Green Photoorganocatalytic Synthesis of Phenols from Arylboronic Acids

Article abstract of DOI:10.1055/s-0036-1591837

A green and cheap protocol for the photocatalytic hydroxylation of arylboronic acids is presented. 2,2-Dimethoxy-2-phenylacetophenone proved to be the best photoinitiator, among a range of organocatalysts in promoting this reaction. This photocatalytic protocol can be expanded into a wide substrate scope of aromatic boronic acids bearing various functional groups, leading to the corresponding phenols in good to high yields under mild reaction conditions, which include water as solvent, light irradiation provided from standard light-bulbs at room temperature.

Full text of DOI:10.1055/s-0036-1591837

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
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A
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I. K. Sideri et al.
Letter
Synlett
Green Photoorganocatalytic Synthesis of Phenols from Arylbo-
ronic Acids
Ioanna K. Sideri
HO
OH
OH
B
O
R1
Errika Voutyritsa
R1
R1
R1
OMe
OMe
catalyst (20 mol%)
Ph
Christoforos G. Kokotos*
Ph
catalyst
R2
R2
DIPEA, H2O
Laboratory of Organic Chemistry, Department of Chemistry,
National and Kapodistrian University of Athens, Panepistimiopolis,
Athens 15771, Greece
hν, 72 h
R3
R3
17 examples, 25–83% yield
environmentally friendly
ckokotos@chem.uoa.gr
mild reaction conditions
photocatalytic
Received: 04.09.2017
Accepted after revision: 01.11.2017
Published online: 24.11.2017
DOI: 10.1055/s-0036-1591837; Art ID: st-2017-d0665-l
Abstract A green and cheap protocol for the photocatalytic hydroxyl-
ation of arylboronic acids is presented. 2,2-Dimethoxy-2-phenylace-
tophenone proved to be the best photoinitiator, among a range of or-
ganocatalysts in promoting this reaction. This photocatalytic protocol
can be expanded into a wide substrate scope of aromatic boronic acids
bearing various functional groups, leading to the corresponding phe-
nols in good to high yields under mild reaction conditions, which in-
clude water as solvent, light irradiation provided from standard light-
bulbs at room temperature.
Key words photoorganocatalysis, hydroxylation, arylboronic acid,
phenols, green chemistry
Errika Voutyritsa (right) is a PhD student at the Department of Chem-
istry of the National and Kapodistrian University of Athens under the
supervision of assistant professor Christoforos G. Kokotos. Errika was
born in Athens, Greece in 1991, studied Chemistry at the Department
of Chemistry, National and Kapodistrian University of Athens, where
she carried out also her MSc on organic synthetic methodology working
on green organocatalytic synthesis with applications in industry. Her re-
search interests lie on organocatalysis and photocatalysis.
Phenols are important synthetic intermediates and are
found in various natural products, pharmaceuticals and
polymers, as the hydroxyl function is central to biological ac-
tivity.1 In addition, phenols are widely employed as inter-
mediates in the production of compounds of industrial sig-
nificance.2 Traditionally, the synthesis of phenols involves
nucleophilic aromatic substitution of aryl halides, but this
approach is limited by the harsh reaction conditions often
required and the preferable activation of the substrates.3
Aromatic boronic acids are directly available, both com-
mercially and by synthesis via the corresponding halides,
and can be smoothly converted into phenols by oxidative
hydroxylation.4 The general method of hydroxylation of bo-
ronic acids includes the use of hydrogen peroxide under ba-
sic conditions5 (Scheme 1, A). In this area, other metal-free
processes have also been developed; using stoichiometric
amounts of oxidants, such as oxone,6 N-oxides,7 MCBPA,8
hydroxylamine,9 TBHP,10 ozone,11 benzoquinone,12 NaClO2,13
and PhI(OAc)2.14 On the other hand, many efficient and
straightforward metal-catalyzed methods have been estab-
lished to convert arylboronic acids into phenol using cop-
per-,15 gold-,16 silver-,17 palladium-,18 and ruthenium-based
Ioanna K. Sideri (left) is a MSc student at the Department of Chemistry
of the National and Kapodistrian University of Athens, under the super-
vision of assistant professor Christoforos G. Kokotos. Ioanna was born in
Athens, Greece in 1995 and obtained her BSc in Chemistry, at the De-
partment of Chemistry, National and Kapodistrian University of Athens,
where she carried out her diploma thesis on organic synthetic method-
ology. Her current research interests lie on photoorganocatalysis.
Christoforos G. Kokotos (middle) is an assistant professor of organic
chemistry at the Department of Chemistry of the National and Kapo-
distrian University of Athens. Christoforos was born in Athens, Greece in
1981, studied chemistry at the Department of Chemistry, National and
Kapodistrian University of Athens and then moved to the School of
Chemistry, University of Bristol, where he carried out his Phd under the
supervision of Professor Varinder Aggarwal. After postdoctoral appoint-
ments at the University of Athens, Greece and Princeton University
(David W. C. MacMillan) working on organocatalysis, he was elected lec-
turer at the Department of Chemistry, National and Kapodistrian Uni-
versity of Athens, Greece in 2010 and then assistant professor in 2016.
His research interest lies on organocatalysis, green and sustainable
chemistry, photocatalysis and their applications for the synthesis of bio-
active compounds and chiral intermediates.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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I. K. Sideri et al.
Letter
Synlett
catalysts19 (Scheme 1, B). Alternatively, other methods de-
scribe an electrochemical approach20 or an aerobic oxida-
tive organocatalytic hydroxylation.21
We have recently concentrated our attention on the de-
velopment of a photoorganocatalytic protocol for various
radical transformations utilizing phenylglyoxylic acid as the
photoorganocatalyst.25 Reported herein are the results of
our aim to develop an efficient, economic, green, and sim-
ple way to convert aryl boronic acids into phenols utilizing
a cheap, small organic molecule as the photocatalyst, water
as the solvent and standard light bulbs as the means of irra-
diation. Thus, we describe the photoorganocatalytic con-
version of aryl boronic acids into phenols utilizing 2,2-di-
methoxy-2-phenylacetophenone as the photocatalyst and
water as the solvent (Scheme 1, final entry).
Our study began by determining the optimum reaction
conditions for the hydroxylation of 4-fluoroboronic acid
(1a). The reaction was performed with a number of cata-
lysts. The majority of the organocatalysts tested in the reac-
tion provided the desired product in low yields (Scheme 2).
2,2-Dimethoxy-2-phenylacetophenone proved to be the
best photocatalyst, leading to the highest yield (Scheme 2).
General Method for the Hydroxylation of Boronic Acids:
R1 R2
R1 R2
NaOH
H2O2
H
R3
A
H
R3
R2B
H
HO
H
Previous Photocatalytic Methods:
OH
visible light
[Ru(bpy)3Cl2] 6H2O (2 mol%)
B
OH
R1
OH
R3
R1
B
R3
iPr2NEt (2.0 equiv), DMF, air
R2
R2
69–96%
N
OH
B
N
S
N
OH
R1
OH
(1 mol%)
C
HO
OH
B
R3
R1
OH
R3
iPr2NEt (5 equiv)
R2
catalyst (20 mol%)
R2
MeCN:H2O (4:1)
DIPEA (7.5 equiv)
H2O, 72 h
O2, hν, 7 h
69–100%
F
1a
F
hν
2a
N
O
OH
D
O
O
O
OH
R1
B
OH
OH
OMe
OMe
Ph
(1 mol%)
Ph
Ph
OH
R3
OMe
Ph
R3
R1
Ph
O
R2
iPr2NEt (5 equiv)
R2
35%
95%
77%
OH
H2O, O2, blue LED, 17 h
55–100%
O
O
This Method:
Ph
Ph
O
OH
OMe
OMe
S
O
OH
Ph
B
OH
OH
R1
40%
58%
(20 mol%)
Ph
62%
R3
R1
R3
Scheme 2 Catalyst optimization for the photocatalytic hydroxylation
DIPEA (7.5 equiv)
R2
R2
of arylboronic acids
H2O, O2, hν, 72 h
Scheme 1 Methodologies for the hydroxylation of arylboronic acids
After identifying 2,2-dimethoxy-2-phenylacetophenone
as the optimum catalyst for this reaction, a variety of or-
ganic solvents was tested, but these afforded product 2a in
lower yields (Table 1, entries 1–7). Water proved to be the
best solvent among several solvents leading to the highest
yield (Table 1, entry 8).
After determining water as the optimum solvent for this
reaction, we proceeded to examine performance with and
without a catalyst, without irradiation and using different
bases. Decreasing the equivalents of the base led to lower
reaction yields (Table 2, entries 1–3). N,N-Diisopropylethyl-
amine (DIPEA) was identified as the optimum base; while
using DBU and triethylamine resulted in lower yields (Table
2, entries 5 and 6). In the absence of the photocatalyst, no
Photocatalysis has proved to be of importance in chemi-
cal synthesis, because of its environmentally friendly cre-
dentials. Consequently, photocatalytic approaches to this
conversion have been reported, using metal complexes
based on ruthenium, iron, and iridium.22 Despite the effi-
ciency of these reactions, the toxicity of the metals under-
mines the green qualities of such a conversion. Thus, metal-
free, efficient photoinduced alternatives for the synthesis of
phenols from aryl boronic acids have been developed utiliz-
ing methylene blue (Scheme 1, C),23 or acridinium salts
(Scheme 1, D).24
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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I. K. Sideri et al.
Letter
Synlett
Table 1 Solvent Optimization for the Photocatalytic Hydroxylation of
Arylboronic Acids
The next step was to explore the scope of the arylboron-
ic acid substrates (Scheme 3).26,27 All the substrates tested
were converted into the corresponding products in good to
high yields. Substitution at the meta or para position had
minimal effect, and the corresponding phenols 2c,d,h,j–l
were obtained in good yields. In contrast, ortho-bro-
mophenylboronic acid and ortho-cyanophenylboronic acid
proved to be poor substrates, since the corresponding prod-
ucts 2i,m were obtained in lower yields. Aryl boronic acids
containing electron-donating groups were transformed into
the relevant phenols 2c–h in good to high yields. On the
other hand, lower yields were observed with electron-with-
drawing groups (2i–o). Finally, our method proved to be
quite effective in the hydroxylation of aliphatic boronic ac-
ids, such as cyclohexyl boronic acid (1p), affording the de-
sired cyclohexyl alcohol (2p) and pyrene boronic acid (2q)
in good yields.
O
OMe
Ph
HO
OH
OMe
B
Ph
OH
(20 mol%)
DIPEA (7.5 equiv)
solvent, 72 h
F
F
hν
1a
2a
Entry
Solvent
MeCN
Yield (%)a
1
2
3
4
5
6
7
8
68
70
57
60
57
69
63
95
MeOH
EtOAc
THF
CHCl3
DMF
O
OMe
CH2Cl2
H2O
HO
OH
Ph
B
OH
R3
OMe
Ph
R1
R2
R1
R2
a Yield was determined by 1H NMR analysis.
(20 mol%)
DIPEA (7.5 equiv)
H2O, O2, hν, 72 h
R3
Table 2 Further Optimization for the Photocatalytic Hydroxylation of
Arylboronic Acids
OH
OH
F
OH
OH
O
OMe
Ph
HO
OH
OMe
B
Ph
OH
OMe
(20 mol%)
2a, 57%
2b, 76%
2c, 41%
2d, 65%
base (1.5–7.5 equiv)
H2O, O2, 72 h
OH
OH
OH
F
OH
F
hν
1a
2a
Entry
Base (equiv)
Yield (%)a
2e, 83%
2f, 61%
2g, 56%
2h, 52%
OH
1
DIPEA (1.5)
DIPEA (3)
62
83
85
95
60
57
0
OH
OH
OH
Br
2
3
DIPEA (5)
Cl
F
Br
4
DIPEA (7.5)
DBU (7.5)
Et3N (7.5)
DIPEA (7.5)
DIPEA (7.5)
DIPEA (7.5)
2j, 55%
2k, 66%
2l, 75%
2i, 72%
5
OH
OH
OH
OH
6
CN
7b
8c
9d
0
CN
NO2
66
2m, 56%
2n, 44%
2o, 28%
2p, 42%
aYield determined by 1H NMR analysis.
b No catalyst was used.
OH
cThe reaction was kept in the dark.
d Reaction carried out under argon.
reaction took place. Moreover, when the reaction was kept
in the dark, 4-fluoroboronic acid did not convert into the
corresponding product (Table 2, entries 7 and 8). When the
reaction was performed under an argon atmosphere, the
yield decreased (Table 2, entry 9).
2q, 45%
Scheme 3 Substrate scope for the photocatalytic hydroxylation of ar-
ylboronic acids
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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I. K. Sideri et al.
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Synlett
Based on previous reports, we propose a mechanism for
this photocatalytic hydroxylation of arylboronic acids as
outlined in Scheme 4. Visible-light irradiation of the cata-
lyst provides the excited intermediate that oxidizes the
base, by a single-electron-transfer (SET) process, to afford
the radical cation of the base and the radical anion of the
catalyst. This radical anion pair interacts with the boronic
acid and water and affords an intermediate that is attacked
by water. Hydrolysis affords the desired product. A second
mechanism involving a superoxide mechanism from oxy-
gen, 24 must also be considered to account for the yield ob-
tained (Table 2, entry 4 vs. entry 9).
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iPr2NEt
O
*
O
OMe
OMe
OMe
OMe
iPr2NEt
Ph
Ph
Ph
Ph
O
ArB(OH)2
H2O
OMe
OMe
Ph
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48, 2713.
Ph
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OH
OH
hydrolysis
Ar B(OH)2
OH
O
B
ArOH
Ar
–OH
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Scheme 4 Proposed mechanism for the photocatalytic hydroxylation
of arylboronic acids
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In conclusion, a green and cheap photoorganocatalytic
pathway for the synthesis of substituted phenols is present-
ed. Starting from a variety of substituted boronic acids, the
desired products were afforded in good to high yields utiliz-
ing 2,2-dimethoxy-2-phenylacetophenone as the photocat-
alyst.
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Funding Information
The authors gratefully acknowledge the Latsis Foundation for finan-
cial support through the program ‘EPISTHMONIKES MELETES 2015’
(PhotoOrganocatalysis: Development of new environmentally-friend-
ly methods for the synthesis of compounds for the pharmaceutical
and chemical industry) and the Laboratory of Organic Chemistry of
the Department of Chemistry of the National and Kapodistrian Uni-
versity of Athens. E.V. would like to thank the National Scholarship
Foundation (IKY) for financial support through a doctoral fellowship. )(
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Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1591837.
S
u
p
p
ortiInfogrmoaitn
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the atmosphere with stirring at room temperature under stan-
dard light-bulb irradiation (2 × 80 W household lamps) for 72 h.
The reaction mixture was quenched with HCl (1 M, 20 mL) and
then extracted with Et2O (20 mL). The organic layer was washed
with brine (20 mL) and dried over Na2SO4. After filtration and
removal of the solvent in vacuo, the crude product was purified
by flash column chromatography (10% EtOAc/PE) to afford the
pure product.
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(27) 4-Fluorophenol (2a)
White solid; mp 43–45 °C; 57% yield (19 mg, 0.17 mmol). 1H
NMR (200 MHz, CDCl3): δ = 6.97–6.85 (2 H, m, ArH), 6.84–6.72
(2 H, m, ArH), 5.85 (1 H, br s, OH). 13C NMR (50 MHz, CDCl3):
δ = 157.3 (d, J = 237.9 Hz), 151.0 (d, J = 2.1 Hz), 116.3 (d, J = 8.1
Hz), 116.0 (d, J = 23.0). 19F NMR (188 MHz, CDCl3): δ = -81.7.
3-Methylphenol (2e)
(26) General Procedure for the Photoorganocatalytic Hydroxyl-
ation of Arylboronic Acids
Colorless oil; 83% yield (27 mg, 0.25 mmol). 1H NMR (200 MHz,
CDCl3): δ = 7.18 (1 H, t, J = 8.2 Hz, ArH), 6.82 (1 H, d, J = 8.2 Hz,
ArH), 6.76–6.67 (2 H, m, ArH), 6.02 (1 H, br s, OH), 2.34 (3 H, s,
CH3). 13C NMR (50 MHz, CDCl3): δ = 155.0, 139.2, 129.4, 121.7,
116.1, 112.3, 21.2.
The boronic acid (0.60 mmol) was placed in a glass vial and dis-
solved in water (1.0 mL), followed by 2,2-dimethoxy-2-phenyl-
acetophenone (30.0 mg, 0.12 mmol). DIPEA (580 mg, 4.50
mmol) was then added. The reaction mixture was left open to
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E