(BeDABCO)2Pd2Cl6 (1-benzyl-4-aza-1- azoniabicyclo[2.2.2]octane chloride) as a highly active catalytic system for the Stille cross-coupling reaction under microwave irradiation

Article abstract of DOI:10.1002/aoc.2958

An efficient catalytic system using (BeDABCO)₂Pd ₂Cl₆ was developed for the Stille cross-coupling reaction. The substituted biaryls were produced in excellent yields in short reaction times using a catalytic amount of this catalyst in DMF at 120 °C. The ionic character of homogeneous catalyst and microwave irradiation and also DMF as microwave-active polar solvent gave higher yields and shorter reaction times than conventional heating. Benzyl DABCO as an efficient ligand and also a quaternary ammonium salt had an efficient stabilizing effect on the Pd(0) species. Copyright 2013 John Wiley & Sons, Ltd. A palladium catalyst is synthesized using N-benzyl DABCO chloride and palladium chloride and the efficiency of this catalytic system is evaluated for Stille reaction. Copyright

Full text of DOI:10.1002/aoc.2958

Full Paper
Received: 6 October 2012
Revised: 4 December 2012
Accepted: 4 December 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2958
(
BeDABCO) Pd Cl (1-benzyl-4-aza-1-
2
2 6
azoniabicyclo[2.2.2]octane chloride) as a highly
active catalytic system for the Stille cross-
coupling reaction under microwave irradiation
a,b
a
a
Abdol R. Hajipour *, Narges Najafi and Fatemeh Rafiee
An efficient catalytic system using (BeDABCO)
2
Pd
2
Cl
6
was developed for the Stille cross-coupling reaction. The substituted
ꢀ
biaryls were produced in excellent yields in short reaction times using a catalytic amount of this catalyst in DMF at 120 C.
The ionic character of homogeneous catalyst and microwave irradiation and also DMF as microwave-active polar solvent gave
higher yields and shorter reaction times than conventional heating. Benzyl DABCO as an efficient ligand and also a quaternary
ammonium salt had an efficient stabilizing effect on the Pd(0) species. Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: palladium catalyst; Stille reaction; biaryls; microwave irradiation
homogeneous metal catalysts in conjunction with microwaves
leads to an increased lifetime of the catalyst. High-speed Stille
Introduction
The palladium-catalyzed cross-coupling of organostannanes with
coupling reaction was carried out successfully under controlled
microwave conditions.
[
1,2]
[18–20]
electrophilic organic halides and triflates
has emerged as a
potent tool for the formation of CÀC coupling reactions in the
In continuation of our recent investigations on the synthesis of
[
3]
production of new materials, natural products, carbohydrate
the palladium catalysts and application of these complexes in the
[4]
[5]
[21]
chemistry and biological research. This cross-coupling reaction
has gained importance due to the growing availability of the
organostannanes, excellent compatibility with a large variety of
microwave-assisted cross coupling reactions,
we now wish to
report the extension of (BeDABCO) Pd Cl homogeneous complex
2
2
6
for the cross-coupling reaction of various aryl halides with phenyl-
tributyltin under microwave irradiation.
[
6–8]
functional groups and their stability in moisture and air.
coupling is a powerful route to the formation of biaryls. Biaryls are
Stille
[
9]
applied as the building block of a wide range of herbicides,
pharmaceuticals, natural and bioactive products, microelec-
trode array, conducting polymers and liquid crystal materials.
[10]
[11]
[12]
[13]
Results and Discussion
In view of the importance of biaryls, a number of effective
palladium catalytic systems have been developed for the Stille
cross-coupling reaction. Generally, the combination of palladium
catalysts with various phosphine ligands results in excellent yields
1
-Benzyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride ([BeDABCO]
[
22]
Cl) was prepared according to our previous work. The reaction
of [BeDABCO]Cl with PdCl in 1:1 ratio in acetone reflux gave the
(BeDABCO) Pd Cl complex. The structure of synthesized cata-
2
[
23]
[14]
2
2
6
and high efficiency.
are expensive and air sensitive, which places significant limits on
However, most of the phosphine ligands
lyst A was suggested as shown in Scheme 1.
[15]
The effect of tetraalkylammonium salts on the activity and sta-
their synthetic applications. Thus the development of new and
[
24]
bility of palladium catalysts has been described by Jeffery.
(BeDABCO) Pd Cl catalyst comprises both N-donor ligand and
quaternary salt. Benzyl DABCO as an efficient ligand and also a
quaternary ammonium salt has efficient stabilizing effect on the
Pd(0) species and prevents aggregation of palladium metal.
efficient phosphine-free palladium catalytic systems remains a
[
16]
2
2
6
potentially promising field for organic synthesis.
2
Pd(OAc) /
1,4-diazabicyclo[2.2.2]octane (DABCO), as a stable, inexpensive
and highly efficient catalyst system, is used in cross-coupling
reactions and displays some properties as ligands to stabilize the
reactive palladium intermediates. 1,4-Diazabicyclo[2.2.2]octane
has a cage-like structure with high steric hindrance, and as a rigid
*
Correspondence to: Abdol R. Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156,
Islamic Republic of Iran. E-mail: haji@cc.iut.ac.ir
[17]
ligand has an important place in organic synthesis.
Transition-metal-catalyzed cross-coupling reactions usually need
long reaction times and an inert atmosphere to complete with tra-
ditional heating. Microwave-assisted couplings under controlled
conditions are generally quicker and cleaner than conventional
heating, reducing reaction times, yielding products in high yields
with fewer side products and increasing the selectivity. The use of
a Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan, 84156, Islamic Republic of Iran
b Department of Pharmacology, University of Wisconsin, Medical School,
Madison, WI 53706-1532, USA
Appl. Organometal. Chem. 2013, 27, 228–231
Copyright © 2013 John Wiley & Sons, Ltd.

Stille coupling reaction
Scheme 1. Synthesis of catalyst A from N-benzyl DABCO chloride and palladium chloride.
The efficiency of this catalytic system was evaluated in Stille
cross-coupling reaction under microwave irradiation. In order to
investigate the efficiency of this catalytic system and optimize the
reaction conditions, a series of screening experiments were carried
out in cross-coupling of 4-bromobenzonitrile with phenyltributyltin
in different solvents and bases under microwave irradiation. The
monitoring system for temperature, pressure, power and reaction
times in a microwave reactor allows for excellent control of reaction
parameters which generally leads to rapid optimization and more
reproducible reaction conditions. The direct control of reaction
mixture temperature was carried out with IR sensors and results
were summarized in Table 1.
palladium species by weak coordination gave the best result. We
also optimized the concentration of catalyst, employing various
amounts of catalyst for this cross-coupling using CsF as base and
DMF as solvent. The low palladium concentration usually led to a
long period of reaction, as increasing the amount of palladium cat-
alyst shortened the reaction time. The best result was obtained
when the Stille coupling reaction was carried out with 0.19 mol%
of catalyst A (Table 1, entry 12).
These optimized reaction conditions were applied in the Stille
cross-coupling reaction of various aryl halides under microwave
irradiation (Table 2). We examined the electronic and steric effects
of various aryl halides bearing electron-donating and electron-with-
drawing groups on the resulting yields and conversion times of the
reactions. The substituent effects in the aryl iodides emerged to be
less significant than in the aryl bromides and the reactivity of aryl
bromides with electron-withdrawing substituent was higher than
that of aryl bromides with electron-donating substituent. I- and
Br-substituted aryl halides are most reactive; however, Cl analogues
are cheaper and more readily available. As expected, the reactivity
of aryl chlorides was lower than that of aryl iodides and bromides as
the Stille coupling reactions of aryl chlorides required longer times
and lower yields resulted (Table 2, entries 17–20). The steric
hindrance of the procedure was examined using 2-, 3- and
The fluoride sources are potential activators for the organotin
[
25]
species and can facilitate Stille cross-couplings. We investigated
the efficacy of fluoride salts such as KF, LiF, NaF, and CsF as a base in
this reaction (Table 1, entries 5–8). Among the selected bases, CsF
was found to be the most effective. The other bases such
as K CO , Cs CO and NaOAc were less effective (Table 1, entries
2
3
2
3
1–4). Several different solvents such as DMF, N-methylpyrrolidone
(NMP), toluene, acetonitrile, dioxane, THF and ethanol were exam-
ined. Among the tested solvents DMF as a microwave-absorbing
polar aprotic solvent having the ability to additionally stabilize
4
-bromoacetophenone as hindered substituted aryls (Table 2,
entries 8–10). Increasing steric hindrance in the vicinity of
the leaving group can cause a decrease in reaction conversion.
Table 1. Optimization of reaction conditions for Stille coupling
under microwave irradiation
a
2
-Bromoacetophenone showed slower reaction times and there-
Entry
Solvent
Base
Catalyst
Temp.
Conversion
fore only a reasonable yield was obtained. The chemoselectivity of
the procedure was examined using chlorobromobenzene deriva-
tives (Table 2, entries 11 and 12). In these reactions Br acted as
the better leaving group. This catalytic complex was compatible
with a wide range of functional groups such as nitro, cyano,
methoxy, halogen and carbonyl on aryl halides.
Microwave irradiation, in comparison to conventional reflux
conditions, led to dramatically reduced reaction times and higher
yields (Table 3). The dramatic rate enhancement was due to the
rapid and uniform heating of the reaction mixture and increased
catalyst lifetime by the elimination of wall effects in microwave
versus oil-bath heating. Microwave irradiation raises the tempera-
ture of the whole volume simultaneously (bulk heating), whereas
in the oil-heated tube the reaction mixture in contact with the
ꢀ
b
(mol%)
( C)
(%)
1
2
3
4
5
6
7
8
9
EtOH
K
K
2
CO
CO
3
0.2
0.2
0.2
0.2
0.2
0.2
0.2
—
60
120
120
120
120
120
120
120
120
120
120
120
120
80
70
86
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
2
3
Cs
2
CO
3
40
NaOAc
KF
60
75
LiF
50
CsF
NaF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
100
Trace
30
0.04
0.09
0.14
0.19
—
10
11
12
13
14
15
16
17
18
19
50
85
100
—
[
26]
vessel wall is heated first.
CH
3
CN
0.2
0.2
0.2
0.2
0.2
0.2
80
THF
90
Trace
65
Conclusions
Toluene
1,4-Dioxane
NMP
100
100
130
60
53
In this work a general protocol was applied to the microwave-
promoted Stille reaction of various aryl halides using (BeDABCO)
PdCl . The catalytic amounts of this ionic complex converted
2 6
60
EtOH
85
various aryl halides to the corresponding biaryls in excellent yields.
The dipole characteristics of the catalyst translate into rapid excita-
tion by microwaves and consequently faster reactions. The combi-
nation of homogeneous complex as catalyst, high dipole moment
a
Reaction conditions: 4-bromobenzonitrile (1 mmol), phenyltributyltin
1.2 mmol), base (2 mmol), solvent (2 ml), catalyst A, 500 W, 3 min.
(
b
GC yield.
Appl. Organometal. Chem. 2013, 27, 228–231
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

A. R. Hajipour et al.
Table 2. Stille reaction of various aryl halides with phenyltributyltin
under microwave irradiation
of quaternary salt part and microwave irradiation increased the
lifetime of the catalyst, improved the yields of the reactions and
decreased reaction times.
a
catalyst A
X +
Sn(Bu)3
R1
CsF, DMF
20 °C, 500 W
R1
1
Experimental
N
N
N
General
N
(
Pd
-)
catalyst A:
Cl
Cl
Cl
Cl
Cl
Pd
All melting points were taken on a Gallenkamp melting apparatus
and are uncorrected. H NMR spectra were recorded at 400 MHz
Cl
1
(
-)
3
in CDCl solution at room temperature (tetramethylsilane was used
as an internal standard) on a Bruker Avance 500 (Rheinstetten,
Germany) and Varian 400 NMR instrument. FT-IR spectra were
recorded on a spectrophotometer (Jasco 680, Japan). Spectra of
solids were carried out using KBr pellets. Vibrational transition fre-
Entry
Ar-X
Biaryl product
Time
Yield
b
(min)
(%)
À1
1
2
3
4
5
6
7
8
9
Ph-I
p-MeO-Ph-I
p-O N-Ph-I
Ph-Ph
p-MeO-Ph-Ph
p-O N-Ph-Ph
1
4
98
92
93
97
90
91
97
94
74
64
98
95
85
83
89
82
78
75
86
80
quencies are reported as wavenumber (cm ). We used a Milestone
microwave (ATC-FO 300, Microwave Labstation GmbH) for synthe-
sis. We also used gas chromatography (BEIFIN 3420 gas chromato-
graph equipped a Varian CP SIL 5CB column: 30 m, 0.32 mm,
0.25 mm) for examination of reaction completion and yields. Palla-
dium acetate, aryl halides and all chemicals were purchased from
Merck and Aldrich and were used as received.
2
2
3
Ph-Br
Ph-Ph
2
p-MeO-Ph-Br
p-MeO-Ph-Ph
6
p-O
2
N-Ph-Br
p-O
2
N-Ph-Ph
4
p-NC-Ph-Br
p-NC-Ph-Ph
3
p-MeOC-Ph-Br
p-MeOC-Ph-Ph
m-MeOC-Ph-Ph
o-MeOC-Ph-Ph
m-Cl-Ph-Ph
7
m-MeOC-Ph-Br
o-MeOC-Ph-Br
m-Cl-Ph-Br
9
General Procedure for the Synthesis of Palladium Salt (Catalyst A):
10
11
12
13
14
15
16
17
18
19
20
12
1
To a mixture of 0.25 mmol (0.0597 g) N-benzyl DABCO chloride and
0.25 mmol (0.044 g) palladium chloride, 3 ml acetone was added in
a round-bottom flask equipped with condenser and placed in an oil
bath under reflux conditions. After 4 h the reaction mixture was
filtered and the resulting solids were washed with water (5ml) to
remove the unreacted N-benzyl DABCO chloride. The yellow-
brownish catalyst was air dried and weighed. The yield of catalyst
o-Cl-Ph-Br
o-Cl-Ph-Ph
2
p-OHC-Ph-Br
1-Br-naphthalene
p-OHC-Ph-Ph
1-Ph-Naphthalene
9-Ph-phenanthrene
2-Ph-pyridine
Ph-Ph
5
7
9-Br-phenanthrene
2-Br-pyridine
Ph-Cl
10
10
12
10
12
12
1
p-OHC-Ph-Cl
p-MeOC-Ph-Cl
p-OHC-Ph-Ph
p-MeOC-Ph-Ph
A with this procedure was 70%. H NMR (500 MHz, DMSO): dppm
3.01 (t, 6H, J= 6.8 Hz, CH
2 2
), 3.29 (t, 6H, J = 6.8 Hz, CH ), 4.48 (s, 2H,
13
p-O
2
NPh-Cl
p-O
2
N-Ph-Ph
CH ), 7.48 (br s, 5H, Harom); C NMR (125 MHz, DMSO): dppm 45.67
2
(
(
2
8
2
CÀNDABCO), 52.55 (CÀNDABCO), 67.54 (CH
2
), 127.96 (Carom), 129.88
arom), 131.09 (Carom), 134.08 (Carom); FT-IR (KBr, cm ): υ 3016,
a
Reaction conditions: aryl halide (1mmol), phenyltributyltin (1.2mmol),
CsF (2mmol), DMF (2ml), catalyst A (0.19 mol%), 120 C, 500 W.
À1
C
ꢀ
991, 1621, 1585, 1457, 1412, 1372, 1318, 1215, 1078, 1054, 906,
b
Isolated yield.
50, 808, 769, 709; UV–visible (DMSO, nm): 275, 309; decomp:
ꢀ
98 C. Anal. Calcd for C26
6 4 2
H38Cl N Pd : C 37.53, H 4.60, N 6.73;
found C 37.10, H 4.37, N 6.41.
General Procedure for the Stille Reaction of Aryl Halides
Table 3. Stille reaction of various aryl halides with phenyltributyltin
under conventional heating conditions using an oil bath
a
A mixture of the aryl halide (1mmol), phenyltributyltin (1.1 mmol)
and CsF (2mmol), (BeDABCO) PdCl (0.19 mmol%) was added to
Entry
Ar-X
Biaryl product
Time
Yield
2
6
DMF (2ml) in a round-bottom flask equipped with condenser and
placed in the Milestone microwave. Initially using a microwave
power of 500 W, the temperature was ramped from room temper-
b
(
h)
(%)
1
2
3
4
5
6
7
8
9
p-MeO-Ph-I
p-MeO-Ph-Ph
2
1
1
3
2
92
97
95
80
85
94
95
72
61
85
ꢀ
p-O
2
N-Ph-I
p-O
2
N-Ph-Ph
ature to 120 C, this taking approximately 40 s, and then held at this
Ph-Br
Ph-Ph
temperature until the reaction was completed. During this time, the
power was modulated automatically to keep the reaction mixture
p-MeO-Ph-Br
p-MeO-Ph-Ph
ꢀ
p-O
2
N-Ph-Br
p-O
2
N-Ph-Ph
at 120 C. The mixture was stirred continuously using an appropri-
p-NC-Ph-Br
p-NC-Ph-Ph
1.5
ate magnet during the reaction. After the reaction was completed,
the mixture was cooled to room temperature and diluted with
water and n-hexane or diethyl ether. The organic layer was washed
p-MeOC-Ph-Br
m-MeOC-Ph-Br
o-MeOC-Ph-Br
9-Br-Phenanthrene
p-MeOC-Ph-Ph
m-MeOC-Ph-Ph
o-MeOC-Ph-Ph
9-Ph-Phenanthrene
5
6
8
7
4
with saturated KF solution and dried over MgSO . The solution was
10
then filtered and the solvent was evaporated using a rotary evapo-
rator. The residue was purified by silica gel column chromatogra-
phy (n-hexane or n-hexane–ethyl acetate (9:1). The biaryl products
are known compounds and were characterized by comparing their
a
Reaction conditions: aryl halide (1 mmol), phenyltributyltin
1.2 mmol), CsF (2 mmol), DMF (2 ml), catalyst A (0.19 mol%), 120 C.
(
b
Isolated yield.
1
13
[23,27]
IR, H and C NMR spectra with those found in the literature.
wileyonlinelibrary.com/journal/aoc
Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 228–231

Stille coupling reaction
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Acknowledgments
1
We gratefully acknowledge the funding support received for this
project from the Isfahan University of Technology (IUT), IR Iran,
and Isfahan Science and Technology Town (ISTT), IR Iran. Further
financial support from the Center of Excellence in Sensor and
Green Chemistry Research (IUT) is gratefully acknowledged.
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Appl. Organometal. Chem. 2013, 27, 228–231
Copyright © 2013 John Wiley & Sons, Ltd.
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