Electrochemical enol ether/olefin cross-metathesis in a lithium perchlorate/nitromethane electrolyte solution

Article abstract of DOI:10.1002/anie.200503656

(Chemical Equation Presented) Electrolytically generated four-membered-ring radical-cation intermediates are involved in the anodic olefin cross-metathesis of enol ethers and olefins in a lithium perchlorate/nitromethane electrolyte solution (see scheme). The reaction mechanism was investigated through deuterium-labeling studies.

Full text of DOI:10.1002/anie.200503656

Angewandte
Chemie
[
10]
Electrochemistry
substrates served as an intramolecular electron donor to
form the cyclobutane ring. Herein,we describe novel electro-
chemical olefin cross-metatheses between anodically acti-
vated enol ethers and aliphatic alkenes.
DOI: 10.1002/anie.200503656
Electrochemical Enol Ether/Olefin Cross-
Metathesis in a Lithium Perchlorate/
Nitromethane Electrolyte Solution**
Initially,the electrolytic intermolecular reaction between
-methoxybut-3-enylbenzene (1) and hex-1-ene (2) was
4
investigated. The anodic oxidation of enol ether 1 involved
a carbon felt anode,a constant potential of 1.43 V (vs. Ag/
AgCl),a carbon felt counter electrode,and a lithium
perchlorate/nitromethane electrolyte solution (1.0m) in an
undivided cell under argon. Remarkably,the anodic activa-
tion of enol ether 1 in the presence of 2 resulted in the
formation of but-3-enylbenzene (3) in 78% yield (Scheme 2).
Teppei Miura, Shokaku Kim, Yoshikazu Kitano,
Masahiro Tada, and Kazuhiro Chiba*
Transition-metal-catalyzed olefin cross-metathesis plays an
important role in the construction of various carbogenic
skeletons by allowing the exchange of substituents between
different olefins via an activated key intermediate consisting
[
1]
of a four-membered ring coordinated to a transition metal.
Alternatively,electrochemical reactions have proven to be a
viable method in triggering reactions between different
Scheme 2. Electrochemical olefin cross-metathesis of 4-methoxybut-3-
enylbenzene (1) and hex-1-ene (2) in lithium perchlorate/nitromethane
electrolyte solution.
[
2]
olefins by reversing the polarity of alkenes and initiating
[
3]
[4]
[5]
radical anion- (or cation- ) based reactions. Electro-
chemically activated olefin cyclodimerization,for example,
can effectively lead to the intermolecular formation of four-
[
6]
membered rings. Moreover,we reported an electrocatalytic
In our experiments,the reaction was deemed complete
À1
intermolecular formal [2 + 2] cross-coupling of electron-rich
after the passage of approximately 1.0 Fmole of electricity.
[
7]
[8]
olefins (Scheme 1). The anodic oxidation of enol ethers
Under different electrolytic conditions,the desired product
was obtained in trace or in unacceptable yields (Table 1). In
the absence of electrolysis,a mixture of 1 and 2 in lithium
perchlorate/nitromethane electrolyte solution at ambient
temperature did not react,even after more than 10 h.
that bear a methoxyphenyl group can be used to trigger an
intermolecular formal [2 + 2] cycloaddition with varied
alkenes. In this case,the intermolecular olefin coupling was
assisted by a unique lithium perchlorate/nitromethane elec-
[
9]
trolyte solution and the methoxyphenyl group of the
Table 1: Electrolytic cross-metathesis of 4-methoxybut-3-enylbenzene (1)
and hex-1-ene (2) under different conditions.
À1
[a]
Electrolyte
Solvent
Electricity [Fmole
]
Yield (3) [%]
LiClO4
LiClO4
LiClO4
LiClO4
CH NO
1.0
0.5
0.3
0
1.0
1.0
1.0
1.0
78
3
2
2
2
2
2
2
[
b]
CH NO
40
30
n.d.
4
3
[
c]
d]
CH NO
3
[
CH NO
3
Me NClO
CH NO
4
4
3
Bu NClO
CH NO
n.d.
n.d.
3
4
4
3
Me NClO
CH CN
4
4
3
LiClO4
CH CN
3
[
[
a] Yields were determined by gas chromatography (n.d.=not detected).
b] 38% of 1 was recovered. [c] 55% of 1 was recovered. [d] More than
99% of 1 was recovered.
Scheme 1. Anodic intermolecular formal [2+2] cross-couplingof ole-
fins.
Furthermore,the reaction was responsive to a stepwise
application of the electricity. Because the amount of the
product for each stage remained essentially unchanged during
the intervals of electrolysis,an external switch of the
electrolysis allows the regulation of the reaction process.
The electrochemical enol ether/olefin cross-metathesis
can also be extended to the combination of simple alkyl enol
ethers and alkenes. For example,anodic oxidation of 1-
methoxy-tridec-1-ene (4) in the presence of 2 afforded the
desired tridec-1-ene (5) in 76% yield (Scheme 3).
[
*] T. Miura, Dr. S. Kim, Prof. Y. Kitano, Prof. M. Tada, Prof. K. Chiba
Laboratory of Bioorganic Chemistry
Tokyo University of Agriculture and Technology
3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509 (Japan)
Fax: (+81)42-360-7167
E-mail: chiba@cc.tuat.ac.jp
[
**] This work was partially supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science,
and Technology and the Japan Society for the Promotion of Science.
To gain insight into the reaction mechanism,the anodic
activation of enol ether 1 was carried out with deuterium-
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 1461 –1463
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Communications
Scheme 3. Electrochemical olefin cross-metathesis between alkyl enol
ether 4 and hex-1-ene (2).
labeled olefin 6 (Scheme 4). The formation of almost
[
11]
completely dideuterated product 7
helped to verify our
proposed cross-metathesis pathway. Moreover,under the
same reaction conditions,an internal olefin,hex-3-ene ( 8),
resulted in the carbon-chain-elongated product 9.
Scheme 5. Proposed reaction mechanism of electrochemical olefin
cross-metathesis.
In conclusion,anodic-oxidation triggers olefin cross-
metathesis reactions between enol ethers and alkenes in a
lithium perchlorate/nitromethane electrolyte solution. Inter-
molecular reactions allow the exchange of carbon fragments
between even simple alkyl enol ethers and alkenes. Because
the oxidation potentials of enol ethers are relatively low,the
reaction may involve inter- and intramolecular enol ether/
olefin cross-metathesis,thus avoiding overoxidation of sub-
strates. In this reaction system,the synergistic effects of the
electrochemical activation of the enol ether and the stabiliza-
tion of the radical-cation intermediates in a suitable electro-
lyte solution should effectively help to form the active key
intermediate. Efforts to expand the scope of this electro-
chemical methodology for the synthesis of various alkenes
and to extend the work to intramolecular reactions are
currently underway.
Scheme 4. Electrochemical olefin cross-metathesis of enol ether 1 with
deuterated olefin 6 or internal olefin 8.
Notably,these electrochemical enol ether/olefin cross-
metathesis reactions also give rise to a trace amount of the
alternate fragment as the enol ether—for example, 4 was
detected in the electrochemical reaction mixture of 1 and 5
along with main product 3. Accordingly,a radical-cation-
assisted fragmentation must have taken place to complete the
formation of the metathesis products. The radical cation
Received: October 15,2005
Published online: January 27,2006
(
fragment A) generated in situ from the enol ether could
Keywords: alkenes · electrochemistry · enol ethers · metathesis ·
oxidation
regenerate the radical cation of the starting enol ether by
intermolecular electron transfer. On the other hand,the
presence of a methoxyphenyl group should complete the
formation of the corresponding four-membered ring. Except
for specific enol ethers with neighboring oxidizable moieties
that exhibit oxidation potentials of reasonable relative values,
most enol ethers,however,should form the cross-metathesis
products without completion of the intramolecular electron-
transfer-assisted cyclobutane formation. In a cyclic voltam-
metry study,the anodic oxidation peak of 1 appeared at
.
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1
.43 V (vs. Ag/AgCl) with no cathodic peak. The oxidation
current of enol ether 1 was observed in the region 0.8–1.55 V
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Nielsen,J. H. P. Utley in Organic Electrochemistry, 4th ed.
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(
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tial of the region (0.8–1.0 V vs. Ag/AgCl) also gave the
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lithium perchlorate/nitromethane gave no clear oxidation
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enol ether,followed by the formation of the key intermedi-
ate—a four-membered radical-cation ring—from the acti-
vated enol ether and alkene (Scheme 5).
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[
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the Supporting Information.
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