Pd-N-Heterocyclic carbene catalysed Suzuki-Miyaura coupling reactions in aqueous medium

Article abstract of DOI:10.24820/ark.5550190.p010.519

A new series of methyl substituted imidazole-based N-heterocyclic carbene (NHC) palladium complexes (PdCl2(L1)NHC(L1 =pyridine) is reported. Structural definitions of Pd-PEPPSI complexes were determined by NMR spectroscopy, elemental analysis and LC-MS sp

Full text of DOI:10.24820/ark.5550190.p010.519

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Arkivoc 2018, part v, 0-0
Pd-N-Heterocyclic carbene catalysed Suzuki-Miyaura coupling reactions in
aqueous medium
Emine Özge Karaca,a Mitat Akkoç,b Sedat Yaşara,b*, İsmail Özdemira,b
aInönü University, Catalysis Research and Application Centre, 44280 Malatya, Turkey
bInönü University, Faculty of Science and Art, Department of Chemistry, 44280 Malatya, Turkey
Email: syasar44@gmail.com
Dedicated to Prof. Kenneth K. Laali on the occasion of his 65th anniversary
Received 02-07-2018
Accepted 05-28-2018
Published on line 06-25-2018
Abstract
A new series of methyl substituted imidazole-based N-heterocyclic carbene (NHC) palladium complexes
(PdCl2(L1)NHC(L1 =pyridine) is reported. Structural definitions of Pd-PEPPSI complexes were determined by
NMR spectroscopy, elemental analysis and LC-MS spectroscopy techniques. To evolve a more efficient
catalytic system for electronically different aryl chloride substrates on the Suzuki cross-coupling reaction,
complexes were used as pre-catalyst. Activity of palladium(II)-NHC complexes screened under mild reaction
conditions in aqueous media. With this catalytic system, the reaction proceeded in moderate or good yields
with low catalyst loading (0.1 mol%).
Pd-PEPPSI 0.1 mol%
+
Cl
B(OH)2
aqueous media
R
R
Keywords: Suzuki-Miyaura cross-coupling reaction, N-heterocyclic carbene, Pd-PEPPSI complex, C-C bond
formation
DOI: https://doi.org/10.24820/ark.5550190.p010.519
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Introduction
The development of N-heterocyclic carbenes (NHC) and their different metal complexes has provided a new
approach in homogeneous catalysis.1 With these developments, numerous metal-NHC complexes with the
6
inclusion of Ag,2 Ru,3 Ir,4 Rh,5 and Pd have been prepared. Although there are numerous metal complexes of
NHCs, Pd-NHC complexes have particular importance due to their robustness regarding air, moisture and high
temperature. The basis of this interest lies in strong σ-donor and weak π-acceptor ability and the ease of
adjusting the steric effects of NHC by nitrogen atoms. Complexes bearing sterically bulky and electron-rich
ligands show enhanced catalytic activity in oxidative addition and reductive elimination reactions, which are
key steps of many catalytic reactions using homogeneous catalysts.7-9 These unique features make these
compounds indispensable strong ligands for transition metals and homogeneous catalytic systems.10-14 Organ
at al. synthesized different types of palladium N-heterocyclic carbene PEPPSI complexes (PEPSSI=Pyridine-
Enhanced Pre-catalyst Preparation Stabilization (and) Initiation) in 2006.15 Then, couple studies were reported
by Doucet, Matt and Cavell groups.16-18 Following these studies, PEPPSI complexes have been extensively
studied, and it has been reported that these complexes exhibit very good catalytic and biological activity.19-26
N
N
RCl
(i)
N
N
N
N
N
N
N
N
Cl-
O
Cl-
Cl-
Cl-
N
O
1b
1a
1c
1d
PdCl2
(ii)
Cl
Pd N
Cl
Cl
Pd N
Cl
Cl
Pd N
Cl
Cl
Pd N
Cl
N
N
N
N
N
N
N
N
O
N
O
2d
2a
2b
2c
85%
73%
86%
79%
Scheme 1. Synthesis method of un-symmetrical imidazole based-NHC precursors and their Pd-PEPPSI
complexes: (i) THF, reflux; (ii) K2CO3, pyridine, 80 C.
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The Suzuki coupling reaction is one of the most preferred reactions for C-C bond formation reactions due
to the mild reaction conditions. Capretta et al. reported the first NHC-based Suzuki protocol.27 Recently,
important advanced studies in the field of well-defined and air-stable palladium-NHC complex-catalyzed
Suzuki-Miyaura reactions were published by Glorius,28 Beller,29 Herrmann,30 Nolan,31 Organ,32 and others.11,33-
38
However, most of these catalytic systems are need to optimise due to the requirement for hazardous
solvents, harsh reaction conditions and high catalyst loading. Catalytic systems that use water as the solvent
are inherently safer processes, and offer significant advantages.39 For example, the poor solubility of Suzuki
products in water is one of the advantages of this type system because of simplify separation of the desired
products from the reaction medium. Considering these important points, to demonstrate the usefulness of
electron-rich imidazol based palladium complexes 2a-d, we investigated the catalytic performance of the
compounds as co-catalysts in Suzuki-Miyaura coupling reactions in aqueous media.
Result and Discussion
The imidazole-based NHC precursors 1a-d were synthesized according to the literature (Scheme 1) and the
spectroscopic data of 1a-d were consistent with the corresponding literature.40,41 Pd-PEPPSI complexes 2a-d
were synthesized using Organ’s method (Scheme 1), i.e. the reaction of NHCs 1a-d with PdCl2 in pyridine at 80
oC in the presence of K2CO3, to provide the NHC palladium complexes 2a-d in 86%, 79%, 85%, 73%
13
respectively. The C{1H} NMR spectra provide information on complex formation, for example an increasing
13
downfield shift of the NCN carbon from 1a-d to 2a-d; i.e. the C{1H} N-C-N shifts of 1a-d and 2a-d were 137.5
and 152.5 ppm, 137.2 and 151.3 ppm, 137.3 and 161.2 ppm, and 137.7 and 151.2 ppm, respectively.
In the first instance, to find the optimum conditions for the Suzuki coupling reaction, an extensive
screening of the reaction conditions was carried out using common mineral bases with different solvent
variations under standard conditions. To assess the influence of the solvent, we used 4-chloroacetophenone
as the substrate and K2CO3 as the base (2a:1 mol%, 4-chloroacetophenone (1 mmol), PhB(OH)2 (1.5 mmol), 80
o
oC, 3h). In all cases, the reactions were heated for 3h at 80 C. After several reactions, the results showed that
this catalytic system is effective with all solvents and bases, but the best one is K2CO3-DMF/H2O. The optimum
yield was obtained with the most polar solvents in an equal ratio of DMF/H2O. The results are summarized in
Table 1, entries 1-12. These optimum results were attributed to water due to its high polarity and the good
solubility of the base in water. Solubility of the base is important to generate water-soluble aryl boronate
derivatives.42 The use of pure DMF, H2O, i-PrOH or 1,4-dioxane afforded lower yields than the use of a mixture
of DMF and water in equal proportions.
Table 1. The effect of solvent and base on yield in the Suzuki coupling reactiona
O
O
2a (0.1 mol%)
OH
+
Cl
B
OH
Solvent, Base
Entry
Solvent
Dioxane(6 mL)
Dioxane (6 mL)
Dioxane(6 mL)
i-PrOH
Base(eq)
Yield [%]
1
2
3
4
5
Na2CO3 (2)
K2CO3 (2)
Cs2CO3 (2)
Na2CO3 (2)
K2CO3 (2)
60
75
30
50
52
i-PrOH
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Table 1. Continued
Entry
6
Solvent
i-PrOH
DMF (6 mL)
Base(eq)
Cs2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
Na2CO3 (2)
Cs2CO3 (2)
Yield [%]
33
7
8
9
10
11
12
13
69
74
98
88
10
70
45
DMF/H2O (4/2 mL)
DMF/H2O (3/3 mL)
DMF/H2O (2/4 mL)
H2O ( 6 mL)
DMF/H2O (3/3 mL)
DMF/H2O (3/3 mL)
a Reaction conditions: 2a (0.1 mol%), 4-chloroacetophenone (1 mmol), Ph(OH)2 (1.5 mmol), 80 oC, 3 h.
This catalytic system led to the investigation of cheap and abundant sodium and potassium carbonate
bases that have poor solubility, except in water. All carbonate bases resulted in sufficient conversions, but
potassium carbonate demonstrated significantly better performance (Table 1, entry 9, 10).
Catalytic data are available in the literature for the Suzuki coupling reaction catalyzed by Pd-NHC derived
complexes in a variety of different conditions.43-45 Therefore, it is hard to make a comparison of palladium
catalysts, but we can make a provisional comparison with the Organ system ((Pd-PEPPSI: 2 mol %,
chloroanisole (1 mmol), PhBF3K (1.0 mmol), 60oC, 24h, MeOH) in the Suzuki-Miyaura reaction.15
A series of activated and non-activated aryl chloride substrates with phenylboronic acid was used under
the optimized conditions described above (Table 2). The results show that complexes 2a-d were sufficiently
active catalysts in the Suzuki coupling reaction, similar to Organ’s catalyst under similar reaction conditions.
When catalytic activity of 2a-d was compared in the Suzuki coupling reaction, complex 2a gave the best
results, in almost each case with different substrates except in the case of chlorobenzene. We attributed these
performance differences to the electron richness of 2a. It is known that electron rich Pd-complexes undergo
oxidative additions more readily. Also, the steric effect of the catalyst facilitates the reductive elimination of
the product from the active catalyst. The general opinion on this issue is that, the steric and electronic
properties need to be equipoise to create a highly active catalyst system.28, 43, 46,47
Table 2. The Suzuki coupling reaction of aryl chloridesa
2a-d (0.1 mol%)
OH
+
R
Cl
R
B
OH
DMF:H2O (1:1)
K2CO3 (2 mmol)
Entry
Aryl chloride
Product
Pd-NHC
Yield [%]
1
2
3
4
5
-
1
2a
2b
2c
2d
94
86
92
90
O
O
Cl
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Table 2. Continued
Entry
Aryl chloride
Product
Pd-NHC
-
Yield [%]
3
6
7
2a
2b
2c
2d
-
98
94
89
84
0
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2a
2b
2c
2d
-
85
79
71
69
1
Cl
O
O
2a
2b
2c
2d
-
82
75
69
72
2
Cl
2a
2b
2c
2d
94
98
80
88
Cl
a Reaction conditions: 1.0 mmol of p-R–C6H4Cl, 1.5 mmol of phenylboronic acid, 2 mmol K2CO3, 0.1 mol% Pd-
NHC (2a-d), water (3 ml)–DMF (3 ml), 80 oC, 3 h.
The efficiency of our catalytic system is better than the literature48-50 within the meaning of the catalyst
loading and aryl chloride substrates.
Conclusions
Herein, we reported synthesize and define highly active, easy to produce and environmentally friendly new
Pd-PEPPSI complexes. Due to the structure of the Pd-PEPPSI complexes, the carbene remains electron-rich
upon coordination to palladium, which makes the palladium-carbon bond strong and stable. This outstanding
property of Pd-PEPPSI complexes creates advantages over other complexes in catalytic cross-coupling
reactions. The catalytic activity of 2a-d was moderate and encouraged us to synthesize additional Pd
complexes that are electronically and structurally different previously reported Pd-PEPPSI complexes.
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Experimental Section
General. Unless stated otherwise, all procedures were carried out under a normal air atmosphere. Chemicals
and solvents were purchased from Sigma Aldrich Co. (Dorset, UK) and used without any purification.1H NMR
13
and C NMR spectra were recorded using a Bruker Avance 400 operating at 400 MHz (1H), 100 MHz (13C) in
CDCl3. Coupling constants (J values) are given in Hertz. NMR multiplicities are abbreviated as follows: s=
singlet, d= doublet, t= triplet, m= multiplet, bs= broad singlet. Melting points were detected by Stuart
automatic melting point apparatus (SMP-40).
General preparation of 1-methyl-3-alkylimidazolium salts
These known compounds were synthesized according to literature and characterized by m.p, H and C
NMR and micro analyses.
40
1
13
1-methyl-3-(2,3,4,5,6-penthamethylbenzyl)imidazolium chloride (1a). This known compound was synthesized
according to literature.40
1-methyl-3-(2,3, 5,6-tetramethylbenzyl)imidazolium chloride (1b). This known compound was synthesized
according to literature.40
o
1
1-methyl-3-(2-morpholinoethyl)imidazolium chloride (1c). Yield: 90%. m.p: 132-133 C. H NMR (399.9 MHz,
o
DMSO-d6,25 C): =9.24 [s, 1H, NCHN], 7.79 [s, 1H, NCHCHN], 7.72 [s, 1H, NCHCHN], 4.31 [t, J=8 Hz, 2H,
CH2CH2N(CH2CH2)2O], 3.89 [s, 3H, NCH3], 3.55 [t, J=4 Hz, 4H CH2CH2N(CH2CH2)2O] 2.68 [t, J=8 Hz, 2H,
13
o
CH2CH2N(CH2CH2)2O], 2.42 [t, J=4 Hz, 4H CH2CH2N(CH2CH2)2O]. C NMR (100 MHz, DMSO-d6,25 C): =137.3,
123.6, 123.2, 67.5, 57.3, 53.3, 46.0, 36.2.
1-methyl-3-(2-methoxyethyl)imidazolium chloride (1d). This known compound was synthesized according to
literature.40
Preparation of the NHC-palladium-pyridine (PEPPSI) complexes 2a-d
In air, a pressure tube was charged with PdCl2 (180 mg, 1 mmol), 1a-d (1.1 mmol), K2CO3 (700 mg, 5 mmol)
o
and 3 mL of pyridine. The reaction mixture was heated with vigorous stirring for 17 h at 80 C then cooled to
room temperature and diluted with dichloromethane (DCM). A short silica column was used for purification.
All volatiles were evaporated. The yellow solid residue was washed with hexane (2x10 mL) and diethyl ether
(2x10 mL). The crystalline yellow solid was used in the Suzuki reaction as obtained.
Dichloro[1-methyl-3-(2,3,4,5,6-penthamethylbenzyl)imidazol-2-ylidene]pyridine palladium(II) (2a). Yield:
o
1
o
86%. m.p: 225.1 C. H NMR (399.9 MHz, DMSO-d6, 25 C): = 2.17 [s, 6H, CH2C6(CH3)5-2,3,4,5,6], 2.19 s, 6H,
CH2C6(CH3)5-2,3,4,5,6], 2.22 [s, 3H, CH2C6(CH3)5-2,3,4,5,6], 4.02 [s, 3H, NCH3], 5.63 [s, 2H, CH2C6(CH3)5-
2,3,4,5,6], 6.36 [s, 1H, NCHCHN], 7.28 [s, 1H, NCHCHN], 7.56, 7.99 and 8.92 [m, 5H, NC5H5]. 13C NMR (100 MHz,
o
DMSO, 25 C): = 17.0, 17.1, 17.4, 38.2, 50.4, 120.4, 123.9, 125.4, 127.7, 133.1, 134.0, 135.9, 139.1, 147.0,
151.9, 152.5 Anal. Calcd. for C21H27Cl2N3Pd: C,50.57; H, 5.46; N, 8.42 Found: C, 50.68; H, 5.60, N, 8.63. LC-MS
(ESI): m/z 427.7 [M-2Cl].
Dichloro[1-methyl-3-(2,3,5,6-tetramethylbenzyl)imidazol-2-ylidene]pyridine palladium(II) (2b). Yield: 79%.
m.p: 202.5 oC. 1H NMR (399.9 MHz, CDCl3, 25 oC): = 2.25 and 2.28 [s, 12H, CH2C6H(CH3)4-2,3,5,6], 4.20 [s, 3H,
NCH3], 5.88 [s, 2H, CH2C6H(CH3)4-2,3,5,6], 6.33 and 6.79 [s, 2H, NCHCHN], 7.06 [s, 1H, CH2C6H(CH3)4-2,3,5,6],
7.40 [m, 2H, NC5H5], 7.81 [m, 1H, NC5H5], 9.08 [m, 2H, NC5H5]. 13C NMR (100 MHz, DMSO, 25 oC): = 15.9, 20.5,
38.0, 49.7, 120.2, 122.2, 123.7, 124.5, 130.2, 132.5, 134.4, 134.7, 135.9, 138.0, 148.5, 149.9, 151.3 Anal. Calcd
for C20H25Cl2N3Pd: C,49.55; H, 5.20; N, 8.67 Found: C, 49.59; H, 5.28, N, 8.78. LC-MS (ESI): m/z 411.3 [M-
2Cl+2H-].
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Dichloro[1-methyl-3-(2-morpholinoethyl)imidazol-2-ylidene]pyridine palladium(II) (2c). Yield: 85%. m.p:
o
1
o
185.3 C. H NMR (399.9 MHz, DMSO-d6, 25 C): = 2.51 [bs, 8H, CH2CH2(N(CH2CH2)2O], 2.97 [t, J=6.4 Hz, 2H,
CH2CH2(N(CH2CH2)2O], 3.56 [t, J=4.4 Hz, 4H, CH2CH2(N(CH2CH2)2O], 4.03 [s, NCH3], 4.54 [t, J=6.4 Hz, 2H,
CH2CH2(N(CH2CH2)2O], 7.37 and 7.40 [s, 2H, NCHCHN], 7.57 [m, 2H, NC5H5], 8.01 [m, 1H, NC5H5], 8.82 [m, 2H,
13
o
NC5H5]. C NMR (100 MHz, DMSO, 25 C): =44.3, 52.0, 57.2, 65.9, 109.6, 120.2, 122.5, 133.7, 137.5, 150.1,
161.2. Anal. Calcd. for C15H22Cl2N4OPd: C, 39.89, H, 4.91, N, 12.40. Found: C, 39.98; H, 5.02; N, 12.66. LC-MS
(ESI): m/z 450.12 [MH-].
Dichloro[1-methyl-3-(2-methoxyethyl)imidazol-2-ylidene]pyridine palladium(II) (2d). Yield: 73%. m.p: 176.9
1
o
oC. H NMR (399.9 MHz, CDCl3, 25 C): = 3.38 [s, 3H, CH2CH2(OCH3)], 3.98 [t, J= 5.2 Hz, 2H, CH2CH2(OCH3)],
4.17 [s, 3H, NCH3], 4.76 [t, J=5.2 Hz, 2H, CH2CH2(OCH3)], 6.9 and 7.11 [s, 2H, NCHCHN], 7.38, 7.80 and 9.00 [m,
5H, NC5H5]. 13C NMR (100 MHz, CDCl3, 25 oC): = 37.9, 50.9, 59.0, 71.9, 122.6, 123.4, 124.5, 151.2. Anal. Calcd.
for C12H17Cl2N3OPd: C, 36.34; H, 4.32; N, 10.59. Found: C, 36.44; H, 4.45; N, 10.78. LC-MS (ESI): m/z 653.3 [2M-
4Cl+2H+].
General procedure for Suzuki Cross-Coupling reaction
In air, 2a-d (0.1 mol%, aryl chloride (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3 (2 mmol) and 3 mL of a
mixture of water and DMF (1:1) were added to a small round-bottom flask and the mixture was heated at 80
oC for an appropriate period of time. The reaction mixture was cooled to room temperature and 10 mL of
water was added to the reaction mixture and extracted with Et2O. The organic phase was dried with MgSO4
and filtrated by short chromatography on silica gel column. Then volatiles were removed under reduced
pressure and yield distribution was determined by GC using undecane as internal standard. The yields are
based on corresponding aryl chlorides. All catalytic reactions were duplicated. All coupling products obtained
via Suzuki-Miyaura coupling reaction are previously reported compounds, and were identified by comparison
of our data with that available in the literature.
Acknowledgements
This work was financially supported by İnönü University Research Fund (BAP: 2016-195).
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