A palladium/perfluoroalkylated pyridine catalyst for sonogashira reaction of aryl bromides and chlorides in a fluorous biphasic system

Article abstract of DOI:10.1002/ejoc.200700287

Palladium perfluorooctanesulfonate [Pd(OPf)₂] catalyses the highly efficient Sonogashira reaction of aryl bromides and chlorides in the presence of a catalytic amount of perfluoroalkylated pyridine as a ligand in a fluorous biphasic system (FBS) composed of toluene and perfluorodecalin. The reaction can be performed under phosphane-, copper- and DMF-free conditions in an air atmosphere. By simple separation of the fluorous phase containing palladium/perfluoroalkylated pyridine catalyst, the reaction can be repeated several times. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200700287

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.200700287
A Palladium/Perfluoroalkylated Pyridine Catalyst for Sonogashira Reaction of
Aryl Bromides and Chlorides in a Fluorous Biphasic System
Wen-Bin Yi,*[a] Chun Cai,[a] and Xin Wang[a]
Keywords: Cross-coupling / Palladium / Fluorinated ligand / Heterogeneous catalysis
Palladium perfluorooctanesulfonate [Pd(OPf)2] catalyses the
highly efficient Sonogashira reaction of aryl bromides and
free conditions in an air atmosphere. By simple separation of
the fluorous phase containing palladium/perfluoroalkylated
chlorides in the presence of a catalytic amount of perfluoro- pyridine catalyst, the reaction can be repeated several times.
alkylated pyridine as a ligand in a fluorous biphasic system
(FBS) composed of toluene and perfluorodecalin. The reac-
tion can be performed under phosphane-, copper- and DMF-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
the most important methods for facile catalyst separation
from the reaction mixture and recycling of the catalyst since
FBS was introduced by Horváth and Rábai in 1994.[21]
Traditional palladium-catalyzed Sonogashira reactions
often require relatively large amounts of catalysts, which
have to be removed from the reaction product. Perfluoro-
labelled Pd complexes offer a solution to this problem since
the perfluoro-labelled catalysts are soluble in fluorous sol-
vents and can be separated from the organic product very
easily by liquid–liquid extractions.[22] However, the previous
studies of the Sonogashira reaction under fluorous biphasic
conditions were related to phosphane palladium cata-
lysts.[22] It is known that catalysts containing phosphane li-
gands are unstable.[22,23] Therefore, the development of
phosphane-free and recyclable palladium catalysts having
high activity and good stability is a topic of enormous im-
portance. In the reported copper-free palladium catalyst
systems, in order to achieve the satisfactory yield either aryl
iodides or aryl bromides bearing electron-withdrawing sub-
stituents, were employed.[15] Quire recently, Hua reported
the Sonogashira reaction of electron-rich aryl bromides
with terminal alkynes using PdCl2(PPh3)2 as a catalyst and
DMF as a solvent.[16] Hell and coworkers prepared a Pd/
MgLa mixed oxide as an efficient catalyst for the Sonoga-
shira reaction of activated chlorides in DMF.[17]
As part of our studies to explore the utility of transition
metal perflates catalyzed reactions in fluorous solvents,[24]
we decided to investigate the application of a novel fluorous
palladium catalyst, palladium(II) perfluorooctanesulfonate
[Pd(OSO2Rf8)2, Rf8 = (CF2)7CF3, Pd(OPf)2] with a per-
fluoroalkylated pyridine ligand 1 [pyridine-3-carbaldehyde
bis(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorode-
cyl)acetal], for Sonogashira reaction of bromides and chlo-
rides in FBS. The coupling process can be performed under
copper- and DMF-free conditions in an air atmosphere.
The fluorous palladium catalyst and perfluoroalkylated
Introduction
The Sonogashira cross-coupling reaction, discovered in
the mid seventies, is a powerful method for C–C bond for-
mation.[1] It can be used in the synthesis of a variety of
compounds,[2] including heterocycles,[3] several natural
products and pharmaceuticals.[4] Besides natural products,
oligomers and polymers have also been prepared via the
Sonogashira reaction.[5] Traditionally, the reaction is carried
out in aprotic, polar solvents such as DMF and DMAC,
with a complex palladium catalyst in conjunction with CuX
(X = Cl, Br, I) as a cocatalyst.[6] However, these solvents
have high boiling points, which make them difficult to be
removed after completion of the reaction. Furthermore,
oxidative homocoupling of acetylenes (Glaser-type reac-
tion) cannot be avoided in copper-mediated reactions,[7] in
which diaryldiacetylene byproducts are generally difficult to
separate from the desired products and copper acetylide is
a potentially explosive reagent.[8]
In recent years numerous modifications have been re-
ported for the Sonogashira coupling procedure, such as
reaction in ionic liquids,[9] reaction in aqueous media,[10]
reaction in microemulsion,[11] polymer-[12]and zeolite-
supported[13] catalytic reaction system, phase-transfer cata-
lytic reaction conditions,[14] various copper-free condi-
tions,[8,9,15–17] use of a variety of promoters,[18] such as Zn,
Mg and Sn and the use of microwave irradiation.[19] The
use of a fluorous biphasic system (FBS) as a phase-separa-
tion and catalyst immobilization technique has also been
developed for the catalytic coupling reaction.[20] Transition-
metal-catalyzed reactions in an FBS have become one of
[a] Chemical Engineering College, Nanjing University of Science &
Technology,
Nanjing, 210094 China
Fax: +86-25-84315030
E-mail: yiwenbin@mail.njust.edu.cn
Eur. J. Org. Chem. 2007, 3445–3448
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W.-B. Yi, C. Cai, X. Wang
SHORT COMMUNICATION
pyridine are stable and easy to prepare. The fluorous phase (Table 2, Entry 1). No byproducts were observed, diaryldi-
containing the active palladium species is easily separated acetylenes, which are the most common byproducts that are
and can be reused several times without a significant loss observed in the reaction with a copper salt cocatalyst were
of catalytic activity.
not detected by GC. Compared with the reported copper-
free methods, our procedure produces better yields in
shorter reaction times.[6,15–17]
Results and Discussion
Table 2. Sonogashira reaction of aryl bromides and chlorides in
FBS.[a]
We prepared Pd(OPf)2 from palladium carbonate
(PdCO3) by stirring it with heptadecafluorooctanesulfonic
acid (Rf8SO3H, PfOH) (Scheme 1), and perfluorinated
pyridine 1 according the method described by Uemura
(Scheme 2).[25] Pd(OPf)2 and 1 were very soluble in
perfluorodecalin (C10F18), perfluoromethylcyclohexane
(CF3C6F11) and perfluorotoluene (CF3C6F5). Compound 1
also showed significant solubility in ether, THF, CHCl3 and
CH2Cl2. However, 1 always appeared to be less soluble in
toluene, hexane and octane. Pd(OPf)2 showed almost no
solubility in traditional organic solvents such as methanol,
CH3CN, CH2Cl2 and toluene.
Entry
X
Y
R
Time
[h]
Yield
[%][b]
1
2
3
4
5
6
7
8
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
H
H
H
Ph
4
5
5
6
2
2
2
2
96 (92)
95
92 (89)
84
97 (94)
94
97
96
94
95 (91)
91
89
n-C5H11
n-C6H13
CH3OCO
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-CH3CO
2-CH3CO
4-NO2
4-CF3
3-CF3
3-CHO
4-F
4-CH3
4-CH3O
2-CH3O
H
Scheme 1.
9
3
2.5
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
8
10
10
16
18
12
12
12
12
12
12
12
10
10
32
32
83 (80)
85
68
H
70[c]
85 (83)
79
4-CH3CO
3-CH3CO
2-CH3CO
4-CHO
3-CHO
2-CHO
4-CF3
4-NO2
2-NO2
4-CH3
4-CH3O
Scheme 2.
84
88 (84)
81 (80)
84
85
87
85
55 (50)
41
Quantitative data on fluorous phase affinities were
sought. The perfluorodecalin/toluene partition coefficients
were determined by GC or ICP according to a previously
reported method.[26,27] These reflect relative as opposed to 24
25
26
absolute solubilities and are summarized in Table 1. In pyr-
idine ligand, two Rf8 pony tails give high fluorous phase
27
affinities (coefficients 99.8:0.2), allowing essentially quanti-
tative recovery. Pd(OPf)2 was retained in the fluorous phase
quantitatively, with the coefficients being Ͼ99.9:Ͻ0.1.
[a] Reaction conditions: halobenzene (10 mmol), acetylene
(12 mmol), ligand 1 (0.2 mmol), Pd(OPf)2 (0.05 mmol), fluorous
solvent (4 mL), toluene (2 mL), NEt3 (15 mmol), 80 °C. [b] GC
yield based on halobenzene. Numbers in parentheses are isolated
yields. [c] 0.06 mmol of Pd(OPf)2 and 0.24 mmol of ligand 1 were
used.
Table 1. Partition Coefficients (24 °C).
Analyte
Perfluorodecalin/toluene
1
99.8:0.2[a]
Ͼ99.9:Ͻ0.1[b]
The efforts were directed to the study of recycling such a
catalytic system using the reaction of bromobenzene with
phenylacetylene. We found that, upon heating at 80 °C, the
Pd(OPf)2
[a] Detected by GC. [b] Detected by ICP.
The Sonogashira reaction of bromobenzene with phenyl- organic phase (upper toluene layer containing bromoben-
acetylene in an FBS composed of toluene and perfluorode- zene and NEt3) is miscible with the fluorous phase [bottom
calin (C10F18) at 80 °C was firstly examined. In a typical perfluorodecalin layer containing Pd(OPf)2 and ligand 1]
experiment, the catalyst and ligand were added to a mixture and the coupling reaction became a monophasic reaction.
of perfluorodecalin and toluene, then a slight excess of the After completion of the reaction, the toluene (upper phase)
acetylene (1.2 equiv.) and a solution of the aryl halide in was colourless, suggesting that palladium-1 complex could
toluene and NEt3 was added. The reaction mixture was dissolve in the fluorous phase. In addition, there is no for-
stirred at 80 °C. The course of the reaction was followed by mation of palladium black in the fluorous phase. The sepa-
GC. Thus, in the reaction of bromobenzene with phenyl- rated fluorous phase could be reused for the next reaction
acetylene in the presence of Pd(OPf)2 and ligand 1, the without any specific treatment and the work up procedure
coupling product was obtained in 96% GC yield after 4 h of recycling was accomplished by simple phase separation.
3446
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Pd/Perfluoroalkylated Pyridine Catalyst for Sonogashira Reaction
SHORT COMMUNICATION
In order to analyze and quantify the catalyst recovery, the robromobenzene with phenylacetylene (Table 2, Entry 11).
reaction rates (= activity) at cycles 1, 3 and 5 were com- The less active electron-rich p-methylbromobenzene
pared. The representative results are shown in Figure 1. It (Table 2, Entry 12) and p-methoxybromobenzene (Table 2,
was found that there was almost no difference in the forma- Entries 13–14) produced lower yields. The aryl chlorides
tion rate of the Sonogashira product at cycles 1, 3 and 5 with electron donating groups such as p-methyl (Table 2,
within a 4 h reaction time, which indicated that there was Entry 26) and p-methoxy (Table 2, Entry 27) afforded the
not only no loss of the catalyst but also no depression of coupling products in 55 and 41% yields, respectively, after
catalytic activity during the recycling. Thus, it can be de- 32 h. Steric effects did not influence the yield significantly;
duced that Ͼ99% of the catalyst should still remain in the for example, in the reaction of o-bromo- (Table 2, Entry 6)
fluorous phase even after the fourth cycle. It is important or o-chloroacetophenone (Table 2, Entry 19) with phenyl-
to study the leaching problem in these catalyst recycling sys- acetylene, the corresponding coupled products were ob-
tems. On the basis of 19F NMR spectroscopic data, no loss tained in 94 and 84% yields, respectively.
of Pd(OPf)2 to the organic phases can be detected. Only
trace amounts of ligand 1 (Ͻ0.1%) and perfluorodecalin
were found by GC–MS in separated organic layer. In fact,
we also investigated the exact amount of palladium and li-
Conclusions
gand 1 in the recovered fluorous phase by ICP and GC,
finding that Ͼ99.7% of palladium and 99.8% of ligand 1
were retained in perfluorodecalin. This result suggests the
robustness of the catalytic system for recycled use.
The fluorous biphasic system of a combination of
Pd(OPf)2 and perfluoroalkylated pyridine 1 showed a high
catalytic activity for Sonogashira coupling reaction of aryl
bomides and activated chlorides. The advantages of our
catalytic system over others are: the reaction can be per-
formed under phosphane-, copper- and DMF- free condi-
tions in an air atmosphere (1), the fluorous palladium cata-
lyst and perfluoroalkylated pyridine are easily accessible
and stable to atmosphere (2), the fluorous phase containing
palladium/perfluoroalkylated pyridine catalyst is easily sep-
arated and can be reused several times without a significant
loss of yield of reaction (3).
Experimental Section
Pd(OPf)2: Heptadecafluorooctanesulfonic acid (Rf8SO3H) was
commercially obtained from ARCOS Co. A mixture of a solution
of Rf8SO3H (1.23 g, 2.5 mmol) in water (5 mL) and palladium car-
bonate (0.17 g, 1.0 mmol) was heated at reflux with stirring for 4 h.
The resulting gelatin-like solid was collected, washed and dried at
160 °C for 24 h in vacuo to give a brown solid (0.84 g, 76%), which
does not have a clear melting point up to 500 °C, but shrinks
around 330 °C and 410 °C. 19F NMR (500 MHz, CF3C6H5): δ =
Figure 1. Sonogshira reaction of bromobenzene with phenylacetyl-
ene in FBS at different cycles.
We then screened other phosphane-free palladium cata-
lysts for the coupling of bromobenzene with phenylacetyl-
ene under the reaction conditions of Pd(OPf)2 in the pres-
ence of ligand 1 in FBS and found that Pd(OAc)2, PdCl2,
PdCO3 and PdCl2(MeCN)2 showed low catalytic activity to
afford diphenylacetylene in 46, 35, 10 and 12% GC yields,
respectively. When the less reactive acetylene, 1-heptyne
(Table 2, Entry 2) and 1-octyne (Table 2, Entry 3) were
used, the coupling product was produced efficiently. The
reaction of an electron-poor alkyne such as methyl pro-
piolate gave 84% GC yield (Table 2, Entry 4).
–126.2, –121.2, –114.2, –81.4. IR (KBr): ν = 1230 (CF ), 1148
˜
3
(CF2), 1080 (SO2), 1061 (SO2), 752 (S–O), 640 (C–S) cm–1.
C16F34O6PdS2 (1104.6): calcd. C 17.38, Pd 9.63; found C 17.29, Pd
9.61.
Typical Procedure for Sonogashira Reaction in an FBS: To a mixture
of Pd(OPf)2 (0.055 g, 0.05 mmol) and toluene (2 mL) in a glass
tube was added ligand 1 (0.204 mg, 0.2 mmol) under vigorous stir-
ring. After a few minutes, perfluorodecalin (4 mL), bromobenzene
Thus, the coupling reaction catalyzed by Pd(OPf)2–1
complex was extended to chlorobenzene, under the above (1.58 g, 10 mmol) and NEt3 (2.1 mL, 15 mmol) were introduced
into the glass tube, and then phenylacetylene (1.24 g, 12 mmol) in
toluene (2 mL) was added. After stirring at 80 °C for 4 h, the mix-
ture was cooled with an ice bath. Then, the fluorous layer on the
bottom was separated for the next reaction. The reaction mixture
(organic phase) was diluted with water (5 mL) and extracted with
hexane (2ϫ10 mL). The combined organic extracts were dried with
Na2SO4 and p-xylene (1.06 g, 10 mmol) was added as an internal
standard for GC analysis. After GC and GC–MS analyses, the sol-
vents and volatiles were removed under vacuum, and then the resi-
due was subjected to column chromatography on SiO2 (cyclo-
reaction conditions. Although the reaction became slower,
chlorobenzene gave 68% of diphenylacetylene after 16 h
(Table 2, Entry 15). Increasing catalyst loading did not im-
prove the yield significantly (Table 2, Entry 16). The effect
of substituents was also examined in this reaction. The re-
sults are summarized in Table 2. Aryl bromides (Table 2,
Entries 5–14) and electron deficient aryl chlorides (Table 2,
Entries 17–25) with aryl alkynes gave the corresponding
aryl-substituted acetylenes in good-to-excellent yields. No-
tably, 91% GC yield was obtained in the reaction of p-fluo- hexane) to give diphenylacetylene as a white solid (1.67 g, 92%).
Eur. J. Org. Chem. 2007, 3445–3448
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W.-B. Yi, C. Cai, X. Wang
SHORT COMMUNICATION
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Acknowledgments
This project was supported by the China Postdoctoral Science
Foundation (20060400942).
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Received: April 1, 2007
Published Online: June 14, 2007
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Eur. J. Org. Chem. 2007, 3445–3448