Accelerated Heck reaction using ortho-palladated complex in a nonaqueous ionic liquid with controlled microwave heating

Article abstract of DOI:10.1002/aoc.1800

The activity of {Pd[C₆H₂(CH₂CH ₂NH₂)-(OMe)₂,3,4] (μ-Br)}₂ complex was investigated in the Heck-Mizoroki C-C cross-coupling reaction under conventional heating and microwave irradiation conditions in molten salt tetrabutylammonium bromide as the solvent and promoter at 130 °C. This complex in these conditions is an active and efficient catalyst for the Heck reaction of aryl iodides, bromides and even chlorides, and also arenesulfonyl chlorides. Copyright

Full text of DOI:10.1002/aoc.1800

Full Paper
Received: 11 January 2011
Revised: 14 March 2011
Accepted: 19 March 2011
Published online in Wiley Online Library: 5 May 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1800
Accelerated Heck reaction using
ortho-palladated complex in a nonaqueous
ionic liquid with controlled microwave heating
Abdol R. Hajipoura,b∗ and Fatemeh Rafieea
The activity of {Pd[C6H2(CH2CH2NH2)-(OMe)2,3,4] (µ-Br)}2 complex was investigated in the Heck–Mizoroki C–C cross-coupling
reaction under conventional heating and microwave irradiation conditions in molten salt tetrabutylammonium bromide as the
solvent and promoter at 130 ◦C. This complex in these conditions is an active and efficient catalyst for the Heck reaction of aryl
c
iodides, bromides and even chlorides, and also arenesulfonyl chlorides. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Keywords: ortho-palladate; homoveratrylamine; catalyst; Heck reaction; ionic liquid
Introduction
the combined use of microwave and ILs will certainly go a long
way towards meeting the increasing demand for environmentally
benign chemical processes.[9]
Owing to the increase in environmental consciousness in chemical
research and industry, the challenge of achieving a sustainable
environment calls for clean procedures that avoid the use
of harmful organic solvents. Ionic liquids (ILs) have received
significant attention owing to their interesting chemical and
physical properties, such as wide liquid range with melting point
around room temperature, good stability in air and moisture,
high solubility including inorganic, organic and even polymeric
materials, and negligible vapor pressure.[1] Ionic liquids are used
as solvents or as co-catalysts for transition metal catalysis. The
solubility of organometallic compounds increases in ILs and
therefore they are best solvents for reactions with homogeneous
catalysts.[2,3] The uses of ILs in transition metal-catalyzed reactions
have many advantages, including improvement of the stability of
the catalyst, assistance in the activation of the catalyst, excellent
immobilization and recyclability, facilitated product isolation and
influence over the selectivity of the reaction.[4]
Microwave-assisted organic synthesis has been known since
1986.[5] This‘nonconventional’syntheticmethodhasshownbroad
applicationsasaveryefficientwaytoacceleratethecourseofmany
organic reactions, producing high yields and higher selectivity,
lower quantities of side products and, consequently, easier work-
up and purification of the products.[6]
Transition metal-catalyzed reactions represent one of the most
important and well-studied reaction types in microwave-assisted
organicsynthesis.[7] Thesereactionstypicallyneedhoursordaysto
reach completion with traditional heating under reflux conditions
and often require an inert atmosphere. The use of microwave
irradiation in transition metal-catalyzed reactions, which are
usually time-consuming, has assumed great importance owing
to the reduction of the reaction times to minutes and the decrease
in discarded byproducts from thermal side-reactions.[8]
The Heck coupling, one of the most important palladium-
catalyzed C–C bond formation reactions, is a process that can
also be described as olefin arylation. The Heck reaction can be
performed not only in organic solvent but also in ionic liquids
or molten salts, where PdII complexes or Pd0 nanoparticles act
as catalysts. The main catalytic cycle is initiated by oxidative
addition of aryl halide to the Pd0 species, which is followed by
the reversible coordination of the olefin to the oxidative addition
product.ThePd0 formedinthemaincatalyticcycle,afterβ-hydride
andreductiveeliminationsteps,caneithercontinueinthecatalytic
cycle or fall back to the nanoparticles reservoir.[10–23]
The Heck reaction has received considerable attention, pri-
marily owing to the enormous synthetic potential to generate
sp2 –sp2 carbon–carbon bonds.[24] However, the reaction suf-
fers from limitations such as needing a relatively large amount
of catalyst (>1 mol%) for reasonable conversions and having
hampered catalyst recycling. Various approaches towards cat-
alyst improvement have been described.[25,26] Most attempts
have been made with activated, electron-poor chloroarenes using
highly basic, sterically hindered phosphanes,[27] N-heterocyclic
carbenes,[28] palladacycles,[29,30] a large excess of coordinat-
ing ligands [for example triphenylphosphane[31] or tris(2,4-
di-tert-butylphenyl)phosphate[32]], heterogeneous Pd/C[33] or
Pd/MgO,[34] or nanostructured palladium clusters.[35]
The Heck reaction is usually performed in polar solvents
like acetonitrile, dimethylformamide (DMF), dimethylacetamide,
∗
Correspondence to: Abdol R. Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156,
I. R. Iran. E-mail: haji@cc.iut.ac.ir
Microwave-assisted organic transformation coupled with ILs
has attracted attention because of its enhanced reaction rates,
easier workup and straightforward purification. The dipole
characteristics of IL translate into rapid excitation by microwaves
and consequently faster reactions. The synergies arising from
a
Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan 84156, I. R. Iran
b
Department of Pharmacology, University of Wisconsin, Medical School, 1300
University Avenue, Madison, 53706-1532, WI, USA
c
Appl. Organometal. Chem. 2011, 25, 542–551
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

Accelerated Heck reaction using ortho-palladated complex
Scheme 1. The Heck cross coupling reaction aryl halides with olefins by orthopalladate complex A.
N-methyl-2-pyrrolidinone (NMP) and 1,4-dioxane.[24] Since it was
Table 1. Optimization of base and solvent under conventional
heating in an oil batha
knownthatsaltadditivescanactivateandstabilizethecatalytically
active palladium species,[36] the use of nonaqueous ionic liquids or
molten salts seemed to be a promising alternative to conventional
molecular solvents.[37] The use of ionic liquids would have an
advantage over conventional reaction conditions if the product
could be distilled from the ‘solvent’ and the still active, stable
catalyst, thus giving the opportunity to recycle the whole
catalyst–solvent system.[11]
The use of tetraalkylammonium salts has made it possible to
obtain very good results in a number of Heck reactions[12,13]
as well as carbonylation and Suzuki reactions.[19] The role of
tetraalkylammonium salts and their interactions with palladium
catalyst have been discussed by many authors. The co-catalytic
effect of tetraalkylammonium salts as phase-transfer agents
has been considered.[38,39] Tetraalkylammonium salts can enter
the catalytic cycle and assist oxidative addition and reductive
elimination during a Pd0/PdII catalytic cycle.[11,40–43]
In a continuation of our recent investigations on the appli-
cation of the new palladacycle complex {Pd[C6H2(CH2CH2NH2)-
(OMe)2,3,4] (µ-Br)}2 (A), in Heck reactions[44,45] and cyanation
reactions,[46] we report here the extension of its utilization in tetra-
butylammonium bromide (TBAB) as a nonaqueous ionic liquid and
under microwave irradiation as a nonconventional green energy
source (Scheme 1).
Temperature Time Conversion
Entry Solvent
Base
(◦C)
(h)
(%)
1
NMP
Et3N
130
130
130
130
130
130
80
6
6
6
2.5
3
6
6
6
6
4
6
4
2
2
6
6
2
0
30
2
NMP
Cs2CO3
Na2CO3
K2CO3
3
NMP
Trace
100
70
4
NMP
5
NMP
CH3COONa
K2CO3
6
DMF
95
7
CH3CN
Toluene
K2CO3
0
8
K2CO3
110
130
130
130
130
130
130
130
130
130
0
9
[bmim]Br K2CO3
[bmim]Br CH3COONa
[bmim]Br Et3N
60
10
11
12
13
14
15
16
17
80
10
TBAB
TBAB
TBAB
TBAB
TBAB
TBAC
K2CO3
70
NaHCO3
CH3COONa
Na2CO3
90
100
70
Cs2CO3
40
CH3COONa
90
a Reaction conditions: 1 mmol aryl halide, 2.5 mmol olefin, 1.1 mmol
base, 0.1 mol% palladacycle A.
both conventional heating and microwave irradiation. We did
not use stronger bases like KOH or KOtBu, because they lead to
decomposition of TBAB in Hofmann-type elimination.[11]
Results and Discussion
A few preliminary experiments were carried out on vinylation of
bromobenzene with styrene using palladacycle A as the catalyst
to optimize the reaction conditions. In order to adjust the effect
of various parameters on the reaction time and yield, the reaction
was carried out in various solvents and organic and inorganic
bases under both conventional heating and microwave irradiation
(600 W, 130 ◦C), as shown in Tables 1 and 2.
In order to study the solvent effect, we compared molten salt
TBABwiththecommonlyusedmolecularsolvents(Tables 1 and 2).
The results showed increased yields and decreased time in molten
[NBu4]Br in competition to NMP as the best molecular solvent
under conditions ceteris paribus. After optimization of base and
solvent, various catalyst concentrations were also tested (Tables 3
and 4), and 0.1 mol% under conventional heating and 0.05 mol%
under microwave irradiation gave the best result.
As demonstrated in Table 1 (entry 14) and Table 2 (entry 12),
NaOAc as base and TBAB as solvent gave the best results under
c
Appl. Organometal. Chem. 2011, 25, 542–551
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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A. R. Hajipour and F. Rafiee
Table 2. Optimization of base and solvent under microwave
Table 4. Optimization of catalyst concentration under microwave
irradiationa
irradiationa
Mol% catalyst
Time (min)
Temperature (◦C)
Conversion (%)
Temperature Time Conversion
Entry Solvent
Base
(◦C)
(min)
(%)
None
0.01
0.03
0.05
0.1
2
2
2
2
2
2
130
130
130
130
130
130
0
65
1
NMP
NMP
NMP
NMP
NMP
DMF
Et3N
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
2
2
2
2
2
2
2
2
2
2
2
1
2
2
2
2
0
30
2
Cs2CO3
Na2CO3
K2CO3
85
3
Trace
100
70
100
100
100
4
5
CH3COONa
K2CO3
0.2
6
70
a Reaction conditions: 1 mmol aryl halide, 2.5 mmol olefin, 1.1 mmol
NaOAc, 3 mmol TBAB and palladacycle A.
7
[bmim]Br K2CO3
[bmim]Br CH3COONa
[bmim]Br Et3N
60
8
55
9
10
10
11
12
13
14
15
16
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAC
K2CO3
90
NaHCO3
CH3COONa
Na2CO3
Cs2CO3
97
the olefin and bromobenzene with 1 mol% PdCl2 needed 13 h to
complete.[11]
100
75
The results showed that aryl halides with either electron-
withdrawing or electron-donating substituents reacted with
olefins rapidly and generated coupled products with excellent
yields. We found that, in comparison to conventional heating
conditions, microwave irradiation reduced the reaction times
from hours to minutes with reasonable yields. The microwave
energy can easily penetrate the particle, and all particles can
be heated simultaneously, thus reducing heat transfer problems.
Also the dipole characteristics of TBAB provide rapid excitation by
microwaves and so decrease the reactions time in comparison to
conventional heating.
45
Et3N
15
CH3COONa
97
a Reaction conditions: 1 mmol aryl halide, 2.5 mmol olefin, 1.1 mmol
base, 0.1 mol% palladacycle A.
Table 3. Optimization of catalyst concentration under conventional
heatinga
Inaddition, theresultslistedinTable 5and 6clearlyshowedthat
thecouplingreactionswithmethylacrylateastheolefinwerefaster
than those with styrene. In the case of 1-bromo-3-chlorobenzene,
although this procedure could active the C–Cl bond, by using a
stoichiometric amount of olefins owing to the higher activity of
Br compared with Cl, only the Br was substituted. In all of the
reactions, only the trans isomers were produced.
Mol% catalyst
Time (h)
Temperature (◦C)
Conversion (%)
None
0.01
0.03
0.05
0.1
2
130
130
130
130
130
130
130
130
0
50
2
2
65
2
90
1.5
2
100
100
100
100
0.2
The most reported catalysts need to be used in high loadings
and they show little or no activity with aryl chloride substrates.
The ideal substrates for coupling reactions are aryl chlorides,
since they tend to be cheaper and more widely available than
their bromide or iodide counterparts. Unfortunately the high C–Cl
bond strength compared with C–Br and C–I bonds disfavors
oxidative addition, the first step in catalytic coupling reactions.
The activity with aryl chloride substrates can be greatly improved
by the use of appropriate additives, typically an ammonium or
phosphonium salt such as TBAB or tetraphenylphosphonium
chloride.[47] FurthermoreasshowninTable 6(entries11, 12, 24and
25) the palladacyclic complex A can be used in the Heck coupling
of even less reactive aryl chloride derivatives with olefines in TBAB
as a nonaqueous ionic liquid with longer reaction times.
Arenesulfonyl chlorides could be used as electrophilic partners
under desulfinylation conditions in place of arylhalides (Table 6,
entry 13 and 26). Arenesulfonyl chlorides are inexpensive
and readily available compounds and more reactive than
the corresponding bromides and chlorides. The reactive Pd0
catalyst undergoes oxidative addition of the SO2 –Cl bond of
the sulfonyl chloride to give first a chloropalladiumII sulfinate
(Ar–SO2 –PdII –Cl). At high temperature (130 ◦C), desulfinylation
occurs rapidly and an R–Pd–Cl complex forms, which is followed
by the reversible coordination of the olefin to it. The Pd0 forms in
the main catalytic cycle, after β-hydride and reductive elimination
steps.[48]
0.4
2
0.6
2
a Reaction conditions: 1 mmol aryl halide, 2.5 mmol olefin, 1.1 mmol
NaOAc, 3 mmol TBAB and palladacycle A.
The melting point of TBAB is in the range 103–104 ◦C;
therefore it needs high temperatures of over 120 ◦C to reduce
the viscosity of the melt to a sufficient extent to make magnetic
stirring possible. We did not apply tetra-n-butylammonium
salts with the noncoordinating anions hexafluorophosphate
or tetrafluoroborate and also tetra-n-butylammonium iodide
because they need higher temperatures owing to their high
melting points. The reaction was better carried out at 130 ◦C in
view of the fact that a higher temperature (150 ◦C) affected the
decomposition of TBAB.
We applied these conditions for Heck cross-coupling reactions
of different types of aryl halides with methylacrylate and styrene
as olefines under both conventional heating and microwave
irradiation, as shown in Tables 5 and 6.
Herrmann and Bo¨hm reported using TBAB in the Heck reaction
with a simple palladium salt such as Pd(OAc)2 or PdCl2. In
comparison with palladacycle A, these reactions took a long
time, for example, the Heck reaction of n-butyl methacrylate as
c
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Appl. Organometal. Chem. 2011, 25, 542–551

Accelerated Heck reaction using ortho-palladated complex
Table 5. Heck reaction of aryl halides under conventional heating conditions in an oil batha
Entry
1
ArX
RꢁCH CH2
Product
Time (h)
1.5
Yield (%)b
93
2
4
84
3
4
5
1
93
93
91
0.75
6
6
7
0.5
1.5
93
96
8
9
2
6
95
85
10
1
93
c
Appl. Organometal. Chem. 2011, 25, 542–551
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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A. R. Hajipour and F. Rafiee
Table 5. (continued)
Entry
11
ArX
RꢁCH CH2
Product
Time (h)
7
Yield (%)b
85
12
6.5
80
a Reaction conditions: 1 mmol aryl halide, 2.5 mmol olefin, 1.1 mmol NaOAc, 3 mmol TBAB, 0.2 mol% palladacycle A and temperature 130 ◦C.
b Isolated yield.
The useful effects of tetrabutylammonium salts as additives Experimental
or solvents in the Heck reaction have been known for a long
time.[49] The co-catalytic effect of tetraalkylammonium salts has
General
1H-NMR spectra were recorded using 500 and 400 MHz in
been assigned as phase-transfer catalysis and increased polarity
of the solvent, which stabilizes ionic and polar transition states in
the catalytic cycle and leads to an increase in the whole reaction
rate. TBAB can also facilitate oxidative addition and reductive
elimination during a Pd0/PdII catalytic cycle. In the literature it has
also been reported that, in the presence of tetraalkylammonium
salts, the Heck reaction can proceed through a hypothetical
PdII/PdIV catalytic cycle in competition with the established
Pd0/PdII mechanism.[11,50] However, a PdII/PdIV mechanism for
the Heck reaction is no longer accepted. On the contrary, direct
NMRmeasurementsandagreatdealofexperimentaldataindicate
that the Heck reaction proceeds via a Pd0/PdII catalytic cycle.[51]
As demonstrated in Tables 1 and 2, comparative study of the
effect exerted by different ionic liquids on the reaction rate
indicates the domination of the quaternary ammonium halides
over imidazolium ionic liquids. Imidazolium ILs also have a high
polarity comparable to those of polar aprotic solvents. It appears
that the Coulombic interaction between cations and anions in ILs
influences the activity catalysts in the Heck reaction. In TBAB, the
bulkiness of the tetrahedral ammonium cation makes this anion
more nucleophilic and available for increased activity and stability
of the palladium catalysts by forcing the bromide away from
the cation.[52] Indeed, the planar structures of [bmim]+ cations,
by binding tightly the halide ions, decrease their availability.[53]
Weak interaction of the bromide anion with tetrabutylammonium
cation in TBAB makes possible the reaction of bromide with the
relatively unstable 14-electron complex L2Pd(0) that achieved
palladacycle A. This reaction would lead to an anionic, more stab+le
CDCl3 solutions at room temperature with a Bruker Avance
500 instrument (Rheinstetten, Germany) and Varian 400 NMR
(Tetramethylsilane (TMS) was used as an internal standard). The
FT-IR spectra were recorded on a spectrophotometer (Jasco-680,
Japan). We used the Milestone Microwave (Microwave Labstation)
forsynthesis.AllchemicalswerepurchasedfromMerckandAldrich
and were used as received.
Synthesis of palladacycle complex (A)
{Pd[C6H2(CH2CH2NH2)-(OMe)2,3,4] (µ-Br)}2 (A) as a palladacycle
complex was prepared according to our previous work.[44]
General procedure for the microwave-assisted Heck cross-coupling
reaction
In a round-bottom flask equipped with a magnetic stirring bar,
to a mixture of NaOAc (1.1 mmol), olefin (2.5 mmol) and aryl
halide (1 mmol) in TBAB (3 mmol), were added 0.1 mol% of
palladacyclecomplex(A).Theflaskwasequippedwithacondenser
for refluxing in microwave irradiation. The mixture was heated
in the microwave oven at 130 ◦C and 600 W. In the case of
conventional heating reactions, the above mixture was heated
at 130 ◦C in an oil bath with 0.5 mol% of palladacycle complex
(A). The progress of the reactions was monitored by thin-layer
chromatography (hexane–EtOAc, 70 : 30). After completing the
reaction, the mixture was diluted with ether and water. The
organic layer was washed with brine, dried over Na2SO4 and
concentrated under reduced pressure. The residue was purified
by recrystallization from ethanol and water. The products were
and catalytically active 16-electron complex [L2Pd(0)Br]− NR4
.
The anionic Pd center in the oxidative addition step facilitates
conversion of Pd0 to PdII. In competing with TBAC, the bromide
ion is more nucleophilic than the chloride and therefore the
reaction proceeds faster in TBAB. In the illustrated procedure,
TBAB can act as a co-catalyst and solvent. The catalyst and solvent
are recycled and used again after distillation of the reactants and
products in a vacuum.
1
characterized by comparing their melting points, IR, H and 13C
NMR spectra with those found in the literature.[23,44]
In the recycling process, after completing the reaction and
distillation of the products in vacuum, acetone was added to the
mixture. Then NaBr could be separated and removed by filtration,
and the catalyst and the [NBu4]Br reused and further recycled.
c
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Appl. Organometal. Chem. 2011, 25, 542–551

Accelerated Heck reaction using ortho-palladated complex
Table 6. Microwave assisted Heck reaction of aryl halides
Entry
1
ArX
RꢁCH CH2
Product
Time (min)
1
Yield (%)b
91
2
3
1
94
85
10
4
4
93
5
6
1
6
94
90
7
8
4
9
91
84
9
40
81
c
Appl. Organometal. Chem. 2011, 25, 542–551
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A. R. Hajipour and F. Rafiee
Table 6. (continued)
Entry
10
ArX
RꢁCH CH2
Product
Time (min)
12
Yield (%)b
89
11
12
4
90
82
20
13
14
4
1
93
94
15
16
12
95
95
2
17
18
83
c
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Appl. Organometal. Chem. 2011, 25, 542–551

Accelerated Heck reaction using ortho-palladated complex
Table 6. (continued)
Entry
18
ArX
RꢁCH CH2
Product
Time (min)
5
Yield (%)b
91
19
17
80
20
21
22
18
92
92
90
4
20
23
30
78
24
25
6
90
80
25
c
Appl. Organometal. Chem. 2011, 25, 542–551
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A. R. Hajipour and F. Rafiee
Table 6. (continued)
Entry
26
ArX
RꢁCH CH2
Product
Time (min)
5
Yield (%)b
90
Conclusion
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The use of dimeric orthopalladate complex {Pd[C6H2
(CH2CH2NH2)-(OMe)2,3,4] (µ-Br)}2 as an efficient catalyst for the
Heck reaction of aryl bromides, aryl iodides, aryl chlorides and
arenesulfonyl chlorides under both traditional heating condition
and microwave irradiation in TBAB as a nonaqueous ionic liquid
was reported. The catalytic amount of palladium complex in TBAB
at 130 ◦C led to C–C bond formation and corresponding products
were produced in good yields. The results from both conditions
were compared. The microwave-absorbing nature of ionic liquids
improve the reaction times. The dipole characteristics of TBAB
translate into rapid excitation by microwaves and consequently
faster reactions. Improved catalyst efficiencies compared with
molecular solvents were observed for reaction of arylhalides with
olefines.
[13] V. Calo, A. Nacci, L. Lopez, N. Mannarini, Tetrahedron Lett. 2000, 41,
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[14] S. Bauquillon, B. Ganchegui, B. Estrine, F. Henin, J. Muzart,
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We gratefully acknowledge the funding support received for
this project from the Isfahan University of Technology, I. R. Iran
and Isfahan Science and Technology Town, I. R. Iran. Further
financial support from the Center of Excellence in Sensor and
Green Chemistry Research, Isfahan University of Technology, is
gratefully acknowledged.
[17] I. Pryjomska-Ray, A.M. Trzeciak, J. Zio´łkowski, J. Mol. Catal. A: Chem.
2006, 257, 3.
[18] Y. Liu, M. Li, Y. Lu, G. H, Gao, Q. Yang, M. Y. He, Catal.Commun. 2006,
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