Development of an electrolytic system using solid-supported bases for in situ generation of a supporting electrolyte from methanol as a solvent

Article abstract of DOI:10.1021/ja0423062

We have successfully developed a novel electrolytic system using solid-supported bases for in situ generation of a supporting electrolyte from methanol as a solvent. Anodic methoxylation of phenyl 2,2,2-trifluoroethyl sulfide using solid-supported bases w

Full text of DOI:10.1021/ja0423062

Published on Web 02/10/2005
Development of an Electrolytic System Using Solid-Supported Bases for in
Situ Generation of a Supporting Electrolyte from Methanol as a Solvent
Toshiki Tajima and Toshio Fuchigami*
Department of Electronic Chemistry, Tokyo Institute of Technology, Yokohama 226-8502, Japan
Received December 22, 2004; E-mail: fuchi@echem.titech.ac.jp
Electroorganic synthesis is one of the most useful methods in
organic synthesis.1 For example, the electrohydrodimerization of
acrylonitrile to adiponitrile, an intermediate to nylon 66, originally
developed by Monsanto Co. in the early 1960s, is now operating
at several sites worldwide. In addition, electroorganic synthesis is
an environmentally friendly process because electrons are inherently
environmentally friendly reagents compared with conventional
oxidizing and reducing reagents. Therefore, electroorganic synthesis
has potential as a “green sustainable” process. However, large
amounts of supporting electrolytes are required in order to provide
sufficient electrical conductivity to solvents for electrolyses.
Therefore, it is necessary to separate the supporting electrolytes
from the solvents after the electrolysis, and the separated supporting
electrolytes generally become industrial waste because they are
mostly unrecyclable. Furthermore, the inefficient separation of
supporting electrolytes from solvents results in a huge consumption
of energy and large amounts of additional industrial waste. To solve
such separation and industrial waste problems, it is most important
to develop a novel electrolytic system not requiring the addition of
any supporting electrolytes.
A capillary gap cell has been developed to minimize addition of
supporting electrolytes.2 On the other hand, solid polymer electro-
lytes3 and thermomorphic biphasic electroorganic synthesis4 have
Figure 1. Cyclic voltammograms of (a) N-methylpiperidine (100 mM)
and polystyrene-supported piperidine (100 mM) in 0.1 M n-Bu4N-BF4/
been developed to eliminate steps for the separation of supporting
electrolytes. Furthermore, very recently, a thin-layer flow cell has
also been developed to conduct electroorganic synthesis without
intentionally added supporting electrolytes.5
MeCN and (b) methanol in the presence of polystyrene-supported piperidine
(0.1 M), recorded at a Pt disk anode (φ ) 0.8 mm). The scan rate was 100
mV s-1
.
n-Bu4N-BF4/anhydrous acetonitrile. As shown in Figure 1a, 1 was
easily oxidized at ca. 1.3 V vs SCE, while 2 was not oxidized at
all, even under stirring. This means that solid-supported bases are
not oxidized at the electrode surface. Next, 2 was added into
methanol (0.1 M based on the concentration of piperidine), and
the cyclic voltammogram of this solution was measured. As shown
in Figure 1b, the oxidation wave for MeO- at ca. 1.0 V vs SCE
and the reduction current for H+ were observed. Therefore, it is
clear that methanol is dissociated into methoxide anions and protons
in the presence of solid-supported bases, and the resulting protons
seem to be the main carrier of an electronic charge.
An ideal electroorganic synthetic system from the viewpoint of
green sustainable chemistry should be an electrolytic system not
requiring the addition of any supporting electrolytes as well as a
totally closed electrolytic system. On the other hand, it is well
known that electron transfer between solid and solid is very
difficult.6 Therefore, it 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, protic organic solvents such as methanol would
be dissociated into anions and protons to some extent (eq 1). This
Next, we investigated anodic R-methoxylation using solid-
supported bases. As a model reaction we chose the anodic
methoxylation of phenyl 2,2,2-trifluoroethyl sulfide (3).7 Anodic
methoxylation processes,8 such as 2,5-dimethoxylation of furans,
anodic benzylic methoxylation of aromatic compounds, and R-meth-
oxylation of organonitrogen compounds, are performed widely in
organic synthesis and on an industrial scale. The overall experi-
mental procedure is illustrated in Figure 2.
Anodic methoxylation of 3 was carried out using various solid-
supported bases. As shown in Table 1, anodic methoxylation using
polystyrene-supported piperidine took place to provide the meth-
oxylated product 4 in low yield (entry 1), while anodic methoxyl-
ation proceeded efficiently using porous polystyrene-supported and
system would be suitable for electroorganic synthesis because the
protons derived from protic organic solvents would act as the main
carrier of an electronic charge. In this system, protic organic solvents
would serve as both a solvent and a supporting electrolyte generated
in situ. Herein, we report a novel electrolytic system using solid-
supported bases for in situ generation of a supporting electrolyte
from a methanol solvent.
At first, the cyclic voltammograms of N-methylpiperidine (1)
and polystyrene-supported piperidine 2 were measured in 0.1 M
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10.1021/ja0423062 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Experimental procedure.
Table 1. Anodic Methoxylation of 3 Using Various
Solid-Supported Bases
Figure 3. Yield of methoxylated product 4 in the reuse of silica
gel-supported piperidine.
that solid-supported bases are recyclable because solid-supported
bases are not subject to oxidative decomposition at the electrode
surface.
In conclusion, we have developed a novel electrolytic system
for anodic methoxylation using solid-supported bases. This system
has many practical advantages and characteristics: (a) an electrolytic
system without intentionally added supporting electrolytes; (b) a
supporting electrolyte generated in situ from a methanol solvent;
(c) simple separation of the methoxylated product and solid-
supported bases by only filtration without neutralization; (d)
electrochemical stability and recyclability of solid-supported bases.
Limitations of this new methodology and the further application
for electroorganic synthesis are now under investigation.
a 19F NMR yield based on the CF3 group using monofluorobenzene as
an internal standard. b Polystyrene. c Porous polystyrene. d Silica gel. e Iso-
lated yield in parentheses.
Acknowledgment. This work was financially supported by
Nissan Motor Co., Mizuho Foundation for the Promotion of
Sciences, The Foundation “Hattori-Hokokai”, and Venture Business
Laboratory, Tokyo Institute of Technology.
silica gel-supported piperidine to provide 4 in good to excellent
yields (entries 2, 3). These results indicate that the solvent
compatibility of solids is highly significant. The methoxylated
product 4 was obtained in 52% yield using silica gel-supported
pyridine, whose basicity is much lower compared with that of
piperidine (entry 4). In this case, silica gel-supported pyridine did
not dissociate methanol into methoxide anions and protons ef-
ficiently since the cell voltage was higher than 50 V. On the other
hand, the methoxylated product 4 was formed in high yields using
strong silica gel-supported bases (entries 3, 5, and 6). Thus, it was
found that relatively strong bases are suitable for the dissociation
of methanol and anodic methoxylation. It is notable that the current
efficiency for the anodic methoxylation of 3 was greatly increased
about 3 times compared with our previous work.7 In addition, as
shown in Figure 2, the methoxylated product and solid-supported
bases were easily separated by only filtration without neutralization.
Anodic methoxylation of 3 was successfully carried out 10 times
by the recycling of silica gel-supported piperidine. In the recycling
process, 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 not
decrease at all upon the reuse of silica gel-supported piperidine. In
addition, the appearance of silica gel-supported piperidine did not
change at all before and after electrolysis. This clearly suggests
Supporting Information Available: Experimental procedures. This
material is available free of charge via the Internet at http://pubs.acs.org.
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