A Novel Thermomorphic System for Electrocatalytic Diels-Alder Reactions

Article abstract of DOI:10.1002/cjoc.201900053

The discovery that lithium bis(trifluoromethane)sulfonamide (LiTFSI)/1-nitropropane (PrNO₂) solution functions as a less polar alternative to lithium perchlorate (LiClO₄)/nitromethane (MeNO₂) solution has led to the develo

Full text of DOI:10.1002/cjoc.201900053

Accepted Article
Title: A Novel Thermomorphic System for Electrocatalytic Diels-Alder Reactions
Authors: Yasushi Imada, Naoki Shida, Yohei Okada, and Kazuhiro Chiba*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2019, 37, 10.1002/cjoc.201900053.
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published online in Early View: http://dx.doi.org/10.1002/cjoc.201900053.
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A Novel Thermomorphic System for Electrocatalytic Diels-Alder
Reactions
Yasushi Imada,a Naoki Shida,a Yohei Okada,b and Kazuhiro Chiba*,a
a Department of Applied Biological Science, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan.
b Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan.
Cite this paper: Chin. J. Chem. 2019, 37, XXX—XXX. DOI: 10.1002/cjoc.201900XXX
Summary of main observation and conclusion The discovery that lithium bis(trifluoromethane)sulfonamide (LiTFSI)/1-nitropropane (PrNO2) solution
functions as a less polar alternative to lithium perchlorate (LiClO4)/nitromethane (MeNO2) solution has led to the development of a novel thermomorphic
system for electrocatalytic Diels-Alder reactions. Methyl cyclohexane (Me-c-Hex) can form a monophasic condition with LiTFSI/PrNO2 solution at room
temperature, enabling the use of hydrophobic dienophiles. After the electrochemical reaction, a biphasic condition can be formed at -50 °C, where the
cycloadducts are selectively recovered from the upper Me-c-Hex phase and the remaining lower LiTFSI/PrNO2 solution can be reused.
the lower electrolyte solution could be reused for further
reactions.
Background and Originality Content
Electrochemical approaches have proven to be powerful
We have been developing reactions triggered by anodic
redox options in the field of synthetic organic chemistry.1
Cathodes and anodes can induce reductive and oxidative single
electron transfer (SET) even under mild conditions by employing
electrons as reagents or catalysts.2 Both reducing and oxidizing
powers can simply be controlled by the potentials applied, with
the equivalents (amount of electricity) used for the reactions
monitored in real time. The choice of cooperative
additives/catalysts3 and electrode materials4 can also be unique
variables that can have a significant impact on the synthetic
outcomes. The only factor that must be considered is that the
solution used for the reaction must have sufficient electrical
conductivity. For this reason, polar organic solvents such as
MeOH, MeCN, or DMF are typically used in combination with a
large amount of supporting electrolyte, which subsequently
requires separation and disposal. Furthermore, substrates must
be soluble in highly polar electrolyte solutions and therefore, the
use of hydrophobic compounds is restricted. Although several
electrochemical reaction systems have been devised that enable
recycling and/or decreasing supporting electrolyte,5 the use of
hydrophobic substrates remains an intrinsic challenge.
oxidative SET8 in highly concentrated lithium perchlorate
(LiClO4)/nitromethane (MeNO2) solution.9 Since the use of this
solution is crucial for most cases, recycling and/or reducing the
amount of LiClO4 are in high demand. Although the c-Hex-based
biphasic systems have found some promising applications in
combination with LiClO4/MeNO2 solution,10 thermomorphic
properties were not exhibited, presumably due to the salting-out
effect. Therefore, the use of hydrophobic substrates in
LiClO4/MeNO2 solution is still restricted. In this context, we
recently demonstrated that nitroethane (EtNO2) could be a
superior alternative to MeNO2 for some reactions.11 Furthermore,
we also found that lithium bis(trifluoromethane)sulfonamide
(LiTFSI) could also be used as a supporting electrolyte instead of
LiClO4 in MeNO2.9 We questioned whether LiTFSI/EtNO2 solution
could be a less polar alternative to LiClO4/MeNO2 solution, which
can form a thermomorphic system with c-Hex. Described herein is
the discovery of a novel thermomorphic system that enables the
recycling of the supporting electrolyte and allows the use of
hydrophobic substrates.
Previously, we found that cyclohexane (c-Hex) can form
thermomorphic systems with typical polar organic solvents,
including MeOH, MeCN, or DMF, where biphasic and monophasic
conditions were reversibly switched over
a
moderate
temperature range.6 While the monophasic condition serves as an
effective homogeneous reaction environment for both
hydrophobic and hydrophilic substrates and/or reagents, selective
separation can be possible in the biphasic condition. We applied
this system to some electrochemical reactions, where the
monophasic condition could be recognized as a less polar
electrolyte solution, enabling the use of hydrophobic substrates
(Figure 1).7 Since hydrophobic products were selectively
recovered in the upper c-Hex phase under biphasic conditions,
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First author et al.
Report
H
C22 45O
Electrochemical Reaction
Product
4
(2 mM)
-Hex/LiTFSI/EtNO2
c
in
Heating
Cooling
c
Phase: -Hex
Upper
Lower
(Yellow)
Phase:
Supporting
Electrolyte
DMF
MeOH, MeCN,
(Blue)
Figure 1. Schematic illustration of electrochemical reactions in
thermomorphic systems.
Figure 2. Cyclic voltammograms of the dienophile (4) in 0.5
LiTFSI/EtNO2 solution with c-Hex.
M
Results and Discussion
The present work began with the investigation of the
LiTFSI/EtNO2 solution using an electrocatalytic Diels-Alder
reaction of trans-anethole (1) with isoprene (2) as a model (Table
1). Previously, we found that the reaction was completed with a
catalytic amount of electricity in LiClO4/MeNO2 or LiTFSI/MeNO2
solutions to afford the cycloadduct (3) in excellent yields (Entries
1 and 2). When the reaction was carried out in 0.5 M LiTFSI/EtNO2
solution, the cycloadduct (3) was also obtained in excellent yield
without any difficulty (Entry 3). Furthermore, c-Hex was found to
We then turned our attention to the use 1-nitropropane
(PrNO2) instead of EtNO2. The reaction of trans-anethole (1) with
isoprene (2) gave the cycloadduct (3) in excellent yield in 0.3 M
LiTFSI/PrNO2 solution (Table 1, Entry 4). Furthermore, c-Hex was
found to form a monophasic condition with LiTFSI/PrNO2 solution,
however, a biphasic condition was not obtained even at the
freezing point of c-Hex. We eventually found that methyl
cyclohexane (Me-c-Hex) was able to form a thermomorphic
system with LiTFSI/PrNO2 solution, causing
a monophasic
form
a thermomorphic system with the same volume of
condition at room temperature and a biphasic condition at
around -50 °C. Notably, the dienophile (4) was soluble in a 1:1
(v/v) mixture of Me-c-Hex and 0.3 M LiTFSI/PrNO2 solution and a
clear oxidation peak was observed in the CV measurement (Figure
3). To our delight, when the reaction of the dienophile (4) with
isoprene was carried out in a 1:1 (v/v) mixture of Me-c-Hex and
0.3 M LiTFSI/PrNO2 solution, the cycloadduct (5) was obtained in
excellent yield with a catalytic amount of electricity. Oxidation
potentials of isoprene (2) and the cycloadduct (5) were found to
be significantly higher than that of the dienophile (4) (Figure S1).
Therefore, the reaction was triggered by anodic oxidative SET of
the dienophile (4), involving a radical cation chain mechanism
(Figure S2). The product was selectively recovered from the upper
Me-c-Hex phase under the biphasic condition at -50 oC (Scheme 1
and Figure 4). It should be noted that the remaining LiTFSI/PrNO2
could be used at least 5 times (Figure 5).
LiTFSI/EtNO2 solution, causing a monophasic condition at room
temperature and a biphasic condition at around -7 °C. In order to
test the feasibility of this system for a hydrophobic substrate, the
dienophile (4) was prepared and used instead of trans-anethole
(1). Unfortunately, the dienophile (4) was barely soluble in a 1:1
(v/v) mixture of c-Hex and 0.5 M LiTFSI/EtNO2 solution and no
significant oxidation peak was observed in cyclic voltammetry (CV)
measurements (Figure 2).
Table 1. Electrocatalytic Diels-Alder reaction of trans-anethole (1).
MeO
MeO
+
2
1
3
Entrya
(%)
98
c
Conditionsb
Yield
1
2
3
4
1.0 M
1.0 M
0.1
0.1
LiClO4/MeNO2
F/mol
F/mol
,
LiTFSI/MeNO2
98
97
98
,
0.5 M LiTFSI/EtNO2 0.2 F/mol
,
0.3 M
0.3
LiTFSI/PrNO2 F/mol
,
a
were
on a
1
Reactions
carried out
1.00 mmol scale of trans-anethole
( )
2
with 3 mol
of
equiv. isoprene
was
carbon felt electrodes at
rt.
using
( )
carried out at 1.0 V
b
vs
in 10 mL of
as an
analysis using benzaldehyde
Electrolysis
Ag/AgCl
nitroalkane.
cDerermined
standard.
1H NMR
internal
by
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Chin. J. Chem. 2019, 37, XXX－XXX
This article is protected by copyright. All rights reserved.

Running title
Chin. J. Chem.
demonstrated by the reaction of the dienophiles (4) with several
dienes (6-11) (Table 2 and Figure S3). To our satisfaction, the
electrocatalytic Diels-Alder reactions took place effectively in a
1:1 (v/v) mixture of Me-c-Hex and 0.3 M LiTFSI/PrNO2 solution.
The cycloadducts (12-17), except the cycloadduct (14), were
obtained in high to excellent yields. In all cases, the reactions
were completed with a catalytic amount of electricity and the
cycloadducts were selectively recovered in the upper Me-c-Hex
phase under the biphasic condition at -50 oC, realizing facile
separation processes. It should be noted that not only the
dienophile (4) but also the dienophile (18) was found to be a
successful substrate for the reaction (Scheme 2).
H
C22 45O
4
(2 mM)
Me-c-Hex/LiTFSI/PrNO2
in
in
LiTFSI/PrNO2
Table 2. Scope of the dienophiles for electrocatalytic Diels-Alder
reactions.a
Figure 3. Cyclic voltammograms of the dienophile (4) in 0.3
LiTFSI/PrNO2 solution with (orange) or without (black) Me-c-Hex.
M
RO
RO
0.3 F/mol
+
R'
Scheme 1. Electrocatalytic Diels-Alder reaction of the dienophile (4).
R'
c
Me- -Hex
-
LiTFSI/PrNO2
4
C22
6 11
RO
RO
=
R
H45
0.3 F/mol
+
c
Me- -Hex
RO
RO
RO
4
2
LiTFSI/PrNO2
5
=
R
H45
C22
98%
12
13
14
, 0%
, 98%
, 91%
Electrochemical Reaction
Cycloadduct
RO
RO
15
16
, 80%
, 81%
Mixing
(at rt)
Cooling
-50 o
(at
C)
RO
c
Phase: Me- -Hex
Upper
Lower
(Gray)
(Blue)
LiTFSI
Phase: LiTFSI/PrNO
2
17
, 84%
Figure 4. Schematic ilustration of electrochemical reactions in
thermomorphic Me-c-Hex-LiTFSI/PrNO2 solution.
a
a
were
on a
0.10 mmol scale of the
Reactions
carried out
dienophile
-
4
( )
6 11
with 3 mol
of dienes
carbon felt electrodes at
rt.
equiv.
using
(
)
98%
97%
96%
93%
91%
Scheme 2. Electrocatalytic Diels-Alder reaction of the dienophile (18).
RO
RO
0.3 F/mol
+
MeO
MeO
c
Me- -Hex
LiTFSI/PrNO2
18
C22
2
19
86%
=
R
H45
Conclusions
In conclusion, we have demonstrated that the LiTFSI/PrNO2
solution can be a less polar alternative to the LiClO4/MeNO2
solution, and it can form a thermomorphic system with Me-c-Hex.
The monophasic condition at room temperature serves as a less
polar electrolyte solution, enabling the use of hydrophobic
dienophiles for electrocatalytic Diels-Alder reactions. The
Figure 5. Reusability of LiTFSI/PrNO2 solution.
Finally, the potential of the thermomorphic system was
Chin. J. Chem. 2019, 37, XXX－XXX
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
This article is protected by copyright. All rights reserved.

First author et al.
Report
cycloadducts could selectively be recovered in the upper
Me-c-Hex phase under the biphasic condition at -50 C, realizing
Advances in Organic Electrochemical C—H Functionalization. Chin. J.
Chem. 2018, 36, 338–352.
o
facile separation processes and the recycling of the supporting
electrolyte. We believe that the thermomorphic system described
herein should find further applications for electrochemical
reactions for hydrophobic substrates.
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Experimental
General Procedure for Electrocatalytic Diels-Alder Reactions.
To a solution of LiTFSI/PrNO2 (5 mL), the dienophile (0.10 mmol in
5 mL Me-c-Hex) and the diene (3 mol equiv.) were added. Two
pieces of carbon felt were inserted into the solution and
electrolysis was performed using an undivided cell with stirring at
a constant potential of 1.0 V vs. Ag/AgCl at room temperature. A
catalytic amount of electric charge was passed through the
solution (the reactions can also be monitored by TLC), followed by
cooling to -50 oC to form a biphasic condition. The upper
Me-c-Hex phase was concentrated in vacuo to give the respective
cycloadducts.
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Acknowledgement
This work was partially supported by JSPS KAKENHI Grant
Numbers 15H04494, 17K19222 (to K.C.), 16H06193, and
17K19221 (to Y.O.).
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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2019
Manuscript revised: XXXX, 2019
Manuscript accepted: XXXX, 2019
Accepted manuscript online: XXXX, 2019
Version of record online: XXXX, 2019
Okada,
Electron-Transfer-Induced Intermolecular [2
Reactions Based on the Aromatic “Redox Tag” Strategy. J. Org. Chem.
Y.;
Nishimoto,
A.;
Akaba,
R.;
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+
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Chin. J. Chem. 2019, 37, XXX－XXX
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First author et al.
Report
Entry for the Table of Contents
Electrochemical Reaction
Page No.
A
Novel
Thermomorphic
System
for
Electrocatalytic Diels-Alder Reactions
Mixing
(at rt)
Cooling
-50 o
(at
C)
c
Phase: Me- -Hex
Upper (Gray)
Lower
Phase: LiTFSI/PrNO
(Brue)
2
Mixing
(at rt)
Cooling
-50 o
(at
C)
Methyl
cyclohexane
can
form
a
thermomorphic
system
with
lithium
bis(trifluoromethane)sulfonamide/1-nitropropane solution, where biphasic and monophasic
conditions were reversibly switched over a practical temperature range. The monophasic
condition serves as a less polar electrolyte solution, enabling the use of hydrophobic dienophiles
for electrocatalytic Diels-Alder reactions.
Yasushi Imada, Naoki Shida, Yohei Okada, and
Kazuhiro Chiba*
www.cjc.wiley-vch.de
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, XXX－XXX
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