Boric acid as highly efficient catalyst for the synthesis of phenols from arylboronic acids

Article abstract of DOI:10.1002/hc.21138

A simple, efficient, and environmentally benign boric acid catalyzed methodology for the ipso-hydroxylation of arylboronic acid to phenol has been developed using aqueous hydrogen peroxide as an oxidizing agent and ethanol as the solvent. The versatility of this protocol is that the reactions were performed at room temperature in short reaction time under metal-, ligand-, and base-free conditions.

Full text of DOI:10.1002/hc.21138

Heteroatom Chemistry
Volume 25, Number 2, 2014
B
oric Acid as Highly Efficient Catalyst for the
Synthesis of Phenols from Arylboronic Acids
1
1
1
1
Kongkona Gogoi, Anindita Dewan, Ankur Gogoi, Geetika Borah,
1,2
and Utpal Bora
1
Department of Chemistry, Dibrugarh University, Dibrugarh 786 004, Assam, India
2Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784 028, Assam, India
Received 6 June 2013; revised 26 November 2013
transformation of diazoarenes and benzyne proto-
cols [2]. However, these methods suffer from major
drawbacks such as harsh reaction conditions and di-
azotization of amino groups to diazoarenes, which
is often not compatible with many other functional
groups. In addition to it, some literature reported
the conversion of aryl halides such as aryl bromides
and chlorides into phenols in the presence of Pd-
based catalysts, phosphine ligands [3], and aryl io-
dides into the corresponding phenols in the pres-
ence of a Cu-catalyst using non-phosphine ligands
ABSTRACT: A simple, efficient, and environmentally
benign boric acid catalyzed methodology for the ipso-
hydroxylation of arylboronic acid to phenol has been
developed using aqueous hydrogen peroxide as an oxi-
dizing agent and ethanol as the solvent. The versatility
of this protocol is that the reactions were performed at
room temperature in short reaction time under metal-,
ligand-, and base-free conditions. ꢀC 2014 Wiley Peri-
odicals, Inc. Heteroatom Chem. 25:127–130, 2014;
View this article online at wileyonlinelibrary.com.
DOI 10.1002/hc.21138
[4] at high temperature. Over the last decade, aryl-
boronic acids and their derivatives have emerged
as an important moiety in a wide range of or-
ganic reactions [5]. Although phenols have been
reported as byproducts in many metal-catalyzed
reactions of arylboronic acids [6], a detailed liter-
ature search revealed only a handful of recent re-
ports available on the direct and efficient conver-
sion of arylboronic acids into phenols. Arylboronic
acids are considered as a suitable and potent pre-
cursor of phenols because they are safe, stable
to heat, moisture, and air, and are commercially
available in many forms substituted by functional
groups. Accordingly, hydroxylation of arylboronic
acids to phenols is reported to be feasible in the pres-
ence of MCPBA–H2O–EtOH [7], acidic Al2O3–H2O2
INTRODUCTION
Phenols and their derivatives are ubiquitous in nu-
merous natural products in monomeric or polymeric
forms. They have vast application potential as a key
synthetic precursor for the construction of pharma-
ceuticals, polymers, and naturally occurring com-
pounds [1]. Over the years, various modifications
have been made to the reaction conditions leading
to novel synthetic methods for their synthesis. Phe-
nols are commonly prepared by nucleophilic aro-
matic substitution of aryl halides, and Cu-catalyzed
[
8], I2–H2O2 [9], CuSO4–phenanthroline [10], H2O2–
poly(N-vinylpyrollidone) [11], NH2OH.HCl:NaOH
12], potassium peroxymonosulfate [13], or aqueous
Correspondence to: U. Bora; e-mail: utbora@yahoo.co.in;
ubora@tezu.ernet.in.
Contract grant sponsor: University Grants Commission, New
Delhi, India.
[
H2O2 [14], clay-entrapped Cu(OH)x [15], aqueous
Contract grant number: 41-254/2012 (SR).
ammonia [16], sodium chlorite [17], and tert-butyl
ꢀ
2014 Wiley Periodicals, Inc.
C
127

128 Gogoi et al.
TABLE 1 Optimization of Reaction Conditions for Boric Acid Mediated ipso-Hydroxylationa
OH
H BO /H O (30% aq)
3
3
2 2
B
OH
OH
solvent, rt
Entry
Oxidant
Solvent
–
Catalyst
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
.
.
.
.
.
.
.
.
.
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O
O
O
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
2
2
2
–
60
60
40
40
35
40
40
40
50
40
40
40
40
60
36 h
05
08
74
70
88
79
80
78
73
81
78
80
81
EtOH
–
–
–
H
3
H
3
H
3
H
3
H
3
H
3
H
3
H
3
H
3
H
3
H
3
H
3
BO
BO
BO
BO
BO
BO
BO
BO
BO
BO
BO
BO
3
3
3
3
3
3
3
3
3
3
3
3
(10 mol%)
(5 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
–
EtOH
MeCN
MeOH
THF
CH
EtOH–H
MeCN–H
MeOH–H
2
Cl
2
1
1
1
1
1
1
0.
1.
2.
3.
4.
5.
2
O (1:1)
2
O (1:1)
O (1:1)
2
THF–H
2
O (1:1)
–
H
2
O
Trace
c
H
2
O
2
–
80
a
Reaction conditions: phenylboronic acid (0.5 mmol), H2O2 (30% aq. 2 mL), and solvent (2 mL); room temperature.
Isolated yields.
The reaction did not reach completion.
b
c
hydroperoxide [18] etc. Aqueous hydrogen peroxide
hydrogen peroxide (2 mL) was stirred at room tem-
perature for 1 h. The result was not encouraging
and only a small amount of phenol was obtained
at the end of the reaction (Table 1, entry 1). When
the reaction was performed using ethanol as the sol-
vent, the yield of phenol 2a was slightly enhanced
(Table 1, entry 2). We interestingly observed that the
reaction furnished the desired product 2a in 74%
yield on introducing boric acid (10 mol%) as cata-
lyst (Table 1, entry 3). It was observed that the reac-
tion showed comparatively reduced yield in the pres-
ence of 5 mol% catalyst. Encouraged by the good
result (Table 1, entry 3), we started to screen the
effect of various solvents on the reaction using aryl-
boronic acid, aqueous hydrogen peroxide, and 10
mol% boric acid as reaction promoter (Table 1, en-
tries 5–13). We were pleased to find that the reaction
using ethanol as the solvent gave the desired phe-
nol in 88% yield. The reaction was found to proceed
smoothly in other solvents as well although showing
variation in yields (Table 1, entries 6–13). Likewise,
under the same reaction condition, 1:1 aqueous sol-
vents such as ethanol, acetonitrile, methanol, and
tetrahydrofuran (THF), respectively, gave compara-
ble results (Table 1, entries 10–13). Interestingly, the
use of dichloromethane as the solvent resulted in a
relatively reduced yield (Table 1, entry 9). The re-
action did not progress in the absence of hydrogen
peroxide in aqueous medium (Table 1, entry 14). The
catalytic activity of boric acid could further be es-
tablished by the observation that phenol (2a) was
is an environmentally benign oxidant that shows a
high efficiency per weight of oxidant and it is rela-
tively easy to handle [19]. Moreover, in recent years,
from the viewpoint of green chemistry, boric acid
has achieved special attention as a catalyst in or-
ganic synthesis because of its excellent solubility in
water, commercial availability, stability, ease to han-
dle, inexpensiveness, and ecofriendly nature. Boric
acid is a useful catalyst that has been successfully
utilized in numerous organic transformations such
as esterification reaction [20], transesterification re-
action [21], aza-Michael addition [22], thia-Michael
addition [23], Biginelli reaction [24], and decarboxy-
lation of cyclic β-enaminoketoesters [25]. Thus, in
this respect, boric acid–H2O2 system and use of in-
nocuous ethanol as the solvent offers a perfect com-
bination for the chosen oxidation reaction. In this
communication, we wish to report the use of eas-
ily available, cheap and environment friendly boric
acid as an excellent promoter to convert arylboronic
acids to phenols at room temperature under metal-,
ligand-, and base-free conditions.
To prove the catalytic activity of boric acid and
study the efficacy of hydrogen peroxide in the hy-
droxylation reactions, we have chosen arylboronic
acid (1a; 0.5 mmol) as a model substrate and the
reactions were performed under aerobic condition
at room temperature. The results are summarized
in Table 1. Initially, we conducted a blank reaction
in which a mixture of 1a (0.5 mmol) and aqueous
Heteroatom Chemistry DOI 10.1002/hc

Boric Acid as Highly Efficient Catalyst for the Synthesis of Phenols from Arylboronic Acids 129
TABLE 2 Boric Acid Catalyzed Synthesis of Phenolsa
stituents were tolerated on the aromatic substrate.
Electron-rich phenols (Table 2, entries 1–6) were
obtained in good yield whereas electron-deficient
phenols (Table 2, entries 7–10) were also isolated
in satisfactory yields, but comparatively long reac-
tion time was required. The current catalyst system
is also effective for aryl boronate esters (Table 2, en-
tries 11 and 12) and heteroaryl boronic acid (Table 2,
entry 13).
OH
OH
H BO /H O (30% aq)
3
3
2
2
B
OH
Solvent, rt
R
R
b,c
Entry
R
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
.
.
.
.
.
.
.
.
.
H
35
25
50
50
45
45
60
65
70
70
88
95
89
85
88
92
91
93
88
80
4-C(CH
3-Me
4-Me
2-OMe
4-OMe
4-CN
4-Cl
4-NO
4-F
3 3
)
A probable mechanistic pathway has been pro-
posed for the ipso-hydroxylation of arylboronic acids
(Scheme 1) following the literature report [12, 26,
27]. Hydrogen peroxide in presence of boric acid
is a known system for oxidation reactions [23]. It
has been reported that the reaction between H O
2
1
1
0.
2
2
O
O
and H3BO3 forms adduct A [27], which then reacts
with phenylboronic acid to form the adduct B. Re-
moval of the adduct D and subsequent phenyl mi-
gration to oxygen atom in the adduct B generates
adduct C. Elimination of water regenerates boronic
acid along with the formation of adduct E. Finally,
hydrolysis of the adduct E leads to the formation of
phenol.
In summary, we have developed a simple
and facile one-pot synthetic method of ipso-
hydroxylation of arylboronic acids to the corre-
sponding phenol using aqueous hydrogen peroxide
as an oxidizing agent, ethanol as the solvent, and
boric acid as a green catalyst. The characteristic of
this protocol is that it is mild, facile, and feasible at
room temperature under metal-, ligand-, and base-
free conditions.
B
1.
60
82
1
1
2.
3.
70
60
83
88
S
B(OH)
2
aReaction conditions: ArB(OH)2 (0.5 mmol), H3BO3 (10 mol%), H2O2
(
30% aq, 2 mL), and ethanol (2 mL); room temperature.
b
c
Yields of isolated products.
All compounds were characterized by 1H NMR, 13C NMR, FT-IR,
and MS.
obtained in good yield in the absence of boric acid
only in extended reaction time (Table 1, entry 15).
To understand the scope and limitation of the
current methodology, a variety of electronically di-
verse arylboronic acids were investigated to obtain
the hydroxylation product, using boric acid as cat-
alyst and ethanol as the solvent, and the results
are summarized in Table 2. With various electron-
donating and electron-withdrawing substituents, the
ipso-hydroxylation products were isolated in good
yield (80%–95%). It was observed that different sub-
ACKNOWLEDGMENTS
This work has been supported by UGC, New Delhi
(No. 41-254/2012 (SR)). A.G. thanks the Department
of Science and Technology, New Delhi for an IN-
SPIRE research fellowship, A.D. acknowledges DST,
New Delhi for financial support.
OH
OH
H
+ O O
H
OH
B OH
OH
H
HO
OH
B OH
OH
δ+ δ−
Ph B
Ph B O O
+
HO
H
B
A
OH
d−
+ H O
OH
B OH
OH
OH
+
Ph OH
Ph O B
+ B(OH)
3
Ph O B
H
−
H O
2
OH
OH
E
C
D
SCHEME 1 A plausible mechanistic pathway.
Heteroatom Chemistry DOI 10.1002/hc

130 Gogoi et al.
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Heteroatom Chemistry DOI 10.1002/hc