Room-Temperature Ionic Liquids (RTILs) as Green Media for Metal- and Base-Free ipso -Hydroxylation of Arylboronic Acids

Article abstract of DOI:10.1055/s-0037-1611894

The oxidative hydroxylation of arylboronic acids to the corresponding phenolic compounds under metal- and base-free aerobic conditions is successfully demonstrated on a greener media. Hydrogen peroxide, as an eco-friendly oxidant, is compatible with green mediates room-temperature ionic liquids (RTIL)s, providing hydroxylation products of arylboronic acids in an efficient manner. The RTIL support is particularly interesting for its reusability.

Full text of DOI:10.1055/s-0037-1611894

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Georg Thieme Verlag Stuttgart · New York
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Synlett
E.-J. Shin et al.
Letter
Room-Temperature Ionic Liquids (RTILs) as Green Media for Metal-
and Base-Free ipso-Hydroxylation of Arylboronic Acids
Eun-Jae Shin
B(OH)2
OH
Cl
[
bmim]Cl
N
N
Gyu-Tae Kwon
H2O2, rt
Seung-Hoi Kim*
X
X
X: alkyl, phenyl, halogen,
carbonyl, nitro, cyano
20 examples
up to 96%
Department of Chemistry, Dankook University, 119 Dandaero,
Cheonan 31116, Republic of Korea
metal- and base-free
kimsemail@dankook.ac.kr
eco-friendly media RTILs
aerobic conditions
[bmim]Cl
functional group tolerance
Received: 26.06.2019
Accepted after revision: 03.07.2019
Published online: 17.07.2019
biocatalysts have been efficiently applied to the ipso-
hydroxylation of arylboronic acids. It is of interest that
10
DOI: 10.1055/s-0037-1611894; Art ID: st-2019-k0331-l
this transformation has been intensively carried out using
hydrogen peroxide as an oxidant because it is a well-known
high-efficiency-per-weight, and environmentally benign,
oxidant. Numerous investigations have been performed to
improve the efficiency and convenience of the process un-
der diverse systems utilizing H O and various supports.
Abstract The oxidative hydroxylation of arylboronic acids to the corre-
sponding phenolic compounds under metal- and base-free aerobic con-
ditions is successfully demonstrated on a greener media. Hydrogen per-
oxide, as an eco-friendly oxidant, is compatible with green mediates
room-temperature ionic liquids (RTIL)s, providing hydroxylation prod-
ucts of arylboronic acids in an efficient manner. The RTIL support is par-
ticularly interesting for its reusability.
2
2
11
12
13
14
15
16
Urea, biosilica PEG-400, H BO , Al O , WERSE,
3
3
2
3
PVD/PVP, iodine,18 Amberlite IR-120, Mont K-10@Ag-
17
19
NPs,20 and silica chloride have been used as supports for
the successful conversion of boronic acids into the desired
phenolic derivatives.
21
Key words room-temperature ionic liquid, ipso-hydroxylation, arylbo-
ronic acid, phenol, hydrogen peroxide
Notwithstanding the good results obtained through the
protocols, continuous efforts have been devoted to improv-
ing the efficiency of the process, with particular focus on
greener reaction conditions. Given the aspects of sustain-
able chemistry, bio-friendly and eco-friendly reaction plat-
forms could be suitable approaches to reducing the eco-
nomic and environmental burden of various chemical pro-
cesses. In our search for more environmentally friendly
routes, we sought potentially greener reaction conditions,
such as the use of a benign oxidant and a sustainable sup-
port under aerobic conditions, to promote the hydroxyl-
ation of arylboronic acid. Thus, we postulated that the com-
bination of hydrogen peroxide and room-temperature ionic
liquids (RTILs) would be a greener reaction system for the
ipso-hydroxylation of arylboronic acid, furnishing the cor-
responding phenols.
The functional group transformation of organic com-
pounds is one of the fundamental pathways used to intro-
duce novel functionality into organic derivatives, and this
approach has been frequently utilized to obtain valuable
target materials in both academic and industrial chemical
processes. Several strategies have been developed to deter-
mine more efficient protocols providing various functional
groups in organic compounds that play significant roles in
synthetic organic chemistry. Among the many organic com-
pounds prepared using this functional group conversion,
phenols have been considered and used as one of the most
1
important organic materials. Therefore, the synthesis of
phenols continues to attract the attention of organic chem-
ists. To obtain phenol derivatives through chemical syn-
thetic routes, the ipso-hydroxylation of phenylboronic acid
has been intensively and widely used under various reac-
To date, the development of green and sustainable
chemical processes with no waste production, maximum
material recycling, and lower financial burden is the biggest
challenge in chemical industries. To address this issue,
chemists began looking for more versatile solvents, eventu-
ally using ionic liquids (ILs) consisting of a substituted het-
2
tion conditions.
For the success of this method, the use of an oxidant
3
4
such as anhydrous trimethylamine N-oxide , OXONE, hy-
5
6
7
pervalent iodine, hydroxylamine or peroxides, photocat-
alyzed hydroxylation, and metal-complex-mediated trans-
formations have been developed. In addition, metal-free
8
9
22
erocyclic cation with an organic or inorganic anion. It is
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
Synlett
E.-J. Shin et al.
Letter
well known that RTILs exist in liquid state at room tempera-
ture and have unique properties such as nonvolatility, ther-
mal stability, solvation ability, and easy recyclability. These
unique characteristic properties render RTILs as more ver-
satile materials, not only as chemical mediates but also as
precious substances in organic synthetic catalysis, solid
support, and nanoparticle formation.23
butyl-4-methylpyridium or 1-butylpyridium as a cationic
component were also examined for the hydroxylation of 1a
under the same conditions. As depicted in Table 1, the re-
sults were nearly identical to those of the previous tests
(Table 1, entries 4–6). The characteristic reactivity of the
anion of the IL was further confirmed using another hydro-
philic RTIL, [tba]Cl (Table 1, entry 7).
Regarding the application of RTILs as a reaction media
in oxidative hydroxylation, an interesting study on the
preparation of a task-specific IL, acid-functionalized mag-
netic ionic liquid ([AcMAIL]), and its application was re-
Table 1 Screening the RTILs for Hydroxylation of Phenylboronic Acid (1a)
HO
OH
B
OH
24
cently reported by Banerjee et al. The prepared [AcMAIL]s
were successfully employed in the ipso-hydroxylation of ar-
ylboronic acids in addition to the regioselective Friedel–
Crafts acylation in the presence of hydrogen peroxide.
RTIL
H2O2 (30% aq)
rt, H2O
1a
2a
24
Initially, building on the previous work of others and
Entrya
Yield (%)b
RTIL (equiv)
Time
25
of our own on achieving the ipso-hydroxylation of arylbo-
ronic acids, we postulated that a combination of hydrogen
peroxide and simple RTILs could be used for the prepara-
tion of phenols through the hydroxylation of arylboronic
acids.
1
2
3
4
5
6
7
8
9
[bmim]PF
6
(0.1)
(0.1)
6 h
92
97
96
90
95
95
96
95
68
64
40
[bmim]BF
4
4 h
[bmim]Cl (0.1)
10 min
6 h
[bmpy]PF (0.1)
6
[bmpy]BF
4
(0.1)
4 h
Bu
PF6
Bu
BF4
Bu
N
N
N
N
N
N
[bpy]Cl (0.1)
[tba]Cl (0.1)
[bmim]Cl (1.0)
[bmim]Cl (0.05)
[bmim]Cl (0.01)
none
10 min
10 min
10 min
12 h
Cl
[bmim]PF6
[bmim]BF4
[bmim]Cl
PF6
BF4
Cl
4
(Bu) N Cl
10
24 h
N
N
N
[tba]Cl
Bu
Bu
Bu
11
48 h
[
bmpy]PF6
[bmpy]BF4
[bpy]Cl
a
Reaction conditions: 1.0 equiv of 1a and 1.0 equiv of H
Isolated yield based on 1a.
2 2
O .
b
Figure 1 RTILs used in the reaction optimization
Our following experiments focused on the effect of the
For developing such a reaction system, we have under-
taken the ipso-hydroxylation of phenylboronic acids using
acid- and metal-free RTILs along with hydrogen peroxide.
As described in Table 1, all of the RTILs used in this study
are commercially and readily available. Moreover, one char-
acteristic nature of RTILs is water-miscibility, which de-
amount of the [bmim]Cl media used. To evaluate this, vari-
able amounts of [bmim]Cl were employed, and the results
are described in Table 1. No significant outcome was ob-
served from increasing the amount of [bmim]Cl up to 1.0
equivalent (Table 1, entry 8). In contrast, when the reaction
was performed with decreased amounts of reaction media,
the reduced isolated yield of 2a and prolonged reaction
times for conversion were observed (Table 1, entries 9 and
10). To verify the role of the IL, the conversion of 1a into 2a
was tested in the absence of IL. Unfortunately, the reaction
did not reach completion even when the reaction time was
prolonged (Table 1, entry 11).
26
pends on the anion species of the IL. Since our reaction
system consists of aqueous H O and water, the initial ob-
2
2
jective is to elucidate the effect of the anions. Thus, several
RTILs (Figure 1) possessing different anions, [bmim]PF6,
[
bmim]BF , and [bmim]Cl, were tested first under the con-
4
ditions described in Table 1. In general, a satisfactory out-
come forming the desired product 2a was obtained regard-
less of the anion species presented in the RTILs. However, it
is of interest that a relatively longer reaction time was re-
quired for the conversion into 2a when less hydrophilic
RTILs were employed (Table 1, entries 1 and 2). Of the RTILs
tested, the most water-miscible media, [bmim]Cl, turned
out to be very effective in our system in terms of the reac-
tion time and isolated yield of desired product 2a (Table 1,
entry 3). To verify the effect of the cationic part of the
RTILs, slightly different types of RTILs possessing either 1-
Based on the results obtained thus far, we inferred that
the cooperation between RTILs and the H O oxidant would
2
2
be a versatile route for the ipso-hydroxylation of arylboron-
ic acids for producing the corresponding hydroxylation
products. Having established the optimum conditions (Ta-
ble 1, entry 3), we further explored the versatility and ap-
plicability of our system by using various arylboronic acids
for the ipso-hydroxylation. The results are presented in
Scheme 1.
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

C
Synlett
E.-J. Shin et al.
Letter
HO
OH
eroaryl boronic acids were also amenable to this protocol.
Quinoline-3-boronic acid and 6-methoxy-3-pyridinylbo-
ronic acid were examined under the same conditions, lead-
ing to the desired products 2q and 2r, respectively. Howev-
er, hydroxylation of 2-furanyl and 3-thienylboronic acids
resulted in negligible yields under identical conditions. The
conversions of both naphthalene-1-boronic acid and naph-
thalene-2-boronic acid also occurred, producing the corre-
sponding products 1-naphthol (2s) and 2-naphthol (2t) at
B
OH
[
bmim]Cl (0.1 equiv)
H2O2 (0.24 mL)
rt, 15 min
1
b–t (3.0 mmol)
2
b–t
OH
OH
H3C
OH
CH3
83% and 91% isolated yields, respectively. Furthermore, we
CH3
explored this method for the alkylboronic acid, octylboron-
ic acid, showing a mixture of unidentified products.
2
b, 88%
2c, 93%
2d, 91%
OH
OH
OH
Even though diverse boronic acids are readily available
and highly cooperative in our system, the tendency of bo-
ronic acids to rapidly decompose and other drawbacks have
limited their applications in organic synthesis.27 To over-
come the shortcomings and expand the scope of our sys-
tem, potassium phenyltrifluoroborate (1u) and phenyl bo-
ronic acid ester (1v) were employed as boronic acid surro-
gates under the same conditions used earlier in this study.
As depicted in Scheme 2, the transformation was unevent-
ful, producing the desired product 2a in good to excellent
yields. The completion of the conversion of these surrogates
into phenol required a slightly prolonged reaction time.
Ph
H
O
2
e, 90%
2f, 88%
2g, 85%
OH
OH
OH
F
F
H3C
O
2
2
2
h, 86%
2i, 84%
2j, 89%
OH
Br
OH
Cl
OH
BF3K
OH
O
O
[
bmim]Cl
H2O2
[bmim]Cl
H O
B
2
2
CN
rt, 30 min
89%
rt, 30 min
97%
k, 88%
2l, 92%
2m, 90%
1u
2a
1v
OH
Scheme 2 Expansion to boronic acid surrogates
MeO
MeO
OH
HO
OH
From the viewpoint of sustainability, recycling and re-
use of reaction media should be investigated to increase the
value of the presented protocol. Accordingly, we turned our
attention to recycling and reuse of the reaction media RTIL.
Since the RTILs are insoluble in organic solvent, the reaction
mixture was extracted with diethyl ether after the initial
reaction was completed. Then, the recycled [bmim]Cl was
directly employed in the consecutive conversion of phenyl-
boronic acid (1a) into phenol (2a) under the same condi-
tions used before. As described in Figure 2, the transforma-
tion proceeded well, as expected, in a fashion similar to the
case of using fresh [bmim]Cl. The conversion was complet-
ed within 10 min in each run to afford 2a in satisfactory
NO2
n, 92%
2o, 90%
2p, 91%
OH
OH
OH
OH
N
N
OMe
2
q, 80%
2r, 84%
2s, 83%
2t, 91%
Scheme 1 Substrate screening
Irrespective of the electronic properties of the arylbo-
ronic acids, the transformation was successful, resulting in
the moderate to high yields of the corresponding phenolic
compounds. Notably, the present catalytic system displayed
a considerable tolerance toward various functional groups
such as carbonyl (2g and 2h), halogens (2i–l), cyano (2m),
nitro (2n), and alkoxy (2o and 2p) functionalities. Although
their products were obtained in relatively low yields, het-
st
nd
rd
yield from the 1 , 2 , and 3 runs together with the slight-
th
th
ly diminished yields of 2a from both the 4 and 5 runs
(Figure 2). The effect of the residents that could be cumulat-
ed from the previous runs in the IL has not yet been fully
investigated.
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

D
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E.-J. Shin et al.
Letter
1
st
96%
OH
HO
OH
HO
O
H
B
–B
O
RTIL
H
2
nd
9
6%
95%
93%
O
H
RTIL
3
rd
th
O
+
IL
H
4
5th
92%
OH
B
+
O
Figure 2 Reusability test
HO
OB(OH)2
–
O
H
H
H2O
At this point, as was stated above, the successful trans-
formation of arylboronic acids to phenolic compounds us-
ing [AcMAIL]s, which contain an iron metal and an acidic
moiety has been revealed. Therefore, it would be import-
ant to investigate whether or not the presence of these sub-
stance affect our system. To elucidate the influence, we set
up the hydroxylation of phenylboronic acid utilizing our re-
2a
–
H2O
Scheme 3 Plausible mechanism for the ipso-hydroxylation of 1a
24
In conclusion, we have developed a simpler and greener
protocol for the ipso-hydroxylation of boronic acids that
produces the corresponding alcoholic derivatives in a satis-
factory manner.28 The combination of eco-friendly oxidant
H O and ready availability of RTILs were sufficient enough
action system along with FeCl or acetic acid, and the re-
3
sults are summarized in Table 2. As depicted, the reaction
with FeCl at room temperature was initialized vigorously,
3
2
2
which rendered monitoring the progress impossible (Table
to convert arylboronic acids into phenols under mild aero-
bic conditions at excellent yields.
2, entry 1) whereas the hydroxylation starting up in an ice-
bath did not occur and low conversion into 2a was observed
even when the temperature was allowed to warm up to
room temperature (Table 2, entry 2). No further investiga-
tions to elucidate this observation were performed in this
study. In contrary, transformation to 2a proceeded smooth-
ly in the presence of acetic acid, but little disappointed out-
come was revealed (Table 2, entry 3). These two experi-
ments clearly indicated that the simple addition of iron
salts or acetic acid was not compatible with our newly de-
veloped system.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611894.
S
u
p
p
orit
n
g Inform ati
o
n
S
u
p
p
orit
n
g Inform ati
o
n
References and Notes
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OH
[bmim]Cl (0.1 equiv)
B
OH
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H2O2 (0.24 mL)
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(
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(
1
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FeCl
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rt
too vigorous
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AcOH (1.0)
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(28) Typical Procedure for the ipso-Hydroxylation
A flask was charged with phenylboronic acid (3.0 mmol),
[bmim]Cl (52.0 mg), and H O (aq 30 wt%, 0.24 mL). Then, the
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(
(
(
(
2
2
mixture was stirred at room temperature in open air for 15 min.
The reaction mixture was extracted with EtOAc (3 × 10 mL). The
combined organic layers were washed with water, then dried
with anhydrous Na SO , and evaporated under reduced pres-
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15) Gogoi, A.; Bora, U. Tetrahedron Lett. 2013, 54, 1821.
16) Saikia, E.; Bora, S. J.; Chetia, B. RSC Adv. 2015, 5, 102723.
17) Prakash, G. K. S.; Chacko, S.; Panja, C.; Thomas, T. E.; Gurung, L.;
Rasul, G.; Mathew, T.; Olah, G. A. Adv. Synth. Catal. 2009, 351,
2
4
sure. The crude mixture was purified by column chromatogra-
phy on silica gel (hexanes/EtOAc).
Phenol (2a)
1
(
(
(
96%, colorless oily liquid. H NMR (400 MHz, CDCl ):  = 7.28
3
(t, J = 8.4 Hz, 2 H), 6.99–6.95 (m, 1 H), 6.89–6.85 (m, 2 H), 4.80
13
(br s, 1 H) ppm. C NMR (100 MHz, CDCl ):  = 155.4, 129.7,
3
120.9, 115.3 ppm.
1567.
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2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E