Recyclable and reusable PdCl2(PPh3) 2/PEG-2000/H2O system for the carbonylative Sonogashira coupling reaction of aryl iodides with alkynes

Article abstract of DOI:10.1039/c3gc42278j

PdCl₂(PPh₃)₂ in a mixture of water and poly(ethylene glycol) (PEG-2000) is shown to be an extremely active catalyst for the carbonylative Sonogashira coupling reaction of aryl iodides with terminal alkynes. The reaction can be conducted under an atmospheric pressure of carbon monoxide at 25 °C with Et₃N as a base, yielding a variety of alkynyl ketones in good to excellent yields. Application of this synthetic method to prepare flavones from o-iodophenol and terminal alkynes was also achieved. The isolation of the products is readily achieved by extraction with diethyl ether, and the PdCl₂(PPh₃)₂/PEG-2000/ H₂O system can be easily recycled and reused six times without any loss of catalytic activity. the Partner Organisations 2014.

Full text of DOI:10.1039/c3gc42278j

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Recyclable and reusable PdCl2(PPh3)2/PEG-2000/
H2O system for the carbonylative Sonogashira
coupling reaction of aryl iodides with alkynes†
Cite this: Green Chem., 2014, 16,
2515
Hong Zhao,a,b Mingzhu Cheng,a Jiatao Zhanga and Mingzhong Cai*a
PdCl2(PPh3)2 in a mixture of water and poly(ethylene glycol) (PEG-2000) is shown to be an extremely
active catalyst for the carbonylative Sonogashira coupling reaction of aryl iodides with terminal alkynes.
The reaction can be conducted under an atmospheric pressure of carbon monoxide at 25 °C with Et3N as
a base, yielding a variety of alkynyl ketones in good to excellent yields. Application of this synthetic
method to prepare flavones from o-iodophenol and terminal alkynes was also achieved. The isolation of
the products is readily achieved by extraction with diethyl ether, and the PdCl2(PPh3)2/PEG-2000/H2O
system can be easily recycled and reused six times without any loss of catalytic activity.
Received 5th November 2013,
Accepted 10th December 2013
DOI: 10.1039/c3gc42278j
www.rsc.org/greenchem
reported
that
the
palladium-catalyzed
carbonylative
Introduction
Sonogashira coupling of terminal alkynes with aryl iodides
The synthesis of alkynyl ketones has attracted considerable under an atmospheric pressure of carbon monoxide could be
interest because of their appearance in many biologically accomplished by using aqueous ammonia as a base in THF at
active molecules and because they are key intermediates in room temperature with or without CuI.23 Recently, Yang and
the synthesis of natural products and drug-like molecules.1–5 co-workers reported that the palladium-catalyzed copper-free
Traditionally, alkynyl ketones are generally prepared via carbonylative Sonogashira coupling of terminal alkynes
the direct coupling reaction of acid chlorides with alkynyl with aryl iodides under an atmospheric pressure of carbon
organometallic reagents such as silver,6 copper,7 sodium,8 monoxide could be achieved by using water as a solvent
lithium,9 cadmium,10 zinc,11 silicon,12 and tin.13 However, at room temperature with 3 equiv. of Et3N as a base.24 The
these methods require the use of dry solvents under an inert carbonylative Sonogashira coupling reaction generally pro-
atmosphere. An alternative synthetic procedure for alkynyl ceeds in the presence of a homogeneous palladium catalyst
ketones is the palladium-catalyzed coupling of acyl chlorides such as Pd(OAc)2, PdCl2(PPh3)2, and PdCl2.22–24 However,
with terminal alkynes in the presence of CuI.14–18 The tran- industrial applications of these homogeneous palladium cata-
sition-metal-catalyzed coupling reaction of alkynes or their lysts remain a challenge because they are expensive and
metalated derivatives, such as alkynylstannanes19,20 and alkynyl- cannot be recycled. Therefore, from the standpoint of green
silanes,21 with organic halides in the presence of CO provides chemistry, the development of a recyclable and reusable cata-
a convenient route to alkynyl ketones under atmospheric lyst system that allows for the highly efficient carbonylative
conditions with atom economy. Kang et al. reported that iodo- Sonogashira coupling of a wide range of substrates (aryl
nium salts can readily undergo carbonylative coupling with iodides and terminal alkynes) is worthwhile.
terminal alkynes in the presence of Pd(OAc)2/CuI catalysts and
As environmentally friendly “green” syntheses are becom-
an atmospheric pressure of carbon monoxide in DME–H2O ing more and more important, it is desirable to avoid the use
(4 : 1) medium at room temperature, affording the desired of any hazardous and expensive organic solvents. There are
alkynyl ketones in good yields.22 Mori and Mohamed Ahmed also significant economical and environmental reasons for
developing recyclable catalytic reactions from both academic
and industrial perspectives. To satisfy both recyclability and
aKey Laboratory of Functional Small Organic Molecule, Ministry of Education and
Department of Chemistry, Jiangxi Normal University, Nanchang 330022, P. R. China.
E-mail: mzcai@jxnu.edu.cn; Fax: (+86)-791-8812-0388
environmental concerns, a more facile method is to immobi-
lize the catalyst in a liquid phase by dissolving it in a nonvola-
tile and nonmixing liquid, such as ionic liquids25–29 and
PEG.30–36 Although ionic liquids offer some advantages, the
tedious preparation of ionic liquids and their environmental
safety is still debated since data on their toxicity and
bSchool of Chemistry and Chemical Engineering, Guangdong Pharmaceutical
University, Guangzhou 510006, P. R. China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3gc42278j
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environmental burden are for the most part unknown. other hand, the phosphine-free PdCl2- or Pd(OAc)2-catalyzed
However, poly(ethylene glycols) (PEGs), possessing negligible carbonylative coupling reaction in PEG-2000/H2O medium was
vapor pressure, are known to be inexpensive, thermally stable, also tested; however, a palladium precipitate was formed very
recoverable, nontoxic compounds which serve as suitable rapidly under the reductive reaction conditions (CO, H2O).
media for environmentally friendly and safe chemical
To compare the effect of PEG in different organic solvents,
reactions.37–42 So far, PEGs have been successfully used the carbonylative Sonogashira coupling reaction was per-
as reaction media for palladium-catalyzed carbon–carbon formed under identical conditions in mixtures of PEG-2000
coupling reactions, such as the Heck reaction,30 the Suzuki– and various organic solvents, including THF, dioxane, CH2Cl2,
Miyaura reaction,31,32 the homocoupling and cross-coupling of toluene, acetonitrile and MeOH (Table 1). The combination of
aryl halides,33 and the direct arylation of 1,2,3-triazoles with PEG-2000 with nonpolar toluene and aprotic polar solvents,
aryl bromides34 with easy recyclability of the solvent and palla- such as THF, dioxane, CH2Cl2, acetonitrile, and MeOH,
dium catalysts. Recently, Wang and co-workers reported a Pd- afforded the desired product in low yields (Table 1, entries
(PPh3)4-PEG 400 catalyzed protocol for the atom-efficient Stille 2–7). On the other hand, the mixture of PEG-2000 with water
cross-coupling reactions of organotins with aryl bromides.43,44 furnished the coupling product in excellent yield (Table 1,
We herein report the application of a PdCl2(PPh3)2/PEG-2000/ entry 8). These results show a significant solvent effect on the
H2O system as an extremely effective and reusable catalytic reaction, and that water is a good solvent for the carbonylative
medium for the carbonylative Sonogashira coupling reaction Sonogashira coupling reaction in the presence of PEG. Our
of aryl iodides with terminal alkynes under an atmospheric next studies focused on the effect of various bases on the
pressure of carbon monoxide.
model reaction in PEG-2000/H2O medium. When 2 equiv. of
ammonia (0.5 M) was used as a base,23 the reaction did not
occur and the starting materials were recovered completely
(Table 1, entry 9). Bu3N also afforded a good yield (Table 1,
entry 10), while other bases such as pyridine, DBU, TMEDA,
and DABCO gave moderate yields (Table 1, entries 11–14). The
use of Na2CO3, K2CO3, Cs2CO3, K3PO4 and NaOAc delivered
the product in very low yields (Table 1, entries 15–19). So, Et3N
was the best choice for the model reaction in PEG-2000/H2O
medium (Table 1, entry 8). In addition, the efficiency of
various chain length PEGs on the reaction was also examined
under the same reaction conditions (Table 1, entries 20–22).
PEG-2000 was superior to PEG-600, PEG-1000 and PEG-4000.
As a result of these studies, we were encouraged to examine
the reaction with a broad range of substrates to determine the
specificity and scope of substrates for this system. Thus, a
variety of aryl iodides were reacted with phenylacetylene, and
the results are listed in Table 2. As shown in Table 2, it is
evident that most of the carbonylative coupling reactions pro-
ceeded smoothly under mild conditions in the PEG-2000/H2O
medium, affording good to excellent yields of the coupling
products and that many functional groups were tolerated in
these reactions. Generally, the aryl iodides with electron-donat-
ing groups, such as methoxy and methyl, gave excellent yields
(Table 2, entries 1–5). Even the reaction of sterically hindered
2-iodoanisole with phenylacetylene gave the desired coupling
product 3c in 94% yield (Table 2, entry 3). The presence of a
strong electron-withdrawing group, such as –NO2, –COMe and
–CO2Me, on the aryl iodide is known to promote the direct
coupling, producing noncarbonylative coupling products. As
Results and discussion
The effect of PEG on the carbonylative Sonogashira coupling
reaction of aryl iodides with terminal alkynes in water was
initially investigated. The carbonylative coupling of 4-iodoani-
sole (2 mmol) and phenylacetylene (2.4 mmol) with CO was
chosen as the model reaction. The reaction was carried out in
water (3 g) in the presence of 5 mol% of PdCl2(PPh3)2 using
Et3N (4 mmol) as base at 25 °C for 24 h under an atmospheric
pressure of carbon monoxide. In the absence of PEG-2000,
the result was not promising, and a low yield was obtained
(Fig. 1). The yield of 1-(4-methoxyphenyl)-3-phenylpropynone
increased with the addition of incremental PEG-2000 and an
excellent yield was obtained when the amount of PEG-2000
reached 3 g. Further increasing the amount of PEG-2000
resulted in a decrease in yield. It was noteworthy that only a
trace amount of the desired product was formed using neat
PEG-2000 as the solvent at 50 °C (Table 1, entry 1). On the
discussed in Mori and Mohamed Ahmed’s research,23
a
mixture of carbonylative and noncarbonylative coupling pro-
ducts were obtained when electron-deficient aryl iodides were
used as substrates in the carbonylative coupling reaction, pre-
sumably because the alkyne reacts too rapidly with the arylpal-
ladium(II) intermediate derived from the rapid oxidative
addition of electron-deficient aryl iodide to palladium(0)
complex without insertion of CO. However, the reaction of
Fig. 1 Effect of PEG-2000 on the carbonylative coupling reaction of
4-iodoanisole with phenylacetylene in water.
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Table 1 Carbonylative coupling reaction of 4-iodoanisole with phenylacetylene in various mediaa
Entry
Base
Solventb
Yieldc (%)
1d
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
NH3 (0.5 M)
Bu3N
Pyridine
DBU
TMEDA
DABCO
Na2CO3
K2CO3
Cs2CO3
K3PO4
NaOAc
Et3N
PEG-2000
13
18
20
35
38
46
55
93
0
86
67
60
63
59
23
25
31
17
11
67
83
78
PEG-2000–THF
PEG-2000–dioxane
PEG-2000–CH2Cl2
PEG-2000–toluene
PEG-2000–MeCN
PEG-2000–MeOH
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-2000–H2O
PEG-600–H2O
Et3N
Et3N
PEG-1000–H2O
PEG-4000–H2O
a Reaction conditions: 4-iodoanisole (2 mmol), phenylacetylene (2.4 mmol), CO (1 atm), base (4 mmol). b The amount of water or solvent was
3 mL, PEG was 3 g. c Isolated yields. d At 50 °C.
1-chloro-4-iodobenzene having an electron-withdrawing substi- carbonylative coupling reaction of terminal alkynes bearing an
tuent afforded the desired coupling product 3h in 82% yield, alkyl substituent with aryl iodides was slow and required the
along with only 10% yield of 4-chlorodiphenylacetylene under use of CuI as a cocatalyst, where moderate to good yields were
optimized conditions (Table 2, entry 8), the same reaction in obtained.23 It is noteworthy that the coupling reaction of aryl
the presence of 5 mol% of PdCl2(dppf) in THF using aq. NH3 iodides with terminal alkynes bearing alkyl substituents cata-
(0.5 M) as base at room temperature for 12 h gave 3h in 67% lyzed by our catalytic system was achieved by performing the
yield.23 The reaction of methyl 4-iodobenzoate provided a 57% reaction without CuI as a cocatalyst. A plausible mechanism is
yield of the carbonylative coupling product 3i and a 32% shown in Scheme 1. First PdCl2(PPh3)2 is reduced to Pd(0)
yield of 4-methoxycarbonyldiphenylacetylene (Table 2, (PPh3)2 by CO and H2O.23,24 The oxidative addition of an aryl
entry 9), but the reaction of 4-nitroiodobenzene with a strong iodide to Pd(0)(PPh3)2 provides arylpalladium(II) complex (A),
electron-withdrawing substituent only gave a trace of desired which is followed by migratory insertion of carbon monoxide
coupling product, and 4-nitrodiphenylacetylene was formed in giving acylpalladium(II) complex (B). Subsequent transmetala-
87% yield. Interestingly, the unprotected 2-aminoiodobenzene tion between B and a terminal alkyne in the presence of a base
can participate in the reaction, and the desired product and reductive elimination of alkynyl ketone (D) from the inter-
was obtained in good yield (Table 2, entry 7). In addition, the mediate (C) regenerates Pd(0)(PPh3)2. We reasoned that in the
reactions of bulky 1-iodonaphthalene and heteroaryl iodides absence of CuI, the less reactive terminal alkyne (compared
such as 2-iodothiophene also proceeded smoothly to give the with its corresponding alkynyl copper ate complex) would
corresponding coupling products 3j and 3k in good yields. easily react with the electron-deficient acylpalladium(II)
This new catalyst system towards the carbonylative Sonoga- complex (B) rather than its precursor arylpalladium(II) complex
shira coupling reaction of aryl iodides is more efficient than (A); therefore, the desired carbonylative coupling reaction
the published PdCl2(PPh3)2/THF/aq. NH3 system.23 The high could be secured. The formation of palladium black was not
efficiency of the PdCl2(PPh3)2/PEG-2000/H2O system is pro- observed during the carbonylative coupling reaction due to the
bably attributed to the properties of PEG as the cosolvent45,46 presence of the phosphine ligand. The TEM measurement
and PTC.
showed that there is no palladium nanoparticle present in the
To further evaluate the reaction, we next examined the car- reaction. So, the real catalytic species should be Pd(0)(PPh3)2
bonylative coupling reaction of alkyl-substituted terminal in the PEG-2000/H2O phase.
alkynes and substituted phenylacetylenes with methyl- and
The results listed in Table 3 show that the carbonylative
methoxy-substituted aryl iodides, and the results are summar- coupling reactions of a variety of aliphatic alkynes, such as
ized in Table 3. Mori and Mohamed Ahmed reported that the 1-octyne, 1-hexyne, 3,3-dimethyl-1-butyne, cyclopropylacetylene
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Table 2 Carbonylative coupling of various aryl iodides with phenylacetylenea
Entry
1
Aryl iodide
Alkyne
Product
Yieldb (%)
93
2
3
91
94
4
5
91
90
6
7
86
83
8
82
57
81
9
10
11
69
a Reaction conditions: aryl iodide (2 mmol), phenylacetylene (2.4 mmol), Et3N (4 mmol), CO (1 atm), water (3 g), PEG-2000 (3 g) at 25 °C for 24 h.
b Isolated yields.
and 3-methoxypropyne, with various aryl iodides proceeded occur under the conditions optimized for the iodides. Aryl
smoothly under mild conditions to afford the corresponding chlorides were also not reactive and the reaction of 4-chloro-
carbonylative coupling products 3l–3w in good to excellent iodobenzene with phenylacetylene could afford selectively
yields (Table 3, entries 1–12). The reactions of even the steri- 1-(4-chlorophenyl)-3-phenylpropynone (3h) in 82% yield
cally hindered 2-iodoanisole with various aliphatic alkynes (Table 2, entry 8).
also gave good to excellent yields (Table 3, entries 2, 6, 9 and 11).
Flavones are an important class of naturally occurring
The reaction of trimethylsilylacetylene with 4-methyliodo- products,47–49 and many methods for their preparations have
benzene afforded the corresponding alkynyl ketone 3x in 82% been published.50–58 Recently, the palladium-catalyzed
yield (Table 3, entry 13). The reactions of the substituted phe- carbonylation of o-iodophenol derivatives with terminal
nylacetylenes, such as 4-methylphenylacetylene with 4-iodoani- alkynes to prepare flavones has attracted considerable
sole and 2-iodoanisole, also proceeded effectively under the interest.59–65 We also investigated the feasibility of preparing
same conditions to yield 90 and 86% of the desired coupling flavones by the sequential carbonylative coupling of o-iodophe-
products 3y and 3z, respectively (Table 3, entries 14 and 15). nol with terminal alkynes to form α,β-unsaturated ketones, fol-
The results above prompted us to investigate the reaction of lowed by an intramolecular cyclization to give flavones in a
aryl bromides, but the carbonylative coupling reaction did not one-pot operation using PdCl2(PPh3)2/PEG-2000/H2O as
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Table 3 Carbonylative coupling of various alkynes with aryl iodidesa
Entry
1
Aryl iodide
Alkyne
Product
Yieldb (%)
91
2
87
3
4
88
90
5
6
85
88
7
8
9
82
91
95
10
11
87
81
12
13
14
86
82
90
15
86
a Reaction conditions: aryl iodide (2 mmol), alkyne (2.4 mmol), Et3N (4 mmol), CO (1 atm), water (3 g), PEG-2000 (3 g) at 25 °C for 24 h. b Isolated
yields.
catalyst system (Scheme 2). The results show that the tandem butyne and trimethylsilylacetylene with o-iodophenol could
carbonylative coupling/intramolecular cyclization reaction of proceed smoothly to afford the desired flavones 4a–4c in
various terminal alkynes such as 1-hexyne, 3,3-dimethyl-1- moderate to good yields.
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good to excellent yields. Application of this synthetic method
to generate flavones from o-iodophenol and terminal alkynes
was also achieved. Furthermore, the PdCl2(PPh3)2/PEG-2000/
H2O system could be recycled and reused six times without
any loss of catalytic activity. This protocol will serve as an
efficient and green way to prepare alkynyl ketones and 2-sub-
stituted flavones. Currently, further efforts to extend the appli-
cation of the system in other palladium-catalyzed
transformations are underway in our laboratory.
Experimental
All chemicals were of reagent grade and used as purchased. All
solvents were dried and distilled before use. The products were
purified by flash chromatography on silica gel. A mixture of
EtOAc and hexane was generally used as eluent. All carbonyla-
tive coupling products were characterized by comparison of
their spectra and physical data with authentic samples. IR
spectra were determined on a Perkin-Elmer 683 instrument.
1H NMR spectra were recorded on a Bruker Avance 400
(400 MHz) spectrometer with TMS as an internal standard in
CDCl3 as solvent. 13C NMR spectra were recorded on a Bruker
Avance 400 (100 MHz) spectrometer in CDCl3 as solvent.
Melting points are uncorrected.
Scheme 1 Possible explanation for the carbonylative coupling
reactions.
Scheme 2 Synthesis of flavones.
General procedure for the carbonylative Sonogashira coupling
in PEG-2000/H2O
To check the reusability of PEG-2000 and the catalyst, the
carbonylative coupling reaction of 4-iodoanisole with phenyl-
A 50 mL round-bottomed flask equipped with a gas inlet tube
and a magnetic stirring bar was charged with PEG-2000 (3 g),
water (3 g), PdCl2(PPh3)2 (0.1 mmol), and Et3N (4 mmol). The
mixture was heated to 50 °C with stirring. The flask was
flushed with carbon monoxide, and the mixture was slowly
cooled to 25 °C and then aryl iodide (2 mmol) and alkyne
(2.4 mmol) were added. After being stirred at 25 °C for 24 h
under CO (1 atm), the resulting suspension was extracted four
times with diethyl ether (4 × 15 mL). The residue of the extrac-
tion was subjected to a second run of the carbonylative coup-
ling reaction by charging with the same substrates (aryl iodide,
terminal alkyne, and Et3N) under the same conditions without
further addition of PdCl2(PPh3)2 or PEG. The combined ether
phase was concentrated under reduced pressure. The residue
was purified by flash column chromatography on silica gel
(hexane–ethyl acetate = 20 : 1).
acetylene was examined in the presence of
5 mol%
PdCl2(PPh3)2. After initial experimentation, the reaction
mixture was extracted with diethyl ether (4 × 15 mL), and the
solidified PdCl2(PPh3)2/PEG-2000/H2O was subjected to
a
second run by charging with the same substrates (4-iodoani-
sole, phenylacetylene, and Et3N) under 1 atm of CO without
any regeneration or addition of PdCl2(PPh3)2. We were gratified
to observe that the PdCl2(PPh3)2/PEG-2000/H2O system could
be recycled and reused six times without any loss of activity.
The results of six runs showed that they were almost consistent
in yields and rates (93%, 91%, 94%, 92%, 93%, and 92%,
respectively, for 24 h). In addition, the leaching of palladium
in the product was also determined. ICP analysis of the extract
found that palladium content was less than 0.65 ppm.
General procedure for the synthesis of flavones
Conclusions
A 50 mL round-bottomed flask equipped with a gas inlet tube
In summary, a highly efficient and reusable PdCl2(PPh3)2/ and a magnetic stirring bar was charged with PEG-2000 (3 g),
PEG-2000/H2O system for the carbonylative Sonogashira coup- water (3 g), PdCl2(PPh3)2 (0.1 mmol), and Et3N (4 mmol). The
ling reaction has been developed. In the presence of mixture was heated to 50 °C with stirring. The flask was
PdCl2(PPh3)2, the carbonylative coupling reactions of
a
flushed with carbon monoxide, and the mixture was slowly
number of terminal alkynes, including alkyl- and aryl-substi- cooled to 25 °C and then o-iodophenol (2 mmol) and alkyne
tuted acetylenes, with various aryl iodides under an atmo- (2.4 mmol) were added. After being stirred at 25 °C for 48 h
spheric pressure of carbon monoxide proceeded smoothly and under CO (1 atm), the resulting suspension was extracted four
efficiently at 25 °C using Et3N as base in a mixture of times with diethyl ether (4 × 15 mL). The combined ether
PEG-2000 and water to afford the desired alkynyl ketones in phase was concentrated under reduced pressure. The residue
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Acknowledgements
We thank the National Natural Science Foundation of China
(no. 21272044) and Key Laboratory of Functional Small
Organic Molecule, Ministry of Education (No. KLFS-KF-201213)
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