Photocatalytic Cycloadditions Enabled by a Lithium Perchlorate/Nitromethane Electrolyte Solution

Article abstract of DOI:10.1002/ejoc.201801280

Photocatalytic cycloadditions involving carbon–carbon bond formation in the absence of an external sensitizer are described. The use of a lithium perchlorate/nitromethane electrolyte solution exhibiting remarkable Lewis acidity is the key for the successf

Full text of DOI:10.1002/ejoc.201801280

A Journal of
Accepted Article
Title: Photocatalytic Cycloadditions Enabled by Lithium Perchlorate/
Nitromethane Electrolyte Solution
Authors: Shingo Nagahara, Hiroki Wakamatsu, Yohei Okada, and
Kazuhiro Chiba
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201801280
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10.1002/ejoc.201801280
European Journal of Organic Chemistry
COMMUNICATION
Photocatalytic Cycloadditions Enabled by Lithium
Perchlorate/Nitromethane Electrolyte Solution
Shingo Nagahara,[a] Hiroki Wakamatsu,[a] Yohei Okada,[b] and Kazuhiro Chiba*[a]
Abstract: Photocatalytic cycloadditions involving carbon-carbon
bond formation in the absence of an external sensitizer are
described. The use of lithium perchlorate/nitromethane electrolyte
solution exhibiting remarkable Lewis acidity is the key for the
successful transformations. Based on the UV-vis measurements, an
electron donor-acceptor complex did not form and nitromethane was
expected to function as the initial sensitizer for the reactions.
Although the mechanisms are still under debate, both electron- and
energy-transfer processes could be involved, both of which are
triggered by excited nitromethane in combination with molecular
oxygen.
reactions between carbon-centred radical cations and carbon
nucleophiles.13 Very recently, we demonstrated that formal [2 +
2] cycloadditions are also carried out by TiO2 photocatalysis in
this electrolyte solution.14 Described herein is our serendipitous
development of sensitizer-free photocatalytic cycloadditions in
this electrolyte solution.
Results and Discussion
Introduction
Light has been recognized as a green and sustainable reagent
to catalyze chemical reactions in the field of organic synthesis.
Since most substrate molecules have absorption maxima at
relatively short wavelengths, typically <250 nm, photosensitizers
are generally required. Photosensitizers can be excited at longer
wavelengths and thus activate non-absorbing substrate
molecules either through electron- or energy-transfer
mechanisms. Stemming from the pioneering works by
MacMillan,1 Yoon,2 and Stephenson,3 the use of transition metal
complexes as sensitizers has occupied a central place in this
field, which is commonly referred to as photoredox catalysis.4
Moreover, the potential of organic photoredox catalysis, using
purely organic sensitizers, has been demonstrated as a metal-
free alternative.5 Nevertheless, sensitizer-free approaches would
be the most straightforward and highly desirable in terms of
atom economy. Such sensitizer-free photocatalytic reactions
have generally been designed based on electron donor-acceptor
complexes that have absorption maxima at relatively longer
wavelengths.6 Furthermore, Melchiorre,7 Glorius,8 Hong,9 Cho,10
and Wang11 have recently achieved this class of transformation
without reliance on these complexes.
We have been developing carbon-carbon bond forming
reactions
by
electrocatalysis
in
lithium
perchlorate
(LPC)/nitromethane (NM) electrolyte solution.12 The electrolyte
solution exhibits remarkable Lewis acidity and facilitates
Table 1. Optimization of the conditions for the [2 + 2] cycloaddition.
[a]
[b]
S. Nagahara, Dr. H. Wakamatsu, Prof. Dr. K. Chiba
Department of Applied Biological Science
Tokyo University of Agriculture and Technology
3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan
E-mail: chiba@cc.tuat.ac.jp
Homepage: http://web.tuat.ac.jp/~bio-org/
Dr. Y. Okada
Department of Chemical Engineering
The present work began with the [2 + 2] cycloaddition between
the aryl vinyl ether (1) and 1-hexene (2) as a model15 under
irradiation conditions at 365 nm (Table 1).16 Although no reaction
took place when relatively weaker bulbs were used, such as 15
W, a bulb of 250 W was found to induce the reaction, affording
the desired cycloadduct (3) in high yield. The use of LPC/NM
solution was found to be the key for the photocatalytic
Tokyo University of Agriculture and Technology
2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejoc.201801280
European Journal of Organic Chemistry
COMMUNICATION
cycloaddition, since no other electrolyte solutions were as
productive as LPC/NM. Although sodium perchlorate might be
equally productive as LPC at high concentration, it is not
sufficiently soluble in NM. We also found that the stereochemical
outcome (dr = ca. 2:1:1:1) of the photocatalytic cycloaddition
was similar to that of the electrocatalytic version (Figure S1),
which can rationally be understood based on both intermolecular
and intramolecular single electron transfer (SET) processes
(Figure 1). Namely, the radical cation of the aryl vinyl ether (1.+)
is generated via an oxidative SET at the anode, which is then
trapped by 1-hexene to give the aromatic radical cation (3.+) that
leads to the neutral form via reductive SET. It is reasonable to
the initial sensitizer for the reaction, however, the mechanism is
still unknown.
During the course of several control experiments, we discovered
that the reaction was significantly inhibited when the LPC/NM
solution was purged with argon prior to use (Scheme S1). Since
molecular oxygen is well-known to function as an electron
acceptor, we propose two possible pathways, involving electron-
and energy-transfer mechanisms (Figure 2). Path A includes
energy transfer from the excited NM to triplet oxygen, generating
singlet oxygen, which then oxidizes the aryl vinyl ether (1) by
electron transfer. However, path B cannot be ruled out, in which
energy transfer from the excited NM to aryl vinyl ether (1) occurs,
generating its excited form (1*) that in turn reduces triplet
oxygen. Both pathways are expected to give the radical cation
assume that
a similar mechanism would occur for the
photocatalytic cycloaddition, however, the SET events involved
in the transformation are as yet unclear.
.-
(1.+) and superoxide radical anion (O2 ), which can also be an
oxidant.
Figure 1. Plausible reaction mechanism for the electrocatalytic [2 + 2]
cycloaddition.
Table 2. Photocatalytic [2 + 2] cycloadditions.
Figure 2. Plausible reaction mechanisms for the photocatalytic [2 + 2]
cycloaddition.
With this mechanistic understanding in hand, the reactions of the
aryl vinyl ether (1) with several alkenes (4, 5) were conducted
under the optimized conditions (Table 2). To our delight, the
photocatalytic cycloadditions took place effectively to afford the
corresponding cycloadducts (6, 7) in high yields. The
photocatalytic cycloadditions were not limited to the [2 + 2] mode.
Several [4 + 2]12b and [3 + 2]17 versions were also possible
(Tables 3 and 4). In all cases, the presence of molecular oxygen
was essential for successful transformations, suggesting that
similar mechanisms as proposed in Figure 2 would be involved.
UV-vis measurements clearly show that neither the aryl vinyl
ether (1) nor 1-hexene (2) had any absorptions at 365 nm, even
at high concentrations (Figure S2). Since a negligible shift was
observed when the substrates were mixed together, an electron
donor-acceptor complex likely did not form. In contrast, NM is
known to have an absorption maximum at a relatively longer
wavelength and indeed, we observed an absorption at 365 nm
(Figure S3). Therefore, it can be concluded that NM functions as
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10.1002/ejoc.201801280
European Journal of Organic Chemistry
COMMUNICATION
external sensitizer. Based on the UV-vis measurements, an
electron donor-acceptor complex did not form and NM was
expected to function as the initial sensitizer for the reactions.
Although the mechanisms are still under debate, both electron-
and energy-transfer processes could be involved, both of which
are triggered by excited NM in combination with molecular
oxygen. We believe that the results described herein will
contribute fundamentally to the design of novel photocatalytic
reactions and the expansion of potential applications in the field
of organic synthesis.
Experimental Section
General Procedure for the Photocatalytic [2 + 2] Cycloadditions. To
a solution of LiClO4 (2.0 M) in CH3NO2 stirred at room temperature were
added the aryl vinyl ether (1) and respective alkenes. The resulting
reaction mixture was stirred at 10 oC under irradiation of a 250 W UV
lamp (365 nm) until the starting material was consumed (checked by thin-
layer chromatography), diluted with water, and extracted with EtOAc. The
combined organic layers were dried over Na2SO4, filtered, and
concentrated in vacuo. Yields were determined by 1H NMR analysis
using benzaldehyde as an internal standard. Silica gel column
chromatography (Hex/toluene = 50/1) gave the desired cycloadduct.
General Procedure for the Photocatalytic [4 + 2] Cycloadditions. To
a solution of LiClO4 (2.0 M) in CH3NO2 stirred at room temperature were
added trans-anethole and respective dienes. The resulting reaction
mixture was stirred at 10 oC under irradiation of a 250 W UV lamp (365
nm) until the starting material was consumed (checked by thin-layer
chromatography), diluted with water, and extracted with EtOAc. The
combined organic layers were dried over Na2SO4, filtered, and
concentrated in vacuo. Yields were determined by 1H NMR analysis
using benzaldehyde as an internal standard. Silica gel column
chromatography (Hex/EtOAc = 30/1) gave the desired cycloadduct.
Table 3. Photocatalytic [4 + 2] cycloadditions.
General Procedure for the Photocatalytic [3 + 2] Cycloadditions. To
a solution of LiClO4 (3.0 M) and AcOH (50 mM) in CH3NO2 stirred at
room temperature were added 4-methoxyphenol and respective alkenes.
o
The resulting reaction mixture was stirred at 10 C under irradiation of a
250 W UV lamp (365 nm) until the starting material was consumed
(checked by thin-layer chromatography), diluted with water, and
extracted with EtOAc. The combined organic layers were dried over
Na2SO4, filtered, and concentrated in vacuo. Yields were determined by
1H NMR analysis using benzaldehyde as an internal standard. Silica gel
column chromatography (Hex/EtOAc = 5/1) gave the desired cycloadduct.
Acknowledgements
This work was partially supported by JSPS KAKENHI Grant
Numbers 15H04494, 17K19222 (to K. C.), 16H06193, and
17K19221 (to Y. O.).
Table 4. Photocatalytic [3 + 2] cycloadditions.
Keywords: photocatalytic • cycloaddition • lithium
perchlorate/nitromethane • molecular oxygen • carbon-carbon
bond formation
Conclusions
In conclusion, we have demonstrated that several photocatalytic
cycloadditions, including [2 + 2], [4 + 2], and [3 + 2], were
enabled by the use of LPC/NM solution in the absence of any
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Entry for the Table of Contents (Please choose one layout)
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COMMUNICATION
Photocatalytic cycloadditions involving
carbon-carbon bond formation in the
absence of an external sensitizer are
described. The use of lithium
Photocatalysis
Shingo Nagahara, Hiroki Wakamatsu,
Yohei Okada, Kazuhiro Chiba*
perchlorate/nitromethane electrolyte
solution exhibiting remarkable Lewis
acidity is the key for the successful
transformations.
Page No. – Page No.
Photocatalytic Cycloadditions
Enabled by Lithium
Perchlorate/Nitromethane Electrolyte
Solution
*one or two words that highlight the emphasis of the paper or the field of the study
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