Borylation of aryldiazonium salts at room temperature in an aqueous solution under catalyst-free conditions

Article abstract of DOI:10.1016/j.tetlet.2017.08.060

A general and convenient borylation reaction of aryldiazonium tetrafluoroborate salts with B₂pin₂ has been developed. In this catalytic system, no catalyst, additional ligands or additives were required. The reaction proceeded smoothly in an aqueous solution, and a variety of arylboronates were isolated in moderate to excellent yields under mild reaction conditions.

Full text of DOI:10.1016/j.tetlet.2017.08.060

Tetrahedron Letters 58 (2017) 3851–3853
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Borylation of aryldiazonium salts at room temperature in an aqueous
solution under catalyst-free conditions
a
a
a
a,b,
⇑
Xinxin Qi , Hao-Peng Li , Jin-Bao Peng , Xiao-Feng Wu
a
Department of Chemistry, Zhejiang Sci-Tech University, Xiasha Campus, Hangzhou 310018, People’s Republic of China
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Straße 29a, 18059 Rostock, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A general and convenient borylation reaction of aryldiazonium tetrafluoroborate salts with B pin has
2
2
Received 19 July 2017
Revised 23 August 2017
Accepted 24 August 2017
Available online 26 August 2017
been developed. In this catalytic system, no catalyst, additional ligands or additives were required. The
reaction proceeded smoothly in an aqueous solution, and a variety of arylboronates were isolated in mod-
erate to excellent yields under mild reaction conditions
.
Ó 2017 Elsevier Ltd. All rights reserved.
Keywords:
Green chemistry
Metal-free
Aryldiazonium salt
Arylboronates
Sustainable
Introduction
chemists.12 Ito and co-workers reported the borylation reaction of
aryl bromides with a silylborane reagent in the presence of an
1
3
Arylboronic acids and their derivatives play an important role in
organic synthesis and have found a wide variety of applications in
alkoxy base. The photoinduced borylation of aryl halides with
diboron reagents were also independently developed by the groups
1
2
14
material science, and medicinal chemisty. Due to their high sta-
bility, low toxicity, and easy handling, they are widely used as ver-
satile building blocks in transition-metal-catalyzed C–C bond and
C–X bond formation reactions.3 Thus, synthetic methods for the
preparation of arylboronates have been extensively developed in
the past few decades. In particular, the reaction of trialkyl borates
with lithium or Grignard reagents is one of the most widely used
approaches to prepare these boron compounds.4 However, this
method usually suffers from limitations such as requiring rigor-
ously anhydrous conditions and narrow functional group toler-
of Fu, Larionov, and Li. Most recently, Jiao and co-workers dis-
closed a pyridine-catalyzed borylation of haloarenes with diboron
1
5
reagents using KOMe as the base. Therefore, the development of
an efficient and environmentally friendly catalytic system for aryl-
boronate formation is necessary, important and in high demand.
Aryldiazonium tetrafluoroborate salts, which can be easily pre-
pared from readily available aryl amines, are a good alternative
to aryl halides and triflates in cross-coupling reactions. Various cat-
alyst systems have been established for their applications in bory-
lation reactions. However, palladium catalysts, light irradiation,
organic peroxides, strong base or acid, or high reaction tempera-
5
ance. Alternatively, the transition-metal-catalyzed borylation
1
6
reaction has become an effective and popular strategy for the syn-
thesis of arylboronates due to its mild reaction conditions and good
substrate compatibility, and a wide range of transition-metal cat-
alysts, including Pd , Ni, Cu, Fe, and others,
explored.
On the other hand, since the end of 20th century, ‘Green’ pro-
cesses have become a hot issue in organic synthesis. Thus, the syn-
thesis of arylboronates without transition-metal catalysts has
drawn continuous attention and represents a challenge for organic
ture are required. With regard to this background, herein, we
describe a general and efficient catalyst-free borylation of aryldia-
6
zonium tetrafluoroborate salts with B
2 2 2
pin in an acetone/H O co-
7
8
9
10
11
have been
solvent under mild reaction conditions.
Recently, gallic acid was reported to activate aryldiazonium
salts and applied as a catalyst in cross-coupling reactions with
1
7
heteroarenes. Thus, we became interested to verify the possibil-
ity of using gallic acid as a catalyst in the borylation of aryldiazo-
nium salts. Initially, phenyldiazonium tetrafluoroborate salts and
2 2 2
B pin were used as model substrates in acetone/H O at 20 °C in
the presence of catalytic gallic acid. To our delight, the correspond-
ing phenylboronate product was obtained in 82% yield (Table 1,
entry 1). Next, various solvents were examined (Entries 2–5);
⇑
Corresponding author at: Department of Chemistry, Zhejiang Sci-Tech Univer-
sity, Xiasha Campus, Hangzhou 310018, People’s Republic of China.
E-mail address: xiao-feng.wu@catalysis.de (X.-F. Wu).
http://dx.doi.org/10.1016/j.tetlet.2017.08.060
0
040-4039/Ó 2017 Elsevier Ltd. All rights reserved.

3
852
X. Qi et al. / Tetrahedron Letters 58 (2017) 3851–3853
Table 1
Table 2
a
2 2
Borylation of aryldiazonium tetrafluoroborate salts with B pin
.a
Screening of reaction conditions.
N
2
BF
4
Bpin
N
BF
Bpin
Catalyst
2
4
2
Acetone/H O (2/1)
B
2
pin
2
B
2
pin
2
R
R
Solvent, Temp.
20 °C
Yield (%)b
59
Entry
Gallic acid (mol%)
Solvent
Temp. (°C)
Yield (%)b
Entry
1
Substrate
N BF
Product
1
2
3
4
5
6
10
10
10
10
10
10
10
5
Acetone/H
Acetone
H O
2
EtOH
2
EtOH/H O (3/1)
Acetone/H
Acetone/H
Acetone/H
Acetone/H
Acetone/H
Acetone/H
Acetone/H
Acetone/H
Acetone/H
2
O (3/1)
20
20
20
20
20
20
20
20
20
20
20
30
50
10
82
11
15
13
79
88
99
92
96
89
76
69
63
71
Bpin
2
4
2
43
N
2
BF
4
Bpin
2
2
2
2
2
2
2
2
2
O (2/1)
O (2/1)
O (2/1)
O (2/1)
O (2/1)
O (2/1)
O (2/1)
O (2/1)
O (2/1)
c
7
c
3
4
5
2
N BF
4
Bpin
Bpin
46
57
54
8
9
c
20
1
1
1
1
1
0c
none
none
none
none
none
N BF
4
2
c
1
c
2
c
3
N
2
BF
4
4
Bpin
Bpin
c
4
a
Reagents and conditions: PhN
10 mol%), solvent (4 mL), 1 h.
Yields were determined by GC using dodecane as an internal standard.
2 4 2 2
BF (0.5 mmol), B pin (1 mmol), gallic acid
(
6
N BF
48
b
2
c
2 2
B pin (1.5 mmol).
O
O
S
7
8
9
N BF
Bpin
Bpin
Bpin
81
80
66
2
4
acetone/H
ratio of acetone/H
in 88% yield with a 2/1 ratio (Entry 6). When 3 equivalents of
pin were utilized in the reaction, phenylboronate was obtained
2
O was shown to be the best solvent. Furthermore, the
S
2
O was tested and the target product was obtained
N BF
2
4
B
2
2
HO
HO
in 99% yield (Entry 7). Furthermore, different catalyst loadings
were tested (Entries 8–10); we found that the reaction outcome
is not influenced by this factor. To our surprise, the desired product
could also be obtained in 89% yield in the absence of gallic acid
N BF
2
4
O
2
N
2
O N
1
1
0
1
N BF
Bpin
Bpin
71
60
2
4
(
Entry 10). Therefore, we turned our attention to this new catalyst-
NC
NC
free borylation system. We found 20 °C to be the best reaction
temperature for this transformation and the yields decreased when
the temperature was changed to 10, 30 or 50 °C (Entries 12–14).
With the optimal reaction conditions in hand,18 we continued
our investigation with regard to the substrate scope of this trans-
formation using a variety of aryldiazonium tetrafluoroborate salts
2 4
N BF
F3C
3
F C
1
1
2
3
N BF
Bpin
74
87
2
4
(
Table 2). Substrates with electron-rich groups, such as methyl,
O
O
N BF
4
2
Bpin
tert-butyl, methoxy, methylthio, and hydroxyl groups, gave the
desired products in moderate to good yields (Entries 2–8). Those
with methyl groups substituted at the para-position worked better
than at the ortho- and meta- positions (Entries 2–3 vs. 4). Electron-
deficient groups, including nitro, cyano, trifluoromethyl, ketone,
and ester groups, also provided the corresponding arylboronate
products in very good yields (Entries 9–13). Remarkably, aryldiazo-
nium tetrafluoroborate salts with halogen substitutions also
worked well and afforded the corresponding products in moderate
to good yields (Entries 14–17). The halogen groups in the products
are important functional group in cross-coupling chemistry. It was
noteworthy that the naphthyl group was also well tolerated and
gave the desired product in 68% yield (Entry 18). It is important
to note that the reaction was totally shut down by the addition
of TEMPO or BHT (2 equiv.).
EtO
2
C
2
EtO C
1
1
4
5
N
2
BF
4
Bpin
58
68
72
55
68
F
F
N BF
2
4
Bpin
Cl
Br
I
Cl
Br
I
16
N
BF
Bpin
Bpin
2
4
1
1
7
8
2 4
N BF
N BF
2
4
Bpin
In conclusion, a catalyst-free borylation of aryldiazonium
2 2
tetrafluoroborate salts with B pin in an aqueous solution has been
a
explored. Under mild reaction conditions, moderate to excellent
yields of the desired arylboronate products were obtained, and a
wide range of functional groups are tolerated.
2 4 2 2 2
Reagents and conditions: ArN BF (0.5 mmol), B pin (1.5 mmol), acetone/H O
(
2/1, 4 mL), 20 °C, 1–2 h.
b
Isolated yields.

X. Qi et al. / Tetrahedron Letters 58 (2017) 3851–3853
3853
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The authors thank the financial supports from NSFC (21472174,
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18. General procedure: Diazonium tetrafluoroborate salts (0.5 mmol) and B
(1.5 mmol) were transferred into an oven-dried tube under air. Then, acetone/
O (2/1, 4 mL) were added into the tube via syringe. The sealed tube was kept
2 2
pin
m
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H
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a
2
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silica gel (petroleum ether/ethyl acetate).