Reusable, polymer-supported, palladium-catalyzed, atom-efficient coupling reaction of aryl halides with sodium tetraphenylborate in water by focused microwave irradiation

Article abstract of DOI:10.1002/adsc.200700361

A rapid and efficient cross-coupling reaction of sodium tetraphenylborate with aryl bromides was carried out in water at 120°C in the presence of a polymer-supported palladium catalyst and potassium carbonate under focused microwave irradiation. All four phenyl groups of sodium tetraphenylborate participated in the reaction and produced polyfunctional biaryls in excellent yields. The polymeric catalyst can be easily separated from the reaction mixture and reused more than 10 times without showing any decrease in activity.

Full text of DOI:10.1002/adsc.200700361

FULL PAPERS
DOI: 10.1002/adsc.200700361
Reusable, Polymer-Supported, Palladium-Catalyzed, Atom-
Efficient Coupling Reaction of Aryl Halides with Sodium
Tetraphenylborate in Water by Focused Microwave Irradiation
Lin Baia,c and Jin-Xian Wanga,b,*
a
Institute of Chemistry and Department of Chemistry, Northwest NormalUniversity, 805 An Ning Road (E.), Lanzhou
730070, Peopleꢀs Republic of China
Fax : (+86)-931-776-8159; e-mail: wangjx@nwnu.edu.cn
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, Peopleꢀs Republic of China
Institute of Green Chemistry Experiment and Teaching, Lanzhou City University, Lanzhou 730070, Peopleꢀs Republic of
b
c
China
Received: July 23, 2007; Revised: November 18, 2007; Published online: January 4, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A rapid and efficient cross-coupling reac- lyst can be easily separated from the reaction mix-
tion of sodium tetraphenylborate with aryl bromides ture and reused more than 10 times without showing
was carried out in water at 1208C in the presence of any decrease in activity.
a polymer-supported palladium catalyst and potassi-
um carbonate under focused microwave irradiation.
All four phenyl groups of sodium tetraphenylborate Keywords: focused microwave heating; polymer-sup-
participated in the reaction and produced polyfunc- ported palladium; sodium tetraphenylborate; Suzuki
tional biaryls in excellent yields. The polymeric cata- cross-coupling; water
Introduction
for the Suzuki coupling is becoming an area of grow-
ing importance.
Homogeneous catalysts have many attractive proper-
The major disadvantages of homogeneous catalytic
ties, such as high chemoselectivity, regioselectivity processes are often due to contamination of the prod-
and/or enantioselectivity of the reaction. However, ucts by palladium residues even after the purification
many homogeneous catalytic systems cannot be com- step and the impossibility to recover the expensive
mercialized because of difficulties associated with sep- catalysts for reuse. The development of heterogene-
arating the products from the catalyst. The Suzuki ous alternatives where the metal is grafted on inor-
coupling, which is one of the most commonly used ganic or organic supports has attracted great interest
methods for biaryl synthesis, has conventionally been in the past years, because it could combine the advan-
performed by homogeneous palladium catalysts in the tages of both homogeneous and heterogeneous cata-
presence of various phosphine ligands.[1] Virtually all lysts in one system.[2,3] It does meet both economical
forms of palladium can be used as precatalysts for the and green chemistry requirements and has proven ef-
highly active reactions, yet catalyst systems based on fective in overcoming these problems.[4,5] Recently, re-
ligand-free palladium salts [PdACHTRE(UGN OAc)2, PdCl2, etc.] ports have appeared on the Suzuki coupling with pal-
and even Pd/C have attracted increased attention due ladium loaded on magnesium-lanthanum mixed
to their low cost.[2,3] Recently, significant progress in oxides,[6] g-Al2O3,[7] and mesoporous silica.[8] Several
the Suzuki cross-coupling reaction has been made in approaches to immobilize palladium catalysts on poly-
several key research areas, such as environmentally mer supports have been described in the literature,
benign media, the design of recoverable and reusable the most common is polystyrene (cross-linked with di-
catalysts, solid-phase synthesis, and the development vinylbenzene), which can bear different functional
of synthetic methodology.[4] From both scientific and groups.[4a,c,d,5] The polystyrene-supported palladium
environmentalpoints of view, deveolpment of a new
catalysts can be seen as a borderline class of catalysts,
catalytic system and environmentally friendly media which retain the advantages of homogeneous catalysts
while securing the ease of recovery and work-up of
Adv. Synth. Catal. 2008, 350, 315 – 320
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Lin Bai and Jin-Xian Wang
FULL PAPERS
heterogeneous catalysts. These catalysts are also ad- under microwave irradiation conditions.[18] Herein, we
vantageous in as much as contamination of the prod- report the PS-supported, Pd(II)-catalyzed, heteroge-
ucts by the phosphine ligand is avoid. The palladium neous reaction of Ph4BNa with 4 equivs. of arylbro-
complex supported on polystyrene resin has indeed mides in water using our methodology. The reaction
achieved excellent results in the Suzuki cross-coupling is performed in a focused microwave synthesis system
reaction of arylboronic acids with aryl halides or tri- so that it is possible to control the temperature, pres-
flates.[9–12] The polystyrene-supported palladium cata- sure, microwave power, and reaction times very easily
lysts were stable to heat and air and were reused mul- and with a high degree of reproducibility.
tiple times with no significant decrease in activity.
However, it was demonstrated that in many cases, the
polymeric catalysts lose the palladium, which leaches Results and Discussion
under the reaction conditions from the surface into
the solution because the reaction was carried out in As a starting point for the development of our micro-
organic solvents (such as DMF, dioxane, toluene, etc.) wave-mediated methodology and the economical re-
and the polymeric catalysts are broken by vigorous, action in water, we initially studied the microwave-
long-lasting stirring.
promoted coupling of 4-bromoacetophenone with
Water has clear advantages as an environmentally Ph4BNa for optimizing reaction conditions, and the
friendly solvent alternative in organic synthesis due to effects of base, reaction time, temperature on the
its low cost, non-flammability, non-toxicity, and the yield of the coupling reaction are summarized in
fact that it is a renewable resource. In addition, the Table 1.
use of water in combination with hydrophilic homoge-
The effect of bases on the reaction was significant
neous metal catalysts offers the potential to easily (Table 1, entries 1–6), because the desired cross-cou-
separate and recycle the metal catalyst from organic pling products were not obtained in any noticeable
product streams.[4a] With its high dielectric constant, amounts in the absence of bases. A comparative reac-
water is also potentially a very useful solvent for the tivity study of bases in the reaction showed that
microwave-assisted Suzuki cross-coupling reac- K2CO3, K3PO4, and Na2CO3 proved to be more effec-
tion.[13,14] Thus, we were interested to see if the combi-
nation of the polystyrene-supported palladium cata-
lyst and water alone as a solvent would show im-
proved activity and stability of the catalyst in micro-
wave-promoted cross-coupling reactions. By using this
method it is possible to shorten the reaction time and
decreased leaching of the catalyst occurs in water.
The unique advantage associated with sodium tetra-
phenylborate, unlike boronic acid or ester and other
reagents, is that it can react with 4 equivalents of elec-
trophilic reagents. From both economical concerns
Table 1. Optimized conditions via the coupling of 4-bromo-
acetophenone with Ph4BNa.[a]
Entry Base [mmol]
Time [min] T [oC] Yield [%][b]
and increased environmentalawareness, this is an
[15]
atom economicalreaction in organic synthesis.
De-
1
2
3
4
5
6
7
8
Na2CO3 (3.5)
K2CO3 (3.5)
K3PO4 (3.5)
NaOH (3.5)
KF (3.5)
15
15
15
15
15
120
120
120
120
120
120
110
125
130
120
120
120
120
120
120
90
95
91
78
84
90
87
94
90
82
90
94
81
91
94
spite this obvious advantage, only a few reactions in-
À
volving Ph4BNa for C C bond formation have been
reported.[13a,16,17] The known cross-coupling reaction of
Ph4BNa with 4 equivalents of aryl halide involves ho-
KF/Al2O3 (3.5) 15
mogeneous palladium catalysts [PdACHTRE(UGN OAc)2, PdCl2] and
K2CO3 (3.5)
K2CO3 (3.5)
K2CO3 (3.5)
K2CO3 (2.5)
K2CO3 (3.0)
K2CO3 (4.0)
K2CO3 (3.5)
K2CO3 (3.5)
K2CO3 (3.5)
15
15
15
15
15
15
12
14
16
with electrophilic coupling reagents such as iodo-
(and bromo-)benzoic acids, p-iodonitrobenzene, and
allyl bromide;[16] 2-bromopyridine, 5-bromoindole;[17]
p-bromoacetophenone, p-bromobenzoic acid and p-
bromomethoxybenzene with no transition-metalcata-
lyst being used under focused microwave irradiation
conditions.[13a] Thus, to expand the scope and reactivi-
ty of Ph4BNa, the development of new and efficient
catalytic protocols is necessary.
We have also reported the efficiency of the poly-
styrene-supported palladium-catalyzed Suzuki cross-
coupling of aryl bromides (1 equiv.) with Ph4BNa
(1 equiv.) using toluene-H2O (10:1) as a co-solvent
9
10
11
12
13
14
15
[a]
Reaction conditions: p-CH3COC6H4Br (1.0 mmol),
Ph4BNa (0.25 mmol), TBAB (0.3 mmol), PS-supported
Pd(II) catalyst (corresponding to 1 mol% palladium),
and water (2 mL); microwave irradiation at 10 W.
Yield of isolated products (average of two runs).
[b]
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Reusable, Polymer-Supported, Palladium-Catalyzed, Atom-Efficient Coupling Reaction
FULL PAPERS
Table 2. Effect of solvents on the coupling of 2-bromonaph-
tive for this coupling. Considering the economical and
environmentaladvantages, we chose K 2CO3 as the
base. The quantity of K2CO3 was also found to be im-
portant (Table 1, entries 2, 10–12). A 3.5 mmol of
K2CO3 in 2 mL water worked most efficiently. The
highest yield of 4-acetylbiphenyl can be obtained at
1208C after 15 min microwave heating (Table 1, en-
tries 2, 3–15).
We have also investigated the effects of solvent and
phase-transfer catalyst (PTC) on a representative re-
action: the coupling of 2-bromonaphthalene with
Ph4BNa (Scheme 1).
thalene with Ph4BNa.[a]
Entry
Solvents
Yield [%][b]
1
2
3
4
5
Benzene-H2O (5 mL-1 mL)
Toluene-H2O (5 mL-1 mL)
H2O (2 mL)
Ethanol-H2O (5 mL-1 mL)
Solvent-free
48
32
93
82
40
[a]
Reaction conditions: 2-bromonaphthalene (1.0 mmol),
Ph4BNa (0.25 mmol), K2CO3 (3.5 mmol), TBAB
(0.3 mmol), PS-supported Pd(II) catalyst (corresponding
to 1 mol% palladium); microwave irradiation at 1208C
for 15 min.
Water is potentially a very useful solvent with its
high dielectric constant for the microwave-assisted,
Suzuki-type coupling reaction of aryl halides with
phenylboronic acid.[13] In our previous report,[18] the
[b]
Yield of isolated products.
reaction could not couple in water using PS-supported chloride (TBAC), tetrabutylammonium bromide
Pd(II) as catalyst under an improved reflux domestic (TBAB),
microwave oven. Toluene-H2O or benzene-H2O was (BTMAC),
found to be an effective solvent for the reaction. (BTEAC),
benzyltrimethylammonium
benzyltriethylammonium
cetyltrimethylammonium
chloride
chloride
bromide
However, aryl bromides cannot completely couple (CTMAB), and polyethylene glycol (PEG) were
with all four phenyl groups of Ph4BNa. The corre- used. The best yields, for the representative reaction,
sponding biaryl products were isolated in good yields, were obtained in the presence of TBAB as catalyst
using a stoichometric ratio of the two reagents of 1:1, (Table 3).
respectively.
The PS-supported Pd(II) catalyst with a Pd/P ratio
The Discover single-mode focused microwave of 1/1 was prepared by the procedure described previ-
system is a new device designed for synthesis. It can ously,[18] using a Merrifield resin with 2% divinylben-
be operate at elevated pressure and temperature zene under ultrasound conditions. The activity of the
using sealed vessels. The focused microwave system is PS-supported Pd(II) catalyst for the Suzuki reactions
able to raise the temperature from ambient to 2508C of arylbromide with sodium tetraphenyblorate in
(pressure is close to 300 psi). In our initial efforts, we water was first measured on a representative reaction
screened the representative reaction of 2-bromonaph- (Scheme 1). Our catalysts were stable for more than 5
thalene (4 equivs.) as
a example with Ph4BNa years at least when stored in air at 208C. They were
(1 equivs.) using benzene-H2O and toluene-H2O as
solvents under focused microwave irradiation
(Table 2, entries 1 and 2). Unfortunately, these sol-
vents failed to promote formation of the correspond-
ing atom-efficient cross-coupling products in high
yields. The experiments proved that the polymeric
catalyst along with K2CO3 in water at 1208C were the
idealreagent combination affording high conversions
of the atom-efficient products. The PS-supported
Pd(II) catalyst was activated in water at high temper-
ature under focused microwave irradiation.
Similar to classical PTC reaction conditions, under
solid-liquid PTC conditions with the use of micro-
waves the role of the catalyst is very important. For
the Suzuki reaction it has been found that, in the ab-
sence of a PTC, the reaction proceeds very slowly or
not at all. During investigation of the effect of PTC
Table 3. Effect of PTC on the coupling of 2-bromonaphtha-
lene with Ph4BNa.[a]
Entry
Phase-transfer catalyst
Yield [%][b]
1
2
3
4
5
6
7
8
TBAC
TBAB
86
93
65
78
74
73
53
36
BTMAC
BTEAC
CTMAB
PEG-400
PEG-1000
None
[a]
Reaction conditions: 2-bromonaphthalene (1.0 mmol),
Ph4BNa (0.25 mmol), K2CO3 (3.5 mmol), PTC
(0.3 mmol), PS-supported Pd(II) catalyst (corresponding
to 1 mol% palladium), and water (2 mL); microwave ir-
radiation at 1208C for 15 min.
on the reaction of arylbromide with Ph BNa, quater-
4
[b]
Yield of isolated products.
nary ammonium salts such as tetrabutylammonium
Scheme 1.
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Lin Bai and Jin-Xian Wang
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Table 5. Comparative experiments with simple Pd catalysts
reused more than 10 times without any decrease in
activity: the polymeric catalyst was placed into the re-
action vessel, which was heated with reactants at
1208C for 15 min under microwave irradiation. The
polymeric catalyst was filtered and washed with sol-
vent. The same catalyst was recycled, carrying out the
identical reaction ten times. The isolated yields of pu-
rified product are showed in Table 4.
on the representative reaction.[a]
Entry
Pd catalyst
Yield [%][e]
1
2
3
4
PS-supported Pd(II)[b]
93
82
94
85
[c]
PdCl2
[c]
Pd
ACHTRE(UNG OAc)2
Pd/C[d]
[a]
Reaction conditions: 2-bromonaphthalene (1.0 mmol),
Ph4BNa (0.25 mmol), K2CO3 (3.5 mmol), TBAB
(0.3 mmol), and water (2 mL); microwave irradiation at
1208C for 15 min.
PS-supported Pd(II) catalyst (corresponding to 1 mol%
palladium).
1 mmol% Pd salt.
1 mmol% Pd/C.
Yield of isolated products.
Table 4. Recycling of polymer-supported Pd(II) catalyst.[a]
Entry
PS-supported Pd(II)
Yield [%]
[b]
1
2
3
4
5
6
7
8
9
10
fresh
93
91
92
91
90
92
93
91
93
92
recycle 1
recycle 2
recycle 3
recycle 4
recycle 5
recycle 6
recycle 7
recycle 8
recycle 9
[c]
[d]
[e]
Conclusions
We have developed a reliable, highly efficient method
for the synthesis of biaryls in excellent yields that in-
volves the use of water as solvent under focused mi-
crowave irradiation. The coupling of microwave tech-
nology with water, polymer-supported palladium cata-
lyst and solid-liquid PTC conditions constitutes a new
and especially efficient, powerful and attractive
method. Particularly valuable features of this method-
[a]
Reaction conditions: 2-bromonaphthalene (1.0 mmol),
Ph4BNa (0.25 mmol), K2CO3 (3.5 mmol), TBAB
(0.3 mmol), PS-supported Pd(II) catalyst (corresponding
to 1 mol% palladium), and water (2 mL); microwave ir-
radiation at 1208C for 15 min.
The polymeric catalyst holding the concentration of ology are its superiority from the point of view of
palladium at 1 mol% gives very high activity in the yield, atom economics, short reaction time, broad sub-
coupling of aryl bromide with Ph4BNa in the presence strate scopes, as well as an environmentally more
of a phase-transfer catalyst, K2CO3, and in water friendly synthesis than the reported methods. From a
under focused microwave irradiation at 1208C. The synthetic perspective, these results open new possibili-
catalyst was found to be insensitive to high tempera- ties for exploiting the cross-coupling reaction, since
ture, and filtration of the catalyst followed by evapo- sodium tetraphenylborate is a cheap and easily avail-
ration of the extracted solvent yielded almost pure able starting material in place of phenylboronic acid
biaryls since neither the phosphine ligand nor the pal- as a phenylating agent.
ladium metal contaminated the products.
To evaluate the activity of the PS-supported Pd(II)
catalyst we also investigated the influence of simple
(classic) homogeneous or heterogeneous Pd catalysts
Experimental Section
on the representative reaction under the applied con-
ditions. It can be seen in Table 5 that the polymeric-
The melting points were determined on a WRS-1 A digital
melting point apparatus. IR spectra were measured for KBr
discs using an Alpha Centauri FT-IR spectrophotometer.
catalyst is almost the same as PdACHTRE(UNG OAc)2 in terms of
activity. To us the polymeric catalyst appeared to be 1H NMR spectra were recorded on a Mercury plus 400 MHz
spectrometer in CDCl3 with TMS as an internalstandard.
13C NMR spectra were obtained at 100 MHz in CDCl3 with
TMS as an internalstandard using a Mercury pul s 400 MHz
spectrometer. EI-MS were measured on an HP5988A mass
spectrometer.
most convenient, since it can be recovered and reused
as shown in the above investigations.
Under the optimized reaction conditions, various
aryl bromides, including some heterocyclic species can
react with Ph4BNa providing cross-coupling products
(2a–t) in excellent yields. From the results shown in
Table 6, it can be seen that a wide array of aryl bro-
mides containing an electron-withdrawing group or an
electron-donating group were efficiently coupled to
provide the corresponding biaryl products in excellent
isolated yields. The aryl bromides can completely
couple with all four phenyl groups of Ph4BNa.
Microwave reactions were conducted using a CEM Fo-
cused Microwaveꢂ Synthesis System (CEM Corp., Mat-
thews, NC). The machine consists of a continuous focused
microwave power delivery system with operator selectable
power output from 0 to 300 W. The pressure controlsystem
uses a load cell for an indirect measurement of the reaction
vesselcontents. The ol ad celis connected to a 10-mL vessel
and senses changes in the externaldefel ction of the septa on
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Reusable, Polymer-Supported, Palladium-Catalyzed, Atom-Efficient Coupling Reaction
FULL PAPERS
Table 6. Cross-coupling of aryl bromides with sodium tetraphenylborate.[a]
Entry
Ar-Br
Products[b]
T [oC]/Time
A
Yield [%][c]
1
2
3
4
5
2a[18,19]
2b[18,20]
2c[18,20]
2d[18,21]
2e[18,22]
120/18
120/18
120/20
120/20
120/18
90
91
89
88
92
6
7
2f[18,23]
2g[18,19]
120/15
120/15
93
89
8
2h[18,19]
120/20
87
9
2i[18,19]
2j[18,19]
2k[18,19]
120/20
120/20
120/15
92
90
92
10
11
12
2l[26]
120/15
88
13
14
15
16
17
2m[18,20]
2n[18,21]
2o[18,19]
2p[18,19]
2q[18,24]
120/15
120/15
120/15
120/15
120/15
91
94
92
95
92
18
19
2r[18,25]
2s[18]
120/15
120/15
120/15
91
92
95
20
2t[18,26]
[a]
Reaction conditions: arylbromides (1.0 mmo)l, Ph 4BNa (0.25 mmol), K2CO3 (3.5 mmol), PS-supported Pd(II) catalyst
(corresponding to 1 mol% palladium), H2O (2 mL).
[b]
[c]
1
All products were characterized by melting point, IR, H NMR, 13C NMR and MS.
Average of isolated yields based on two runs.
top of the sealed pressure vessel. The sensor housing incor-
ature on the bottom of the vessel. The sensor is vessel
volume independent and is used in a feedback loop with the
on-board computer to controlthe temperature rise rate and
controlpoint of the vesselcontents. Temperature is pro-
grammable from 25–2508C. All experiments were per-
formed using a stirring option whereby the contents of the
porates a capture and release mechanism to secure the reac-
tion in the cavity. Pressure is programmable from 0–300 psi
(0–21 bar). The temperature controlsystem uses a non-con-
tact, infrared sensor to measure temperature. It is located
below the microwave cavity floor and measures the temper-
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asc.wiley-vch.de
319

Lin Bai and Jin-Xian Wang
FULL PAPERS
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In a 10-mL glass tube were placed aryl bromide (1.0 mmol),
Ph4BNa (0.25 mmol), K2CO3 (3.5 mmol), PS-supported
Pd(II) (corresponding to
1 mol% palladium), TBAB
(0.3 mmol), 2 mL H2O, and a magnetic stir bar. The vessel
was sealed with a septum and placed into the microwave
cavity. Microwave irradiation of 10 W was used, the temper-
ature being ramped from room temperature to 1208C. Once
1208C was reached, the reaction mixture was held at this
temperature for 15 min. After cooling the mixture to room
temperature, the reaction vesselwas opened and the con-
tents poured into a separating funnel. Ethyl acetate-acetone
(5:1 v/v, 10 mL) was added and the organic materialwas dis-
solved. The mixture was filtered, and polymeric catalyst was
extracted twice with ethylacetate-acetone (5 mL”2). The
water layer was separated and the organic phase was
washed with saturated NaCl(5 mL). The organic phase was
dried over anhydrous MgSO4. The solvent was removed by
evaporation under reduced pressure to afford the biaryls.
The product was recrystallized from 95% ethanol or puri-
fied by column chromatography on silica gel using petrole-
um ether-ethylacetate (30:1 v/v) as the eulent to give the
analytically pure product.
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All products are known compounds. The compounds were
1
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Supporting Information
Details of the characterization of all compounds can be
founded in the Supporting Information.
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Acknowledgements
The work is supported by the National Natural Science Foun-
dation of China (NO. 20272047, 20572086) and the Gansu
Natural Science Foundation of China (NO. 3ZS061 A25–059,
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