Scalable electrochemical synthesis of diaryliodonium salts

Article abstract of DOI:10.1039/d1ob00457c

Cyclic and acyclic diaryliodonium are synthesised by anodic oxidation of iodobiaryls and iodoarene/arene mixtures, respectively, in a simple undivided electrolysis cell in MeCN-HFIP-TfOH without any added electrolyte salts. This atom efficient process does not require chemical oxidants and generates no chemical waste. More than 30 cyclic and acyclic diaryliodonium salts with different substitution patterns were prepared in very good to excellent yields. The reaction was scaled-up to 10 mmol scale giving more than four grams of dibenzo[b,d]iodol-5-ium trifluoromethanesulfonate (>95%) in less than three hours. The solvent mixture of the large-scale experiment was recovered (>97%) and recycled several times without significant reduction in yield.

Full text of DOI:10.1039/d1ob00457c

Organic &
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Scalable electrochemical synthesis of
diaryliodonium salts†
Cite this: Org. Biomol. Chem., 2021,
19, 4706
Mohamed Elsherbini and Wesley J. Moran
*
Cyclic and acyclic diaryliodonium are synthesised by anodic oxidation of iodobiaryls and iodoarene/arene
mixtures, respectively, in a simple undivided electrolysis cell in MeCN–HFIP–TfOH without any added
electrolyte salts. This atom efficient process does not require chemical oxidants and generates no chemi-
cal waste. More than 30 cyclic and acyclic diaryliodonium salts with different substitution patterns were
prepared in very good to excellent yields. The reaction was scaled-up to 10 mmol scale giving more than
four grams of dibenzo[b,d]iodol-5-ium trifluoromethanesulfonate (>95%) in less than three hours. The
solvent mixture of the large-scale experiment was recovered (>97%) and recycled several times without
significant reduction in yield.
Received 9th March 2021,
Accepted 4th May 2021
DOI: 10.1039/d1ob00457c
rsc.li/obc
in one-pot.11 Most of these synthetic approaches to iodonium
salts are under acidic conditions, although some neutral or
Introduction
Owing to their ready availability, low toxicity, unique chemical basic conditions have been developed. In addition, synthesis
properties and structural diversity, hypervalent iodine reagents under flow conditions has been achieved.12
have found tremendous applications in organic synthesis.1
Hypervalent iodine compounds are often lauded in the lit-
Iodonium salts,2 a class of hypervalent iodine reagents with two erature as mild, “green” reagents and are considered eco-
carbon ligands, have practical synthetic applications as oxi- friendly alternatives to various chemical oxidants, especially
dants,3 photosensitisers in photopolymerisation4 and electro- heavy metal oxidising agents. Unfortunately, syntheses typi-
philic group transfer reagents in a wide range of metal-catalysed cally require the utilisation of toxic and/or hazardous, and
and metal-free cross coupling reactions,5 in addition to their sometimes expensive, chemical oxidants, usually in large
medical and pharmaceutical applications.6 Diaryliodonium excess. This is accompanied by the generation of waste, which
salts Ar2I+X− are, typically, highly stable compounds that have increases their ecological footprint and reduces the “green-
been extensively studied for over a century. Their high electro- ness” of the overall process. This problem can, in theory, be
philicity and the superior leaving group ability of iodoarenes alleviated by the development of electrochemical syntheses of
stand behind their versatility as arylating agents for a wide hypervalent iodine reagents.13 Unsurprisingly, the electro-
range of nucleophiles.7 In addition, they serve as efficient aryne chemical synthesis14 of hypervalent iodine reagents is a
synthons.8 Furthermore, cyclic diaryliodonium salts provide a current hot topic,13,15 however the published methods for the
handle for the rapid construction of polycyclic heterocyclic and electrochemical synthesis of diaryliodonium salts are scarce
carbocyclic compounds of various classes.9
and suffer from limited substrate scopes and low current
Over the long history of diaryliodonium salts, various syn- efficiencies. In addition, published procedures suffer from the
thetic routes have been developed and improved over time to formation of side products and complex mixtures as a result of
achieve high levels of efficiency and selectivity.1a,7 Typically, the reaction medium; usually, a mixture of H2SO4/AcOH/Ac2O,
the synthesis of diaryliodonium salts involves oxidation of that leads to acylation of electron-rich aromatic substrates.16
iodoarenes to the corresponding λ3-iodanes followed by ligand Herein, we report a simple, scalable “green” electrochemical
exchange with arenes or organometallic arenes and anion synthesis of cyclic and acyclic diaryliodonium salts with wide
exchange if necessary. Extended reaction cascades that involve scope. A very recent report on the electrochemical synthesis of
initial iodination of arenes with molecular iodine are also aryliodine(III) reagents including some diaryliodonium salts
known.10 The reaction sequence can be achieved stepwise or emerged during the final preparation of this work.17
Results and discussion
Department of Chemistry, University of Huddersfield, Queensgate, Huddersfield HD1
3DH, UK. E-mail: w.j.moran@hud.ac.uk
Diaryliodonium salts are air and moisture stable compounds,
but their physical properties and synthetic utility are strongly
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00457c
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affected by the nature of the anionic part of the molecule. 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) enhanced the solu-
Previously published electrochemical methods for their prepa- tion conductivity, and the electrolysis could be completed
ration utilised H2SO4/AcOH/Ac2O as the reaction medium but leading to the formation of the iodonium salt 2a in 70% yield
these suffered from low yields and side product formation. In after passing a charge of 2.5 F (entry 2). Despite this encoura-
addition, isolation was problematic as the initially formed ging result, the cell voltage was still high, 12 V at the beginning
hydrogen sulfate salts required an additional ion exchange of the electrolysis which dropped gradually to 5 V at the end of
step. Usually this was achieved with halide salts such that the the electrolysis. This problem was alleviated by using a solvent
iodonium salts were isolated and characterised as halides, mixture of HFIP and MeCN (4 : 1) leading to the formation of
mainly iodides. Unfortunately, the synthetic utility of diarylio- 2a in excellent yield (95%, entry 3).
donium salts with halide counterions is limited due to their
Interestingly, running the electrolysis in acetonitrile
extremely poor solubility in organic solvents and the nucleo- without fluorinated alcohol (entry 4) led to the isolation of salt
philicity of the halide anions. On the other hand, diaryliodo- 2a in 80%, which is not significantly different from using a
nium triflates are easily soluble in organic solvents and are 1 : 1 mixture of HFIP and MeCN (entry 5, see the ESI† for more
widely used in organic synthesis.9,18 Thereby, this work results of solvent screening). Increasing the current density
focuses on the direct electrochemical synthesis of diaryliodo- from 1.8 mA cm−2 to 2.7 and 3.6 mA cm−2 whilst maintaining
nium triflates and hence triflic acid (TfOH) was the acid of the same total charge transfer (entries 6 and 7) led to a
choice for the process optimisation.
decrease of the reaction yield to 88 and 89%, respectively.
The oxidation of 2-iodobiphenyl (1a) to the corresponding Reducing the charge passed to the theoretical amount (2.0 F)
cyclic diaryliodonium triflate 2a was attempted using the led to a significant decrease of the reaction yield (78%, entry 8)
Electrasyn device with an undivided cell setup. In accordance while passing a charge of 2.25 F (entry 9) led to the formation
with several previous reports on the anodic oxidation of of 2a in 86% yield. Doubling the concentration of 1a (entry 10)
iodoarenes into the corresponding hypervalent iodine did not negatively affect the reaction yield, as 2a was isolated
reagents, we initiated our studies using a glassy carbon (GC) in 96% yield, while increasing the concentration to 0.1 M
anode and a platinum (Pt) cathode.13 The initial attempt at (entry 11) was accompanied by a gradual increase in the cell
electrolysis of a solution of 1a (0.15 mmol) in 2,2,2-trifluoro- voltage due to the precipitation of the product during the elec-
ethanol (TFE, 5 mL) containing TfOH (0.75 mmol, 5 equiv.) trolysis which led to a drop in the yield to 85% (see the ESI†
applying a constant current of 5 mA (Table 1, entry 1) was for more experiments). Applying the optimum conditions
unsuccessful as the cell voltage increased rapidly to reach the (entry 10) but using 3 or 4 equivalents of TfOH did not lead to
cell limit (30 V) within a few minutes. Replacing TFE with any reduction in the yield (entry 12) but was accompanied by a
significant increase in the cell voltage.
To explore the scope of substrates, a wide range of iodobiar-
yls with various substitution patterns in both rings were syn-
thesised and subjected to the optimised reaction conditions
(Table 1, entry 10). The results (Scheme 1) showed that the
Table 1 Optimization of the electrochemical oxidation of 2-iodobiphe-
nyl (1a) to the corresponding iodonium triflate 2a a
reaction proceeded smoothly in most cases giving the desired
cyclic diaryliodonium salts 2 in very good to excellent yields.
Under the standard reaction conditions 2-iodobiphenyl
afforded the corresponding iodonium triflate 2a in 96% yield,
replacing triflic acid with tosylic acid and trifluoroacetic acid
afforded the tosylate and trifluoroacetate analogues in 93%
1a
Current Charge 2a yield
Entry Solvent
[mol L−1
]
[mA]
[F]
[%]
and 70%, respectively, showing the flexibility of the method
for producing the desired iodonium salts with different
counter anions. The relatively lower yield in the case of tri-
fluoroacetic acid could be attributed to the poor conductivity
of the reaction solution; the addition of water (150 μL) was
required to improve the conductivity and complete the electro-
lysis. Substrates without substituents on the ArI moiety furn-
ished the desired products (2b–k) in very good to excellent
yields regardless of the nature of the substituent in the arene
part, with excellent tolerance of cyano, ketone, halogen, alkyl,
and aryl substituents. 2-Fluorodibenzo[b,d]iodol-5-ium tri-
fluoro-methanesulfonate (2b) was obtained in 88% using 3′-
fluoro-2-iodo-1,1′-biphenyl (1b), containing the fluorine in the
arene moiety, while the analogous substrate 5-fluoro-2-iodo-
1,1′-biphenyl (1b′) containing the fluorine in the ArI moiety led
to a slight decrease in the yield (82%). Highly electron-rich
1
2
TFE
HFIP
0.03
0.03
5
5
5
5
—
—
70
95
80
83
88
89
78
86
96
85
95
2.5
2.5
2.5
2.5
2.5
2.5
2.0
2.25
2.5
2.5
2.5
3
4
5
6
7
8
9
10
11
12b
HFIP–MeCN (4 : 1) 0.03
MeCN 0.03
HFIP–MeCN (1 : 1) 0.03
HFIP–MeCN (4 : 1) 0.03
HFIP–MeCN (4 : 1) 0.03
HFIP–MeCN (4 : 1) 0.03
HFIP–MeCN (4 : 1) 0.03
HFIP–MeCN (4 : 1) 0.06
HFIP–MeCN (4 : 1) 0.1
HFIP–MeCN (4 : 1) 0.06
5
7.5
10
5
5
5
5
5
a Electrolyses were carried out with an Electrasyn 2.0, using a 5 mL
glass vial equipped with a glassy carbon (GC) anode and a Pt cathode;
electrode immersed area: 2.8 cm2. Reaction conditions: 2-iodobiphenyl
(1a) dissolved in 5 mL solvent, TfOH (5 equiv.), constant current elec-
trolysis. b TfOH (3 or 4 equiv.). TFE: 2,2,2-trifluoroethanol; HFIP:
1,1,1,3,3,3-hexafluoro-2-propanol.
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Scheme 1 Reaction scope of the electrochemical synthesis of cyclic diaryliodonium salts 2. a TfOH replaced with TsOH·H2O (2 equiv.). b TfOH
replaced with CF3CO2H (5 equiv.). c Using substrate 1b’. d Obtained using Pt–Pt with alternating polarity.
substrates were problematic. The methoxy-substituted product sition/polymerisation was observed under both conditions.
2l was obtained in just 44% yield, as the experiment could not Substrates with one methyl group in the ArI moiety led to pro-
be completed. Electrolysis was stopped after passing only 1.25 ducts 2n and 2p–r in excellent yields, higher than 90% in all
F due to unwanted polymerisation and electrode passivation cases regardless of the nature of the substituent in the arene
that led to the gradual increase of the cell voltage till the cell moiety. In addition, electron-deficient substrates with elec-
limit; a problem that has been reported previously for electro- tron-withdrawing groups in both parts of the molecule per-
lysis of electron-rich aryliodides.13a,19 The same problem was formed very well and gave the corresponding iodonium salts
observed in the case of 2m and 2o. Fortunately, this problem (2u–2x) in high yields (79%–91%). All substrates containing
was alleviated by replacing the glassy carbon anode with plati- chlorine substituents gave the desired iodonium salts in excel-
num and alternating the polarity of the electrolysis each 20 lent yields (>90%). The lowest yields obtained (59% and 75%)
seconds. Under these modified conditions, the electrolysis for products 2k and 2j having Ph and tBu substituent, respect-
could be completed without problems and product 2l was ively maybe attributed to the low solubility of the corres-
obtained in a very good yield (87%). Also, product 2o was ponding substrates in the reaction medium.
obtained in very good yield (81%) under the modified con-
Electron-rich heterocyclic substrates proved challenging.
ditions, however product 2m was not obtained and decompo- Compounds S1y, S1z, and S1aa for example underwent poly-
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merisation/decomposition and the corresponding heterocyclic 1-bromo-4-iodobenzene in the presence of toluene gave 5g in a
iodonium salts were not obtained under either the standard or relatively lower yield (70%). Replacing toluene with mesitylene
the modified reaction conditions. The anodic oxidation of achieved the corresponding iodonium salt 5h in higher yield
pyrrole typically leads to deposition to pyrrole black and other (81%), which was formed in lower yield (58%) when using iodo-
polymeric materials.20 In addition, these electron-rich hetero- mesitylene and bromobenzene. Using a more electron rich aryl-
cyclic systems are prone to polymerisation under acidic iodide (4-iodotoluene) and electron-rich arene (mesitylene) led
conditions.21
to the formation of 5j in excellent yield (97%). Like the
Although the original focus of this work was the electro- methoxy-containing cyclic product 2m, the acyclic product 5k
chemical synthesis of cyclic diaryliodonium salts, the excellent was not obtained and decomposition/polymerisation was
results achieved encouraged us to explore the general applica- observed upon electrolysis of 2-methoxyiodobenzene in the
bility of the developed method and extending the reaction presence of benzene as the arene counterpart. The competition
scope to acyclic diaryliodonium salts (Scheme 2). Under the between oxidation/intramolecular coupling and oxidation/inter-
same reaction conditions (Table 1, entry 10), diphenyliodonium molecular coupling was studied by anodic oxidation of 2-iodobi-
trifluoromethanesulfonate (5a) was obtained in 55% yield from phenyl (1a) in the presence of one equivalent of each of
the reaction of iodobenzene and 1.0 equivalent of benzene. benzene and mesitylene. The experiment using benzene
Increasing the amount of benzene to 1.2, 1.5 and 2.0 equiva- showed the exclusive formation of the cyclic iodonium salt 2a in
lents led to a gradual improvement of the reaction outcome, 95% yield without any trace of the acyclic product 5l. While the
forming 5a in 67%, 80% and 88% yield, respectively. The reac- reaction using mesitylene led to the formation of the acyclic
tion scope was studied using 1.5 equivalents of the arene. product 5m as a major product along with a small percentage
Electrolysis of iodobenzene without addition of any other arene of the cyclic product 2a in a 10 : 1 ratio.
component led to the isolation of (4-Iodophenyl)(phenyl)iodo-
nium trifluoromethanesulfonate (5b) in excellent yield (91%).
The scalability of the developed method was demonstrated
using a beaker-type cell equipped with three parallel 5 × 5 cm
Anodic oxidation of the relatively electron-deficient 4-fluoro- electrodes [anode (GC)-cathode (Pt)-anode (GC), see the ESI†
iodobenzene in the presence of benzene, toluene and mesity- for details]. Electrolysis of 10 mmol of 2-iodobiaryl (1a) apply-
lene furnished the corresponding diaryliodonium salts 5c,d,f in ing a total current of 92 mA (46 mA on each anode, 1.84 mA
high yields, while using electron-deficient arene counterpart, cm−2) led to the isolation of 4.17 g of product 2a (97%) in
namely, benzotrifluoride did not lead to successful formation of 5.8 hours (2.5 F) with excellent purity (99.9%) (Fig. 1, experi-
the desired iodonium salt 5e. The electrochemical oxidation of ment 1). Clearly demonstrating the practicality of the devel-
oped method at the gram-scale. Performing the same experi-
ment using m-chloroperbenzoic acid (mCPBA) on the same
scale requires 3.5 g mCPBA (15.0 mmol) and produces equi-
valent amount of benzoic acid as chemical waste.22 To improve
the “greenness” of the whole process and to reduce its ecologi-
cal footprint more, the solvent system of the first large-scale
experiment was collected using a rotary evaporator and
recycled several times. The solvent recovery efficiency was very
Fig. 1 Scale-up/solvent recycling data. Purity of product 2a was deter-
mined by 1H NMR using 1,4-dibromobenzene as an internal standard;
experiment 6: the recovered solvent was dried over MS before use;
experiment 7 was performed applying 92 mA on each anode (double
current density: 3.68 mA cm−2) and fresh solvent mixture.
Scheme 2 Substrate scope of acyclic diaryliodonium triflates 5. a Using
iodomesitylene and bromobenzene. b Product 2a was obtained instead.
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high (97–99%) and the recovered solvent was used for three Moran group members for helpful discussions and assistance.
more cycles. In these cases, the formation of 2a was observed Many thanks to Dr S. Ward for mass spectrometric analysis
in excellent yield and purity (Fig. 1, experiments 2–4).
and Dr N. McLay for NMR analysis.
A subsequent fifth cycle showed a significant reduction of
the product purity (85%), but without significant reduction of
the actual yield (yield x purity %). We supposed that this
reduction of purity could be due to the increase of the water
content of the solvent with recycling and collection. To test
this hypothesis, the solvent recovered from experiment
number five was dried using molecular sieves prior to use,
which led to a regain of the high purity and yield of the
product 2a (Fig. 1, experiment 6). In addition, doubling the
current density from 1.84 mA cm−2 to 3.68 mA cm−2, applying
a total current of 184 mA (92 mA on each anode), did not have
a significant negative impact on the reaction outcome and
furnished 2a in high purity and yield, while cutting the reac-
tion time in half to 2.9 hours (Fig. 1, experiment 7).
Notes and references
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Conclusions
In conclusion, the synthesis of a wide range of cyclic and
acyclic diaryliodonium salts was achieved in high to excellent
yields via anodic oxidation of iodobiaryls and iodoarene/arene
mixtures using a simple undivided cell. The reaction proceeds
smoothly in a mixture of HFIP, MeCN and TfOH under con-
stant current electrolysis, avoiding the use of added salts such
as Bu4NBF4, Bu4NClO4 or LiClO4 typically used in batch-type
electrolyses as supporting electrolytes. Using triflic acid as a
volatile added electrolyte in addition to its role as a reactant
rendered the isolation of the formed iodonium salts very
simple and efficient. The flexibility of the method was proven
by obtaining iodonium salts with other counterions by simply
replacing triflic acid with the desired acid. Electrolysis of elec-
tron-rich heterocyclic substrates was unsuccessful with this
setup and led to polymerisation/decomposition of the starting
materials without the isolation of the desired heterocyclic
iodonium salts. The easy scalability of the developed method
was demonstrated by running
a large-scale experiment
(10 mmol, 33 folds) leading to the isolation of more than four
grams of product 2a (>97%) with excellent purity (>99%). In
addition, the solvent system of the large-scale experiment was
easily recovered (>97%) using a rotary evaporator and recycled
several times without significant reduction of the reaction
yield, improving the overall efficiency, economy, and the eco-
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