Electrochemical-induced hydroxylation of aryl halides in the presence of Et3N in water

Article abstract of DOI:10.1039/d1ob00931a

A thorough study of mild and environmentally friendly electrochemical-induced hydroxylation of aryl halides without a catalyst is presented. The best protocol consists of hydroxylation of different aryl iodides and aryl bromides by water solution in the presence of Et₃N under air, affording the target phenols in good isolated yields. Moreover, aryl chlorides were successfully employed as substrates. This methodology also provides a direct pathway for the formation of deoxyphomalone, which displayed a significant anti-proliferation effect.

Full text of DOI:10.1039/d1ob00931a

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Electrochemical-induced hydroxylation of aryl
halides in the presence of Et3N in water†
Cite this: Org. Biomol. Chem., 2021,
19, 6417
Li Yang,a Qinglong Zhuang,b Mei Wu,c Hua Long,c Chen Lin,c Mei Linc and
Received 12th May 2021,
Accepted 23rd June 2021
a,c
Fang Ke
*
DOI: 10.1039/d1ob00931a
rsc.li/obc
A thorough study of mild and environmentally friendly electro- to be a typical and practical tool toward phenol derivatives
chemical-induced hydroxylation of aryl halides without a catalyst (Scheme 1c).7 Recently, Fier reported a Pd- or Cu-catalysed
is presented. The best protocol consists of hydroxylation of method to prepare phenols with benzaldehyde oxime as a
different aryl iodides and aryl bromides by water solution in the hydroxide surrogate. It was found that an inorganic base such
presence of Et3N under air, affording the target phenols in good as Cs2CO3 would promote the deprotonation of the oxime
isolated yields. Moreover, aryl chlorides were successfully ether.8 However, transition-metal-catalyzed methods often
employed as substrates. This methodology also provides a direct require specially designed ligands, low chemoselectivity in the
pathway for the formation of deoxyphomalone, which displayed a presence of both Csp2–X and Csp3–X bonds with a strong base
significant anti-proliferation effect.
and/or longer times, and they exhibit typically poor perform-
ance with synthetically useful aryl chlorides.9 Despite these
recent advances, these protocols suffer from one or more dis-
advantages, such as elevated temperature, use of a catalyst and
a large amount of an oxidant, and need for ways to remove
metal-related impurities from the products.
Recently, electrochemical synthesis has emerged as a power-
ful and environmentally friendly activation protocol,10 and
some interesting reactions have led to the cleavage of the
carbon–halogen bond by single electron transfer to the haloar-
ene substrate, which represents a new mode of carbon–halogen
bond activation in a transition-metal-free manner.11 In the past
few decades, the catalyst-free electrochemical reduction of aryl
halides to afford aryl radicals has been reported, but it can only
be applied in limited transformations.12 Last but not least, reac-
tions performed in aqueous media fit well the principles of
green chemistry under electrochemical conditions.13 The appli-
cation of aqueous protocols for direct hydroxylation under mild
conditions is doubtless of great significance.14,15
Phenols and their derivatives are prevalent structures in
numerous natural products, pharmaceuticals, polymers and
agrochemicals as well as versatile intermediates in organic
chemistry.1 For example, there exist phenol structures in some
molecules such as riccardin, Pristiq and thyroxine (Fig. 1).2
The classical method for the preparation of phenols is
based on the decomposition of cumene hydroperoxide with
sulfuric acid (Hock process),3 which is limited merely to
phenol, and suffers from drawbacks such as a multistep
process and low overall efficiency (Scheme 1a). Thus, a much
simpler and more efficient process that proceeds under mild
conditions is desired.4 The oxidative hydroxylation of aromatic
boronic acids (Scheme 1b) has attracted attention since aro-
matic boronic acids have low toxicity and are stable and readily
available.5 Moreover, the transition-metal-catalyzed hydroxy-
lation of aryl halides is a viable strategy to construct phenols
regiospecifically.6 Among the different catalysts, palladium- or
copper-catalyzed hydroxylation of aryl halides has been proven
In 2019, Li and Zhang reported a novel Nidabpy/g-C3N4
cooperative catalytic system for the hydroxylation reaction with
aFaculty of Material and Chemical Engineering, Yibin University, Yibin 644000,
China
bSchool of Stomatology, Fujian Medical University, Fuzhou 350122, China
cInstitute of Materia Medica, School of Pharmacy, Fujian Provincial Key Laboratory
of Natural Medicine Pharmacology, Fujian Medical University, Fuzhou 350122,
China. E-mail: kefang@mail.fjmu.edu.cn; Fax: +86-591-22862016;
Tel: +86-591-22862016, +86-831-3545095
†Electronic supplementary information (ESI) available: Material and equipment,
experimental procedures, and compound characterization. See DOI: 10.1039/
d1ob00931a
Fig. 1 Organic molecules containing hydroxyl moieties.
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formed in 93% yield (Table 1, entry 1). The reaction produced
trace amounts of the product in the absence of Et3N or in the
presence of either K2CO3 or NaOH instead of Et3N (Table 1,
entries 2, 7 and 8). When the reaction was performed without
the use of an electric current or under a N2 atmosphere, a trace
amount of the product was obtained (Table 1, entries 3 and
12). Lower yields were observed when Bu4NBF4 was used as an
electrolyte or when a C cathode and a C anode were used
(Table 1, entries 4–6). This result may be because a Pt cathode
is more susceptible to reduction than a C cathode. Less
product was oxidized by a Pt anode because of its lower poten-
tial. After screening the selected solvents, we found that water
was crucial for this reaction. The reaction in H2O/EtOH (1 : 1)
or H2O/MeCN (1 : 1) under the same conditions resulted in
lower yields (Table 1, entries 9 and 10). Finally, a lower reac-
tion yield was obtained when the current was 5 mA (Table 1,
entry 11). It is worth noting that changing the constant current
has a significant impact on the reaction.
With the optimized conditions determined, the scope of
aryl halides was then explored. As shown in Table 2, different
aryl halides such as aryl iodides, aryl bromides and even aryl
chlorides were transformed into the corresponding products
in low to excellent yields ranging from 72% to 95% (Table 2).
To our delight, when ortho-substituted aryl chlorides with a
strong electron-donating group were examined using our
system, the coupled product was obtained in moderate yields.
From the above results, we can make the following con-
clusions: the order of reactivity was PhI > PhBr > PhCl by using
this catalyst system. A broad range of functional groups such
as methyl, methoxy, hydroxyl, halide, butyl, cyano, aldehyde,
nitro, acetyl, and carboxyl groups were compatible with the
reaction conditions to deliver the corresponding products in
moderate to good yields, emphasizing the generality and excel-
lent functional group tolerance of this reaction. We also found
that aryl halides with electron-withdrawing groups provided
lower yields than those with electron-donating groups. For
example, 4-iodotoluene and 4-methoxyiodobenzene could
afford the corresponding products 4-cresol 2b and 4-methoxy-
phenol 2c in 94% and 95% yields, respectively, and 4-iodoni-
trobenzene could afford the corresponding product 4-nitro-
phenol 2k in 85% yield. Furthermore, the sterically hindered
aryl halides had little influence on the outcome of the reac-
tion, for example, p-iodotoluene and o-iodotoluene could
afford the corresponding products 4-cresol 2b and 2-cresol 2n
in 94% and 91% yields, respectively. Moreover, aryl halides
with a heterocyclic ring also proved to be suitable substrates
(2y, 2z, and 3a). Additionally, the synthesis of 4-hydroxybenzoic
acid 2m and 2-hydroxybenzoic acid 2q required 3 equiv. of
Et3N. Upon extending the reaction scope to also include 1,4-
Scheme 1 Different routes for the synthesis of phenols.
water by using less activated aromatic halides in the presence
of Et3N.16 In this reaction system, Et3N provides electrons as a
weak base and converts itself into Et3N radicals to promote the
reaction process. Inspired by this synthesis strategy and to
develop environmentally friendly protocols,17c,d we report an
efficient and practical electrochemical-induced hydroxylation
of aryl halides using an easily accessible weak organic base
and ambient air at room temperature in an aqueous medium;
a wide array of aryl chlorides was converted to the respective
phenols.
To test our proposal, iodobenzene (1a) was chosen as the
model substrate to screen the optimal reaction conditions
(Table 1). The hydroxylation of 1a was conducted in water
under electrolysis using a Pt anode and a Pt cathode in an
undivided cell at 15 mA for 6 h in the presence of Et3N (2
equiv.) and Bu4NPF6 (1 equiv.) as an electrolyte under air. To
our delight, the hydroxylated product 2a was successfully
Table 1 Optimization of the reaction conditionsa
Entry
Variation from the reaction conditions
Yieldb
1
2
None
No Et3N
93
Trace
Trace
79
63
69
Trace
Trace
42
33
37
3
4
5
No electric current
Bu4NBF4 instead of Bu4NPF6
C cathode
6
7
8
9
10
11
12
C anode
K2CO3 instead of Et3N
NaOH instead of Et3N
H2O/EtOH (1 : 1) as the solvent
H2O/MeCN (1 : 1) as the solvent
5 mA instead of 15 mA
Under a N2 atmosphere
diiodobenzene as
a substrate, the target compound 3b
afforded only 66% yield.
To gain insight into the reaction mechanism, we conducted
a series of cyclic voltammetric analyses (Fig. 2). As depicted by
curve A, we found that the reduction peak of iodobenzene is
0.61 V and the substrate iodobenzene 1a had obvious revers-
ible redox peaks without Et3N, which indicated that iodoben-
Trace
a Reactions conditions: 1 (0.50 mmol), Et3N (2.0 equiv.), Bu4NPF6 (1.0
equiv.), under air, H2O (3 mL), undivided cell, Pt anode (10.0 mm ×
10.0 mm), Pt cathode (10.0 mm × 10.0 mm), constant current (15 mA),
6 h. b Isolated yield.
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Table 2 Substrate scopea,b
Fig. 2 Cyclic voltammetry experiments were performed at room temp-
erature in a standard three electrode system.
Scheme 2 Proposed mechanism for this transformation.
radical quickly reacts with molecular oxygen to produce peroxy
radical intermediate C, which reacts with Et3N to afford inter-
mediate D and phenol. After that, intermediate D undergoes
single electron transfer to produce amide E. And iodine anions
are converted into iodine ion at the anode.
To further demonstrate the practicality of the protocol that
we have developed, the reaction was scaled up (10 mmol).
Under the optimised reaction conditions, the reaction pro-
ceeded smoothly to provide phenol 2 in 87% yield (Scheme 3).
a Reaction conditions: 1 (0.50 mmol), Et3N (2.0 equiv.), Bu4NPF6 (1.0
equiv.), under air, H2O (3 mL), undivided cell, Pt anode (10.0 mm ×
10.0 mm), Pt cathode (10.0 mm × 10.0 mm), constant current (15 mA),
6 h. b Isolated yield. c 3.0 equiv. Et3N. d 1,4-Diiodibenzene as the
substrate.
zene can react spontaneously under an electric current. The
addition of Et3N resulted in the disappearance of the oxidation
wave in curve B; by comparing curves A and B, we found that
the current of the curve changed significantly from the revers-
ible redox to the irreversible redox, indicating that a reaction
occurred.
Based on the above results, a possible mechanism for the
electrochemical-induced hydroxylation of iodobenzene is pro-
posed in Scheme 2.10,17a,b,18,19 At the beginning, iodobenzene
is reduced by the cathode to generate radical anion A, which
undergoes fragmentation to give aryl radical B. Then the Scheme 3 Scaled-up reactions.
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Acknowledgements
This project was financially supported by the Fujian Provincial
Foundation (No. FJNMP-201902, FJNMP-2020004, 2020J01622,
2019R11020034-1, and 2020J01627), the Research Fund of
Fujian Medical University (No. 2018QH2021) and the Natural
Science Foundation of Yunnan Province (No. 202001AT070068).
Scheme 4 The synthesis of deoxyphomalone (D1).
Notes and references
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In conclusion, we have developed an electrochemical-induced
hydroxylation strategy with the use of green ambient air as the
oxygen source, providing a mild access to functionalized
phenols. In addition, the use of Et3N that enables a challen-
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Conflicts of interest
There are no conflicts to declare.
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