N-Methylphthalimide-substituted Benzimidazolium salts and PEPPSI Pd-NHC complexes: Synthesis, characterization and catalytic activity in carbon-carbon bond-forming reactions

Article abstract of DOI:10.3762/bjoc.12.9

A series of novel benzimidazolium salts (1-4) and their pyridine enhanced precatalyst preparation stabilization and initiation (PEPPSI) themed palladium N-heterocyclic carbene complexes [PdCl₂(NHC)(Py)] (5-8), where NHC = l-(N-methylphthalimide)-3-alkylbenzimidazolin-2-ylidene and Py = 3-chloropyridine, were synthesized and characterized by means of ¹H and ¹³C{¹H} NMR, UV-vis (for 5-8), ESI-FTICR-MS (for 2, 4, 6-8) and FTIR spectroscopic methods and elemental analysis. The synthesized compounds were tested in Suzuki-Miyaura cross-coupling (for 1-8) and arylation (for 5-8) reactions. As catalysts, they demonstrated a highly efficient route for the formation of asymmetric biaryl compounds even though they were used in very low loading. For example, all compounds displayed good catalytic activity for the C-C bond formation of 4-tert-butylphenyl-boronic acid with 4-chlorotoluene.

Full text of DOI:10.3762/bjoc.12.9

N-Methylphthalimide-substituted benzimidazolium salts and
PEPPSI Pd–NHC complexes: synthesis, characterization and
catalytic activity in carbon–carbon bond-forming reactions
Senem Akkoç*1,2,§, Yetkin Gök3, İlhan Özer İlhan2 and Veysel Kayser*1,¶
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 81–88.
1Faculty of Pharmacy, The University of Sydney, 2006 Sydney,
doi:10.3762/bjoc.12.9
Australia, 2Department of Chemistry, Faculty of Sciences, Erciyes
University, Talas Street, 38039 Kayseri, Turkey and 3Department of
Chemistry, Faculty of Arts and Sciences, Inönü University, 44280
Malatya, Turkey
Received: 07 November 2015
Accepted: 05 January 2016
Published: 15 January 2016
Email:
This article is part of the Thematic Series "N-Heterocyclic carbenes".
Guest Editor: S. P. Nolan
Senem Akkoç* - senem.akkoc@sydney.edu.au; Veysel Kayser* -
veysel.kayser@sydney.edu.au
*
Corresponding author
© 2016 Akkoç et al; licensee Beilstein-Institut.
License and terms: see end of document.
§
3
¶
Tel: +61 2 9351 2330; Tel: +90 352 437 52 62; Fax: +90 352 437 49
3
Tel: +61 2 9351 3391; Fax: +61 2 9351 4391
Keywords:
arylation; benzimidazolium salts; catalysis; N-heterocyclic carbene;
PEPPSI complex; Suzuki–Miyaura cross-coupling reaction
Abstract
A series of novel benzimidazolium salts (1–4) and their pyridine enhanced precatalyst preparation stabilization and initiation
PEPPSI) themed palladium N-heterocyclic carbene complexes [PdCl2(NHC)(Py)] (5–8), where NHC = 1-(N-methylphthalimide)-
-alkylbenzimidazolin-2-ylidene and Py = 3-chloropyridine, were synthesized and characterized by means of 1H and
3C{1H} NMR, UV–vis (for 5–8), ESI-FTICR-MS (for 2, 4, 6–8) and FTIR spectroscopic methods and elemental analysis. The
(
3
1
synthesized compounds were tested in Suzuki–Miyaura cross-coupling (for 1–8) and arylation (for 5–8) reactions. As catalysts,
they demonstrated a highly efficient route for the formation of asymmetric biaryl compounds even though they were used in very
low loading. For example, all compounds displayed good catalytic activity for the C–C bond formation of 4-tert-butylphenyl-
boronic acid with 4-chlorotoluene.
Introduction
The use of N-heterocyclic carbenes (NHCs) as ligands was the last two decades, and they had a wide range of applications
started by Wanzlick [1] and Öfele [2] almost fifty years ago. in different fields, particularly in homogeneous/heterogeneous
There have been major advances in the design and synthesis of catalysis [3-8] and bioorganometallic chemistry [9-11]. This is
metal complexes containing N-heterocyclic carbene ligands in because NHC complexes are easily obtained by deprotonating
81

Beilstein J. Org. Chem. 2016, 12, 81–88.
Results and Discussion
imidazolium or benzimidazolium salts and most are relatively
Synthesis of N-methylphthalimide
stable in air and moisture. They are weak π-acceptors and
strong σ-donors and can form strong M–C bonds with tran- substituted benzimidazolium salts
sition metal ions compared to trivalent phosphine ligands New benzimidazolium salts 1–4, which are carbene precursors,
[
12,13].
were synthesized by N-alkylbenzimidazole and various alkyl
halides in DMF (Scheme 1). These salts, especially containing
As catalysts, palladium N-heterocyclic carbene (Pd–NHC) com- benzyl and 3-methylbenzyl groups, were obtained in very high
plexes display remarkable activities in coupling reactions [5,14- yields (81–97%) as white or cream solids. The salt containing
1
7]. Among various Pd–NHC complexes such as the 2-morpholinoethyl group was obtained in a much lower
[
[
Pd(NHC)(dmba)Cl] (dmba = N,N-dimethylbenzylamine) and yield of 62%.
Pd(NHC)(Im)Cl2] (Im = imidazole) [18,19], [PEPPSI
Pd–NHC] complexes are prevalent due to the combination of The benzimidazolium salts include an acidic NCHN proton,
efficiency and versatility [20,21]. The synthesis conditions for which can be deprotonated easily to form an NHC, at the C2
these complexes are generally mild and do not require an inert position of the benzimidazole ring. The sharp salt peak indi-
atmosphere. The steric and electronic parameters are also easily cating the synthesis of a benzimidazolium salt came quite
modified by attaching substituents. PEPPSI Pd–NHC downfield at δ 10.13, 11.10, 9.65 and 10.83 ppm in the
complexes have been used in different coupling reactions such 1H NMR spectra for 1–4, respectively. The NCHN peaks of the
as Mizoroki–Heck cross-coupling [22,23], Suzuki–Miyaura carbene precursors were observed at δ 144.56, 144.69, 143.55
cross-coupling [24,25], Sonogashira [26] and arylation and 145.26 ppm in the 13C{1H} NMR spectra for 1–4, respec-
reactions [27].
tively. The formation of the benzimidazolium salts was also
evident through their IR spectra, which showed peaks at 1562.2,
There are suitable precatalyst scaffolds, which were developed 1558.4, 1562.2 and 1554.5 cm−1 for the ν(CN) bond of 1–4, re-
by Nolan [28], Organ [21] and Buchwald [29]. To find more spectively. Compounds 2 and 4 among these salts were further
effective catalysts containing an Organ type scaffold among characterized by using high-resolution mass spectrometry
these precatalyst scaffolds, we synthesized four new pure (HRMS). The mass spectra demonstrated m/z peaks at 382.16
N-methylphthalimide substituted benzimidazolium salts (1–4) and 391.18 for the cationic moieties against the calculated value
and their PEPPSI Pd–NHC complexes (5–8) in this study. The of m/z 382.43 and 391.44 for 2 and 4, respectively.
structures of all compounds were confirmed by various spec-
troscopic methods (1H and 13C{1H} NMR, UV–vis, ESI- Synthesis of PEPPSI Pd–NHC complexes
FTICR-MS, FTIR) and elemental analysis. The PEPPSI Our aim was to synthesize novel Pd–NHC complexes contain-
Pd–NHC complexes were tested for catalytic activities both in ing a PEPPSI skeleton. The target PEPPSI Pd–NHC complexes
direct arylation and Suzuki–Miyaura cross-coupling reactions. 5–8 were successfully synthesized by using carbene precursors
The catalytic activities of benzimidazolium salts were only 1–4, PdCl2 and K2CO3 as a base in 3-chloropyridine
tested in a Suzuki–Miyaura cross-coupling reaction. The com- (Scheme 1). The colors of the obtained solid complexes were
pounds were found to be very efficient in the symmetric and either yellow or cream. These complexes, which are stable both
asymmetric C–C bond-forming reactions.
in solution and in solid states against air, light and moisture,
Scheme 1: Synthesis of benzimidazolium salts and their PEPPSI Pd–NHC complexes.
82

Beilstein J. Org. Chem. 2016, 12, 81–88.
were obtained in low yields of 25–60%. All complexes with a tion spectra are indicative of a charge transfer, and to deconvo-
benzimidazolium moiety display characteristic signals for the lute the shoulder or ripple of the absorption spectra, we applied
3
-chloropyridine ligand in the 1H and 13C{1H} NMR spectra. the second derivative analysis (Figure 1B). The second deriva-
For the 1H NMR spectra of the metal complexes, sharp peaks in tive spectra were noisy, as can clearly be seen after smoothing
the lower field belonging to the benzimidazolium salts (NCHN) three positive peaks (at 315, 345 and 430 nm) and two negative
were not observed between δ 10 and 12 ppm. The FTIR data peaks (at 380 and 405 nm). In the second derivative analysis,
clearly indicated the presence of ν(CN) at 1508.2, 1446.5, 1394.4 a negative band has a minimum at the same wavelength as
and 1444.6 cm−1 for the PEPPSI Pd–NHC complexes 5–8, re- the maximum on the main absorption spectrum. With this in
spectively. The formation of a C–N module in the benz- mind, there are at least two absorbance bands buried between
imidazole ring correlated with a shift of IR (CN) band. The 350–430 nm.
complexes 6–8 were further characterized by means of HRMS
Catalytic activity of PEPPSI Pd–NHC com-
which showed m/z peaks at 637.02, 691.99 and 574.99 for the
cationic moieties against the calculated values of m/z 636.84, plexes as catalysts in arylation reaction
6
91.84 and 574.95, respectively. Unfortunately, we were unable First, we performed the arylation reaction between the 4-bromo-
to yield a proper single crystal from these compounds for X-ray acetophenone, which is electron-deficient, with 2-n-butylthio-
diffraction. Unlike the salts, the metal complexes showed absor- phene without the catalyst at 110 °C for 1 h in DMAc as sol-
bance in UV–vis experiments.
vent and the reaction resulted in only a 1% yield. When we
attempted the same reaction using 8 as a catalyst at 130 °C, the
efficiency of the reaction was 27% (Table 1, entry 3). For cata-
Absorption spectroscopy studies
The absorption spectra of the complexes were recorded in lyst 5, the yield dropped to 9%, the temperature was decreased
DMSO and are shown in Figure 1A.
to 110 °C and the procedure time was increased from 1 h to
.5 h (Table 1, entry 2).
1
Only metal complexes show an absorbance above 330 nm and
have the highest absorption, whereas salts did not display any When 4-bromoacetophenone was used as a substrate with cata-
absorbance except for an intense peak below 330 nm. There- lyst 6, sp2–sp2 C–C bond formation with 2-n-butylfuran was
fore, we only discuss the metal complexes here. For the metal achieved with a yield of 49% in just 1 h (Table 1, entry 5). The
complexes, the spectra are characterized by a broad band be- product was obtained in a much lower yield when the same
tween 350 and 430 nm and display a strong absorption below reaction was carried out with 5 as a catalyst. Compounds 7 and
3
50 nm. Complex 5 exhibits the overall highest absorption, 8 as catalysts were displaying better results than complexes 5
whereas the overall absorption of complex 7 seems to be the and 6 for the same reaction (Table 1, entries 6 and 7). When we
lowest of the six investigated complexes. Very broad absorp- employed electron-neutral bromobenzene as a substrate instead
Figure 1: (A) UV–vis absorbance spectra were taken in DMSO. (B) The second derivative of the compound 5 calculated from A. Other metal
complexes have similar second derivative bands but are omitted for simplicity.
83

Beilstein J. Org. Chem. 2016, 12, 81–88.
Table 1: PEPPSI Pd–NHC catalyzed direct intermolecular arylation of heteroaryl derivatives with various aryl bromidesa,b.
Entry
R
X
S
Time (h)
Temp. (°C)
Comp.
Yield (%)
1
2
3
4
5
6
7
8
9
CH3(C=O)-
1
110
110
130
110
110
110
110
110
80
–
5
8
5
6
7
8
8
7
7
7
5
1
1.5
1
9
27
14
49
83
89
97
71
84
98
79
O
1
1
1
1
H-
S
1
O
21
21
21
1
10
11
12
90
110
130
CH3O-
S
aReaction conditions: 2-n-butylthiophene or 2-n-butylfuran (2 mmol), 4-bromoacetophenone, bromobenzene or 4-bromoanisole (1 mmol), PEPPSI
Pd–NHC 5–8 (1 mol %), KOAc (1 mmol), DMAc (2 mL), 80–130 °C, 1 or 21 h. Product purity was checked by GC and NMR. bYields were calculated
according to aryl bromides.
of 4-bromoacetophenone, C–C bond formation (2-butyl-5- The Suzuki–Miyaura reaction was carried out using electron-
phenylthiophene) was achieved in 97% yield using catalyst 8 at rich (4-chloroanisole, 4-chlorotoluene), electron-poor
1
10 °C with a reaction time of 1 h (Table 1, entry 8). When (4-chloroacetophenone, 4-chloronitrobenzene) and electron-
bromobenzene was used with catalyst 7 at 80 °C for 21 h, the neutral (chlorobenzene) substrates. We studied the catalytic ac-
product 2-butyl-5-phenylfuran was obtained in 71% yield. By tivity of compound 1 regarding C–C bond coupling of
increasing the reaction temperature by 10 degree centigrade, as 4-iodoacetophenone as a substrate with phenylboronic acid
shown in entry 10, the result showed a reaction yield that was (Table 2, entry 2). This catalytic system showed a better perfor-
1
3% higher than that of entry 9. However, when we further in- mance for aryl iodide than for aryl chlorides except chloroben-
creased the temperature by more than 30 degrees to 110 °C, a zene (Table 2, entries 1–17). 4-Acetylbiphenyl was obtained in
maximum yield of 98% was obtained (Table 1, entry 11). In the 99% yield and 100% conversion at 80 °C and 1 h (Table 2,
reaction catalyzed by 5, 2-butyl-5-(4-methoxyphenyl)thiophene entry 2). The best results were obtained when chlorobenzene
was obtained in 79% yield (Table 1, entry 12).
was used as a substrate (Table 2, entries 18–21) whereas least
favorable results were obtained when the electron-poor sub-
strates were used. Among the employed carbene precursors,
compound 4 gave the best result to acquire the 4-nitrobiphenyl
Catalytic activity of synthesized compounds
in Suzuki–Miyaura cross-coupling reaction
The Suzuki–Miyaura cross-coupling reaction, which has mostly product (Table 2, entries 8 and 9). Compounds 2 and 4 gave
been performed in organic solvents until recently, can now very good results to get the 4-methoxybiphenyl product from
be performed using green solvents under mild conditions [29- among the used carbene precursors (Table 2, entries 11 and 13).
3
4]. We used H2O with DMF as solvents in different propor-
tions in this work. We preferred to use a 1:1 ratio in the Melvin et al. found the coupling of phenylboronic acid with
Suzuki–Miyaura reaction as there was not much difference 4-chlorotoluene with a yield of approximately 45% within
between the obtained results when the ratios used were 3:1 or 90 minutes at room temperature and reaction conditions of
1
:1. To optimize the reaction conditions, a series of experi- 1.0 mol % PEPPSI-IPr, K2CO3, MeOH/THF (19:1). They ob-
ments with different bases such as KOH, NaOH and K2CO3 tained the same coupling product with a yield of around 50%
were conducted at different temperatures to provide the cou- when they changed the amount of PEPPSI-IPr to 0.5 mol %, the
pling of the C–C bond of different substrates with phenyl- base to KOt-Bu and extended the reaction time to 2 h [35].
boronic acid. The best results were obtained with the base However, we obtained the same product in much higher yields
KOH. We also employed different time periods ranging from 1 such as 90 and 93% under reaction conditions of 1 mol % of
to 3 h. When the reaction time was extended, the yield Pd(OAc)2, 1 mol % of 1 and 2, KOH, DMF/H2O within a short
increased in a linear manner.
period of time (1 h) at 80 °C.
84

Beilstein J. Org. Chem. 2016, 12, 81–88.
Table 2: Suzuki–Miyaura cross-coupling reaction of phenylboronic acid with aryl chloridesa,b.
Entry
1
R-C6H4-X
LHX
Base Time (h) Temp (°C) DMF (mL) H2O (mL)
Yield (%)
41
Conv. (%)
53
1
KOH
KOH
1
1
80
80
3
3
1
1
2
1
99
100
3
4
5
6
7
8
9
1
2
2
3
3
4
4
KOH
KOH
KOH
KOH
KOH
KOH
KOH
1
2
3
2
3
2
3
80
80
80
80
80
80
80
3
2
2
2
2
2
2
1
2
2
2
2
2
2
27
34
74
25
30
75
75
42
40
77
62
77
76
97
10
11
12
13
1
2
3
4
KOH
KOH
KOH
KOH
1
2
2
2
80
80
80
80
3
3
3
3
1
1
1
1
52
94
23
95
68
99
55
98
14
15
16
17
1
2
3
4
KOH
KOH
KOH
KOH
1
1
1
1
80
80
80
80
3
2
2
2
1
2
2
2
90
93
44
12
92
99
92
99
18
19
20
21
1
1
1
1
KOH
NaOH
K2CO3
KOH
2
2
2
1
70
70
70
80
2
2
2
3
2
2
2
1
99.9
99.8
96
99.9
99.8
98
99.9
100
aReaction conditions: p-R-C6H4Cl (1.0 mmol), Pd(OAc)2 (1.0 mol %), phenylboronic acid (1.5 mmol), base (2.0 mmol), 1–4 (1 mol %), DMF/H2O,
0–80 °C, 1–3 h. bYields were calculated according to aryl chlorides by using GC or GC–MS.
7
The synthesized carbene precursors consist of an electron- The C–C bond formation of thianaphthene-2-boronic acid with
neutral group (benzyl), electron-donating group (3-methyl- 4-chloronitrobenzene using an in situ formed Pd–NHC com-
benzyl) and electron-withdrawing group (N-methylphthalimide) plex as catalyst resulted in low yields and conversions (Table 3,
on the benzimidazolium salts. These groups are important for entries 9–12). The usage of the morpholinoethyl substituted
the catalytic performance and to understand the electronic effect benzimidazolium compound 4 with Pd(OAc)2 resulted in higher
of the ligands. In the presence of the catalysts formed in the in yields of 2-(4-nitrophenyl)benzo[b]thiophene compared to the
situ medium, the coupling of 4-methoxy-1-chlorobenzene with usage of compounds 1–3.
2
,5-dimethoxyphenylboronic acid was performed with overall
low yields (Table 3, entries 1–4). While carbene precursor 2 Generally, the PEPPSI Pd–NHC complexes showed similar
showed the lowest catalytic activity, 3 gave the highest catalytic catalytic activity with in situ formed Pd–NHC complexes under
activity (Table 3, entries 2 and 3). We acquired excellent the same experiment conditions (Table 3 and Table 4). The
catalytic activity results using 4-tert-butylphenylboronic acid 2-morpholinoethyl substituted Pd–NHC complex 8, on the other
which is a derivative of phenylboronic acid (Table 3, entries hand, displayed very low activity compared to the other three
5
–8). All compounds were found to be very effective for this complexes for the coupling of 2,5-dimethoxyphenylboronic
coupling reaction and the product yield was between 94–100%. acid with 4-methoxy-1-chlorobenzene (Table 4, entry 4). High
85

Beilstein J. Org. Chem. 2016, 12, 81–88.
Table 3: Suzuki–Miyaura cross-coupling reaction of boronic acid derivatives with aryl halidesa,b.
Entry
Aryl halide
Derivatives of boronic acid
LHX
Product
Yield (%) Conv. (%)
1
2
3
1
2
3
35
15
77
68
26
80
4
4
24
33
5
6
7
8
1
2
3
4
94
99
96
99.7
99
98
99.9
100
9
1
2
3
4
10
8
38
15
7
10
11
12
6
45
48
aReaction conditions: 4-methoxy-1-chlorobenzene, 4-chlorotoluene, 4-chloronitrobenzene (1.0 mmol), Pd(OAc)2 (1.0 mol %), 2,5-dimethoxyphenyl-
boronic acid, 4-tert-butylphenylboronic acid, thianaphthene-2-boronic acid (1.5 mmol), NaOt-Bu (2.0 mmol), 1–4 (1 mol %), DMF/H2O (1:1, 4 mL),
80 °C, 2 h. bYields were calculated according to aryl chlorides by using GC or GC–MS.
Table 4: Suzuki–Miyaura cross-coupling reaction of boronic acid derivatives with aryl chloridesa,b.
Entry
Aryl chloride
Derivatives of boronic acid PEPPSI Pd–NHC
Product
Yield (%) Conv. (%)
1
2
3
5
6
7
56
51
59
67
55
77
4
8
9
25
5
6
7
8
5
6
7
8
92
95
99
99
93
98
99.9
100
9
5
6
7
8
3
1
5
9
10
11
12
35
14
55
30
aReaction conditions: 4-methoxy-1-chlorobenzene, 4-chlorotoluene, 4-chloronitrobenzene (1.0 mmol), 2,5-dimethoxyphenylboronic acid, 4-tert-
butylphenylboronic acid, thianaphthene-2-boronic acid (1.5 mmol), NaOt-Bu (2.0 mmol), 5–8 (1 mol %), DMF/H2O (1:1, 4 mL), 80 °C, 2 h. bYields
were calculated according to aryl chlorides by using GC or GC–MS.
86

Beilstein J. Org. Chem. 2016, 12, 81–88.
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4.
5.
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sized and their structures were confirmed via 1H and
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Acknowledgments
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This work was financially supported by TUBITAK
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1059B141400496) and Erciyes University Research Fund. We
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