Development of a novel environmentally friendly electrolytic system by using recyclable solid-supported bases for in situ generation of a supporting electrolyte from methanol as a solvent: Application for Anodic methoxylation of organic compounds

Article abstract of DOI:10.1002/chem.200500340

We have successfully developed a novel environmentally friendly electrolytic system using recyclable solid-supported bases for in situ generation of a supporting electrolyte from methanol as a solvent. It was found that solid-supported bases are electrochemically inactive at an electrode surface. It was also found that solid-supported bases dissociate methanol into methoxide anions and protons. Therefore, in the presence of solid-supported bases, it was clarified that methanol serves as both a solvent and a supporting electrolyte generated in situ. Anodic methoxylation of various compounds with solid-supported bases was carried out to provide the corresponding methoxylated products in good to excellent yields with a few exceptions. The methoxylated products and the solid-supported bases were easily separated by only filtration, and the desired pure methoxylated products were readily isolated simply by concentration of the filtrates. The separated and recovered solid-supported bases were recyclable for several times.

Full text of DOI:10.1002/chem.200500340

DOI: 10.1002/chem.200500340
Development of a Novel Environmentally Friendly Electrolytic System by
Using Recyclable Solid-Supported Bases for In Situ Generation of a
Supporting Electrolyte from Methanol as a Solvent: Application for Anodic
Methoxylation of Organic Compounds
[
a]
Toshiki Tajima and Toshio Fuchigami*
Abstract: We have successfully devel-
oped a novel environmentally friendly
electrolytic system using recyclable
solid-supported bases for in situ gener-
ation of a supporting electrolyte from
methanol as a solvent. It was found
that solid-supported bases are electro-
chemically inactive at an electrode sur-
face. It was also found that solid-sup-
ported bases dissociate methanol into
methoxide anions and protons. There-
fore, in the presence of solid-supported
bases, it was clarified that methanol
serves as both a solvent and a support-
ing electrolyte generated in situ.
Anodic methoxylation of various com-
pounds with solid-supported bases was
carried out to provide the correspond-
ing methoxylated products in good to
excellent yields with a few exceptions.
The methoxylated products and the
solid-supported bases were easily sepa-
rated by only filtration, and the desired
pure methoxylated products were read-
ily isolated simply by concentration of
the filtrates. The separated and recov-
ered solid-supported bases were recy-
clable for several times.
Keywords: electrochemistry · elec-
tron transfer
synthesis green chemistry
solid-supported base
·
electroorganic
·
·
Introduction
because they are mostly unrecyclable except for some indus-
[2]
trialized cases. Furthermore, the inefficient separation of
the supporting electrolytes from the solvents requires a
huge consumption of energy and causes large amounts of
additional industrial waste.
Electroorganic synthesis is one of the most useful methods
in organic synthesis and is even applied in large-scale indus-
[1]
trial processes. For example, the electrohydrodimerization
of acrylonitrile to adiponitrile, an intermediate to nylon 66,
originally developed by Monsanto Co. in early 1960s, is op-
erating at several sites worldwide now. Furthermore, electro-
organic synthesis has recently attracted much interest as an
environmentally friendly method because electrons are in-
herently environmentally friendly reagents compared with
conventional oxidizing and reducing reagents. However,
large amounts of supporting electrolytes are necessary to
provide sufficient electrical conductivity to the solvents for
electrolyses. Therefore, after the electrolyses, separation of
the supporting electrolytes is required, and the separated
supporting electrolytes generally become industrial waste
In order to solve such separation and industrial waste
problems, a capillary gap cell has been developed to mini-
[3]
mize addition of supporting electrolytes. On the other
[4]
hand, a polymeric electron carrier system, solid polymer
[
5]
[6]
electrolytes, an aqueous silica gel disperse system, and
[7]
thermomorphic biphasic electroorganic synthesis
have
been developed to eliminate the steps for the separation of
supporting electrolytes. Furthermore, recently, electrochemi-
[8]
[9]
cal microreactors and a thin-layer flow cell have also
been developed to conduct electroorganic synthesis without
intentionally added supporting electrolytes. To our knowl-
edge, an ideal electroorganic synthetic system from the
viewpoint of green sustainable chemistry should be an elec-
trolytic system not requiring the addition of any supporting
electrolytes. Although electrochemical microreactors and a
thin-layer flow cell realize such an ideal electroorganic syn-
thetic system, they require special equipments.
[
a] T. Tajima, Prof. Dr. T. Fuchigami
Department of Electronic Chemistry
Tokyo Institute of Technology, Nagatsuta
Midori-ku, Yokohama 226-8502 (Japan)
Fax : (+81)45-924-5489
On the other hand, it is well known that electron transfer
[10]
E-mail: fuchi@echem.titech.ac.jp
between a solid and a solid is very difficult. Therefore, it
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Chem. Eur. J. 2005, 11, 6192 – 6196

FULL PAPER
can be expected that solid-supported bases play the role of
bases in bulk solutions as well as electrochemically inactive
reagents at an electrode surface. In the presence of solid-
supported bases such as amines, protic organic solvents such
as methanol would be dissociated into anions and protons to
some extent [Eqs. (1) and (2)]. This system would be suita-
ble for electroorganic synthesis because the protons derived
from protic organic solvents would act as the main carrier of
[11]
an electronic charge via ammonium ions [Eq. (2)]. There-
fore, in this system, protic organic solvents would serve as
both a solvent and a supporting electrolyte generated in
situ. Herein, we report a novel environmentally friendly
electrolytic system using recyclable solid-supported bases
for in situ generation of a supporting electrolyte from meth-
[12]
anol as a solvent and its application to anodic methoxyla-
tion of various organic compounds.
Figure 1. Cyclic voltammograms of a) N-methylpiperidine (0.1m) and
Results and Discussion
polystyrene-supported piperidine (0.1m) in 0.1m nBu
4 4
NBF /MeCN (c:
1
, c: 2), and b) methanol in the presence of polystyrene-supported pi-
peridine (0.1m) recorded at a Pt disk anode (f=0.8 mm). The scan rate
Electrochemical properties of solid-supported bases: At
first, we examined the electrochemical properties of solid-
supported bases. The cyclic voltammograms of N-methylpi-
peridine (1) and polystyrene-supported piperidine 2 were
measured in 0.1m nBu NBF /anhydrous acetonitrile. As
À1
was 100 mVs
.
4
4
shown in Figure 1a, 1 was easily oxidized at about 1.3 V
versus SCE, while 2 was not oxidized at all even under stir-
ring. This means that solid-supported bases are not oxidized
at the electrode surface. Then, 2 was added into methanol
(
0.1m based on the concentration of piperidine), and the
cyclic voltammogram of this solution was measured. As
À
shown in Figure 1b, the oxidation wave for MeO at about
+
1
.0 V versus SCE and the reduction current for H were ob-
served. Therefore, it is clear that solid-supported bases dis-
sociate methanol into methoxide anions and protons, and
the resulting protons seem to act as the main carrier of an
electronic charge.
Figure 2. Experimental procedure.
Then, anodic methoxylation of a substrate was carried out.
After the electrolysis, the corresponding methoxylated prod-
uct and the solid-supported bases were easily separated by
only filtration without neutralization. Finally, the desired
pure methoxylated product was readily isolated simply by
concentration of the filtrate.
At first, anodic methoxylation of 3 was carried out by
using various solid-supported piperidines. As shown in
Table 1, anodic methoxylation of 3 with polystyrene-sup-
ported piperidine provided the corresponding a-methoxylat-
ed product 4 in low yield (entry 1). In this case, the cell volt-
age was higher than 50 V since polystyrene-supported piper-
idine did not dissociate methanol into methoxide anions and
protons efficiently. This means that the piperidine, which is
inside in the polystyrene, does not interact with methanol
Anodic methoxylation of phenyl 2,2,2-trifluoroethyl sulfide
using solid-supported bases: Next, we investigated anodic
methoxylation using solid-supported bases. As a model reac-
tion we chose the anodic methoxylation of phenyl 2,2,2-tri-
[13]
fluoroethyl sulfide (3).
Anodic methoxylation processes
such as 2,5-dimethoxylation of furans, anodic benzylic me-
thoxylation of aromatic compounds, and a-methoxylation of
organonitrogen compounds, are performed widely in organic
[14]
synthesis and on an industrial scale. The overall experi-
mental procedure is illustrated in Figure 2. As shown in
Figure 2, at first, solid-supported bases were added into
methanol, and this solution was stirred for 1 h in order to
dissociate methanol into methoxide anions and protons.
Chem. Eur. J. 2005, 11, 6192 – 6196
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6193

T. Fuchigami and T. Tajima
using silica gel-supported pyridine and imidazole; their ba-
sicities are much lower compared with that of piperidine
Table 1. Anodic methoxylation of 3 with various solid-supported piperi-
dines.
(
5
entries 1, 2). In these cases, the cell voltage was higher than
0 V since silica gel-supported pyridine and imidazole did
not dissociate methanol into methoxide anions and protons
efficiently. On the other hand, the methoxylated product 4
was formed in good to high yields using relatively strong
silica gel-supported bases (entries 3–6). However, the cell
voltage in entry 6 was higher than those in entries 3–5. This
can be explained as follows. Silica gel-supported B in entry 6
can dissociate methanol into methoxide anions and protons
efficiently; however, silica gel-supported B strongly captures
the protons because B has a high basicity. Therefore, the
equilibrium (2) shifts to the left, and the resulting proton
mobility is much lower compared with those of the other
[
a]
Entry
Solid
n
Yield [%]
[
b]
1
2
3
PS
PS
1
2
3
39
[
c]
[e]
92 (87)
76 (73)
[
d]
SiO
2
1
9
[
3
a] F NMR yield based on the CF group using monofluorobenzene as
an internal standard. [b] Polystyrene. [c] Porous polystyrene. [d] Silica
gel. [e] Isolated yield in parentheses.
[15]
bases.
Thus, it was found that relatively strong bases,
because polystyrene is not swelled in methanol. On the
other hand, anodic methoxylation of 3 proceeded efficiently
to provide 4 in good to excellent yields with porous polystyr-
ene-supported and silica gel-supported piperidine (entries 2,
which have conjugated acid acidities close to that of metha-
nol (pK 15.5), are suitable for the dissociation of methanol
a
and anodic methoxylation. It is notable that the current effi-
ciency for the anodic methoxylation of 3 was greatly in-
creased about three times compared with our previous work
3). These results indicate that the solvent compatibility of
[13]
solids is highly significant.
using a conventional supporting electrolyte, Et NOTs.
4
Next, anodic methoxylation of 3 was carried out with vari-
ous silica gel-supported bases. As shown in Table 2, the me-
thoxylated product 4 was obtained in moderate yields by
Recyclability of solid-supported bases: The recyclability of
solid-supported bases was also examined. Anodic methoxy-
lation of 3 was carried out 10 times by the recycling of silica
gel-supported piperidine. In the recycling process, the silica
gel-supported piperidine was easily separated and recovered
by only filtration, and reused (Figure 2). As shown in
Figure 3, the yield of 4 was always more than 70% and did
Table 2. Anodic methoxylation of 3 with various silica gel-supported
bases.
[
a]
Entry
1
Base
Yield [%]
52
2
3
51
65
Figure 3. Yield of methoxylated product 4 in the reuse of silica gel-sup-
ported piperidine.
[
b]
4
89 (84)
not decrease at all upon the reuse of silica gel-supported pi-
peridine. In addition, the appearance of silica gel-supported
piperidine did not change at all before and after electrolysis.
This clearly suggests that solid-supported bases are recycla-
ble for many times because solid-supported bases are not
subject to oxidative decomposition at the electrode surface.
5
6
76 (73)
70
1
9
Generality of the electrolytic system using solid-supported
[
a] F NMR yield based on the CF
3
group using monofluorobenzene as
an internal standard. [b] Isolated yield in parentheses.
bases: The generality of the new electrolytic system was
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Chem. Eur. J. 2005, 11, 6192 – 6196

Anodic Methoxylation
FULL PAPER
demonstrated. As shown in Table 3, anodic methoxylation
of carbamates 5a and 6a was carried out to provide the cor-
responding a-methoxylated products 5b and 6b in excellent
yields (entries 1, 2 in Table 3).
ported bases is effective for the wide range of anodic me-
thoxylation except for some substrates whose oxidation po-
tentials are higher than that of methanol.
Anodic 2,5-dimethoxylation of
Table 3. Anodic methoxylation of carbamates and furans with silica gel-supported piperidine.
furan (7a) proceeded to pro-
vide 7b in 78% yield as a ster-
eoisomeric mixture (entry 3 in
Table 3). In this case, although
7
a was completely consumed,
À2
À1
[a]
Entry Substrate
1
Current density [mAcm
5
]
Electricity [Fmol
5
]
Product
Yield [%]
no by-product was formed.
Then, anodic 2,5-dimethoxyla-
tion of furans 8a and 9a was
also attempted to provide 8b
and 9b in excellent yields (en-
tries 4, 5 in Table 3). Anodic
methoxylation of furans gener-
94
98
2
10
5
À
+
ally requires a Br /Br media-
tor.
[
b]
[
16]
3
4
10
10
7
7
78 (cis/trans 1:1)
On the other hand, sur-
prisingly, anodic methoxylation
of furans proceeded smoothly
without a Br /Br mediator in
this electrolytic system (en-
tries 3–5 in Table 3).
[b]
96
À
+
[
b]
5
10
7
94
Next, as shown in Table 4,
anodic methoxylation of phe-
nols 10a and 11a was carried
out to provide the correspond-
ing ring methoxylated products
[
a] Isolated yield. [b] Stereoisomeric mixture.
Table 4. Anodic methoxylation of substituted benzenes with silica gel-supported piperidine.
10b and 11b in excellent yields
(
entries 1, 2 in Table 4). Anodic
methoxylation of 1,4-dimethox-
ybenzene (12a) also proceeded
to provide the corresponding
ring dimethoxylated product
À2
À1
[a]
Entry Substrate
1
Current density [mAcm
10
]
Electricity [Fmol
3
]
Product
Yield [%]
12b in high yield (entry 3 in
94
91
88
Table 4). Furthermore, anodic
benzylic methoxylation of p-
methoxytoluene (13a) took
place to provide the benzylic
mono- and dimethoxylated
products 13b and 13c in 58 and
2
5
7
3
4
5
5
4
6
31%
yields,
respectively
(
entry 4 in Table 4). However,
[
f]
58
anodic benzylic methoxylation
of 14a and 15a did not proceed
smoothly and 14a and 15a were
mostly recovered after the elec-
trolyses, because the oxidation
potentials of 14a and 15a are
higher than that of methanol
[f]
3
1
[
g]
5
6
10
10
10
10
18
(
entries 5, 6 in Table 4). There-
[
h]
0
fore, methanol seems to be
mainly oxidized at the anode.
From these results, it was dem-
onstrated that the new electro-
ox
ox
ox
[
a] Isolated yield. [b] Oxidation peak potential (E
p
ox
)=1.72 V vs SCE. [c] E
p
=2.14 V vs SCE. [d] E
p
=
2.36 V vs SCE. [e] Oxidation peak potentials (E
p
) of 13a, 14a, and 15a (0.01m) were measured in 0.1m
1
À1
nBu
4
NBF
4
/MeCN. The scan rate was 100 mVs . [f] H NMR yield based on the benzylic proton using toluene
lytic system by using solid-sup- as an internal standard. [g] 74% of 14a were recovered. [h] 93% of 15a were recovered.
Chem. Eur. J. 2005, 11, 6192 – 6196
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6195

T. Fuchigami and T. Tajima
Conclusion
Sports, Science and Technology. We also thank Nissan Motor Co.,
Mizuho Foundation for the Promotion of Sciences, The Foundation “Hat-
tori-Hokokai”, and Venture Business Laboratory, Tokyo Institute of
Technology, for their financial support.
We have successfully developed a novel environmentally
friendly electrolytic system for anodic methoxylation using
recyclable solid-supported bases. This system has many prac-
tical advantages and characteristics: a) an electrolytic
system without intentionally added supporting electrolytes;
b) a supporting electrolyte generated in situ from methanol
as a solvent; c) easy and simple separation of methoxylated
products and solid-supported bases by only filtration with-
out neutralization; d) simple isolation of methoxylated prod-
ucts; e) electrochemical stability and recyclability of solid-
supported bases. It is hoped that this new electrolytic system
will make significant contributions to green sustainable
chemistry and open a new aspect of electroorganic synthesis.
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1
19
3
JNMEX-270 ( H: 270 MHz, F: 254 MHz) spectrometer in CDCl . The
1
19
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[
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[
17]
ing to the literature. All reagents except for 3 were purchased from
commercial suppliers and used without further purification. All solid-sup-
ported bases were purchased from Aldrich. As a typical example, the
[
À1
loading of silica gel-supported piperidine was 1.1 mmolg . In addition,
silica gel size was 40–63 mm.
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puter-controlled electrochemical system (ALS/CHI 600). Cyclic voltam-
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2
(
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3
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of a substrate was monitored by TLC. After the electrolysis, the electro-
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1
19
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3
19
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H NMR spectroscopy using toluene as an internal standard material.
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1
Acknowledgements
7
This work was financially supported by a Grant-in-Aid for Scientific Re-
search (No. 17750144) from The Japanese Ministry of Education, Culture,
Received: March 25, 2005
Published online: August 1, 2005
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Chem. Eur. J. 2005, 11, 6192 – 6196