A practical microreactor for electrochemistry in flow

Article abstract of DOI:10.3762/bjoc.7.127

A microreactor for electrochemical synthesis has been designed and fabricated. It has been shown that different reactions can be carried out successfully using simple protocols.

Full text of DOI:10.3762/bjoc.7.127

A practical microreactor for electrochemistry in flow
Kevin Watts1, William Gattrell2 and Thomas Wirth*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 1108–1114.
1Cardiff University, School of Chemistry, Park Place, Cardiff CF10
3AT, UK and 2Prosidion Ltd, Watlington Road, Oxford OX4 6LT, UK
doi:10.3762/bjoc.7.127
Received: 06 June 2011
Accepted: 21 July 2011
Published: 15 August 2011
Email:
Thomas Wirth* - wirth@cf.ac.uk
* Corresponding author
This article is part of the Thematic Series "Chemistry in flow systems II".
Guest Editor: A. Kirschning
Keywords:
diaryliodonium compounds; electrochemistry; flow chemistry;
microreactor
© 2011 Watts et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A microreactor for electrochemical synthesis has been designed and fabricated. It has been shown that different reactions can be
carried out successfully using simple protocols.
Introduction
Electrochemical reactions offer a clean route to the formation of sion layers of the electrodes overlap or become "coupled". This
anion and cation radical species from neutral organic molecules. allows ions to be electrogenerated and play the role of the
Electrons can be added or removed from the substrates under supporting electrolyte.
mild conditions without the need for chemical oxidizing or
reducing reagents which might complicate the reaction Different microreactor systems have already been developed for
sequence [1]. A major advantage of electrochemical methods is chemistry in this area and have been successfully employed in
the reduced formation of side products, as no chemical reagents the conduction of electrochemical reactions without any added
are necessary. Electrochemical methods therefore provide a electrolyte [4,5]. Atobe et al. constructed a thin layer flow cell
better environment for subsequent reactions involving the elec- from platinum and/or glassy carbon plates (3 × 3 cm), sep-
trochemically generated reactive species [2]. Traditional elec- arated by adhesive tape (80 µm), with a space left in between to
trochemical batch methods can suffer from several drawbacks. act as the channel, and the devices were sealed with epoxy resin
The electrical field in the cell is heterogeneous and often [6]. They reported the self-supported, paired electrosynthesis of
supporting electrolytes have to be used, which must be re- 2,5-dimethoxy-2,5-dihydrofuran from furan with excellent
moved after the reaction. As a result, only a few electrochem- yields and flow rates of up to 0.5 mL·min−1. A microflow
ical processes for the production of organic compounds have system where the current flow and liquid flow are parallel was
been commercialized [3]. In microreactors, the distances reported by Yoshida et al. [7]. Two carbon fibre electrodes were
between electrodes can be very small such that the two diffu- separated by a hydrophobic porous PTFE membrane (75 µm
1108

Beilstein J. Org. Chem. 2011, 7, 1108–1114.
thickness). The substrate solution was fed into the anodic flow is complete within seconds, and the extremely fast 1:1
chamber and flowed through the membrane into the cathodic mixing in a micromixer, together with efficient heat transfer in
chamber, where it would leave as products. The carbon fibre the microsystem, seems to be responsible for the dramatic
electrodes used in this design allow for a much greater surface increase in the product selectivity. Cycloadditions using the
area than the empty space between electrodes in the setup N-acyliminium ions as heterodienes with a variety of
reported by Atobe. The anodic methoxylation of 4-methoxy- dienophiles, such as alkenes and alkynes, give the corres-
toluene was carried out in the electrochemical cell under a ponding [4 + 2] cycloaddition products in high yields [10]. The
constant current of 11 mA with a flow rate of 2 mL·h−1 same authors also reported a successful polymer synthesis using
resulting in a 90% conversion. A simpler configuration of elec- the same technique by adding a monomer to the initiator
trodes was reported by Haswell et al. [8]. Two platinum elec- (N-acyliminium ions) followed by micromixing and the addi-
trodes with a surface area of 45 mm2 were positioned with an tion of the terminator (diisopropylamine) in a subsequent
inter-electrode gap of either 160 µm or 320 µm, and C–C bond micromixer. This was considered to be a superior technique to
forming reactions based on the electro-reductive coupling of the batch method as the molecular weight distribution of the
activated olefins and benzyl bromide derivatives were reported. polymer 3 decreased in the flow system [11].
The best result obtained was a 98% formation of the product at
a flow rate of 10–15 µL·min−1 with a current of 0.6 mA.
Yoshida et al. also reported the electrochemical iodination of
aromatic compounds by elemental iodine followed by a subse-
Yoshida reported a method of producing carbocations such as 1 quent reaction with aromatic compounds. This sequential
in the absence of a suitable nucleophile (the "cation pool" method has one main advantage over the batch method, in that
method). This is an unconventional method because the ions the polyiodination problem for highly reactive aromatic com-
generated are unstable and usually need to be trapped immedi- pounds, based on disguised chemical selectivity, can be avoided
ately after generation. In the "cation flow" method, a carbocat- by the micromixing of a solution of preformed iodine cations
ion is generated continuously in a flow system by low tempera- (I+) and a solution of an aromatic compound, increasing the
ture electrolysis. The generation of the cation can be monitored yield of mono iodination product from 38% to 85% [12].
by a FTIR spectrometer in the flow system. The electrochemi- Nishiyama et al. reported the synthesis of hypervalent iodine
cally generated N-acyliminium ions can then be used in compounds using electrochemistry [13]. Electrochemical
different subsequent reactions (Scheme 1).
microreactors for investigations of laminar flow have also been
reported recently [14,15].
The first reaction studied was a Friedel–Crafts reaction of
aromatic compounds. In comparison with the "cation pool" Results and Discussion
method, the "cation flow" method was far more successful for The target of this research is to develop a simple and practical
this reaction, producing the monoalkylated product 2 in 92% microreactor in which to carry out electrochemical reactions in
yield in contrast to the batch method, which led to a 1:1 mix- flow. A small gap between the electrodes, to avoid the neces-
ture of mono- and dialkylation products [9]. The reaction in sity of electrolytes, and a simple and robust setup, were the
Scheme 1: Electrochemically generated N-acyliminium ions 1 and subsequent reactions.
1109

Beilstein J. Org. Chem. 2011, 7, 1108–1114.
leading principles in the design of the reactor. Herein we
describe the construction of such a simple microreactor, some
initial test reactions and the novel application to the continuous
synthesis of diaryliodonium compounds. A microflow electro-
chemical reactor made out of two aluminium bodies (50 mm
diameter, 25 mm height) was manufactured. The electrodes are
constructed of two PTFE plates (35 mm diameter, 4 mm height)
onto which 0.1 mm platinum foil electrodes [16] are mounted.
Scheme 2: Electrolysis of furan.
The wires for connection to the potentiostat are fixed into the able reaction for a flow reactor due to the large amount of
PFTE plate under the platinum. The electrodes are held apart by carbon dioxide and hydrogen that is formed at the anode during
a FEP (fluorinated ethylene propylene) foil of variable thick- the decarboxylation reaction. This was partially overcome by
ness, into which a rectangular reaction channel is cut (3 × 30 neutralising the solution with triethylamine as base, hence
mm) giving an overall channel volume of 23 µL (FEP foil 254 carbon dioxide gas was not liberated during the reaction [18].
µm thick), and the whole device is held together by steel screws The reactions were carried out in acetonitrile instead of
and wing nuts. The opened device is shown in Figure 1.
methanol to reduce the amount of side-products that could be
formed due to the reduction of methanol at the cathode. The
Known electrochemical reactions were used as test reactions for Kolbe electrolysis of 6a has also been described as a batch reac-
the electrochemical microreactor. The oxidative methoxylation tion, with a solid base, providing the product 7a in 44% yield
of furan 4 in methanol (Scheme 2) is a very clean reaction with [19]. This means that the reaction conditions in the electro-
no extra reagents necessary. The product 5, obtained in an ap- chemical microreactor were comparable to batch synthesis. The
proximate 1:1 syn:anti ratio, was identified by 1H NMR and reaction conditions described for 6a were also successful for
GC/MS and showed that our system functioned satisfactorily 2,2-diphenylacetic acid (6b) and even an asymmetric reaction
for electrochemical reactions. A yield for this reaction was not
determined, but full conversion was achieved as established by
GC/MS. The reaction shown in Scheme 2 has previously been
performed in batch electrolysis leading to product 5 in 78%
yield [17].
In addition, Kolbe-type reactions were investigated in flow.
2-Phenylacetic acid (6a) was used as a substrate and yields of
up to 40% of 1,2-diphenylethane (7a) were obtained with the
device depicted in Figure 1; the reactions are shown in
Scheme 3: Kolbe electrolysis of phenylacetic acids 6 in flow.
Scheme 3. The Kolbe reaction did not seem to be a very suit-
Figure 1: Electrochemical microreactor.
1110

Beilstein J. Org. Chem. 2011, 7, 1108–1114.
product could be formed through a mixture of phenylacetic acid a second electron [23]. The solvent system consisted of acetoni-
(6a) and diphenylacetic acid (6b), although in smaller yield.
trile, sulfuric acid (2 M) and acetic anhydride, which was
reported to increase the selectivity of the coupling reaction [21].
Hypervalent iodine compounds can be used in organic The sulfuric acid acts both as a counter reaction to the oxi-
syntheses as mild, non-toxic and highly selective reagents. dation at the anode, with proton reduction, and simultaneously
Iodine(III) reagents with two carbon ligands are known as iodo- provides a counter ion for the positively charged iodonium salt.
nium salts. These salts are attractive alternatives to oxidants and It also acts as an electrolyte in the reaction described above. A
catalysts based on heavy metals, as they have similar properties general procedure for the synthesis of both, symmetric and
to those of heavy metal complexes and can, therefore, be used asymmetric iodonium salts, was developed, as shown in
in similar reaction pathways, and are beneficial for organic syn- Scheme 4, and yields of up to 72% were obtained.
thesis due to their low toxicity and cost [20].
Initially, the conditions of the reaction were optimized with
Iodonium salts are currently being used in three main types of iodotoluene 8a (R1 = 4-Me) and toluene (9a) (R2 = Me). A
reactions, namely ligand exchange, reductive elimination and current of 30 mA led to a yield of 72%, whereas with higher
ligand coupling. This is due to their highly electron deficient currents (35, 40, or 45 mA) the yield dropped to 50% for iden-
nature and high dissociation rates that make them excellent tical flow rates. The increased formation of (diacetoxyiodo)
leaving groups. Symmetric salts are usually more desirable than arene derivatives as side products was observed in these cases.
asymmetric compounds purely for the reason that this avoids In Table 1 the results of the experiments using different combi-
problems that can occur with selectivity in aryl-transfers. If a nations of iodoarenes 8 and arenes 9 are summarized.
diaryliodonium salt is asymmetric it is usually the more elec-
tron deficient ligand that is transferred [21].
Various yields were obtained in these experiments and not all
reaction conditions were optimized. In some reactions the
The formation of these compounds usually follows a two-step conversion was not complete and longer reaction times would
procedure. The first step is the oxidation of an iodoarene, which have been necessary to achieve higher yields. Depending on the
can then take part in an acid-catalyzed coupling reaction with nature of the iodoarene 8, traces of the corresponding diace-
another aryl compound. However, there are now more one-pot toxyiodo compounds were also observed as side products. After
procedures to diaryliodonium salts known that involve both oxi- the reaction, treatment of the reaction mixture with potassium
dation and ligand exchange directly from the aryl and iodoarene iodide was required, as the resulting diaryliodonium iodides 11
starting materials. The electrochemical oxidation of an are insoluble in the reaction mixture (in contrast to the
iodoarene in the presence of another arene provides a quite diaryliodonium hydrogensulfates 10) and can thus be easily sep-
general and simple one-step approach to the synthesis of arated and purified by filtration and washing.
diaryliodonium salts [22].
Conclusion
We describe herein a simple procedure for the flow synthesis of The successful development of a microreactor for different
diaryliodonium salts using the electrochemical microreactor types of electrochemical reactions is described. The device has
device described above. The products were obtained in good the advantage of being very easily dismantled, allowing
yields and only minimal work-up was required after the reac- cleaning when reactions cause a blockage in the channel. It is
tion. The reaction takes place with an iodoarene that is oxidized also possible to change the internal volume of the channel
at the anode. The radical cation then undergoes a reaction with quickly by changing the channel height through the foil thick-
the other arene to form an intermediate, followed by the loss of nesses.
Scheme 4: Synthesis of diaryliodonium salts 11 in flow.
1111

Beilstein J. Org. Chem. 2011, 7, 1108–1114.
Table 1: Products and yields in the electrochemical generation of diaryliodonium compounds performed in an electrochemical microreactor (30 mA) at
a flow rate of 80 µL/min (residence time: 17 s) at 25 °C.
Entry
1
R1
R2
Product
Yield [%]
72
4-Me
Me
11a
11b
11c
11d
11e
2
4-Me
4-Me
H
Et
iPr
Me
Et
51
60
44
39
19
64
36
25
18
3
4
5
H
6
H
iPr
t-Bu
Me
Et
11f
7
H
11g
8
3-Me
3-Me
3-Me
11h
11i
11j
9
10
iPr
1112

Beilstein J. Org. Chem. 2011, 7, 1108–1114.
Experimental
Di-p-tolyliodonium iodide (11a) [24]
General: Melting points were obtained in open capillary tubes Collection of 3 mL, yield: 100 mg (72%); colourless solid; mp
and are uncorrected. 1H NMR and 13C NMR spectra were 162–164 °C; 1H NMR (400 MHz, CDCl3) δ 2.27 (s, 6H), 7.08
recorded on a AV-400 Bruker spectrometer, in the solvents (d, J = 8 Hz, 4H), 7.74 (d, J = 8.3 Hz, 4H); HRMS–EI (m/z): [M
indicated, at 400 and 100 MHz, respectively. Mass spectra (m/z) − I]+ calcd for C14H14I, 309.0135; found, 309.0130.
and HRMS were recorded under the conditions of electron
impact (EI). All reactions were monitored by thin-layer chroma- (4-Ethylphenyl)(p-tolyl)iodonium iodide (11b) [25]
tography which was performed on precoated sheets of silica gel Collection of 5 mL, yield: 114 mg (51%); colourless solid; mp
60. The galvanostatic reactions were performed with a HEKA 156–158 °C; 1H NMR (400 MHz, CDCl3) δ 1.13 (t, J = 7.6 Hz,
510 galvanostat/potentiostat. The electrodes were cleaned with 3H), 2.27 (s, 3H), 2.56 (q, J = 7.6 Hz, 2H), 7.09 (m, 4H), 7.77
acetone after each reaction; no polishing was required. (m, 4H); HRMS–EI (m/z): [M – I]+ calcd for C15H16I,
However, with earlier reactions using KF as an electrolyte in 323.0291; found, 323.0297.
solution the electrodes did need polishing as they were slightly
blackened. Methanol and acetonitrile were dried over 4 Å mole- (4-Isopropylphenyl)(p-tolyl)iodonium iodide (11c)
cular sieves. All other chemicals were used as purchased [24]
without further purification.
Collection of 5 mL, yield: 139 mg (60%); colourless solid; mp
154–157 °C; 1H NMR (400 MHz, CDCl3) δ 1.14 (d, J = 6.9 Hz,
6H), 2.28 (s, 3H), 2.81 (td, J = 7.0, 13.9 Hz, 1H), 7.11 (m, 4H),
Furan oxidation
A 0.05 M solution of furan (34 mg, 0.5 mmol) in methanol (10 7.76 (m, 4H); HRMS–EI (m/z): [M – I]+ calcd for C16H18I,
mL) was introduced into the electrochemical device (channel 337.0448; found, 337.0440.
dimensions: 3 mm × 30 mm × 100 µm, volume 9 µL) through a
syringe pump (flow rate 80 µL/min; residence time: 7 s) with an Phenyl(p-tolyl)iodonium iodide (11d) [24]
applied current of 2 mA (current density: 1.11 mA/cm2) and Collection of 5 mL, yield: 93 mg (44%); colourless solid; mp
collected at the outlet to give 5 after removal of the solvent. The 135–139 °C; 1H NMR (400 MHz, CDCl3) δ 2.27 (s, 3H), 7.12
product was identified by GC/MS m/z (EI): M+ 129.1.
(d, J = 8.1 Hz, 2H), 7.30 (m, 2H), 7.48 (t, J = 7.4 Hz, 1H), 7.77
(dd, J = 2.0, 8.7 Hz, 2H), 7.88 (dd, J = 1.0, 8.3 Hz, 2H);
HRMS–EI (m/z): [M – I]+ calcd for C13H12I, 294.9978; found,
Kolbe electrolysis
A 0.1 M solution of 2-phenylacetic acid (6a) (136 mg, 1 mmol) 294.9973.
in acetonitrile (10 mL), with 10 mol % triethylamine (13 µL)
was introduced into the electrochemical device (channel dimen- (4-Ethylphenyl)(phenyl)iodonium iodide (11e)
sions: 3 mm × 30 mm × 127 µm, volume 11 µL) through a Collection of 5 mL, yield: 82 mg (39%); colourless solid; mp
syringe pump (flow rate 40 µL/min; residence time: 17 s) with 159–161 °C; 1H NMR (400 MHz, CDCl3) δ 1.14 (t, J = 7.6 Hz,
an applied current of 4 mA (current density: 2.22 mA/cm2) and 3H), 2.57 (q, J = 7.7 Hz, 2H), 7.12 (d, J = 8.4 Hz, 2H), 7.29 (t, J
collected at the outlet to give 7a with 40% yield (72 mg, = 7.7 Hz, 2H), 7.44 (t, J = 7.4 Hz, 1H), 7.79 (d, J = 8.3 Hz, 2H),
0.4 mmol). The product was identified by GC/MS m/z (EI): 7.88 (d, J = 7.5 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 14.9,
M+ 183.3.
28.6, 117.5, 121.1, 131.1, 131.2 (2C), 131.5 (2C), 134.5 (2C),
134.8 (2C), 148.1; HRMS–EI (m/z): [M – I]+ calcd for C14H14I,
309.0140; found, 309.0130.
General procedure for the synthesis of iodonium
salts 11
A solution of aryl iodide 8 (0.1 M) and aryl compound 9 (0.3 (4-Isopropylphenyl)(phenyl)iodonium iodide (11f)
M) was prepared in 2 M H2SO4/acetonitrile with 25% acetic Collection of 5 mL, yield: 88 mg (19%); colourless solid;
anhydride and introduced into the electrochemical device mp 146–148 °C; 1H NMR (400 MHz, CDCl3) δ 1.15 (d,
(channel dimensions: 3 mm × 30 mm × 254 µm, volume 23 µl) J = 6.9 Hz, 1H), 2.82 (td, J = 6.9, 13.8 Hz, 1H), 7.14 (d,
through syringe pumps (flow rate 80 µL/min; residence time: J = 8.4 Hz, 2H), 7.30 (t, J = 7.8 Hz, 1H), 7.44 (t, J = 4.4 Hz,
17 s) with an applied current of 30 mA (current density: 16.67 1H), 7.79 (d, J = 8.4 Hz, 2H), 7.89 (d, J = 7.6 Hz, 2H);
mA/cm2) and collected at the outlet. The solvent was then re- 13C NMR (126 MHz, CDCl3) δ 23.6 (2C), 34.0, 117.4, 121.2,
moved, water was added (3 mL) and the iodide was precipi- 129.9 (2C), 131.1, 131.5 (2C), 134.6 (2C), 134.7 (2C), 152.7
tated by addition of KI (166 mg, 1 mmol) to give the ppm; HRMS–EI (m/z): [M – I]+ calcd for C15H16I, 323.0291;
diaryliodonium salts 11.
found, 323.0298.
1113

Beilstein J. Org. Chem. 2011, 7, 1108–1114.
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Collection of 5 mL, yield: 148 mg (64%); colourless solid; mp
162–164 °C; 1H NMR (400 MHz, CDCl3) δ 1.23 (s, 9H), 7.34
(m, 4H), 7.48 (t, J = 7.4 Hz, 1H), 7.83 (m, 2H), 7.92 (td, J =
1.7, 2.9 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 30.0 (3C),
34.1, 116.5, 120.4, 127.8 (2C), 130.0, 130.4 (2C), 133.4 (2C),
133.7 (2C), 153.7; HRMS–EI (m/z): [M – I]+ calcd for C16H18I,
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Collection of 5 mL, yield: 78 mg (36%); colourless solid; mp
85–89 °C; 1H NMR (400 MHz, CDCl3) δ 2.27 (s, 6H), 7.09 (d,
J = 8.1 Hz, 2H), 7.19 (m, 2H), 7.65 (d, J = 7.9 Hz, 1H), 7.72 (s,
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(4-Ethylphenyl)(m-tolyl)iodonium iodide (11i)
16.Pt electrodes made from Pt foil 99.95%, thickness 0.1 mm (Goodfellow
Cambridge Ltd).
Collection of 5 mL, yield: 57 mg (25%); colourless solid; mp
124–127 °C; 1H NMR (400 MHz, CDCl3) δ 1.14 (t, J = 7.6 Hz,
3H), 2.28 (s, 3H), 2.57 (q, J = 7.6 Hz, 2H), 7.12 (d, J = 8.5 Hz,
2H), 7.21 (m, 2H), 7.67 (d, J = 7.9 Hz, 1H), 7.74 (s, 1H), 7.80
(d, J = 8.4 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 13.9, 20.4,
27.6, 116.1, 119.7, 130.2, 130.3 (2C), 130.6, 131.2, 133.6 (2C),
133.9, 141.1, 147.1; HRMS–EI (m/z): [M – I]+ calcd for
C15H16I, 323.0291; found, 323.0298.
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152–155 °C; 1H NMR (400 MHz, CDCl3) δ 1.14 (d, J = 6.9 Hz,
2H), 2.27 (s, 3H), 2.81 (td, J = 6.9, 13.8 Hz, 1H), 7.13 (d, J =
8.4 Hz, 2H), 7.20 (m, 2H), 7.68 (d, J = 8.0 Hz, 1H), 7.75 (s,
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131.0, 133.6 (2C), 134.0, 140.9, 151.4; HRMS–EI (m/z): [M –
I]+ calcd for C16H18I, 337.0448; found, 337.0444.
License and Terms
Acknowledgements
We thank Prosidion Ltd and Chemistry Innovation/EPSRC for
generous support and the EPSRC National Mass Spectrometry
Service Centre, Swansea, for mass spectrometry data.
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
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productions (<100 t/year) such as 1,4-dihydronaphthalene
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