TBAF-assisted palladium-catalyzed Suzuki reaction in water under the ligand and base-free conditions

Article abstract of DOI:10.2174/157017812801322408

Tetrabutyl ammonium fluoride (abbreviated as TBAF)-assisted palladium-catalyzed Suzuki reaction in neat water has been demonstrated under the ligand and base-free conditions. The cross-coupling of aryl or heteroaryl bromides with arylboronic acids generated the corresponding products in good to excellent yields in the presence of low concentration of palladium acetate in combination with TBAF under air.

Full text of DOI:10.2174/157017812801322408

3
96
Letters in Organic Chemistry, 2012, 9, 396-400
TBAF-Assisted Palladium-Catalyzed Suzuki Reaction in Water Under the
Ligand and Base-Free Conditions
Xiang-Mei Wu* and Yong-Bing Gu
Department of Chemistry, College of Science, Lishui University, Lishui, Zhejiang 323000, P.R. China
Received April 04, 2011: Revised April 22, 2011: Accepted April 29, 2012
Abstract: Tetrabutyl ammonium fluoride (abbreviated as TBAF)-assisted palladium-catalyzed Suzuki reaction in neat
water has been demonstrated under the ligand and base-free conditions. The cross-coupling of aryl or heteroaryl bromides
with arylboronic acids generated the corresponding products in good to excellent yields in the presence of low concentra-
tion of palladium acetate in combination with TBAF under air.
Keywords: Arylboronic acid, palladium, suzuki reaction, TBAF, water.
INTRODUCTION
RESULTS AND DISCUSSIONS
The Suzuki coupling reaction is a powerful and versatile
method for the generation of unsymmetrical biaryls which
has found extensive use in the synthesis of natural products,
pharmaceuticals and advanced materials [1]. Although palla-
dium compounds, usually complexes with phosphane ligands
were used as catalyst precursors in Suzuki reaction, phos-
phane-free catalysts have received great interest recently as a
less complicated, environmentally friendly alternative to
phosphane-containing systems and significant advances have
been made in the past few years in developing such catalysts,
including the use of N-heterocyclic carbenes [2],
ary(heteroaryl)oximes [3], arylimines [4] and sulfur-
containing ligands or palladacycles [5]. However, the syn-
theses of these related catalysts or ligands are often time
consuming, difficult, and/or require the use of expensive
starting materials, which significantly limits their industrial
applications. On the other hand, since Casalnuovo’s initial
Previous reports have shown that the addition of phase
transfer catalysts, such as tetrabutylaminium bromide, can
greatly improve yields of Suzuki reactions in both water and
polar organic solvents [8]. Unfortunately, the use of an ex-
cess amount of base is usually indispensable. And inspired
by the recent findings that N-arylation and benzylation of
benzazoles with aryl and benzyl halides underwent smoothly
2
in the presence of CuBr and TBAF under the ligand, base
and solvent-free conditions [9], we attempt to apply TBAF to
Suzuki reaction in water without the presence of any other
ligand and base.
Initially, the coupling of 4-bromotoluene (1.0 mmol) and
phenylboronic acid (1.2 mmol) was chosen as the model
reaction and the reaction was performed in water in the pres-
ence of PdCl (0.5 mol%) and TBAF (2.0 mmol) at 80ºC
2
under air (at 1atm). As expected, the desired product was
obtained in satisfactory yield of 70% within only 3h and ele-
vated reaction temperature resulted in higher yield (Table 1,
entry 1). Nevertheless, considering the boiling point of wa-
ter, a reaction temperature of 95ºC was found to be optimal
and the yield was enhanced to 88% (Table 1, entry 2). Sub-
sequently, the effect of the palladium source on the Suzuki
report
of
TPPMS
(triphenylphosphine
monosul-
fonate)/Pd(OAc)
2
catalyzed cross-coupling reactions in
aqueous system [6], there has been a great deal of effort de-
voted to the development of Suzuki reaction in aqueous
phase including water-organic mixed solvents and neat wa-
ter, and many water-soluble ligands have been successfully
applied in the Suzuki reaction as useful promoters [7]. How-
ever, due to environmental, economical, and safety concerns,
the requirement of simple and practical method for the Su-
zuki coupling reaction in water remains a highly desirable
goal. Herein, we reported the TBAF-assisted Pd-catalyzed
Suzuki reaction under the ligand and base-free conditions in
neat water.
reaction was examined. Compared with Pd
dium on activated carbon, PdCl and [Pd(CH
more satisfactory results and Pd(OAc) proved to be the best
choice among the palladium source tested (Table 1, entries
-6). We also optimized the amount of Pd catalysts and
2
(dba)
3
and palla-
2
3
CN)
2
]Cl gave
2
2
3
TBAF required for the cross-coupling. A respectable 90%
yield of the product was observed upon decreasing the cata-
lyst loading to 0.3 mol% and similar product yield was ob-
tained upon increasing the catalyst loading to 1.0 mol% (Ta-
ble 1, entries 7 and 8). In the absence of TBAF, the result
was not promising, and two equivalents of TBAF were ade-
quate to perform the reaction. Therefore, TBAF may play
concurrently two roles as the phase transfer catalyst and the
base under the Suzuki reaction conditions. In addition, we
*
Address correspondence to this author at the Department of Chemistry,
College of Science, Lishui University, Lishui, Zhejiang 323000, P. R.
China; Tel: +86-578-2271250; E-mail: wxm7162@yahoo.com.cn
1
875-6255/12 $58.00+.00
© 2012 Bentham Science Publishers

TBAF-Assisted Palladium-Catalyzed Suzuki Reaction
Letters in Organic Chemistry, 2012, Vol. 9, No. 6
397
a
Table 1. Cross-Coupling of 4-Bromotoluene with Phenylboronic Acid in Different Conditions
Br
B(OH)2
catalyst
+
TBAF, H O
2
b
Entry
Catalyst
T (ºC)
Yield (%)
1
2
3
4
5
6
PdCl
2
2
80
95
95
95
95
95
95
95
70
88
92
85
56
78
90
91
PdCl
Pd(OAc)
2
[Pd(CH
3
CN)
2
]Cl
2
2
Pd (dba)
3
Pd on activated carbon
c
7
Pd(OAc)
Pd(OAc)
2
2
d
8
a
Reaction conditions: 4-bromotoluene(1.0 mmol); phenylboronic acid (1.2 mmol); TBAF
b
c
(
(
2.0 mmol); H
2
O(1.0 ml); Pd(OAc)
2
(0.005 mmol); 95°C; 3h. Isolated yield. Pd(OAc)
2
d
0.003 mmol). Pd(OAc)
2
(0.01 mmol).
have discovered that almost no activity difference was ob-
served no matter the reaction was carried out in the inert
atmosphere or in air. Thus, the optimal reaction conditions
EXPERIMENTAL
General
for the Suzuki reaction are the combination of Pd(OAc)
mol%) with TBAF at 95ºC for 3h in air.
2
(0.5
All commercial reagents were used as received without
1
purification, and all solvents were of reagent grade. H NMR
1
3
and C NMR spectra were recorded at 300 MHz and 75
MHz with Bruker Avance 300 spectrometer using CDCl as
To examine the versatility of this procedure, the Suzuki
reaction of phenylboronic acid with aryl bromides containing
electron-donating and electron-withdrawing substituents
were investigated. The results are listed in Table 2. It shows
that the conversions are satisfactory under the present reac-
tion conditions. For bromobenzene and activated bromoare-
nes, excellent yields of cross-coupling products were isolated
3
solvent and tetramethylsilane as internal standard. Melting
points were obtained of a XT4A melting point apparatus and
were uncorrected. Gas chromatography mass spectra
GC/MS) were recorded on an Agilent Technologies 6890 N
instrument with an Agilent 5975 N mass detector (EI) and a
(
HP5-MS 30 mꢀ0.25 mm capillary apolar column.
(
Table 2, entries 1 and 2). Meanwhile, unactivated bro-
moarenes, such as 4-bromotoluene and 4-bromoanisole, also
reacted with arylboronic acids to generate the corresponding
products in good yields (Table 2, entries 3 and 4). Further-
more, heteroaromatic bromides such as 3-bromothiophene,
General Experimental Procedure
A mixture of aryl or heteroaryl bromide (1.0 mmol),
arylboronic acid (1.2 mmol), TBAF (2.0 mmol), Pd(OAc)
0.5 mol%) and H O (1.0 ml) was taken in a 25 ml two-neck
2
2
5
-bromothiophene, 2-methyl-5-bromothiophene and 2-ethyl-
-bromothiophene also proved to be highly efficient cou-
(
2
flask. The mixture was heated at 95°C in air for 3h by mag-
netic stirring. After cooling to room temperature, the product
pling substrates and satisfactory results were obtained (Table
, entries 5-8) under identical conditions, which provides a
2
was diluted with H
ml). The extracts were combined and washed by brine
3ꢀ8ml), dried over MgSO , filtered, and evaporated, and
2
O (5 ml) and extracted with EtOAc (4ꢀ5
useful way for the synthesis of aryl-substituted thiophene
heterocycles which are constituents of a variety of important
classes of pharmacologically active compounds. The opti-
mized reaction conditions were also applied to the Suzuki
reaction of substituted phenylboronic acids with bromoben-
zene and the results are also summarized in Table 2. Various
electron-donating and electron-withdrawing groups such as -
(
4
purified by chromatography on silica gel to obtain the de-
sired products with petroleum ether/ethyl acetate (v/v=9:1～
1
00:1). The products were characterized by their spectral and
analytical data and compared with those of the known com-
pounds.
3 3 3
CH , -Br, -Cl, -F, -COCH and -CF were well tolerated to
give the desired unsymmetrical biaryls in good to excellent
yields (Table 2, entries 9-17).
Characterization Data of All the Products
In summary, the cross-coupling of aryl or heteroaryl
bromides with arylboronic acids in water generated the cor-
responding products in good to excellent yields in the pres-
ence of TBAF and low catalytic amounts of palladium ace-
tate without any ligand and base.
Biphenyl[10]
1
White solid, mp 67-70°C; H NMR (300 MHz, CDCl
3
): ꢁ
7
8
1
.28 (t, J = 7.5 Hz, 2H), 7.37 (d, J = 8.3 Hz, 4H), 7.53 (d, J =
.3 Hz, 4H); C NMR (CDCl
28.7, 141.2; GC-MS (EI, m/z): 154 [M ].
13
3
, 75MHz): ꢁ 127.1, 127.2,
+

3
98 Letters in Organic Chemistry, 2012, Vol. 9, No. 6
Wu and Gu
a
Table 2. Cross-Coupling of Aryl and Heteroaryl Bromides with Arylboronic Acids
Pd(OAc)2
Ar1 B(OH)2
Ar
Ar1
Ar
Br
+
TBAF, H O
2
b
Entry
Substrates
Product
Yield(%)
B(OH)2
1
Br
93
95
Br
F
B(OH)2
2
F
B(OH)2
B(OH)2
B(OH)2
3
4
5
6
7
8
9
H
3
C
Br
92
86
68
76
72
77
93
91
87
H
CO
Br
H
3
CO
3
S
S
Br
S
Br
B(OH)2
S
B(OH)2
S
Br
S
S
Br
B(OH)
2
S
B(OH)2
B(OH)2
B(OH)2
Br
1
1
0
1
H
3
C
Br
Br
Br
Br
Br
F
F
B(OH)
2
F
1
1
1
2
3
4
86
88
86
F
B(OH)2
Cl
Cl
B(OH)2
Cl
Cl
B(OH)2
1
1
1
5
6
7
Br
Br
Br
H
3
COC
82
75
70
H COC
3
F3C
B(OH)2
3
F C
CF3
F3C
Br
B(OH)2
Br
a
b
2 2
Reaction conditions: aryl or heteroaryl bromide(1.0 mmol); arylboronic acid(1.2 mmol); TBAF(2.0 mmol); H O(1.0 ml); Pd(OAc) (0.005 mmol); 95°C; 3h. Isolated yield.

TBAF-Assisted Palladium-Catalyzed Suzuki Reaction
Letters in Organic Chemistry, 2012, Vol. 9, No. 6
399
3
-Fluorobiphenyl[11]
Pale-yellow solid, mp 26-27°C; H NMR (300 MHz,
4-Chlorobiphenyl[18]
White solid, mp 75-77°C; H NMR (300 MHz, CDCl
1
1
3
): ꢀ
1
3
CDCl
1
1
3
): ꢀ 7.01-7.61(m, 9H); C NMR (CDCl
3
, 75MHz): ꢀ
7.34-7.38 (m, 1H), 7.39-7.46 (m, 4H), 7.52 (d, J = 8.6 Hz,
13
13.9, 114.2, 122.8, 127.1, 127.9, 128.9, 130.2, 139.9, 143.5,
3
2H), 7.55 (d, J =8.0 Hz, 2H); C NMR (CDCl , 75MHz): ꢀ
+
61.6, 164.8; GC-MS (EI, m/z): 172 [M ].
127.0, 127.6, 128.3, 128.4, 128.9, 133.4, 139.7, 140.0; GC-
+
MS (EI, m/z): 188 [M ].
4
-Methylbiphenyl[12]
1
3-Chlorobiphenyl[19]
White solid, mp 46-47°C; H NMR (300 MHz, CDCl
3
): ꢀ
1
2
1
(
.38 (s, 3H), 7.24 (d, J = 7.5 Hz, 2H), 7.30 (t, J = 7.5 Hz,
H), 7.41 (t, J = 7.5 Hz, 2H), 7.49 (d, J = 7.8 Hz, 2H), 7.58
d, J = 7.5 Hz, 2H); C NMR (CDCl
Colourless oil; H NMR (300 MHz, CDCl
3
): ꢀ 7.31-7.41
1
3
(m, 3H), 7.42-7.48 (m, 3H), 7.55-7.59 (m, 3H); C NMR
(CDCl , 75MHz): ꢀ 125.3, 127.1, 127.2, 127.3, 127.9, 128.9,
130.0, 134.7, 139.9, 143.1; GC-MS (EI, m/z): 188 [M ].
1
3
3
, 75MHz): ꢀ 21.2,
3
+
1
27.0, 127.2, 128.8, 129.6, 137.1, 138.4, 141.2; GC-MS (EI,
+
m/z): 168 [M ].
4
-Acetylbiphenyl[18]
White solid, mp 120-121°C; H NMR (300 MHz,
CDCl ): ꢀ 2.63 (s, 3H), 7.41-7.47 (m, 3H), 7.62-7.69 (m,
3
4H), 8.03 (d, J = 5.2 Hz, 2H); C NMR (CDCl , 75MHz): ꢀ
4
-Methoxybiphenyl[10]
White solid, mp 90-91°C; H NMR (300 MHz, CDCl
.84 (s, 3H), 6.97 (d, J = 8.7 Hz, 2H), 7.25-7.29 (m, 1H),
1
1
3
): ꢀ
3
1
3
3
7
7
1
1
3
.38-7.44 (m, 2H), 7.51-7.56 (m, 4H); C NMR (CDCl
5MHz): ꢀ 55.4, 114.2, 126.7, 126.8, 128.2, 128.7, 133.7,
40.8, 159.1; GC-MS (EI, m/z): 184 [M ].
3
,
26.7, 127.2, 127.3, 128.2, 128.9, 129.0, 135.8, 139.8, 145.7,
197.7; GC-MS (EI, m/z): 196 [M ].
+
+
3
,5-Bis(trifluoromethyl)biphenyl[20]
3
-Phenylthiophene[10]
1
Colourless oil; H NMR (300 MHz, CDCl
3
): ꢀ 7.45-7.53
1
13
Pale-yellow solid, mp 88-90°C; H NMR (300 MHz,
(m, 3H), 7.59-7.62 (m, 2H), 7.85 (s, 1H), 8.01 (s, 2H); C
NMR (CDCl , 75MHz): ꢀ 127.2, 128.8, 129.2, 131.8, 132.3,
138.2, 143.3; GC-MS (EI, m/z): 290 [M ].
CDCl
7
1
3
): ꢀ 7.26 (m, 1H)
.60 (d, J = 7.2 Hz, 2H); C NMR (CDCl
21.6, 127.5, 128.2, 128.3, 129.3, 136.4, 139.7; GC-MS (EI,
ꢁ
3
1
3
+
3
, 75MHz): ꢀ
+
2-Bromobiphenyl[21]
m/z): 160 [M ].
-Phenylthiophene[13]
Pale-yellow solid, mp 35-36°C; H NMR (300 MHz,
CDCl ): ꢀ 7.08 (m, 1H), 7.27-7.32 (m, 3H), 7.35-7.40 (m,
H), 7.62 (d, J = 7.2 Hz, 2H); C NMR (CDCl
23.1, 124.8, 125.9, 127.5, 128.1, 128.9, 133.9, 143.9; GC-
1
Colorless oil; H NMR (300 MHz, CDCl
3
): ꢀ 7.12 (m,
2
1
H), 7.26 (m, 2H), 7.33 (t, J=7.0 Hz,1H), 7.36 (m, 4H), 7.59
(d, J=7.2 Hz, 1H); C NMR (CDCl , 75MHz): ꢀ 122.5,
3
127.3, 127.5, 127.8, 128.9, 129.2, 131.3, 133.1, 141.1, 142.3;
GC-MS (EI, m/z): 233[M ].
1
13
3
ꢁ
1
3
+
2
1
3
, 75MHz): ꢀ
+
MS (EI, m/z): 160 [M ].
ACKNOWLEDGEMENT
2
-Ethyl-5-phenylthiophene[14]
This work was supported by the Natural Science Founda-
tion of Zhejiang Province (NO. Y407240).
1
Oil; H NMR (300 MHz, CDCl
3
): ꢀ 1.34 (t, J = 7.5 Hz,
3
H), 2.84 (q, J = 7.5 Hz, 2H), 6.75 (d, J = 3.6 Hz, 1H), 7.12
(
7
2
d, J = 3.5 Hz, 1H), 7.24 (m, 1H), 7.34 (t, J = 7.5 Hz, 2H),
13
.56 (d, J = 7.2 Hz, 2H); C NMR (75 MHz, CDCl
3
): ꢀ 15.9,
CONFLICT OF INTEREST
3.6, 122.7, 124.3, 125.5, 127.0, 128.8, 134.8, 141.6, 147.2;
+
Declared none.
GC-MS (EI, m/z): 188 [M ].
-Methyl-5-phenylthiophene[15]
Pale yellow solid, mp 39-41°C; H NMR (300 MHz,
CDCl ): ꢀ 2.40 (s, 3H), 6.65 (d, J = 3.2 Hz, 1H), 7.04 (d, J =
.3 Hz, 1H), 7.18 (m, 1H), 7.28 (t, J = 7.2 Hz, 2H), 7.50 (d, J
2
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