Catalyst- And solvent-free: Ipso -hydroxylation of arylboronic acids to phenols

Article abstract of DOI:10.1039/c9ra07201b

A catalyst-free method for the hydroxylation of arylboronic acids to form the corresponding phenols with sodium perborate as the oxidant was developed using water as the solvent. Under the reaction conditions, the yield of phenol reached 92% at only 5 min. Moreover, the reaction could be conducted without a catalyst under the solvent-free condition, the efficiency of which was as high as that of a liquid-phase reaction. Using a microcalorimeter, the reaction was found to be an exothermic reaction. The reaction mechanism was investigated and understood via DFT calculations, which revealed that it was a nucleophilic reaction.

Full text of DOI:10.1039/c9ra07201b

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Catalyst- and solvent-free ipso-hydroxylation of
arylboronic acids to phenols†
Cite this: RSC Adv., 2019, 9, 34529
ab
c
Xiufang Yang,ab Xulu Jiang,a Weitao Wang,
Qi Yang,
Yangmin Maab
*
ab
*
and Kuan Wang
A catalyst-free method for the hydroxylation of arylboronic acids to form the corresponding phenols with
sodium perborate as the oxidant was developed using water as the solvent. Under the reaction conditions,
the yield of phenol reached 92% at only 5 min. Moreover, the reaction could be conducted without
a catalyst under the solvent-free condition, the efficiency of which was as high as that of a liquid-phase
reaction. Using a microcalorimeter, the reaction was found to be an exothermic reaction. The reaction
mechanism was investigated and understood via DFT calculations, which revealed that it was
a nucleophilic reaction.
Received 8th September 2019
Accepted 11th October 2019
DOI: 10.1039/c9ra07201b
rsc.li/rsc-advances
the reaction solvents. The concept of green chemistry motivates
researchers to develop a more efficient and cost-effective reac-
Introduction
tion system with environment benign solvents or a solvent-free
reaction system for the ipso-hydroxylation of arylboronic acids
to phenol and its derivatives via C–B bond cleavage.
Phenol and its derivatives are important compounds used in
industries, such as pharmaceuticals, agrochemicals, polymers,
and natural antioxidants.1,2 Due to the important applications
of phenols, a number of methods have been developed for the
synthesis of phenol and its derivatives. Traditionally, phenol
syntheses involves the hydroxylation of aryl halides using
hydroxide salts,3 pyrolysis of sodium salts of benzene sulfonic
acid,4 oxidation of cumene,5 hydrolysis of diazonium salts,6 and
direct hydroxylation of benzene,7 which need harsh reaction
conditions. Therefore, exploring new greener methodologies for
the synthesis of phenol derivatives with better efficiency and
less waste generation is an attractive research direction.
Catalyst-free ipso-hydroxylation reaction has gained attrac-
tion in this regard. PEG-400,23 lactic acid,24 WERSA (Water
Extract of Rice Straw Ashes)25 and dimethyl carbonate26 have
been employed as reaction media for the catalyst-free ipso-
hydroxylation reaction with H2O2 as the oxidant. However, the
storage and transportation of H2O2 need additional safety
measures. Hydrogen peroxide–solid adducts have received
considerable attention in oxidation chemistry due to their
storage stability, ready availability, and low cost.19,27 Sodium
perborate (SPB), one such solid adducts, is an inexpensive large-
scale industrial oxidant widely used in washing powder and
bleaching. Moreover, the borate in SPB can help buffer, stabilize
against the decomposition of H2O2 and activate nucleophilic
oxidation.28 SPB has been widely used as a green oxidizing agent
for organic oxidation reactions.29–32 It was employed as the
oxidant in the oxidation of organoboranes and gave satisfying
yields.33–35 Inspired by these, we expected SPB to act as an
oxidant in the catalyst-free ipso-hydroxylation of aryl boronic
acids to phenols through the release of nucleophilic species.
Herein, we have reported a new methodology for quick and
efficient synthesis of phenol derivatives with arylboronic acids
Arylboronic acids are explored as a new source for the
synthesis of phenol derivatives due to their easy availability and
stability, and the synthetic route is known as the ipso-hydrox-
ylation of arylboronic acids that occurs via C–B bond cleavage.
An oxidant, such as O2,8 H2O2,9 NaClO2,10 NH2OH,11
12
(NH4)2S2O8 N-oxides,13 is essential for the ipso-hydroxylation
reaction (Scheme 1). Catalysts, such as noble metal complexes,14
transition metal oxides,9,15 and noble metal catalysts,16 are oen
employed in the reaction system besides a base additive, such
as NaOH17 and NaCO3.18 Moreover, some organic solvents, such
as CH3OH,19 CH3CH2OH,18 CHCl3,20 DMF,21 THF,22 are used as
aCollege of Chemistry & Chemical Engineering, Shaanxi University of Science &
Technology, Xi'an, Shaanxi, 710021, China. E-mail: wwt1806@163.com;
wangkuan@sust.edu.cn
bShaanxi Key Laboratory of Chemical Additives for Industry, Shaanxi University of
Science & Technology, Xi'an, Shaanxi, 710021, China
cCollege of Chemistry and Materials Science, Northwest University, Xi'an, Shaanxi,
710127, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra07201b
Scheme 1 Ipso-hydroxylation of arylboronic acid to phenols.
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and sodium perborate. The reaction could be conducted under
a catalyst-free condition, as well as a solvent-free condition, at
room temperature. The DH was determined by microcalorim-
etry, as well as thermokinetics analysis. In addition, the mech-
anism was revealed using DFT calculations.
Catalyst- and solvent-free ipso-hydroxylation of arylboronic
acids to phenol derivatives
The above reaction could be conducted under a catalyst- and
solvent-free (solid state condition) reaction condition. Typically,
arylboronic acid (1 mmol) and SPB (2 mmol) were added into
a mortar and ground for 10 min. Aer the reaction, the solid
mixture was dissolved in 5 mL H2O and then acidied. The
following steps were the same as those in the ipso-hydroxylation
of arylboronic acid to phenol derivatives.
Experimental section
Materials
Commercial reagents were used as received without additional
purication.
DFT computations
Characterization
DFT computations were used to verify the proposed mechanism
pathway. Geometry optimization and frequency analysis were
performed in a water solvent with the conductor-like polarizable
continuum model (CPCM)36,37 using M06-2X/6-311+G(2d,p).
Intrinsic reaction coordinate (IRC) computations validated the
connections between the reactants, transition states, and
products. All calculations were performed in Gaussian 09,38 and
the images of the optimized structures were generated and
displayed using the CYLview soware.39
The X-ray diffraction (XRD) patterns were obtained by an X-ray
diffractometer (Rigaku IV) operated with Cu-Ka radiation at
40 kV and 40 mA, the scanning mode of 2 theta/theta, the
scanning type of continuous scanning, and a scanning range
from 3ꢀ to 90ꢀ at a scanning rate of 8ꢀ minꢁ1 1H NMR (400
.
MHz) was recorded with a Bruker spectrometer (ADVANCE III).
The calorimetric experiment was performed in a Tian–Calvet
type differential microcalorimeter Setaram C80 at a constant
temperature of 20.00 ꢀC. The phenylboronic acid solvent
(60 mg, 1.0 mL H2O) was placed in a stainless steel sample cell.
When it reached equilibrium, a container with sodium perbo-
Results and discussion
rate (77 mg, 1.0 mL H2O) was pushed down . As a result, the Commercial phenylboronic acid and SPB were employed for
solvents were mixed at 298.15 K, and the heat ow of the reac- ipso-hydroxylation reactions in various solvents and solvent-free
tion was recorded with the increase of time.
reaction conditions. As shown in Table 1, the reaction could
proceed in both protic and aprotic solvents, but the yield of
phenol tended to vary with different solvents. Among the
aprotic solvents, it was found that the yield of phenol was higher
with tetrahydrofuran, acetone, and ethyl acetate than with
acetonitrile (Table 1, entry 4–7). This indicated that solvents
with large electron density atoms tended to favor the reaction.
Therefore, it was postulated that the nucleophilicity of the
solvent might affect the reaction. The protic solvents tended to
be more efficient in the reaction (Table 1, entry 1–3). However,
too many protons provided by inorganic acids disfavored the
Catalyst-free ipso-hydroxylation of phenylboronic acid to
phenol
Typically, in a 50 mL round-bottomed ask, a mixture of phe-
nylboronic acid (5 mmol), Na2BO3$4H2O (5 mmol), and 10 mL
of water was stirred at room temperature under an anaerobic
condition. Aer the completion of the reaction, the reaction
mixture was acidied with HCl solution. The solution was
diluted to 25 mL in a volumetric ask. The concentration of
phenol yielded in the solution was measured by high perfor-
mance liquid chromatography (HPLC, WAYEE LC 3000-2 Series
instrument), which calculated the HPLC yield of phenols.
Table
1 Ipso-hydroxylation of phenylboronic acid in different
solventsa
Catalyst-free ipso-hydroxylation of arylboronic acids to phenol
Entry
Solvents
Yieldb (%)
derivatives
The synthesis procedure was the same as that for the ipso-
hydroxylation of phenylboronic acid to phenol, except the
substrate amounts were changed. Arylboronic acid (1 mmol)
and SPB (2 mmol) were employed. The reaction was monitored
by thin layer chromatography (TLC) using plates precoated with
silica gel 60 GF-254. Aer the completion of the reaction, the
reaction mixture was acidied with HCl solution. Then 30 mL
diethylether was added and extracted with (3 ꢂ 50) mL of
saturated ammonium chloride solution. The organic layer was
dried over anhydrous Na2SO4. The solvent was removed in
1
2
3
4
5
6
7
CH3OH
CH3CH2OH
H2O
CH3CN
THF
Acetone
Ethyl acetate
H2O
91
84
92
23
77
67
72
62
92
95
8c
9d
10e
H2O
Solvent-free
a
Reaction conditions: phenylboronic acid, 5 mmol; SPB, 5 mmol;
b
a rotary evaporator under reduced pressure. The crude product solvent, 5 mL; room temperature; 5 min. HPLC yield of phenol.
c
d
With the addition of HCl aqueous solution. With the addition of
was puried by column chromatography (hexane/ethylacetate,
9 : 1) on silica (100–200 mesh) to get the desired product. The
e
NaHCO3 as the base. Solid phase reaction, grinding in mortar at
room temperature for 5 min.
1
products were identied by H NMR.
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Table 2 Ipso-hydroxylation of phenylboronic acid under different
reaction (Table 1, entry 8). When NaHCO3 was employed as an
additive in the reaction, the yield of phenol was not affected.
This is because the SPB aqueous solution was basic with a pH of
about 10.1.40 Therefore, the reaction did not require further
base addition.
conditionsa
nphenylboronic
acid : nSPB
Temperature
(ꢀC)
Entry
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
5 : 5
5 : 5
5 : 5
5 : 5
5 : 5
5 : 4
5 : 4.5
5 : 6
0
5
5
5
25
60
5
5
5
93
92
92
92
92
74
85
98
A white solid was found in the organic solvent when the
reaction completed. Aer the reaction mixture was acidied, the
white solid was dissolved, collected and identied as NaCl and
H3BO3 by XRD (Fig. 1). These indicated that the white solid was
sodium borate, which was easy to be separated in the solid form
(Scheme S1†). Therefore, the reaction process was green from
the reaction materials to the product. Moreover, it was inter-
esting to nd that the reaction could be carried out in the solid
phase by just grinding the reactants at room temperature,
which gave a yield as high as that of the liquid phase reaction
(Table 1, entry 10).
25
35
25
25
25
25
25
a
Reaction conditions: phenylboronic acid 5 mmol; SPB; H2O, 5 mL.
HPLC yield of phenol.
b
With the optimized solvents in hand, the reaction condition
was studied. The reaction could proceed from 0 ꢀC to 35 ꢀC with
minimal changes in the yield (Table 2, entry 1–3). In this reac-
tion system, the reaction was highly efficient with the yield of
phenol as high as 92% at just 5 min. On prolonging the reaction
time (Table 2, entry 2, 4 and 5), the yield did not vary, which
indicated that the reaction was completed within 5 min. These
implied that the reaction was kinetically favored. The solubility
of phenylboronic acid was not good in water. However, it totally
dissolved as the reaction proceeded. With an increase in the
amount of SPB (Table 2, entry 2, 6–8), the yield of phenol
increased. When the amount of SPB was 1.2 equivalent, the
yield of phenol could reach 98%, which was higher than that
obtained with 1.0 equivalent SPB. This indicated that the effi-
ciency of SPB was not 100%.
high DH indicated that the reaction was thermodynamically
favored. The second broad peak was ascribed to the reaction
between the salts of different boron compounds since the yield
of phenol did not change with time aer the 5 minutes of
reaction time.
The reaction order and rate constant were obtained by the
thermokinetic equation:41,42
ꢀ
ꢁ
ꢀ
ꢁ
1
dHt
Ht
ln
¼ ln k þ n ln 1 ꢁ
HN dt
HN
To realize the reaction nish time, we employed microcalo-
rimetry to monitor the reaction online (Fig. 2). The reaction
started when the two liquids were mixed at 3.57500 h. At
3.64236 h, the exothermic maximum was reached. The result
revealed that the reaction nished within 4.04 min. As shown in
Fig. 2, the rst exothermic peak was integrated to obtain the
reaction heat DH of ꢁ71.7 kJ molꢁ1 at 298.15 K. The relatively
Fig. 2 Variation of heat-flow as a function of time in the title reaction
at 298.15 K.
where, HN is the enthalpy of the whole process, Ht represents
the enthalpy at time t, k is the rate constant, n is the reaction
dHt
order,
is the heat production at time t. The linear rela-
dt
tionship between ln
ꢀ
ꢁ
ꢀ
ꢁ
1
dHt
Ht
HN
and 1 ꢁ
is shown in
HN dt
Fig. 3. The reaction order was 1 (n ¼ 1.05).
The generality of this methodology was tested with different
substituted arylboronic acids (Table 3). Despite the insolubility
of the arylboronic acid substrates in water, the ipso-hydroxyl-
ation reactions could proceed with water as the solvent. Aryl-
boronic acids with methyl and methoxyl substituents at ortho-,
meta- and para-positions afforded the corresponding phenols
with 81–87% yield (2a–2f). Halogen substituted phenols (2h–2j)
Fig. 1 The XRD spectra of the solid collected from the reaction
mixture after acidification. The reaction solvents were (a) CH3CN, (b)
THF, (c) CH3OH, (d) CH3COCH3, and (e) CH2Cl2. The XRD spectra of (f)
NaCl (JCPDS: 75-0306) and (g) H3BO3 (JCPDS: 73-2158) are given for
reference.
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Table 3 Oxidation of substituted arylboronic acids to corresponding
phenols using SPBa
Entry Substrates
R
Products
Yieldb (%) Yieldb,c (%)
1
2
1a
1b
2-Me
87
82
84
98
3-Me
ꢀ
ꢁ
ꢀ
ꢁ
1
dHt
Ht
HN
Fig. 3 The linear relationship between ln
and ln 1 ꢁ
:
HN dt
3
4
1c
4-Me
82
81
80
81
were obtained with satisfactory yields. Substituents, such as
nitrile, t-Bu, and i-Pr, also gave the desired products with
excellent yield (2g, 2k–2m). In general, arylboronic acids with
either electron-withdrawing or electron-donating substituents
underwent the ipso-hydroxylation reaction, resulting in satis-
factory yields. Interestingly, the reaction could be conducted in
the solid phase (solvent-free) with only the reactants, namely
arylboronic acids and SPB. The yields of the corresponding
phenols from the solvent-free reaction condition were as high as
the yields from the reactions in the water solvent (Table 3).
To reveal the reaction mechanism, radical scavengers were
added in the reaction system to testify whether the reaction was
a radical reaction (Table 4). When different radical scavengers
were added into the reaction system, the yield of phenol did not
change. This indicated that the reaction was not radical-
involved.
1d
2-OMe
5
6
7
1e
1f
3-OMe
4-OMe
3-NO2
82
86
89
86
78
1g
80d
8
9
1h
1i
4-F
80
80
81d
81d
It is widely accepted that SPB can release H2O2 and sodium
borate in dilute aqueous solutions, followed by an equilibrium
state27 (Scheme 2). At alkaline pH, hydrogen peroxide or the
perhydroxyl anion (HO2ꢁ) is responsible for the oxidization
4-Cl
10
11
12
1j
4-Br
83
88d
91
ꢁ
activity. At low pH, H2O2 is the main species, while the HO2
species dominant at high pH.40 The pH of the SPB aqueous
solution was about 10.1, which indicated that the reaction had
taken place by the nucleophilic attack of HO2ꢁ, which has very
high nucleophilicity.
1k
1l
4-CN
4-t-Bu
80
80d
77
The mechanism underlying the ipso-hydroxylation of aryl-
boronic acids with SPB was postulated, as shown in Scheme 3.
At alkaline pH, hydrogen peroxide (H2O2) or the perhydroxyl
anion (–O2H) is responsible for the oxidization activity. The
possible mechanisms proposed are the H2O2 oxidative pathway
or the –O2H oxidative pathway, which were analyzed via density
functional theory (DFT) calculations. All geometry optimiza-
tions were performed in the water solvent with the conductor-
like polarizable continuum model (CPCM) using the M06-2X
functional with a basis set of 6-311+G(2d,p). A schematic
depiction of the two pathways is shown in Scheme 3. Mean-
while, the optimized geometries and the corresponding relative
energies are shown in Fig. 4 and S1.†
13
1m
4-i-Pr
88d
74
a
Reaction conditions: arylboronic acid, 1 mmol; SPB, 2 mmol; solvent,
b
water 4 mL; room temperature; reaction time, 10 min. Isolated yield.
c
Yield from solid phase reaction by grinding reactants in mortar at
room temperature for 10 min. d Reaction time was 20 min.
a very high activation free energy (DGs) of 76.7 kcal molꢁ1
(Fig. S1†). Once the barrier is conquered, the hydroxylation
product (P) is nally generated with an exergonicity of
In the H2O2 oxidative pathway, phenylboronic acid (Rc) and
H2O2 approach each other to reach a transition state TSH-1 with
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Table
4 Ipso-hydroxylation of phenylboronic acid with different
radical scavengersa
Entry
Radical scavenger
Yield (%)
1
2
3
4
None
90
90
90
90
tert-Butanol
BHTb
TEMPOc
a
Reaction conditions: phenylboronic acid 5 mmol, SPB 5 mmol, H2O 10
b
c
mL, reaction time
5
min.
Butylated hydroxytoluene. 2,2,6,6-
Tetramethylpiperidine-1-oxyl.
Scheme 2 The equilibrium for SPB aqueous solution.
Fig. 4 DFT computed schematic energy diagram and the corre-
sponding optimized geometries in the –O2H oxidative pathway.
Consequently, the target product
P
is released with
104.1 kcal molꢁ1 relative to the zero-point surface of
exergonicity.
The calculated DGs of the rate-limiting step in the –O2H
oxidative pathway (Int-1 / Int-2) is much lower than that in the
H2O2 oxidative pathway (IntH-1 / IntH-2). Therefore, the
oxidation reaction of arylboronic acids through –O2H is more
favourable both kinetically and thermodynamically, which is in
very good agreement with the experimental observations. From
the mechanism, it can be found that too much H+ would lead to
low pH, which would result in the low concentration of –O2H
and a relatively low yield of phenol. From Scheme 3, the reaction
rate could be expressed as the following equation, according to
the rate-determination-step approximation:
d½phenylboronic acidꢃ
Scheme 3 Possible mechanism for ipso-hydroxylation of arylboronic
acids with SPB.
¼ ꢁk½ꢁO2Hꢃ½phenylboronic acidꢃ
dt
The phenylboronic acid reaction was a rst order reaction,
which was consistent with the thermokinetics analysis.
94.7 kcal molꢁ1. However, the unusually high barrier indicates
that the oxidation reaction of Rc with H2O2 is unlikely to occur.
In organoboronic acids, the boron atoms adopt sp2 hybrid-
ization.16 As shown in Fig. 4, when the nucleophile –O2H
approaches the boron atom in Rc, a boron “ate” complex is
generated with rehybridization to form sp3 boron (Int-1) in the
–O2H oxidative pathway. Starting with Int-1, the C–B bond is
dissociated due to the high electron density on the boron, fol-
lowed by aryl migration to the adjacent acceptor atom of oxygen
with unchanged conguration to generate intermediate Int-2.
The DGs in this step (Int-1 / Int-2) is 28.8 kcal molꢁ1 via
a transition state TS-1. Next, the C–B bond in Int-2 is elongated
to form intermediate Int-3 via a transition state TS-2 with a DGs
of 25.1 kcal molꢁ1. With an H2O molecule approaching Int-3,
the hydrolysis step occurs from intermediate Int-4 through TS-3
to form the P/ꢁB(OH)4 complex (Int-5) (DGs ¼ 8.8 kcal molꢁ1).
Conclusions
A convenient and safe method for the ipso-hydroxylation of
arylboronic acids to corresponding phenols was established
with SPB as the oxidant. With water as the solvent and without
solvents, the yields of phenols were satisfactory. Moreover, the
reaction was both thermodynamically and kinetically favored.
The mechanism of the reaction was found to be nucleophilic
attack in the presence of –O2H.
Conflicts of interest
There are no conicts to declare.
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