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

Article abstract of DOI:10.1002/aoc.1799

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 [Pd(PPh₃)₂Cl₂/NaOAc/PEG 600] as an efficient catalytic system. Copyright

Full text of DOI:10.1002/aoc.1799

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
[Pd(PPh3)2Cl2/NaOAc/PEG 600] as an efficient catalytic system. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Keywords: palladium; arylboronates; aryl halides; borylation; bis(pinacolato)diboron; PEG 600
Introduction
Experimental Section
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.
Aryl boronic acids and their esters are important synthetic
targets because they have been extensively applied as pre-
cursors in organic reactions,[1] such as the Suzuki–Miyaura
reaction,[2] additionstocarbonylcompounds,iminesandnitriles,[3]
heteroatom couplings[4] and multicomponent Petasis-borono
Mannich reactions,[5] to construct new carbon–carbon, car-
bon–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 develop-
ment owing to their low toxicity and stability in air and water.[6]
Consequently,differentmethodshavebeendevelopedforthesyn-
thesis of these compounds. Among them, the palladium-catalyzed
cross-coupling reaction of pinacolborane,[7] 4,4,6-trimethyl-1,3,2-
dioxaborinane (MPBH)[8] or bis(pinacolato)diboron[9] with or-
ganic electrophiles remains one of the simplest and most
direct approaches. Owing to the importance of arylboronic es-
ters 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.
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)2Cl2 (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
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 en-
vironmental, economic and safety standpoints.[10] It is non-
toxic, inexpensive, nonvolatile, biologically acceptable and
eco-friendly, and allows many useful organic transforma-
tions 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 cou-
pling reaction of aryl halides and bis(pinacolato)diboron in PEG
600 (Scheme 1).
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, 509cm−1;1HNMR(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
Appl. Organometal. Chem. 2011, 25, 537–541
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

J. Lu et al.
O
O
O
O
O
O
R
Pd(PPh3)2Cl2, NaOAc
PEG 600, 90 oC
R
B
B
B
X
+
1
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
O
O
O
O
O
Catalyst, NaOAc
PEG 600
B
Br
B B
+
Entry
Catalyst (mol%)
Temperature (◦C)
Time (h)
Conversion (%)
Yield (%)b
1
2
No
90
90
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
Pd(OAc)2 (5 mol%)
3
Pd(PPh3)4 (5 mol%)
90
4
Pd(PPh3)2Cl2 (5 mol%)
Pd(PPh3)2Cl2 (2 mol%)
Pd(PPh3)2Cl2 (7 mol%)
Pd(PPh3)2Cl2 (10 mol%)
Pd(PPh3)2Cl2 (5 mol%)
Pd(PPh3)2Cl2 (5 mol%)
Pd(PPh3)2Cl2 (5 mol%)
Pd(PPh3)2Cl2 (5 mol%)
Pd(PPh3)2Cl2 (1 mol%)
Pd(PPh3)2Cl2 (1 mol%)
90
5
90
6
90
7
90
8
70
9
80
10
11
12
13
100
110
100
110
a 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);
13C NMR (125 MHz, CDCl3) δC 22.9 (CH3), 24.7 (4CH3), 43.4 (CH2),
83.8 (C-O), 127.9 (ArCH), 130.7 (ArCH), 133.7 (ArCH), 133.9 (ArCH),
137.4 (ArC), 170.0 (C O) (13C signals of boron-bound quaternary
carboncouldnotbeobservedowingtoboron-boundatomsbeing
very broad[19]). Anal. calcd for C15H22BNO3: 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 1H 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); 13C 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 C13H17BO3: 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; 1H NMR (500 MHz, DMSO-
d6) 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); 13C 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 C14H22BNO4S: C, 54.03; H, 7.13; N, 4.50.
Found: C, 53.95; H, 6.98; N, 4.66.
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,
somepalladiumcatalystswerescreenedandPd(PPh3)2Cl2 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
1-[4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]ethanone
(3m)
White solid, m.p. 78–79 ◦C; IR (KBr): 2964, 1682, 1508, 1360, 1267,
1
1144, 1094, 858, 656 cm−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); 13C 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 C14H19BO3: C, 68.32; H, 7.78.
Found: C, 68.16; H, 7.95.
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 537–541

Synthesis of arylboronates
Table 2. Screening of bases and solvents for borylation of bromobenzenea
O
O
O
O
O
O
Pd(PPh3)2Cl2, base
Solvent, 90 oC
Time (min)
Br
B
B
B
+
Entry
Base
Solvent
Conversion (%)
Yield (%)b
1
2
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
Et3N
Toluene
THF
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
3
EtOH
4
Dioxane
DMF
5
6
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
PEG 600
7
Na2CO3
K2CO3
KOH
8
9
10
11
12
13
14
NaOH
NaHCO3
Ca(OH)2
KOAc
NaOAc
a Reaction conditions: bromobenzene (1 mmol), bis(pinacolato)diboron (1.5 mmol), NaOAc (4.0 mmol), Pd(PPh3)2Cl2 (0.05 mol), solvent (5 ml), 90 ◦C.
b Yield of isolated product. c Reaction was carried out at reflux.
when dioxane dimethylformamide (DMF) and tetrahydrofuran
(THF)wereusedassolvents. However, thebestresultwasobtained
whenthereactionwasperformedinPEG600. Also, theeffectofthe
Table 3. Palladium-catalyzed synthesis of arylboronates in PEG 600
Entry
1
Aryl halides
Product
Time (h)
2.5
Yield (%)a
93
base on the reaction was investigated using different bases such
as Et3N, Na2CO3, K2CO3, KOH, NaOH, NaHCO3, Ca(OH)2, KOAc and
NaOAc. The results clearly indicated that NaOAc was superior to
otherbasesintermsofyieldandreactiontime.Strongerbasessuch
as Na2CO3 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
systemforthisreactioninvolvedPd(PPh3)2Cl2 (5 mol%)andNaOAc
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 electron-
rich 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 ortho-
substituent.
3a
O
O
I
2
3
3b
3c
2.5
2.5
90
88
O
I
I
N
H
H
N
O
S
O
4
5
6
3d
3e
3f
2.5
2.5
8.0
90
92
78
Br
Br
Br
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
7
8
3g
3h
5.0
85
73
Br
Br
HO
HO
10.0
9
3i
10.0
65
NH2
Br
c
Appl. Organometal. Chem. 2011, 25, 537–541
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

J. Lu et al.
Table 3. (Continued)
Table 3. (Continued)
Entry
10
Aryl halides
Product
Time (h)
8.0
Yield (%)a
81
Entry
27
Aryl halides
Product
Time (h)
8.0
Yield (%)a
71
3j
3r
H2N
N
Br
Br
Cl
Cl
CHO
11
3k
8.0
82
28
3s
12.0
64
O
H
Br
OH
a Yield of isolated product.
12
13
3l
6.0
6.0
78
82
O2N
3m
O
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.
Br
14
15
3n
3o
6.0
8.0
85
86
O
The reusability of the Pd(PPh3)2Cl2/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
simpleextractionofthereactionmixturewithdiethyletherandthe
Pd(PPh3)2Cl2/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]
OMe
Br
Br
O
Me
16
17
3p
3q
10.0
8.0
60
65
CHO
Br
Br
CHO
Conclusion
Inconclusion, wehavedemonstratedanovelandefficientstrategy
for synthesis of arylboronic esters by coupling reaction of aryl
halides and bis(pinacolato)diboron using a catalytic system that
employs Pd(PPh3)2Cl2 along with NaOAc as base in PEG 600.
The operational simplicity, the use of commercially available
Pd(PPh3)2Cl2 with no need for any other ligands, and the wide
scope of substrates are the advantages of this method.
18
19
3r
3s
8.0
80
73
Br
Br
CHO
10.0
O
H
OH
Cl
20
21
3j
10.0
10.0
72
70
H2N
Acknowledgments
3m
O
We thank the National Natural Science Foundation of China
(21072042 and 20872025), and the Natural Science Foundation of
Hebei Province (B2011205031) for financial support.
Cl
Cl
22
23
3n
3t
9.0
71
56
O
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