Copper-free Sonogashira cross-coupling reactions catalyzed by an efficient dimeric C,N-palladacycle in DMF/H2O

Article abstract of DOI:10.3906/kim-1502-86

Very small concentrations of a dimeric C,N-palladacycle were used as an efficient homogeneous catalyst for Sonogashira cross-coupling reactions between various aryl halides and phenylacetylene. The catalytic reactions were performed without the need for copper in DMF/H₂O. This catalytic system shows excellent yields for aryl iodides and bromides and even in the case of aryl chlorides.

Full text of DOI:10.3906/kim-1502-86

Turk J Chem
Turkish Journal of Chemistry
(2015) 39: 1199 – 1207
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1502-86
Research Article
Copper-free Sonogashira cross-coupling reactions catalyzed by an efficient dimeric
C,N-palladacycle in DMF/H2 O
Kazem KARAMI∗, Nasrin HAGHIGHAT NAEINI
Department of Chemistry, Isfahan University of Technology, Isfahan, Iran
Received: 16.02.2015
•
Accepted/Published Online: 12.05.2015
•
Printed: 25.12.2015
Abstract: Very small concentrations of a dimeric C,N-palladacycle were used as an efficient homogeneous catalyst for
Sonogashira cross-coupling reactions between various aryl halides and phenylacetylene. The catalytic reactions were
performed without the need for copper in DMF/H2 O. This catalytic system shows excellent yields for aryl iodides and
bromides and even in the case of aryl chlorides.
Key words: C,N-palladacycle, homogeneous catalyst, Sonogashira reactions, copper-free, aqueous-organic solvents
1. Introduction
Palladacycles are very important materials in organometallic chemistry and have attracted significant interest
due to their applications in organic synthesis, materials science, photochemistry, and the pharmaceutical
industry.1−6 The use of C,N-palladacycles as an efficient catalyst for carbon–carbon bond forming reactions
such as Heck, Suzuki, and Sonogashira reactions has been well explored in past decades.7−14
Among these, the Sonogashira reaction, involving the coupling of aryl or vinyl halides with terminal
alkynes, has usually been performed using a palladium complex as catalyst and many of them in the presence
of a catalytic amount of copper salts as a cocatalyst.15−19
The copper salts play an important role to form an intermediate copper acetylide that subsequently
transmetallates to the palladium center. However, this method has the following drawbacks: 1) using environ-
mentally unfriendly copper salts that waste production and the necessity of separation after the reaction; 2)
formation of homocoupling products of the terminal alkynes by the in situ formed copper acetylides under the
reaction conditions.20,21
Interestingly, the utility of the “copper-free” Sonogashira protocol has subsequently been rediscovered
independently by a number of researchers in recent years.22,23 These copper-free catalytic reactions have been
developed with palladium catalyst containing hindered phosphines, palladacycles, and N-heterocyclic carbenes
(NHC).24−36 The Sonogashira reaction in the absence of copper can be performed in water medium.37 Water
or aqueous solution is economically and environmentally preferable to organic solvents for metal-catalyzed
reactions.
Various aryl halides can be used in the Sonogashira reaction but the bond strengths of C–Cl and C–Br are
stronger than those of C–I, and for this reason their reactivity decreases in the order ArI > ArBr > ArCl.38−41
∗Correspondence: karami@cc.iut.ac.ir
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However, aryl chlorides and bromides are cheaper, more readily available, and more practical; therefore, they
can be used as suitable substrates for coupling reactions in comparison with their iodide analogues.42
In order to find more efficient C,N-palladacycles, the catalytic activity of the palladacycle [(Pd{(κ2 -
C,N)-(3-(dimethylaminomethyl)indole)} µ-OAc)2 ] (Figure) was investigated in the homogeneous copper-free
Sonogashira reaction of phenylacetylene with aryl halides in mixed aqueous-organic solvents.
Figure. C,N-palladacycle used as precatalyst.
2. Results and discussion
C,N-palladacycle was prepared according to our previous work (Figure).43 For this purpose, 3-(dimethylamino-
methyl)indole and Pd(OAc)2 were mixed in a 1:1 molar ratio in toluene and stirred at 60 ◦ C for 24 h to produce
five-membered C,N-palladacycle [(Pd{(κ2 -C,N)-(3-(dimethylaminomethyl)indole)} µ-OAc)2 ].
In order to show the effectiveness of the C,N-palladacycle in organic synthesis, initial studies were
performed upon the cross-coupling reaction of iodobenzene with phenylacetylene as a Sonogashira model system
and the effects of base, solvent, and temperature in the presence of 0.1 mol% of the catalyst were studied (Table
1). The results showed that K2 CO3 and DMF:H2 O (1:1) were the most effective base and solvent at 100 ◦ C,
respectively (Table 1, entry 8).
Since bromobenzene and chlorobenzene are cheaper and more readily available than iodobenzene and
hence are synthetically more useful, we tested the reaction of bromobenzene and chlorobenzene with pheny-
lacetylene under optimized conditions and found that they were not reactive enough and only 36% and <5%
yields of diphenylacetylene were obtained, respectively. However, by changing the temperature from 100 ◦ C to
120 ◦ C and increasing the catalytic amount of the C,N-palladacycle from 0.1 mol% to 0.4 mol%, bromobenzene
could be coupled with phenylacetylene in high yields (Table 2, entry 6). In addition, the result was improved
by increasing the reaction time to 16 h in the presence of 0.1 mol% of the catalyst (Table 2, entry 6). Moreover,
chlorobenzene could be coupled with phenylacetylene in the presence of 1 mol% of the catalyst at 120 ◦ C (Table
3, entry 1). Furthermore, by increasing the reaction time from 6 h to 22 h for 0.1 mol% and 0.4 mol% of the
catalyst, higher yields could be obtained (Table 3, entries 4 and 5).
After the optimized conditions were found, we explored the application of the C,N-palladacycle as an
efficient catalyst for copper-free coupling of phenylacetylene with various aryl halides containing electron-
withdrawing or -donating substituents. The results are summarized in Table 4.
As expected, the electron-rich and electron-poor aryl iodides reacted with phenylacetylene in high yields
in optimized conditions (Table 4, entries 1–5). For the aryl bromides and aryl chlorides with relatively low
reactivity, the activate substrates with electron-withdrawing character coupled with phenylacetylene in excellent
yields (Table 4, entries 9, 10, and 14) and nonactivated aryl bromides and aryl chlorides were less reactive and
required longer reaction times (Table 4, entries 7, 8, and 16).
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Table 1. Optimization of the reaction conditions for the Sonogashira reaction of iodobenzene with phenylacetylene.a
Entry Conv. (%)b Time (h) Temp (◦C) Base
Solvent
DMF
EtOH:H2O (2:1)
DMF:H2O (1:1)
CH3CN
1
75
3
4
3
3
3
3
3
3
3
3
3
3
3
3
100
100
100
100
100
100
100
100
100
100
100
100
100
rt
Et3N
2
3
4
5
6
7
8
9
< 5
85
< 5
41
75
38
90
21
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
NaOH
DMF
DMF:H2O (1:2)
DMF:H2O (1:4)
DMF:H2O (1:1)
DMF:H2O (1:1)
10
11
12
13
14
83
45
85
85
K3PO3.3H2O DMF:H2O (1:1)
CH3COONa
KOH
CS2CO3
K2CO3
DMF:H2O (1:1)
DMF:H2O (1:1)
DMF:H2O (1:1)
DMF:H2O (1:1)
< 5
a Reaction conditions: Iodobenzene (1 mmol), phenylacetylene (1.5 mmol), base (2 mmol), solvent (4 mL), [(Pd{(κ2 -
C,N)-(3-(dimethylaminomethyl)indole)} µ-OAc)2 ] (0.001 mmol). b The reactions were monitored by GC.
Table 2. Optimization of the reaction conditions for the Sonogashira reaction of bromobenzene with phenylacetylene.a
Entry Cat (mol%) Base
Temp (◦C) Time (h) Conv. (%)c
1
2
3b
4
5
6
0.1
0.1
0.1
0.1
0.1
0.4
CS2CO3 100
6
6
6
6
15
6
24
67
34
36
87
97
K2CO3
K2CO3
K2CO3
K2CO2
K2CO3
120
100
100
100
120
a Reaction conditions: bromobenzene (1 mmol), phenylacetylene (1.5 mmol), base (2 mmol), DMF:H2 O (1:1) (4 mL).
b Base (4 mmol). c The reactions were monitored by GC.
Table 3. Optimization of the reaction conditions for the Sonogashira reaction of chlorobenzene with phenylacetylene.a
Entry Conv. (%)b Time (h) Temp (◦C) Cat (mol%)
1
2
3
4
5
90
35
< 5
68
4
6
6
22
22
120
120
100
100
120
1
0.4
0.1
0.1
0.4
85
a Reaction conditions: chlorodobenzene (1 mmol), phenylacetylene (1.5 mmol), K2 CO3 (2 mmol), DMF:H2 O (1:1)
(4 mL). b The reactions were monitored by GC.
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KARAMI and HAGHIGHAT NAEINI/Turk J Chem
Table 4. Sonogashira reactions of various aryl halides with phenylacetylene.
Entry Ar–X
Product
Time (h) Conv. (%)d Yield (%)f
1
3
3
3
5
3
90a
91a
98a
85a
91a,e
85
87
96
81
89
2
3
4
5
15
6
87a
97b
84
95
6
39
7
6
8
42b
67b
65
97
81
8
9
6
6
98b
87b
10
11
12
13
6
6
74b
70
63
65b,e
22
22
4
68a
85b
90c
67
82
89
14
15
16
4
4
7
95c
30c
40c
94
25
38
aReaction conditions: aryl halide (1 mmol), phenylacetylene (1.5 mmol), K2CO3 (2 mmol), DMF:H2O (1:1) (4 mL), Pd catalyst (0.001
mmol), 100 °C. bPd catalyst (0.004 mmol), 120 °C. cPd catalyst (0.01 mmol), 120 °C. d"e reactions were monitored by GC.
ePhenylacetylene (3 mmol). fIsolated yield.
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2.1. Comparison with other studies
The catalytic performance of C,N-palladacycle for the Sonogashira reaction was compared with some of the
previous reported results using different types of palladium complexes.23,44−49 As shown in Table 5, compared
with other palladium complexes, C,N-palladacycle exhibited comparable yields with low catalyst loading. In
comparison with previous works, this catalytic system provides higher turnover frequency (TOF) in the copper-
free Sonogashira reaction.
In conclusion, we successfully employed a C,N-palladacycle as an efficient catalyst for the Sonogashira
reactions of various aryl halides in aqueous solution. In these reactions, very small concentrations of the
palladium catalyst (0.1–1 mol%) were used and the corresponding products were obtained. The coupling
reactions of phenylacetylene with both electron-withdrawing and electron-donating aryl halides afforded the
desire products in good to excellent yields under copper-free conditions.
3. Experimental
All chemicals such as commercial Pd(OAc)2 , solvent, and other chemicals were purchased from Merck Chemical
Company and Aldrich and used as received.
Fourier transform infrared (FT-IR) spectra were obtained in KBr pellets with a Jasco FT/IR 680 plus
instrument. NMR spectra were measured on a Bruker spectrometer at 400.13 MHz (1 H). UV-Vis spectra were
recorded on a JASCO V-750 spectrophotometer. Elemental analysis was performed on a LECO CHNS-932
apparatus. Conversions were monitored using an Agilent Technologies 6890N gas chromatograph equipped with
a flame ionization detector (FID) and an HB-50+ column (length = 30 m, inner diameter 320 µm, and film
thickness = 0.25 µm). Products were identified by comparison with authentic samples.
3.1. Synthesis of C,N-palladacycle
C,N-palladacycle was prepared by the method reported in our earlier work.
UV–visible (CH2 Cl2 , λmax , nm): 291 (ε = 41,700 M−1 cm−1), 240 (ε = 61,400 M−1 cm−1). FT-IR
(KBr, νmax , cm−1): 3429 s (NH), 1577vs, 1413s (CO). 1 H NMR (400 MHz, CDCl3): δ 1.96 (3H, s, MeCO2),
2.31 (3H, s, Me), 2.29 (3H, s, Me), 2.99 (2H, s, CH2), 5.93 (1H, s, NH), 7.28–7.38 (4H, m, C6 H4). 13 C NMR
(CDCl3): δ 23.79 (MeCO2), 40.00 (Me), 41.37 (CH2), 124.18 (C1), 115.91 (C2), 128.70 (C3), 129.96 (C4),
117.85, 118.88, 118.93, 129.93 (Caromatic). Anal. Calcd for C26 H34 N4 O5 Pd2 (%): C, 44.91; H, 4.93; N, 8.06.
Found (%): C, 44.43; H, 4.97; N, 7.63.
3.2. General procedure for the Sonogashira reaction
A 50-mL round-bottom flask was charged with aryl halide (1 mmol), phenylacetylene (1.5 mmol), K2 CO3 (2
mmol), DMF:H2 O (1:1) (4 mL), and the C,N-palladacycle (0.1 mol% Pd). The mixture was stirred at 100 ◦ C
for the desired reaction time. The reaction was monitored by GC.
After the reaction was complete, the mixture was cooled to room temperature and diluted with EtOAc
and H2 O. The product was extracted with EtOAc and the organic phase dried over MgSO4 , filtered, and
concentrated. The arylalkynes obtained could be purified by silica gel column chromatography (hexane:EtOAc).
The arylalkyne products were known compounds and were characterized from their 1 H NMR and 13 C NMR.
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KARAMI and HAGHIGHAT NAEINI/Turk J Chem
3.3. Characterization of the products of Sonogashira cross-coupling reactions
1. Diphenylacetylene (Table 4, entries 1, 6, and 13): 1 H NMR (400 MHz, CDCl3): δ 7.59–7.62 (m, 2H),
7.47–7.50 (m, 1H), 7.40–7.45 (m, 2H). 13 C NMR (CDCl3): δ 133.3, 132.5, 121.6, 129.1, 82.4.50
2. 1-Methyl-4-(phenylethynyl)benzene (Table 4, entry 2): 1 H NMR (400 MHz, CDCl3): δ 7.55–7.60 (m, 2H),
7.45–7.50 (m, 2H), 7.34–7.41 (m, 3H), 7.15–7.19 (d, J = 7.8 Hz, 2H), 2.31 (s, 3H). 13 C NMR (CDCl3):
δ 140.9, 132.5, 131.3, 129.8, 128.9, 128.2, 124.5, 123.1, 89.2, 87.8, 22.2.50
3. 1-Nitro-4-(phenylethynyl)benzene (Table 4, entries 3, 9, and 14): 1 H NMR (400 MHz, CDCl3): δ 8.23
(d, J = 8.8 Hz, 2H), 7.61 (d, J = 8.8 Hz, 2H), 7.47–7.57 (m, 2H), 7.24–7.36 (m, 3H). 13 C NMR (CDCl3):
δ 144.2, 132.8, 132.3, 131.3, 128.9, 128.2, 126.4, 125.1, 93.3, 87.3.50
4. 1-Amino-4-(phenylethynyl)benzene (Table 4, entries 4 and 16): 1 H NMR (400 MHz, CDCl3): δ 7.59–7.68
(m, 2H), 7.38–7.29 (m, 5H), 6.70 (m, 2H), 3.54 (s, 2H). 13 C NMR (CDCl3): δ 149.36, 133.91, 132.22,
129.22, 127.70, 122.81, 112.22, 112.91, 89.29, 87.32.51
5. 1,4-bis(phenylethynyl)benzene (Table 4, entries 5 and 12): 1 H NMR (400 MHz, CDCl3): δ 7.50–7.55 (m,
8H), 7.36–7.43 (m, 6H). 13 C NMR (CDCl3): δ 132.9, 131.4, 129.9, 128.8, 122.1, 120.2, 91.3, 88.1.52
6. 1,3-Dimethyl-4-(phenylethyne)benzene (Table 4, entry 7): 1 H NMR (400 MHz, CDCl3): δ 7.60 (d, J =
8.2 Hz, 1H), 7.30–7.47 (m, 5H), 7.10 (s, 1H), 6.92 (d, J = 8.2 Hz, 1H), 2.41 (s, 3H); 2.32 (s, 3H).53
7. 1-methyl-3-(phenylethynyl)benzene (Table 4, entries 8): 1 H NMR (400 MHz, CDCl3): δ 7.60–7.67 (m,
2H), 7.45–7.54 (m, 5H), 7.29 (t, J = 7.5 Hz, 1H), 7.17 (d, J = 7.3 Hz, 1H), 2.42 (s, 3H). 13 C NMR
(CDCl3): δ 138.21, 134.11, 131.36, 129.22, 128.54, 128.52, 128.37, 128.0, 123.11, 122.45, 87.63, 86.19,
22.36.52
8. 4-(phenylethynyl)benzaldehyde (Table 4, entry 10): 1 H NMR (400 MHz, CDCl3): δ 9.96 (s, 1H), 7.75–
7.84 (m, 2H), 7.56–7.66 (m, 2H), 7.45–7.52 (m, 2H), 7.30–7.37 (m, 3H). 13 C NMR (CDCl3): δ 193.3,
138.2, 133.5, 132.4, 130.6, 129.4, 128.8, 127.3, 121.2, 93.3, 88.4.52
9. 1-Nitro-2-(phenylethynyl)benzene (Table 4, entry 11). 1 H NMR (400 MHz, CDCl3): δg 8.11 (m, 1H),
7.70 (m, 1H), 7.68 (m, 2H), 7.52 (m, 1H), 7.41 (m, 1H), 7.35 (m, 3H). 13 C NMR (CDCl3): δg46.59
133.97, 132.80, 132.11, 128.98, 128.64, 128.12, 125.44, 122.42, 119.11, 99.22, 85.21.54
10. 1,3-Diphenylpropyne (Table 4, entry 15): 1 H NMR (400 MHz, CDCl3): δ 7.40–7.47 (m, 4H), 7.33 (t,
J = 7.6 Hz, 2H), 7.22–7.31 (m, 4H), 3.80 (s, 2H). 13 C NMR (CDCl3): δg137.21, 132.13, 129.2, 128.84,
128.33, 128.21, 127.22, 124.44, 87.3, 84.22, 26.11.55
Acknowledgment
This investigation was supported by the Department of Chemistry, Isfahan University of Technology (IUT).
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