A mild and efficient protocol for the ipso-hydroxylation of arylboronic acids

Article abstract of DOI:10.1016/j.tetlet.2013.01.079

An ipso-hydroxylation of arylboronic acids to phenols using aqueous H ₂O₂ as oxidant and acidic alumina as catalyst has been reported in this study. The reusability of the catalyst is also investigated.

Full text of DOI:10.1016/j.tetlet.2013.01.079

Tetrahedron Letters 54 (2013) 1821–1823
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Tetrahedron Letters
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A mild and efficient protocol for the ipso-hydroxylation of arylboronic acids
⇑
Ankur Gogoi, Utpal Bora
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An ipso-hydroxylation of arylboronic acids to phenols using aqueous H O as oxidant and acidic alumina
2
2
Received 13 December 2012
Revised 16 January 2013
Accepted 20 January 2013
Available online 1 February 2013
as catalyst has been reported in this study. The reusability of the catalyst is also investigated.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Phenols
Aryl boronic acid
Acidic alumina
Hydrogen peroxide
Hydroxylation
For many years phenols are considered as versatile precursors
to a large collection of polymers, drugs, herbicides, and antioxi-
dants both in simple and in polyphenolic forms and consequently
is considered as a stoichiometric, environmentally acceptable oxi-
dant that shows a high efficiency per weight of oxidant.17 In the lit-
erature many reports are available where alumina is used as a
potential reusable catalyst and as support in many organic reac-
1
of enormous interest. Existing protocol for their synthesis involves
2
18
the nucleophilic substitution of activating aryl halide, Cu-catalyzed
tions. In continuation of our efforts to develop an efficient proto-
3
11
transformation of diazoarene to phenols, or Pd-catalyzed conver-
col for the direct transformation of arylboronic acids to phenols,
4
sion of aryl halides to phenols using phosphine ligands. Due to
we herein wish to report the direct ipso-hydroxylation of arylbo-
inactive nature of the precursor used in these cases, harsh reaction
conditions are often necessary hence not compatible with many
other functional groups. In addition Pd based catalytic systems
are very expensive hence not suitable for large scale industrial syn-
thesis. So the developments of precursor which can significantly
afford these moieties in mild conditions are still desirable. Recently
arylboronic acid and its derivative have emerged as a potential
ronic acids to phenols by using H
Al as a reusable catalyst (Fig. 1).
Our study begins with the optimization of the reaction param-
eter, for which we choose phenylboronic acid (1 mmol) as a model
substrate and the reactions are carried out in room temperature
under aerobic conditions. The results obtained are summarized in
2 2
O (30% aqueous) and acidic
2 3
O
2 2
Table 1. Use of H O alone as oxidant yields only a trace amount
5
synthon for a wide array of organic reactions. Among them direct
of phenol (Table 1, entry 1) in 1 h. When we used silica as a cata-
lyst, the yield of the product was significantly increased to 78% (Ta-
ble 1, entry 2). Similar result was also obtained in case of fume
silica (Table 1, entry 3). But the yield of phenol was drastically de-
creased when molecular sieves were used as catalyst (Table 1, en-
try 4). We are pleased to find that incorporation of acidic alumina
increased the isolated yield to 92% (Table 1, entry 5). However neu-
tral and basic alumina gave comparatively lower yield of the prod-
uct (Table 1, entries 6 and 7). To evaluate the optimum amount of
acidic alumina required for the hydroxylation reaction we per-
formed some test reactions under controlled condition using dif-
ferent amounts of acidic alumina (Table 1, entries 8 and 9) and
20 mg of the catalyst was found to be optimum for the effective
conversion. Further decreasing of the amount resulted in lower
yield of phenol (Table 1, entry 10). However increase in the amount
of catalyst loading does not improve the yield of phenol (Table 1,
entry 11). During our study we found that 1.5 mL of hydrogen
conversion of arylboronic acids to different functional groups such
as aryl halides, nitoarenes,7 arylamines,8 aryl sulphons, and
6
9
1
0–16
phenols
is important. The direct transformation of arylboronic
acids to phenols was achieved by using CuSO
H O –poly (N-vinylpyrolidone), NH OH, potassium
2 2 2
peroxymonosulfate, aq H
1
0
4
–phenanthrolin,
1
1
12
13
2
I –H
O
2 2,
1
4
15
16
2
O
2,
Amberlite IR-120 resin, etc.
Although these methods often show excellent activity, however,
in the majority of cases the principal drawbacks associated
1
2
with these protocols are use of harmful toxic organic solvent,
ligands/base, long reaction time,
1
0
13,15
and nonreusability of the
catalyst. In order to circumvent these problems considerable atten-
tion has been directed toward the search for more convenient
methods for the synthesis of phenols. Aqueous hydrogen peroxide
⇑
Corresponding author. Fax: +91 373 2370323.
E-mail addresses: utbora@yahoo.co.in, utpal06@gmail.com (U. Bora).
0
040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.079

1
822
A. Gogoi, U. Bora / Tetrahedron Letters 54 (2013) 1821–1823
Table 2
Acidic Al O
2
3
a
B(OH)2
OH
Acidic alumina promoted synthesis of phenol
R
H O (30%aq)
2
2
R
Acidic Al O3
2
B(OH)2
OH
2 2
Figure 1. ipso-Hydroxylation of arylboronic acid to phenol using aqueous H O as
oxidant and acidic alumina as catalyst.
R
H O (30%aq)
2
2
R
Entry
R
Time (min)
Yieldb,c (%)
Table 1
1
2
3
4
5
5
6
7
8
9
H
10
15
20
05
10
14
15
12
20
25
13
25
15
35
92
95
91
94
98
90
86
91
89
86
87
89
90
83
Optimization of reaction condition for acidic alumina promoted ipso-hydroxylation of
p-Me
p-OMe
p-NO
p-t-butyl
p-COMe
p-F
phenylboronic acida
2
Catalyst
B(OH)2
OH
H O (30%aq)
2
2
2
a
p-Cl
1
a
o-NO
o-OMe
m-NO
m-OMe
m-Me
2
Entry
Catalyst (30 mg)
—
Oxidant
Time (min)
Yieldb (%)
10
2
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
2
2
2
2
2
2
2
2
2
2
2
2
60
45
50
60
12
30
15
12
12
60
12
12
60
60
04
78
79
32
92
81
89
92
92
78
92
92
65
16
11
12
13
SiO
2
Fume silica
Molecular sieves
Thiophene-2-boronic acid
a
b
c
Reaction conditions: phenylboronic acid (1 mmol), aqueous H
Isolated yields.
2 2
O 30% 1.5 mL.
Acidic Al
Basic Al
Neutral Al
Acidic Al
Acidic Al
Acidic Al
Acidic Al
Acidic Al
Acidic Al
Acidic Al
2 3
O
O
2 3
All the compounds were characterized by 1H NMR, 13C NMR, FT-IR, and GC–MS.
2
O
3
2
2
2
2
2
2
2
O
O
O
O
O
O
O
3
3
3
3
3
3
3
(25 mg)
(20 mg)
(15 mg)
(35 mg)
(20 mg)
(20 mg)
(20 mg)
10
11
12
13
14
Table 3
a
Reuse of the acidic alumina in the synthesis of phenol
c
d,e
e
Acidic Al O
2
3
B(OH)2
OH
H O (30%aq), rt
2
2
a
2 2
Reactions conditions phenylboronic acid (1 mmol), H O (30% aq, 2 mL), cata-
lyst 30 mg unless otherwise noted.
Yieldab (%)
Entry
Run
Acidic Al
2
O
3
(mg)
Time (min)
b
Isolated yields.
c
1
2
3
4
5
1st
20
19
17
16
12
10
10
12
13
20
92
92
90
89
70
1
1
.5 mL of H was used.
2
O
2
d
e
2nd
3rd
4th
5th
mL of H O was used.
2 2
Reaction does not reach completion.
peroxide was sufficient for the effective conversion (Table 1, en-
tries 12 and 13). When acidic alumina was used with water we ob-
served 16% of isolated phenol within 1 h (Table 1, entry 14).
Next with optimized reaction condition we investigate the
scope of our protocol with a series of electronically diversified aryl-
boronic acids. The results obtained are summarized in Table 2.
We were pleased to find that our protocol tolerates a wide range
of functionalities. Arylboronic acids with electron donating and
a
Reaction conditions: phenylboronic acid (1 mmol), aqueous H
Isolated yields.
2
O
2
(30%, 1.5 mL).
b
wide range of functional groups. In addition, short reaction time,
metal, ligand, base, and solvent-free conditions make this method-
ology suitable for future applications.
1
9
withdrawing groups such as OMe, Me, t-butyl, F, Cl, NO
2
, COMe
Acknowledgments
at para position afford excellent yield of substituted phenols (Ta-
ble 2, entries 2–7). No significant effect was observed for sterically
demanding ortho substituents and good yield of isolated product
was observed (Table 2, entries 8 and 9). Meta substituted arylbo-
ronic acid as well as heteroaryl boronic acid also underwent ipso-
hydroxylation in excellent yield under the current reaction condi-
tions (Table 2, entries 10–13).
We wish to thank the Department of Science and Technology,
New Delhi (No. SR/FT/CS-0098/2009) and the University Grants
Commission, New Delhi (F. No. 41-254/2012 (SR) for the financial
support. A.G. thanks the Department of Science and Technology,
for an INSPIRE research fellowship.
Reusability of any catalyst makes it even more attractive. In or-
der to investigate the reusability of the acidic alumina we per-
formed the hydroxylation of phenylboronic acid under the same
References and notes
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1
9
reaction condition. After the first cycle, the catalyst was filtered
and washed several times with diethyl ether followed by water.
The recovered catalyst was dried in oven at 100 °C for overnight
and used as catalyst for the ipso-hydroxylation of arylboronic acids.
Effectiveness of the catalyst was found to be up to 5 cycles as rep-
resented in Table 3. It is clear from the reusability test that up to
four cycle ipso-hydroxylation takes place without a significant de-
crease in the yield.
In conclusion we have developed a reusable, mild, and efficient
protocol for the ipso-hydroxylation of arylboronic acids to phenols
using aqueous peroxide as oxidant and acidic alumina as catalyst.
Due to mild reaction condition our protocol is compatible with a
2
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9
, 3405.
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1
1
1
2 4
Na SO and evaporated in a rotary evaporator under reduced pressure. The
crude was purified by column chromatography on silica gel (hexane:ethyl
acetate, 9:1) to afford the desired product. The purity of the compound was
confirmed by 1H NMR, C NMR, FT-IR, MS and melting point data.
13