryl halides.
4.5. X-ray crystallographic data
Single crystal of complex 2g suitable for X-ray analysis were obtained by slow diffusion of n-pentane into a
-1
dichloromethane solution of the complex 2g. Empirical formula = C38
H47Br
2
N
3
Pd; Mr = 811.99 g.mol ; crystal system =
monoclinic; space group = P2
1
/n system; cell lengths, a = 17.8083(16), b = 12.3036(10), c = 18.2077(15) Å; cell angles,
o
o
3
-3
-1
α = 90 , β = 111.745(2)°,
γ
= 90 ; cell volüme = 3853.1(6) Å . Z = 4, d = 1.400 g.cm , µ = 2.583 mm . F(000) = 1648; T =
296 K. The crystal size (0.36 × 0.28 × 0.18 mm) was studied with a Bruker Kappa ApexII CCD diffractometer using Mo-
α
K radiation (λ = 0.71073 Å). The number of measured reflections are 22908 with 8267 independent reflections. The
observed [I > 2σ(I)] reflections are 3325. The structure was solved by direct methods with the SIR97 program, [42] and
2
then refined with full-matrix least-square methods based on F SHELXL2014/6 (Sheldrick, 2015) [43]. All non-hydrogen
atoms were refined isotropically first and then with anisotropic atomic displacement parameters. The large anistropic
thermal parameters and wrong C-C bond distances etc for tert-butyl lead to consider disorder. The terminal C-atoms of
tert-butyl were treated as disordered over two set of sites with occupancy ratio of 0.565(6): 0.435(6) and having equal
anistropic thermal parameters within the groups. The number of restraints used were 57. The H atoms were finally
2
included in their calculated positions. A final refinement on F , with 8267 unique intensities and 389 parameters,
2
2
2
converged at ωR(F ) = 0.1546, [R(F > 2σ(F )] = 0.059 for 3325 observed reflections with I > 2σ(I). The value of weight
2
2
2
2
o o c
parameter is w = 1/[σ(F ) + (0.0546P) ], where P = [F + 2(F ) ]/3 and goodness of fit S = 0.946.
CCDC 1567377 (for 2g) contains the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre.
1
13
Supporting Information (see footnote on the first page of this article): for the H NMR, C NMR and IR spectra of
palladium-NHC-PEPPSI complexes (2a-h).
Acknowledgements
This work was financially supported by the Technological and Scientific Research Council of Turkey TÜBİTAK-
BOSPHORUS (109T605) and the İnönü University Research Fund (İ.Ü. B.A.P. 2015/41). BC thanks to TÜBA for
support.

ACCEPTED MANUSCRIPT
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Table 1. Selected bond lengths [Å] and angles [°] for complex 2g.
Bond lengths
Bond angles
177.4(3)
Pd1-C6
Pd1-N1
Pd1-Br1
Pd1-Br2
N1-C1
N1-C5
N2-C6
N2-C13
N3-C6
N3-C28
1.952(7)
2.087(6)
2.426(1)
2.420(1)
1.320(1)
1.330(1)
1.341(8)
1.451(9)
1.364(9)
1.470(8)
C6-Pd1-N1
C6-Pd1-Br1
C6-Pd1-Br2
N1-Pd1-Br1
N1-Pd1-Br2
Br1-Pd1-Br2
C5-N1-Pd1
C1-N1-Pd1
N2-C6-Pd1
N3-C6-Pd1
89.1(2)
88.8(2)
91.9(2)
90.4(2)
174.37(4)
121.9(5)
121.4(6)
127.6(5)
125.7(5)

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Table 2. Influence of the reaction conditions for palladium-NHC-PEPPSI catalyzed direct C5-arylation of 1-methylpyrrole-2-
[
a]
carboxaldehyde with 4-bromoacetophenone and 4-chloroacetophenone.
Pd-NHC-PEPPSI 2h
o
[b]
Entry
X
Solvent
Base
Time [h]
Temp. [ C]
Yield [%]
[mol-%]
1
2
3
4
5
6
7
8
9
5
5
5
5
5
5
5
5
5
5
5
5
5
1
0.5
1
1
1
1
1
1
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
DMAc
NMP
Toluene
DMAc
NMP
Toluene
DMAc
NMP
Toluene
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
Cs
Cs
Cs
2
2
2
CO
CO
CO
3
2
2
2
2
2
2
2
2
2
2
2
1
0.5
1
1
1
2
150
150
120
150
150
120
150
150
120
120
90
120
120
120
120
120
120
120
120
120
120
57
41
34
49
37
21
93
70
52
93
71
92
53
90
69
-
3
3
KOPiv.
KOPiv.
KOPiv.
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
10
11
12
13
14
15
16
17
18
19
20
21
-
5
16
42
74
76
10
15
20
[a] Conditions: 1-methylpyrrole-2-carboxaldehyde (2.0 mmol), aryl halide (1.0 mmol), base (1.5 mmol), solvent (2 mL). [b] Yields were calculated
according to aryl halide by GC and GC-MS.

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[
a,b]
Table 3. Direct C5-arylation of 1-methylpyrrole-2-carboxaldehyde by using aryl halides.
[
c]
Yield [%]
Entry
Aryl halide
Catalyst
Product
X = Br
X = Cl
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
62
54
43
75
70
68
72
77
47
59
47
67
56
61
67
88
38
47
41
79
59
68
79
90
50
41
34
66
56
59
64
69
40
43
29
64
50
57
60
68
70
65
51
67
63
61
70
73
59
57
43
65
70
59
63
72
59
67
61
71
61
70
72
74
57
62
42
61
54
63
60
67
51
48
32
53
48
54
62
63
2g
2h
2a
2b
2c
2d
2e
2f
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
2g
2h
2a
2b
2c
2d
2e
2f
2g
2h
2a
2b
2c
2d
2e
2f
2g
2h
33
34
35
36
37
38
39
40
2a
2b
2c
2d
2e
2f
2g
2h
[
a] Conditions: Pd-NHC-PEPPSI 2a-h (0.01 mmol), 1-methylpyrrole-2-carboxaldehyde (2.0 mmol), aryl halide (1.0 mmol), KOAc (1.5 mmol), DMAc
2 mL), 120 °C. [b] Reaction time: 1 h for aryl bromides, 15 h for aryl chlorides. [c] Yields were calculated according to aryl halide by GC and GC-
MS.
(

ACCEPTED MANUSCRIPT
Figure 1. Examples of biologically active pyrrole derivatives.

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Figure 2. Perspective view of the molecular structure of 2g.

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Scheme 1. General structure of recently reported palladium-bis(NHC) complexes.

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Scheme 2. Synthesis of the benzimidazolium salts (1a-h).

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Scheme 3. Synthesis of the palladium-NHC-PEPPSI complexes (2a-h).

ACCEPTED MANUSCRIPT
•
A series of new palladium-NHC-PEPPSI complexes (2a-h) with general formula [PdBr (NHC)(Py)]
2
were synthesized, (NHC = N-heterocyclic carbene; PEPPSI = pyridine-enhanced precatalyst
preparation, stabilisation, and initiation).
1
13
•
•
The obtained palladium-NHC-PEPPSI complexes were characterized by NMR ( H and C), FT-IR
spectroscopy and microanalysis techniques.
These palladium-NHC-PEPPSI complexes were used as catalysts in the direct C5-arylation of 1-
methylpyrrole-2-carboxaldehyde by aryl halides.