Synthesis of Phenols via Metal-Free Hydroxylation of Aryl Boronic Acids with Aqueous TBHP

Article abstract of DOI:10.1155/2020/1543081

An alternate procedure for oxidative hydroxylation of aryl boronic acids with aqueous TBHP to access phenols is described. The protocol tolerated various functional groups substituted with aromatic rings. The reaction was performed in water and free from transition metal oxidants.

Full text of DOI:10.1155/2020/1543081

Hindawi
Journal of Chemistry
Volume 2020, Article ID 1543081, 7 pages
https://doi.org/10.1155/2020/1543081
Research Article
SynthesisofPhenolsviaMetal-FreeHydroxylationofArylBoronic
Acids with Aqueous TBHP
Tanveer MahmadAlli Shaikh
Department of Chemistry, College of Natural & Computational Sciences, Mekelle University, Mekelle, Ethiopia
Correspondence should be addressed to Tanveer MahmadAlli Shaikh; tanveerchem1@gmail.com
Received 16 April 2020; Revised 1 June 2020; Accepted 2 June 2020; Published 27 June 2020
Academic Editor: Toyonobu Usuki
Copyright © 2020 Tanveer MahmadAlli Shaikh. 'is is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
An alternate procedure for oxidative hydroxylation of aryl boronic acids with aqueous TBHP to access phenols is described. 'e
protocol tolerated various functional groups substituted with aromatic rings. 'e reaction was performed in water and free from
transition metal oxidants.
methods employing metal catalysts, Cu(OAc)2–H2O2 [57],
CuSO4–phenanthroline [58], CuCl2–micellar systems [59],
Cu2O–NH3 [60], [Ru(bpy)3Cl2]·6H2O [61], Al2O3–H2O2
[62], and H3BO3–H2O2 [63] has been developed. On the other
hand, the metal-free oxidative process are also competitive,
Oxone [64, 65], nBu4NHSO5 [66], NH2OH [67], H2O2-
poly(N-vinylpyrrolidone) [68], I2–H2O2 [69], Amberlite IR-
120–H2O2 [70], N-oxides [71], MCPBA [72], NaClO2 [73],
photoredox catalysis [74, 75], electrochemical oxidation
[76, 77], (NH4)2S2O8 [78], PEG-400-H2O2 [79], WERSA-
H2O2 [80], WEBPA-H2O2 [81], nanoparticles of Ag [82],
Cu2O [83], and Fe2O3/silica gel [84], and TBHP/Cl3CCN [85].
Despite these efficient oxidative processes, developing a new
methodology free from metal oxidants and organic solvents is
highly desirable. As part of our research interest involving
metal-free oxidation reactions [86], herein, a new protocol for
the direct hydroxylation of aryl boronic acids with TBHP in
the aqueous medium is reported (Scheme 1).
1. Introduction
In the past few decades, phenols have received great at-
tention in modern synthetic chemistry [1–3], since ever
Runge and Laurent made the first discovery in 1834 and
1841, respectively [4, 5], with regard to this motif, which is
frequently found in natural products [6–8], flavonoids
[9, 10], and pharmaceutically important compounds [11–17]
associated with certain bioactivities, such as antibacterial
[18–20], antifungal, antibiotic [21], anti-inflammatory
[22, 23], antiviral [24], anxiolytic [25, 26], and antioxidant
[27–33] activities.
In addition, they serve as a powerful building block and
precursor in metal-catalyzed cross-coupling reactions [34–39]
and synthesis of oxy-functionalized derivatives including
o-alkenyl phenols [40], benzofurans and dibenzofurans
[41–44], chromanones [45], coumarins [46–49], glycosides
[50], and benzolactone [51], Representative examples of
commercially important drugs are presented in Figure 1.
Conventional methods for the large-scale synthesis of phenols
include the Hock process, diazotization of aromatic amines,
and nucleophilic substitution reactions. Academicians have
focused on the development of alternative approaches, for
example, C-H activation of arenes [52, 53] and oxidation of
C-Si bonds [54] and C-halo bonds [55, 56]. Recently, the
direct hydroxylation of aryl boronic acids to phenols has
gained a lot of attention. In this context, a variety of oxidative
2. Results and Discussion
During a study of the Pd-Cu catalyzed cross-coupling re-
action with phenylboronic acid as one of the components,
surprisingly the reaction took a different course and resulted
phenol as a new product. 'is unexpected product was
observed after addition of aqueous TBHP as part of workup.
'en, it was our interest to find detailed investigation of this

2
Journal of Chemistry
OH
O
HO
O
O
OH
HO
HO
O
OH
H3C
N
H
OH
Cafeic acid
Paracetamol
Apigenin
OH
OH
H3CO
HO
OMe
N
Eugenol
Cherylline
Figure 1: Bioactive compounds with phenol moiety.
Table 1: Solvent study: hydroxylation of phenylboronic acida.
OH
aqueous. TBHP
B (OH)2
Temp. (°C) Time (h) Yield (%)b,c
R
Entry
Solvent
R
KtOBu, H2O, 50°C
1
2
3
4
5
6
7
8
THF
THF
0
1
1
1
3
3
3
1
3
NR
Trace
22
25
25
25
70
50
25
70
Scheme 1: Hydroxylation of aryl boronic acids.
THF : H2O
CH3CN : H2O
DMF : H2O
THF : H2O
H2O
10
>5d
reaction. In order to find suitable reaction conditions,
various reaction parameters have been considered and they
are presented in Tables 1 and 2.
25
18
H2O
47
Table 1 describes the solvent effect for the hydroxylation
reaction. We commenced our investigation by subjecting
phenylboronic acid as a model substrate and aqueous 70%
TBHP as an oxidant. Traces of phenol were observed under
the reaction temperature of 0–25°C in the presence of
Cs2CO3 as a base and THF solvent (Table 1, entries 1-2).
When the reaction was performed in a 1 :1 mixture of THF :
H2O solvent, to our surprise, only 22% of phenol was ob-
served (Table 1, entry 3). However, combination of aceto-
nitrile or dimethylformamide with water did not favor this
hydroxylation process, although the temperature was raised
to 70°C (entries 4-5). When the reaction was stirred in water
as solvent, 47% of yield of the product was observed at 70°C
(entry 8). It is important to conclude that water is necessary
to form homogeneous reaction mixture.
aReaction conditions: phenylboronic acid (1 mmol), aqueous TBHP
(1 mmol), Cs2CO3 (0.5 mmol), and solvent stirred at designated temper-
ature and time; bisolated yield; cproducts were characterized by MP, FTIR,
and 1H- and 13C-NMR; dtraces of the product when CH3CN and pyridine
solvents were used.
to give any product (entries 11–12). A significant im-
provement noticed with combination of aqueous TBHP and
potassium tert-butoxide as an effective base in water pro-
vided high yield (92%) of phenol at 50°C (entry 14). 'us, the
optimized reaction conditions for this hydroxylation process
involved phenylboronic acid (1 mmol), aqueous TBHP
(2 mmol), TBHP (2), KOtBu (1 mmol), and H2O (1 mL)
heated to 50°C (entry 14).
Having the optimized reaction conditions in hand, we
then pursued the scope of this hydroxylation process using a
variety of substituted aryl boronic acids, and the results are
presented in Table 3. Initially, the substrates bearing halides
such as bromo, chloro, and iodo were tested, and as ex-
pected, the corresponding hydroxylated product was ob-
tained in excellent yields (Table 3, entries 2–4). 'e aryl
substitution did not alter the reaction, for example, the
reaction of 4-phenyl-, 2-phenyl-, and naphthyl-boronic
acids resulted 4-phenylphenol, 2-phenylphenol, and 2-
naphthol in 90–92% yields, respectively (entries 5–7). Simple
methyl groups at para positions proceed smoothly to
give para-Cresol in 89% yields (entry 8). Some limitations
appear when a strong electron-donating group such as
para-methoxy boronic acid was subjected to the
Considering the role of the oxidant and base in the
hydroxylation process, we then studied ratios of the oxidant
and base, and the findings are outlined in Table 2. An initial
study was conducted using 2 :1 ratio of the oxidant and base
in water solvent heated to 70°C (Table 2, entry 2). Although
50% of the desired product was obtained, substantial amount
of unreactive phenylboronic acid was isolated. Changing the
temperature and base K2CO3 did not favor the oxidative
process (entries 3-4). On the other hand, organic bases such
as pyridine, triethyl amine, and diethyl amine completely
ruled out hydroxylation (entries 5–7). To our surprise, an
improved yield of phenol (76%) was observed with NaOH
(entry 8). Variations in temperature (25–80°C) resulted
45–60% yield of the product (entries 9–10). 'e control
experiment either in the absence of an oxidant or base failed

Journal of Chemistry
3
Table 2: Optimization of hydroxylation parametersa.
Entry
Oxidant (equiv.)
Base (equiv.)
Temp. (°C)
Time (h)
Yield (%)b,c
1
TBHP (1)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
—
Cs2CO3 (1)
Cs2CO3 (1)
K2CO3 (0.5)
K2CO3 (1)
Pyridine (1)
Et3N (1)
70
70
60
25
60
60
60
50
25
80
50
50
50
50
3
5
47
50d
25
2
3
5
4
5
10
5
10
5
NR
NR
22
6
7
Et2NH (1)
NaOH (1)
NaOH (1)
NaOH (1)
NaOH (1)
—
5
8
5
76
9
5
45
10
11
12
13
14
2
60
5
NR
Traces
80
TBHP (2)
TBHP (2)
TBHP (2)
5
KOH (1)
5
KOtBu (1)
5
92(87)e
aReaction conditions: phenylboronic acid (1 mmol), aqueous TBHP base, and water (1 mL) stirred at designated temperature and time; bIsolated yield after
workup; Products were characterized by MP, FTIR, and 1H- and 13C-NMR; dTHF was added to mix reactants. Yield after 2 hours.
c
e
Table 3: Substrate scope for the direct hydroxylation of aryl boronic acidsa.
Entry
1
Substrate
Product
OH
Yield (%)b,c
92
OH
B
OH
B (OH)2
B (OH)2
B (OH)2
B (OH)2
OH
90
93
87
92
2
3
4
5
Br
Cl
I
Br
Cl
I
OH
OH
OH
Ph
Ph
Ph
Ph
90
6
B (OH)2
OH
B (OH)2
OH
85
89
7
8
B (OH)2
B (OH)2
B (OH)2
B (OH)2
OH
H3C
H3CO
O2N
H3C
H3CO
O2N
OH
OH
OH
OH
60
95
9
10
11
12
47
H3CO2C
HO2C
NC
ndd
B (OH)2
NC
B(OH)2
OH
10e
13
N
N
aReaction conditions: aryl boronic acid (1 mmol), aqueous TBHP (2 mmol), TBHP (2 mmol), KOtBu (1 mmol), and H2O (1 mL), 50°C; bisolated yield;
1
e
cproducts were characterized by MP, FTIR, and H- and 13C-NMR; dthe desired product was not isolated; Characterized by FTIR.

4
Journal of Chemistry
hydroxylation; the corresponding para methoxyphenol was
isolated in 60% yield (entry 9), in addition to the side
product because of the possibility of overoxidation. 'e
presence of a strong electron-withdrawing substituent –NO2
on aryl boronic acid quantitatively converted to the para-
nitrophenol in excellent yield (entry 10). However, the
expected 4-hydroxy-methyl-benzoate (entry 11) could be
observed only in moderate yield when ester-boronic acid
was tested; the corresponding acid was also formed during
the course of hydroxylation.
sulphate. 'e ethyl acetate was evaporated using a rotary
evaporator to yield the corresponding phenol in pure form.
4.2. Spectral Data. Phenol (Table 2, entry−11): colorless solid;
MP 41-42°C (lit. 41-42°C); FTIR (KBr, cm ) 692, 815, 1065,
1
1220, 1365, 1470, 1592, 1920, 2350, and 3325; H-NMR
(400 MHz, CDCl3): δ 7.28–7.22 (m, 2H), 6.96–6.91 (m, 1H),
6.85–6.82 (m, 2H), and 5.03 (s, 1H); 13C-NMR (100 MHz,
CDCl3): δ 155.0, 129.7, 121.0, and 115.3.
4-Bromophenol (Table 2, entry 2): colorless solid; MP
60−62°C; FTIR (KBr, cm−1) 604, 810, 1030, 1207, 1405, 1500,
3. Conclusion
1
1595, and 3402; H-NMR (400 MHz, CDCl3): δ 5.10 (br s,
1H), 6.71 (d, J � 8.5 Hz, 2H), and 7.30 (d, 2H J � 8.5 Hz, 2H);
13C-NMR (100 MHz, CDCl3): δ 113.1, 117.3, 132.7, and
154.4.
An alternate method for the direct hydroxylation of aryl
boronic acid to phenol promoted by aqueous TBHP as an
oxidant using metal- and solvent-free conditions has been
described. 'e products were obtained in good to excellent
yields. It is important to note that the reaction works only
with aryl boronic acids and not with heteroaryl or base-
sensitive functional groups. 'e methodology is free from
chromatographic purification because the desired products
were obtained in pure form after acidic workup. Since this
protocol is free from chromatographic purification, it could
be extended for large-scale synthesis.
4-Chlorophenol (Table 2, entry 3): white solid; MP
44–45°C; FTIR (KBr, cm−1) 635, 715, 820, 1089, 1220, 1352,
1490, 2850, 2900, 3240, and 3335; 1H-NMR (400 MHz,
CDCl3): δ 4.76 (br s, 1H), 6.73 (d, J � 8.5, 2H), and 7.20 (d,
J � 8.5, 2H); 13C-NMR (100 MHz, CDCl3): δ 116.2, 125.3,
129.2, and 153.2.
4-Iodophenol (Table 2, entry 4): FTIR (KBr, cm−1) 802,
1
1001, 1202, 1218, 1410, 1442, 1590, and 3395; H-NMR
(400 MHz, CDCl3): δ 4.87 (br s, 1H), 6.55–6.61 (m, 2H), and
7.45–7.51 (m, 2H); 13C-NMR (100 MHz, CDCl3): δ 82.6,
118.1, 138.5, and 155.2.
4. Materials and Methods
4-Phenylphenol (Table 2, e−n1try 5): yellow-brown solid;
MP 165–167°C; FTIR (KBr, cm ) 640, 750, 810, 1195, 1225,
1370, 1440, 1510, 1600, 1605, 3005, 3010, and 3395; 1H-NMR
(400 MHz, CDCl3 : DMSO-d6): δ 6.78 (dd, J � 6.0, 2.0, 2H),
7.16–7.20 (m, 1H), 7.30 (dd, J � 6.0, 2.0, 2H), 7.41 (dd, J � 8.0,
2.0, 2H), and 7.45 (d, J � 5.0, 2H); 13C-NMR (100 MHz,
CDCl3 : DMSO-d6): δ 116.4, 127.2, 128.8, 129.5, 133.5, 142.0,
and 158.0.
FTIR spectra of the synthesized compounds were recorded
on a Shimadzu 4000 instrument using the KBr pellet within
the range of 400 10 to 4000 10 cm−1. 1H-NMR and 13C-
NMR spectra of all compounds were recorded on a Bruker
NMR spectrometer using 400 and 100 MHz, respectively.
Analysis of NMR was carried out using deuterated solvent
CDCl3 and DMSO TMS as an internal standard. Chemical
shift values are expressed in δ (ppm).
All the starting materials, chemicals, and reagents were
commercially available and used without further purifica-
tion unless otherwise stated. TBHP was purchased from
Merck, while potassium tert-butoxide and boronic acids
were procured from Sigma Aldrich and Rankem, India,
respectively. TLC plates and commercial grade solvents such
as ethyl acetate and petroleum ether were purchased from
Rankem, India. Other reagents such as Cs2CO3, K2CO3,
KOH, NaOH, CH3CN, and DMF are of AR grade and from
SD Fine, India.
2-Phenylphenol (Table 2, entry 6): FTIR (KBr, cm−1)
696, 745, 805, 1175, 1405, 1425, 1596, 3012, 3020, 3420, and
3521; 1H-NMR (400 MHz, CDCl3): δ 4.96 (br s, 1H),
6.92–7.02 (m, 2H), 7.20–7.29 (m, 2H), 7.34–7.40 (m, 1H),
and 7.43–7.50 (m, 4H); 13C-NMR (100 MHz, CDCl3): δ
116.0, 120.9, 127.8, 128.0, 129.1, 129.1, 129.2, 130.2, 137.0,
and 152.2.
2-Naphthalenol (Table 2, entry 7): white solid; MP
122–124°C; FTIR (KBr, cm−1) 720, 805, 815, 940, 1180, 1395,
1
1600, and 3240; H-NMR (400 MHz, CDCl3): δ 5.02 (br s,
1H), 7.02–7.10 (m, 2H), 7.24–7.29 (m, 1H), 7.38 (d, J � 7.2,
1H), 7.62 (d, J � 8.3, 1H), and 7.76–7.71 (m, 2H); 13C-NMR
(100 MHz, CDCl3): δ 109.0, 117.2, 123.2, 126.0, 126.2, 127.3,
128.5, 129.6, 134.1, and 153.0.
4.1. General Procedure for Hydroxylation. To a 5 mL screw
top sample vial equipped with a magnetic stirrer bar,
phenylboronic acid (0.2 mmol, 1 equiv), water (1 mL), and
potassium tert-butoxide were introduced and stirred at
room temperature. To this mixture, 70% aqueous TBHP was
added with continuous stirring, and the reaction mixture
was further heated at 50 oC for 5 h. 'e formation of phenol
was monitored with the help of TLC. After completion of the
reaction, the vial was cooled to room temperature, followed
by addition of aqueous 6N HCl. 'e resultant mixture was
extracted with ethyl acetate. 'e organic layer was washed
with cold water and then dried over anhydrous sodium
P-Cresol (Table 2, entry 8): FTIR (KBr, cm−1) 725, 925,
1235, 1260, 1461, 1610, 1700, 2545, 2920, and 3345; 1H-NMR
(400 MHz, CDCl3): δ 2.24 (s, 3H), 4.66 (br s, 1H), 6.70 (d,
J � 8.5 Hz, 2H), and 7.05 (d, J � 8.4 Hz, 2H); 13C-NMR
(100 MHz, CDCl3): δ 20.2, 115.0, 130.1, 130.4, and 153.1.
4-Methoxyphenol (Table 2, entry 9): colorless; FTIR
(KBr, cm−1) 725, 805, 1030, 1175, 1225, 1432, 1602, 2830,
2952, and 3375; 1H-NMR (400 MHz, CDCl3): δ 3.72 (s, 3H),
4.75 (br s, 1H), and 6.73–6.80 (m, 4H); 13C-NMR (100 MHz,
CDCl3): δ 55.7, 115.0, 116.2, 149.0, and 153.2.

Journal of Chemistry
5
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4-Nitrophenol (Table 2−,1entry 10): yellow solid; MP
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1605, 1697, 2535, 2915, and 3340; 1H-NMR (400 MHz,
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'e author declares that there are no conflicts of interest.
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'e author gratefully thanks the Department of Chemistry
and College of Natural and Computation Science, Mekelle
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