Rapid and green synthesis of phenols catalyzed by a deep eutectic mixture based on fluorinated alcohol in water

Article abstract of DOI:10.1016/j.jfluchem.2013.12.006

A new deep eutectic mixture based on choline chloride and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was prepared and was found to be effective for the rapid transformation of aryl/heteroaryl boronic acids to the corresponding phenols in water using hydrogen peroxide as the oxidant. Broad substrate compatibility, metal- and additive-free conditions as well as reusability of the catalyst made this procedure more environmentally benign.

Full text of DOI:10.1016/j.jfluchem.2013.12.006

Journal of Fluorine Chemistry 158 (2014) 44–47
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Short Communication
Rapid and green synthesis of phenols catalyzed by a deep eutectic
mixture based on fluorinated alcohol in water
Liang Wang *, Dong-yan Dai, Qun Chen, Ming-yang He *
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
A new deep eutectic mixture based on choline chloride and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)
was prepared and was found to be effective for the rapid transformation of aryl/heteroaryl boronic acids
to the corresponding phenols in water using hydrogen peroxide as the oxidant. Broad substrate
compatibility, metal- and additive-free conditions as well as reusability of the catalyst made this
procedure more environmentally benign.
Received 4 November 2013
Received in revised form 10 December 2013
Accepted 14 December 2013
Available online 21 December 2013
ß 2013 Elsevier B.V. All rights reserved.
Keywords:
Deep eutectic mixture
Hexafluoroisopropanol
Aryl/heteroaryl boronic acids
Phenols
1
. Introduction
have also been achieved in various reaction system including
2 2 2 2 2 2 2
aqueous H O [12], H O -I [13], H O -poly(N-vinylpyrrolidone)
Phenol compounds and derivatives have been found in
[14], H -Amberlite IR-120 resin [15], N-oxides [16] and KOH/
2 2
O
numerous natural products and they frequently serve as key
synthetic intermediates for construction of more complex
structures [1]. A routine method for preparation of phenols
compounds involves either the nucleophilic substitution of
activated aryl halides or metal-catalyzed transformations
of diazoarenes [2,3]. However, these methods suffer from
drawbacks such as harsh conditions or the incompatibilities of
the substrates. In recent years, metal-catalyzed hydroxylation of
aryl halides has emerged as an attractive alternative for the
preparation of phenols and impressive progress has been
achieved by several groups [4–10].
On the other hand, aromatic boronic acid derivatives are
powerful synthons in organic chemistry due to the advantages
such as non-toxic, stable to heat, air and moisture, and easy
availability. Recent investigations also clearly showed that
aromatic boronic acids could be transformed to phenols via
copper-catalyzed hydroxylation or oxidative hydroxylation in the
presence of a wide variety of catalysts or reagents. For example,
TBHP [17]. However, these methods have some disadvantages such
as long reaction times, a large amount of oxidants or reagents, or
the use of toxic chlorinated organic solvent. Thus, a rapid, metal-
and additive-free, and eco-friendly protocol is still in demand.
More recently, deep eutectic solvents (DESs) have invoked
enormous interest as a green and potential solvent or catalyst in
organic reactions [18]. DESs are mainly prepared by combining a
cationic salt with a hydrogen-bond donor. One of the most explicit
examples is the mixing one mole of choline chloride (ChCl) with
two moles of urea (with melting points of 247 8C and 133 8C,
respectively), which results in a deep eutectic solvent with a room
temperature melting point. It is similar to conventional ionic
liquids in terms of low vapor pressure and low flammability. In
addition, they are biodegradable, non-toxic, inexpensive and
reusable. Moreover, unlike ionic liquids, these solvents do not
require a preliminary purification step. Their ability to serve as
catalysts as well as solvents has also been explored in the field of
synthetic organic chemistry [19–23].
Wang et al. has reported CuSO
4
-catalyzed hydroxylation of
It is well known that HFIP exhibits high hydrogen bonding
donor ability, low nucleophilicity, high ionizing power and the
ability to solvate water. Its effect on organic transformations [24–
27] and notably the activation of hydrogen peroxide for oxidation
reactions [28–30] are also well documented. Thus, as a part of our
program for developing new DESs, we believe that HFIP is a good
hydrogen-bond donor and can form deep eutectic mixture with
choline chloride. Herein, we report the preparation of a new DES
(ChCl/HFIP). Its catalytic activity and reusability toward the
aromatic boronic acids at room temperature, albeit using a
stoichiometric strong base (KOH) and a nitrogen ligand [11].
The metal-free oxidative hydroxylation of aromatic boronic acids
*
Corresponding authors. Tel.: +86 519 86330263; fax: +86 519 86330251.
E-mail addresses: lwcczu@126.com (L. Wang), hemingyangjpu@yahoo.com
(M.-y. He).
0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2013.12.006

L. Wang et al. / Journal of Fluorine Chemistry 158 (2014) 44–47
45
Scheme 1. ChCl/HFIP catalyzed the synthesis of phenols in water.
Scheme 3. Oxidative hydroxylation of arylboronic acid.
preparation of phenols from arylboronic acids is also investigated
ethanol and tetrahydrofuran, the yield was only slightly increased.
Surprisingly, the yields were greatly elevated in the presence of
(
Scheme 1).
10 mol% of ChCl/HFIP in both protic and aprotic solvents, although
2
. Results and discussion
variations in yields were observed (Table 1, entries 5–7). When the
reaction proceeded in the absence of any solvent, a 87% yield of 2a
was obtained (Table 1, entry 8). The influences of ChCl/HFIP and
Firstly, the DES (ChCl/HFIP) was readily prepared by mixing the
choline chloride with HFIP at 50 8C till a clear solution was
obtained (Scheme 2). Several ratios of ChCl/HFIP were examined
and 1:1.5 was selected as the best ratio, which was confirmed by
NMR analysis. DSC analysis also clearly showed that the melting
point of ChCl/HFIP was ꢀ37.2 8C. Moreover, ChCl/HFIP was very
stable and could be easily stored on shelf without decomposition.
With this DES in hand, we started to optimize the reaction
conditions using phenylboronic acid 1a as the substrate (Table 1).
aqueous H
that the combination of 10 mol% of ChCl/HFIP and 5 equiv of
aqueous H was the best choice, giving 2a in 95% yield with 100%
selectivity (Table 1, entry 10). When HFIP was used alone, more
catalyst was required but the yield decreased (Table 1, entry 14).
Other oxidants such as oxone and TBHP were also employed,
however, poor yields were obtained after 1 h. Notably, increasing
the amount of DES inhibited the reaction, which was attributed to a
slow mass transfer that caused by the viscosity of the mixture
(Table 1, entries 17–20) [19]. Although the viscosity was reduced
2 2
O were also evaluated, and the results clearly showed
2 2
O
When a mixture of 1a (1 mmol) and aqueous H
2 2
O (2 mL, ca.
2
0 equiv) was stirred at room temperature for 1 h, only a trace
amount of phenol 2a was detected (Table 1, entry 1). When the
2 2
by adding more aqueous H O , the reaction became violent and the
reaction was carried out in other solvents such as acetonitrile,
selectivities decreased.
To evaluate the scope and limitations of the current procedure,
a series of arylboronic acids were tested under the optimized
reaction conditions from Table 1 (entry 10). The results were
summarized in Scheme 3.
Ingeneral, arylboronicacidswitheitherelectron-withdrawing or
3 3 2
electron-donating substituents such as halide, OCH , CF , CHO, NO ,
Ac, OBnunderwent thereactioningoodtoexcellent yields(80–95%).
The conversion of electron-rich arylboronic acids was complete
within 5 min, while relatively longer reaction time (30–60 min) was
required for the electron-deficient arylboronic acids. It was worth
noting that arylboronic acids bearing halides at para position
provided the corresponding phenols 2d and 2e in excellent yields, as
thesecompoundscould beutilizedforsubsequentfunctionalization.
Moreover, the oxidation-sensitive substituent such as 2g tolerated
the conditions and no overoxidation was observed. The steric effect
was also negligible, giving the products 2k–2o in good to excellent
yields. Finally, theheteroarylboronicacidssuchasthiopheneboronic
acid was examined and showed good compatibilities with the
conditions (2p).
To further demonstrate the practicality and efficiency of the
developed protocol, a scale-up reaction was performed using
phenylboronic acid as substrate (2.44 g, 20 mmol). The reaction
was carried out in ice bath with slowly addition of the hydrogen
peroxide in order to avoid the over-oxidation of phenol. The solid
product was precipitated in 89% yield under this condition and
could be easily isolated by simple filtration. Moreover, one of the
most important advantages employing DES as solvent or catalyst is
the recyclability. Thus, a batch of reaction was performed in a
20 mmol scale to examine the recycling process. The recovery was
very simple involving evaporation of the water after isolation of
product by filtration. The deep eutectic mixture was reused
without obvious loss in activity in five consecutive runs (89%, 89%,
Scheme 2. Preparation of ChCl/HFIP.
Table 1
a
Optimization of reaction conditions.
B(OH)2
OH
oxidant
:
ChCl/HFIP, solvent
1
a
2a
b
Entry
ChCl/HFIP
amount)
Oxidant
(equiv)
Solvent
–
Time
Yield
(%)
(
(min)
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
2
(20)
(20)
(20)
(20)
(20)
(20)
(20)
(20)
(10)
(5)
60
60
60
60
30
30
30
5
Trace
<10
<10
<10
82
–
3
CH CN
–
EtOH
THF
–
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
5 mol%
20 mol%
20 mol%
10 mol%
10 mol%
2 mL
3
CH CN
EtOH
THF
–
75
63
87
H
H
H
H
H
H
H
H
–
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
5
94
10
11
12
13
14
15
16
17
18
19
20
5
95
(2)
5
65
(5)
5
70
(5)
5
87
c
(5)
5
82
88%, 88%, and 86% yield, respectively) (Scheme 4).
Oxone (5)
TBHP (5)
60
60
60
60
30
30
Trace
16
H
H
H
H
2
O
2
O
2
O
2
O
2
2
2
2
(2)
40
2 mL
(5)
–
67
2 mL
(10)
(20)
–
71
2 mL
–
82
a
Reaction conditions: phenylboronic acid (1 mmol), solvent (2 mL), oxidant, room
temperature.
b
Isolated yields.
c
HFIP as catalyst.
Scheme 4. Reusability of ChCl/HFIP.

4
6
L. Wang et al. / Journal of Fluorine Chemistry 158 (2014) 44–47
Scheme 5. Plausible reaction mechanism.
As for the reaction mechanism, Berkessel et al. did a detailed
completion of the reaction (indicated by TLC), the reaction mixture
was extracted with EtOAc (3 ꢁ 10 mL). The organic layer was
concentrated and the resulting crude products were purified by
column chromatography on silica gel using PE/EtOAc as eluent to
provide the desired products.
mechanistic investigation of epoxidation of olefins by hydrogen
peroxide in the presence of HFIP and multiple H-bond networks
were believed to be the key factor [31]. Similarly, in this reaction, it
was speculated that the DES could effectively activate the
hydrogen peroxide through multiple H-bond networks. Later,
the activated hydrogen peroxide attacked the boronic acid to
generate intermediate A. Subsequent migration of the phenyl
4.4. Gram-scale synthesis of 2a and recovery of ChCl/HFIP
group from boron to oxygen atom of H
ester B, which was then hydrolyzed to give the final product phenol
Scheme 5) [32].
2
O
2
generated boronate
A reaction flask was charged with phenylboronic acid (2.44 g,
20 mmol), water (40 mL) and ChCl/HFIP (10 mol%, 40 drops), and
(
the mixture was stirred in an ice bath. To this, 30% H
2 2
O (5 equiv,
5
mL) was added slowly. After addition of H , the mixture
2 2
O
3
. Conclusion
became clear and the product was gradually precipitated at 0 8C.
After filtration of the product, the water phase was further
extracted with EtOAc (3 ꢁ 20 mL). The solid product was re-
dissolved in organic layer, followed by concentration and
purification by column chromatography. To the aqueous phase,
a small amount of manganese dioxide was added to decompose the
excess hydrogen peroxide. After filtration of the manganese
dioxide, the catalyst ChCl/HFIP was readily recovered by evapora-
tion of the water and reused for the next run.
In summary, a rapid, green and practical oxidative hydroxyl-
ation of aryl/heteroaryl boronic acids was developed. The reactions
proceeded at room temperature and provided the corresponding
phenols in good to excellent yields with a few minutes. The DES
(ChCl/HFIP) could be recovered by simple filtration and evapora-
tion and reused for five runs without obvious loss of its activity.
The broad substrate compatibility, metal- and additive-free
conditions as well as gram-scale synthesis made this procedure
more environmentally benign. Moreover, the formation of DES
with ChCl also provided a new way to recover the HFIP.
4.4.1. Phenol (2a)
1
3
H NMR (300 MHz, CDCl ) d 7.33 (dd, J = 11.1, 4.2 Hz, 2H), 7.06
1
3
(
d, J = 7.4 Hz, 1H), 7.00 (dd, J = 9.1, 7.9 Hz, 2H); C NMR (75 MHz,
4
. Experimental
CDCl 155.1, 130.0, 121.0, 115.1.
3
) d
4
.1. Method and apparatus
4.4.2. p-Cresol (2b)
H NMR (300 MHz, CDCl ) d 7.03 (d, J = 6.3 Hz, 2H), 6.73 (d,
3
3
J = 6.3 Hz, 2H), 4.79 (s, 1H), 2.27 (s, 3H); C NMR (75 MHz, CDCl ) d
1
1
3
All reagents were obtained from local commercial suppliers and
used without further purification. Melting points were determined
153.4, 130.3, 130.2, 115.3, 20.7.
1
13
with a WRS-1B apparatus and were uncorrected. H and C NMR
spectra were recorded on a Bruker Advance 300 analyzer. All the
products are known compounds and were identified by comparing
of their physical and spectra data with those reported in the
literature.
4.4.3. 4-Methoxyphenol (2c)
1
H NMR (300 MHz, CDCl
3
)
d
6.90–6.72 (m, 4H), 5.62 (s, 1H), 3.77
153.6, 149.6, 116.2, 115.0, 56.0.
1
3
3
(s, 3H); C NMR (75 MHz, CDCl ) d
4
.4.4. 4-Chlorophenol (2d)
1
4.2. General procedure for preparation of ChCl/HFIP
H NMR (300 MHz, CDCl
3
)
d
7.23–7.14 (m, 1H), 6.81–6.73 (m,
152.3, 132.2, 129.3,
1
3
1
H), 5.94 (s, 1H); C NMR (75 MHz, CDCl
3
)
d
Choline chloride (139.6 g, 100 mmol) and 1,1,1,3,3,3-hexa-
121.9, 116.3, 110.3.
fluoro-2-propanol (25.2 g, 150 mmol) were placed in a round
bottom flask and stirred at 50 8C. After 3 h, a homogenous colorless
liquid (164.8 g, 100%) formed, which was used directly for the
reactions without purification.
4.4.5. 4-Bromophenol (2e)
1
H NMR (300 MHz, CDCl
3
)
d
7.37–7.28 (m, 2H), 6.77–6.67 (m,
154.7, 132.6, 117.3,
1
3
2H), 5.51 (s, 1H); C NMR (75 MHz, CDCl
3
) d
113.0.
4.3. General procedure for synthesis of compound 2
4
.4.6. 4-(Trifluoromethyl)phenol (2f)
1
A mixture of arylboronic acid (1.0 mmol), 30% H
2
O
2
(5 equiv,
3
H NMR (300 MHz, CDCl ) d 7.51 (d, J = 8.4 Hz, 2H), 6.99–6.78
1
3
0
.5 mL), water (2 mL) and ChCl/HFIP (10 mol%, 2 drops) was stirred
3
(m, 2H), 5.82 (s, 1H); C NMR (75 MHz, CDCl ) d 158.2, 127.4
at room temperature for the time indicated in Scheme 2. After
(q, JC–F = 3.8 Hz), 126.3, 123.2, 115.6.

L. Wang et al. / Journal of Fluorine Chemistry 158 (2014) 44–47
47
1
3
4
.4.7. 4-Hydroxybenzaldehyde (2g)
5.29 (s, 1H); C NMR (75 MHz, CDCl
3
)
d
152.5, 137.2, 130.4, 129.5,
1
H NMR (300 MHz, CDCl
3
)
d
9.85 (s, 1H), 7.93–7.70 (m, 2H),
3
129.4, 129.3, 128.3, 128.0, 121.0, 116.0.
1
7
1
.11–6.91 (m, 2H), 6.80 (s, 1H); C NMR (75 MHz, CDCl
62.0, 132.7, 129.8, 116.2.
3
) d 191.5,
4.4.16. Thiophen-2-ol (2p)
1
3
H NMR (300 MHz, CDCl ) d 7.58 (td, J = 4.32, 2.04 Hz, 1H), 6.41
4
.4.8. 4-Nitrophenol (2h)
(td, J = 4.42, 1.42 Hz, 1H), 4.14 (t, J = 1.76 Hz, 2H).
1
3
H NMR (300 MHz, CDCl ) d 8.32–8.08 (m, 2H), 7.07–6.85 (m,
13
2
1
H), 6.71 (s, 1H); C NMR (75 MHz, CDCl
15.9.
3
)
d
161.8, 141.7, 126.4,
Acknowledgements
This project was financially supported by the National Natural
Science Foundation of China (No. 21302014) and the Natural
Science Foundation for Colleges and Universities of Jiangsu
Province (13KJB150002).
4
.4.9. 1-(4-Hydroxyphenyl)ethanone (2i)
1
3
H NMR (300 MHz, CDCl ) d 7.96–7.85 (m, 2H), 7.70 (s, 1H),
13
6
1
.99–6.88 (m, 2H), 2.56 (s, 3H); C NMR (75 MHz, CDCl3)
61.4, 131.2, 129.6, 115.7, 26.4.
d 198.6,
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.4.10. 4-(Benzyloxy)phenol (2j)
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13
3
.12 (dd, J = 8.7, 2.5 Hz, 1H), 5.45 (s, 1H); C NMR (75 MHz, CDCl )
3
290–3293.
29] A. Berkessel, M.R.M. Andreae, H. Schmickler, J. Lex, Angew. Chem. Int. Ed. 4 (2002)
481–4484.
[30] R. Neimann, K. Neumann, Org. Lett. 2 (2000) 2861–2863.
153.4, 134.7, 130.0, 129.0, 127.9, 126.6, 123.8, 117.9, 109.7.
[
4
0
4
.4.15. 1,1 -Biphenyl]-2-ol (2o)
[
[
31] A. Berkessel, J.A. Adrio, J. Am. Chem. Soc. 128 (2006) 13412–13420.
32] P. Gogoi, P. Bezboruah, J. Gogoi, R.C. Boruah, Eur. J. Org. Chem. 2013 (2013)
7291–7294.
1
H NMR (300 MHz, CDCl
3
) d 7.53–7.46 (m, 4H), 7.46–7.37 (m,
1
H), 7.29 (dd, J = 12.1, 4.6 Hz, 2H), 7.03 (dd, J = 11.4, 4.2 Hz, 2H),