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Animproved procedure for the synthesis of arylboronates by palladium-catalyzed coupling reaction of aryl halides and bis(pinacolato)diboron in polyethylene glycol.

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Full Paper
Received: 31 January 2011
Revised: 21 March 2011
Accepted: 26 March 2011
Published online in Wiley Online Library: 5 May 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1799
An improved procedure for the synthesis
of arylboronates by palladium-catalyzed
coupling reaction of aryl halides and
bis(pinacolato)diboron in polyethylene glycol
Jie Lu, Zhong-Zhi Guan, Jian-Wu Gao and Zhan-Hui Zhang∗
A new protocol has been developed for the synthesis of arylboronates by a coupling reaction of aryl halides
and bis(pinacolato)diboron using bis(triphenylphosphine)palladium dichloride/sodium acetate/polyethylene glycol 600
c 2011 John Wiley & Sons, Ltd.
[Pd(PPh3 )2 Cl2 /NaOAc/PEG 600] as an efficient catalytic system. Copyright Keywords: palladium; arylboronates; aryl halides; borylation; bis(pinacolato)diboron; PEG 600
Introduction
Appl. Organometal. Chem. 2011, 25, 537–541
Melting points were determined using an X-4 apparatus and are
uncorrected. IR spectra were recorded with a Shimadzu FTIR-8900
spectrometer. NMR spectra were taken with a Bruker DRX-500
spectrometer. Elemental analyses were carried out on a Vario EL III
CHNOS elemental analyzer.
General Procedure for the Preparation of Arylboronate Esters
A mixture of aryl halide (1 mmol), bis(pinacolato)diboron
(1.5 mmol), NaOAc (4.0 mmol) and Pd(PPh3 )2 Cl2 (0.05 mmol) in
PEG 600 (5 ml) was stirred in an oil bath at 90 ◦ C for an appropriate
time under nitrogen. The progress of the reaction was monitored
by thin-layer chromatography using hexane and ethyl acetate
as eluents. After completion, the reaction mixture was cooled to
room temperature and extracted with diethyl ether (10 ml). The
ether layer was washed with brine, and then dried with anhydrous
sodium sulfate. The solvent was removed in vacuo, and the crude
reaction mixture was purified by chromatography on silica gel
(hexane–ethyl acetate).
Representative Spectral and Analytical Data
N-[3-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl]
acetamide (3b)
White solid, m.p. 130–131 ◦ C; IR (KBr): 3258, 2982, 1643, 1566,
1437, 1360, 1144, 1078, 968, 851, 712, 509 cm−1 ; 1 H NMR (500 MHz,
CDCl3 ) δH 1.26 (s, 12H, CH3 ), 1.92 (s, 3H, CH3 ), 4.34 (d, J = 5.5 Hz,
2H, CH2 ), 5.66 (br s, 1H, NH), 7.26 (t, J = 7.5 Hz, 1H, ArH), 7.30 (d,
∗
Correspondence to: Zhan-Hui Zhang, The College of Chemistry and Material
Science, Hebei Normal University, Shijiazhuang 050016, China.
E-mail: zhanhui@126.com
The College of Chemistry and Material Science, Hebei Normal University,
Shijiazhuang 050016, China
c 2011 John Wiley & Sons, Ltd.
Copyright 537
Aryl boronic acids and their esters are important synthetic
targets because they have been extensively applied as precursors in organic reactions,[1] such as the Suzuki–Miyaura
reaction,[2] additions to carbonyl compounds, imines and nitriles,[3]
heteroatom couplings[4] and multicomponent Petasis-borono
Mannich reactions,[5] to construct new carbon–carbon, carbon–nitrogen, carbon–phosphine and carbon–oxygen bonds.
In addition, the synthesis of arylboronic esters is a current topic
of research in molecular recognition and pharmaceutical development owing to their low toxicity and stability in air and water.[6]
Consequently, different methods have been developed for the synthesis of these compounds. Among them, the palladium-catalyzed
cross-coupling reaction of pinacolborane,[7] 4,4,6-trimethyl-1,3,2dioxaborinane (MPBH)[8] or bis(pinacolato)diboron[9] with organic electrophiles remains one of the simplest and most
direct approaches. Owing to the importance of arylboronic esters from pharmaceutical, industrial and synthetic points of
view, the development of a more practical and economical
method for the preparation of these compounds is still highly
desirable.
In recent years, the use of polyethylene glycol (PEG)
as a reaction medium has gained considerable interest in
organic synthesis owing to its many advantages from environmental, economic and safety standpoints.[10] It is nontoxic, inexpensive, nonvolatile, biologically acceptable and
eco-friendly, and allows many useful organic transformations to be performed under mild reaction conditions, such
as coupling,[11] addition,[12] substitution,[13] condensation,[14]
oxidation,[15] reduction[16] and multicomponent reactions.[17] In
a continuation of our interest in developing practical and new
synthetic methodologies,[18] we report herein an efficient and
convenient procedure for the synthesis of arylboronates by coupling reaction of aryl halides and bis(pinacolato)diboron in PEG
600 (Scheme 1).
Experimental Section
J. Lu et al.
O
R
X +
1
O
Pd(PPh3) 2Cl2, NaOAc
B B
O
O
R
B
O
PEG 600, 90 oC
O
3
2
Scheme 1. Palladium-catalyzed synthesis of arylboronates in PEG 600.
Table 1. Investigation of catalysts and temperature effects on the reaction of bromobenzene and bis(pinacolato)diborona
O
Br +
O
B B
O
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13
a
Catalyst, NaOAc
O
O
B
O
PEG 600
Catalyst (mol%)
Temperature (◦ C)
Time (h)
Conversion (%)
Yield (%)b
No
Pd(OAc)2 (5 mol%)
Pd(PPh3 )4 (5 mol%)
Pd(PPh3 )2 Cl2 (5 mol%)
Pd(PPh3 )2 Cl2 (2 mol%)
Pd(PPh3 )2 Cl2 (7 mol%)
Pd(PPh3 )2 Cl2 (10 mol%)
Pd(PPh3 )2 Cl2 (5 mol%)
Pd(PPh3 )2 Cl2 (5 mol%)
Pd(PPh3 )2 Cl2 (5 mol%)
Pd(PPh3 )2 Cl2 (5 mol%)
Pd(PPh3 )2 Cl2 (1 mol%)
Pd(PPh3 )2 Cl2 (1 mol%)
90
90
90
90
90
90
90
70
80
100
110
100
110
8.0
3.0
3.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
6.0
7.0
0
72
53
96
89
91
86
71
82
94
91
86
88
0
65
35
90
82
81
75
58
65
86
60
72
45
Reaction conditions: bromobenzene (1 mmol), bis(pinacolato)diboron (1.5 mmol), NaOAc (4.0 mmol), PEG 600 (5 ml). b Yield of isolated product.
J = 7.5 Hz, 1H, ArH), 7.61 (s, 1H, ArH), 7.64 (d, J = 7.5 Hz, 1H, ArH);
13 C NMR (125 MHz, CDCl ) δ 22.9 (CH ), 24.7 (4CH ), 43.4 (CH ),
3 C
3
3
2
83.8 (C-O), 127.9 (ArCH), 130.7 (ArCH), 133.7 (ArCH), 133.9 (ArCH),
137.4 (ArC), 170.0 (C O) (13 C signals of boron-bound quaternary
carbon could not be observed owing to boron-bound atoms being
very broad[19] ). Anal. calcd for C15 H22 BNO3 : C, 65.48; H, 8.06; N,
5.09. Found: C, 65.62; H, 7.90; N, 4.92.
4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde (3r)
White solid, m.p. 55–56 ◦ C; IR (KBr): 2978, 2849, 1705, 1564, 1508,
1360, 1271, 1205, 1140, 1090, 1015, 800, 648 cm−1 ; 1 H NMR
(500 MHz, CDCl3 ) δH 1.37 (s, 12H, CH3 ), 7.86 (d, J = 8.0 Hz, 2H, ArH),
7.96 (d, J = 8.0 Hz, 2H, ArH), 10.05 (s, 1H, CHO); 13 C NMR (125 MHz,
CDCl3 ) δC 24.8 (4CH3 ), 84.3 (C–O), 128.7 (ArCH), 135.2 (ArCH), 138.1
(ArC), 192.6 (C O) (no C-B signal). Anal. calcd for C13 H17 BO3 : C,
67.28; H, 7.38. Found: C, 67.47; H, 7.20.
N-[3-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl]
methanesulfonamide (3c)
White solid, m.p. 118–119 ◦ C; IR (KBr): 3290, 2980, 1437, 1348,
1267, 1155, 1078, 964, 849, 714 cm−1 ; 1 H NMR (500 MHz, DMSOd6 ) δH 1.30 (s, 12H, CH3 ), 2.87 (s, 3H, CH3 ), 4.16 (s, 2H, CH2 ), 7.36 (t,
J = 7.5 Hz, 1H, ArH), 7.45 (d, J = 7.5 Hz, 1H, ArH), 7.54 (s, 1H, ArH),
7.58 (d, J = 7.5 Hz, 1H, ArH), 7.68 (br s, 1H, NH); 13 C NMR (125 MHz,
DMSO-d6 )) δC 25.1 (4CH3 ), 40.2 (CH3 ), 46.4 (CH2 ), 84.2 (C–O), 128.4
(ArCH), 131.3 (ArCH), 133.7 (ArCH), 134.4 (ArCH), 138.8 (ArC) (no
C-B signal). Anal. calcd for C14 H22 BNO4 S: C, 54.03; H, 7.13; N, 4.50.
Found: C, 53.95; H, 6.98; N, 4.66.
1-[4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]ethanone
(3m)
538
White solid, m.p. 78–79 ◦ C; IR (KBr): 2964, 1682, 1508, 1360, 1267,
1144, 1094, 858, 656 cm−1 ; 1 H NMR (500 MHz, CDCl3 ) δH 1.28 (s,
12H, CH3 ), 2.54 (s, 3H, CH3 ), 7.82 (d, J = 8.0 Hz, 2H, ArH), 7.86 (d,
J = 8.0 Hz, 2H, ArH); 13 C NMR (125 MHz, CDCl3 ) δC 24.8 (4CH3 ), 26.7
(CH3 ), 84.2 (C–O), 127.2 (ArCH), 134.9 (ArCH), 139.0 (ArC), 198.4
(C O) (no C-B signal). Anal. calcd for C14 H19 BO3 : C, 68.32; H, 7.78.
Found: C, 68.16; H, 7.95.
wileyonlinelibrary.com/journal/aoc
Results and Discussion
Initially, the efficacy of various palladium catalysts was tested for
the model reaction of bromobenzene and bis(pinacolato)diboron
(2) under different reaction conditions. As indicated in Table 1,
some palladium catalysts were screened and Pd(PPh3 )2 Cl2 seemed
to be the best choice (Table 1, entries 2–4). Next, we optimized
the catalyst loading for the model reaction by varying the catalyst
amount from 2 to 10 mol%. It was observed that increasing the
catalyst loading up to 5 mol% increased the yield of 3d, but
beyond 5 mol%, it did not improve, rather, it decreased. It should
also be pointed out that no desired product was observed in
the absence of a catalyst (Table 1, entry 1). We also examined
the influence of temperature on the reaction yield. The yields
increased gradually with increasing the reaction temperature
from 70 to 90 ◦ C. Furthermore, further increase of the temperature
neither increased the yield nor shortened the reaction time.
Subsequently, the above reaction was examined in various
solvents. Toluene, EtOH and tetrahydrofuran (THF) afforded low
yields (Table 2, entries 1–3), while better results were obtained
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 537–541
Synthesis of arylboronates
Table 2. Screening of bases and solvents for borylation of bromobenzenea
O
Br +
O
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
a
b
O
O
Pd(PPh3)2Cl2, base
B B
B
Solvent, 90 oC
O
O
Base
Solvent
Time (min)
Conversion (%)
Yield (%)b
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
Et3 N
Na2 CO3
K2 CO3
KOH
NaOH
NaHCO3
Ca(OH)2
KOAc
NaOAc
Toluene
THF
EtOH
Dioxane
DMF
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
2.5
2.5
2.5
2.5
2.5
3.0
3.0
4.0
2.5
4.0
4.0
2.5
4.0
2.5
42
61
58
82
86
29
72
84
75
49
68
32
79
96
20
43c
35c
64c
70
15
50
76
50
26
51
18
58
90
Reaction conditions: bromobenzene (1 mmol), bis(pinacolato)diboron (1.5 mmol), NaOAc (4.0 mmol), Pd(PPh3 )2 Cl2 (0.05 mol), solvent (5 ml), 90 ◦ C.
Yield of isolated product. c Reaction was carried out at reflux.
Appl. Organometal. Chem. 2011, 25, 537–541
Table 3. Palladium-catalyzed synthesis of arylboronates in PEG 600
Entry
Aryl halides
1
O
Product
Time (h)
Yield (%)a
3a
2.5
93
3b
2.5
90
3c
2.5
88
3d
2.5
90
3e
2.5
92
3f
8.0
78
3g
5.0
85
3h
10.0
73
3i
10.0
65
O
I
2
O
I
N
H
3
O H
N
S
O
I
4
Br
5
Br
6
Br
7
Br
HO
8
HO
9
Br
NH2
Br
c 2011 John Wiley & Sons, Ltd.
Copyright 539
when dioxane dimethylformamide (DMF) and tetrahydrofuran
(THF) were used as solvents. However, the best result was obtained
when the reaction was performed in PEG 600. Also, the effect of the
base on the reaction was investigated using different bases such
as Et3 N, Na2 CO3 , K2 CO3 , KOH, NaOH, NaHCO3 , Ca(OH)2 , KOAc and
NaOAc. The results clearly indicated that NaOAc was superior to
other bases in terms of yield and reaction time. Stronger bases such
as Na2 CO3 promoted further the Suzuki–Miyaura reaction of 3d
with bromobenzene, resulting in the formation of approximately
20% of the homocoupling product biphenyl. Thus, the optimal
system for this reaction involved Pd(PPh3 )2 Cl2 (5 mol%) and NaOAc
in PEG 600 at 90 ◦ C.
To evaluate the substrate scope of this reaction, a variety
of aryl halides were reacted with bis(pinacolato)diboron under
optimized reaction conditions, and the results are given in Table 3.
As expected, the reaction proceeded efficiently with aryl iodides
and gave the corresponding arylboronates in high yields (Table 3,
entries 1–3). This method was found to be equally effective for
conversion of aryl bromides with electron-deficient and electronrich groups. It is noteworthy to mention that those substrates
bearing a substituent in the ortho position gave the expected
product in slightly lower yield and required longer reaction times.
This may be attributable to the larger steric hindrance of orthosubstituent.
Aryl chlorides are the most attractive set of substrates
owing to their low cost and ready availability. So far there
have been few reports for the efficient borylation of an aryl
chloride with bis(pinacolato)diboron.[20] Gratifyingly, we found
that this catalyst system was efficient for the reaction of
methyl 4-chlorobenzoate and bis(pinacolato)diboron, furnishing
the corresponding coupling product in 71% yield (Table 3, entry
22). Furthermore, this method was also general for a wide range
of aryl chlorides with electron-withdrawing or electron-releasing
groups. In these cases, the reaction occurred smoothly and the
desired products were obtained in good yields. It is obvious that
this method was compatible with many functional groups, such as
wileyonlinelibrary.com/journal/aoc
J. Lu et al.
Table 3. (Continued)
Entry
10
Table 3. (Continued)
Aryl halides
H2N
Br
Product
Time (h)
Yield (%)a
3j
8.0
81
Aryl halides
27
Cl
3k
11
Entry
8.0
82
28
N
Br
12
13
3l
3m
O
6.0
6.0
78
O
3n
6.0
85
3o
8.0
86
3p
10.0
60
3q
8.0
65
OMe
Br
15
Br
O
Me
16
CHO
Br
17
Br
3r
8.0
71
3s
12.0
64
H
CHO
a
Yield of isolated product.
82
Br
14
Yield (%)a
OH
Br
O 2N
Time (h)
CHO
O
Cl
Product
ester, ketone, aldehyde, nitro, dialkylamine, amides and hydroxy
in the substrates. However, a small amount of dehalogenated and
homocoupling byproducts were formed in some cases.
The reusability of the Pd(PPh3 )2 Cl2 /NaOAc/PEG 600 system
for the model reaction was also investigated. After completion
of the reaction, the reaction mixture was cooled to room
temperature, the coupling product was conveniently isolated by
simple extraction of the reaction mixture with diethyl ether and the
Pd(PPh3 )2 Cl2 /NaOAc/PEG 600 system was subjected to a second
run by charging with bromobenzene and bis(pinacolato)diboron.
The results of the three runs indicated that the reactivity of the
catalyst was reduced (85, 72 and 53%). A possible reason for this
was that the salt byproducts generated in the coupling reaction,
retarded the proceeding of the coupling reaction.[21]
Conclusion
18
3r
8.0
80
3s
10.0
73
3j
10.0
72
O
3m
10.0
70
O
3n
9.0
71
Br
CHO
19
O
Br
H
OH
20
H2N
Cl
21
Acknowledgments
Cl
22
Cl
NO 2
3t
12.0
56
3l
10.0
72
3p
12.0
51
3q
12.0
61
Cl
24
O2N
25
Cl
CHO
Cl
540
26
Cl
CHO
We thank the National Natural Science Foundation of China
(21072042 and 20872025), and the Natural Science Foundation of
Hebei Province (B2011205031) for financial support.
References
OMe
23
In conclusion, we have demonstrated a novel and efficient strategy
for synthesis of arylboronic esters by coupling reaction of aryl
halides and bis(pinacolato)diboron using a catalytic system that
employs Pd(PPh3 )2 Cl2 along with NaOAc as base in PEG 600.
The operational simplicity, the use of commercially available
Pd(PPh3 )2 Cl2 with no need for any other ligands, and the wide
scope of substrates are the advantages of this method.
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animproved, arylboronates, pinacolato, reaction, couplings, polyethylene, glycol, aryl, catalyzed, procedur, synthesis, diboron, palladium, halide, bis
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