Cycloalkane-based thermomorphic systems for organic electrochemistry: An application to Kolbe-coupling

Article abstract of DOI:10.1016/j.tet.2012.04.112

The discovery that cycloalkanes can form thermomorphic systems with typical polar organic solvents has led to the development of less-polar electrolyte solutions. Their mixing and separation can be regulated reversibly at a moderate temperature range. The phase switching temperature can be controlled by changing the solvent compositions. While biphasic conditions are maintained below the phase switching temperature, conductive monophasic conditions as less-polar electrolyte solutions are obtained above the phase switching temperature. After the electrochemical transformations, biphasic conditions are reconstructed below the phase switching temperature, facilitating the separation of cycloalkane where hydrophobic products or designed hydrophobic platforms are selectively partitioned. Several polar organic solvents, including acetonitrile, methanol, and pyridine, can be used in this system according to the requirement of the reactions.

Full text of DOI:10.1016/j.tet.2012.04.112

Tetrahedron 68 (2012) 5857e5862
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Cycloalkane-based thermomorphic systems for organic electrochemistry:
an application to Kolbe-coupling
Yohei Okada, Kazuya Kamimura, Kazuhiro Chiba *
Department of Applied Biological Science, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 April 2012
Received in revised form 26 April 2012
Accepted 28 April 2012
Available online 4 May 2012
The discovery that cycloalkanes can form thermomorphic systems with typical polar organic solvents has
led to the development of less-polar electrolyte solutions. Their mixing and separation can be regulated
reversibly at a moderate temperature range. The phase switching temperature can be controlled by
changing the solvent compositions. While biphasic conditions are maintained below the phase switching
temperature, conductive monophasic conditions as less-polar electrolyte solutions are obtained above
the phase switching temperature. After the electrochemical transformations, biphasic conditions are
reconstructed below the phase switching temperature, facilitating the separation of cycloalkane where
hydrophobic products or designed hydrophobic platforms are selectively partitioned. Several polar or-
ganic solvents, including acetonitrile, methanol, and pyridine, can be used in this system according to the
requirement of the reactions.
Keywords:
Cycloalkane
Thermomorphic
Less-polar electrolyte solution
Phase switching temperature
Kolbe-coupling
Ó 2012 Elsevier Ltd. All rights reserved.
Briefs
is needed to impart electrical conductivity to polar organic solvents,
Less-polar electrolyte solutions have been constructed using
cycloalkane-based thermomorphic systems.
which requires subsequent separation and disposal. Second, re-
action substrates should be soluble in polar electrolyte solutions
because electron transfer takes place only at the surface of the
electrodes; thus, the use of hydrophobic substrates is restricted.
Third, insoluble hydrophobic products generated during the elec-
trochemical transformations might form polymeric films at the
surface of electrodes that suppress electric current severely, known
1. Introduction
The solid-phase technique comprises a substantial fraction of
current research in both the academic and industrial fields, offering
a great advantage with regards to compound separation and has
also proven to be effective for automated synthesis and combina-
torial chemistry. Generally, reaction substrates are bound to a solid-
phase, facilitating their separation from the reaction mixture to
enable efficient multistep chemical transformations, especially for
peptide synthesis.1 Meanwhile, the immobilization of reaction
catalysts on a solid-phase realizes their consecutive recycling to
establish green chemical transformations, which also serves as
4
as electrode passivation. To satisfy the above criteria, several
electrochemical reaction systems have been devised for improve-
5
ment of the electrochemical transformation productivities.
The biphasic technique has also dominated the landscape of
modern synthetic chemistry to provide a facile separation of
6
compounds by simple liquideliquid extraction, including fluorous
7
and ionic systems. The fluorous biphasic technique is based on the
insolubility of perfluorinated hydrocarbons with both polar and
less-polar organic solvents; thus, fluorous compounds, including
reaction substrates, products, and catalysts, or designed fluorous
platforms, are preferentially dissolved in the fluorous phase, re-
alizing their rapid separation. Moreover, thermomorphic systems
that change thermally from biphasic conditions to monophasic
conditions have also been developed to achieve unique separation
2
promising applications in combination with a flow strategy. From
this aspect, electrodes are regarded as immobilized redox reagents
in electrochemical transformations. Their potentials can be simply
3
controlled to regulate electron transfer at their surfaces. Although
not only various functional group transformations, but also a wide
variety of carbonecarbon bond forming reactions have been ac-
complished, electrochemical approaches have the following es-
sential requirements. First, a large amount of supporting electrolyte
8
techniques. For example, a 1:1 (v/v) mixture of heptane and 90%
aqueous ethanol are immiscible, forming biphasic conditions at
ꢀ ꢀ
5 C. While heating to 70 C is sufficient to make this system
2
miscible, constructing monophasic conditions, the initial biphasic
ꢀ
conditions are reformed at 25 C. Therefore, the use of designed
*
Corresponding author. Tel.: þ81 42 367 5667; fax: þ81 42 360 7167; e-mail
water soluble polymer platforms leads to a versatile scheme for
address: chiba@cc.tuat.ac.jp (K. Chiba).
0
040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.04.112

5858
Y. Okada et al. / Tetrahedron 68 (2012) 5857e5862
compound separation, in which the compound can be fully re-
covered from a 90% aqueous ethanol phase even if it is distributed
throughout the monophasic conditions at 70 C, enabling homo-
Table 1
Phase switching temperatures of cycloalkane-based thermomorphic systems using
acetonitrile as a typical polar organic solvent
ꢀ
Cycloalkane
Phase switching temperature ( C)a
ꢀ
geneous chemical transformations.
We have been developing various chemical transformations in
cyclohexane-based thermomorphic systems, which stem from the
discovery that cyclohexane can form thermomorphic systems with
typical polar organic solvents and their mixing and separation can
Cyclopentane (1)
Cyclohexane (2)
Cycloheptane (3)
Cyclooctane (4)
cis-1,3-Dimethylcyclohexane (5)
trans-1,3-Dimethylcyclohexane (6)
29
53
59
75
60
59
9
be regulated reversibly at a moderate temperature range. For ex-
ample, a 1:4 (v/v) mixture of cyclohexane and nitromethane shows
a
Biphasic conditions composed of 1:3 (v/v) mixtures of several cycloalkanes
1e6) and acetonitrile as a typical polar organic solvent (20 mL) were gradually
ꢀ
biphasic conditions at 25 C, which forms monophasic conditions at
(
ꢀ
6
0
C to serve as an effective homogeneous reaction field, while
heated with stirring to form monophasic conditions at the temperature.
ꢀ
cooling to 25 C reforms the initial biphasic conditions. On the basis
of these findings, we have also demonstrated several electro-
chemical transformations in cyclohexane-based multiphasic elec-
trolyte solutions.10 In these systems, while electrolyte solutions are
partially miscible with cyclohexane to construct multiphase elec-
trolyte solutions, supporting electrolytes are selectively dissolved
in polar organic solvents. Thus, hydrophobic products are contin-
uously removed from the electrolyte solutions to suppress their
overoxidations and to enable consecutive recycling of supporting
electrolytes. In light of previous studies, we sought to realize less-
polar electrolyte solutions assisted by cycloalkane-based thermo-
morphic systems so that completion of their mixing and separation
would be regulated reversibly at a moderate temperature range,
satisfying the above-mentioned essential requirements of electro-
chemical transformations.
Table 2
Phase switching temperatures of cycloalkane-based thermomorphic systems using
methanol as a typical polar organic solvent
Phase switching temperature ( C)a
ꢀ
Cycloalkane
b
Cyclopentane (1)
Cyclohexane (2)
Cycloheptane (3)
Cyclooctane (4)
d
40
66
c
d
a
Biphasic conditions composed of 1:1 (v/v) mixtures of several cycloalkanes
1e4) and methanol as a typical polar organic solvent (20 mL) were gradually heated
(
with stirring to form monophasic conditions at the temperature.
b
c
ꢀ
Miscible even at 25 C.
Immiscible even around at the boiling point of the solvent.
separation. These results provided us an incentive to construct less-
polar electrolyte solutions enabled by the cycloalkane-based ther-
momorphic systems.
2
. Results and discussion
The present study began with the investigation of the thermo-
On the basis of these observations, we investigated numerous
compositions of electrolyte solutions that showed practical ther-
momorphic properties in combination with cycloalkanes to realize
less-polar electrolyte solutions. As a supporting electrolyte, potas-
sium hydroxide was added to a 1:1:2 (v/v/v) mixture of pyridine,
methanol, and acetonitrile (saturated), which was then combined
with an equal volume of several cycloalkanes (1e4). It has been
demonstrated that heating to moderate temperatures was enough
to create monophasic conditions as less-polar electrolyte solutions
morphic properties of cycloalkanes. For this purpose, 1:3 (v/v)
mixtures of several cycloalkanes (1e6) and acetonitrile as typical
polar organic solvents were prepared (Scheme 1). All cycloalkanes
(1e6) were immiscible with acetonitrile to form biphasic condi-
ꢀ
tions at 25 C, which were then gradually heated. It was observed
that all cycloalkanes (1e6) exhibited thermomorphic properties
and were miscible with acetonitrile to form monophasic conditions
at each phase switching temperature (Table 1). The hydrophobicity
of cycloalkanes (1e6) is expected to increase with the number of
carbons. As expected, a reasonable increase in the phase switching
temperatures was observed for non substituted cycloalkanes (1e4).
On the other hand, the phase switching temperatures of both cis-
(Table 3). Furthermore, lithium perchlorate could also be employed
as a supporting electrolyte (0.1 M) to realize less-polar electrolyte
solutions assisted by the cycloalkane-based thermomorphic sys-
tems, which completely reformed the initial biphasic conditions
(5) and trans-1,3-dimethylcyclohexane (6) were relatively low that
ꢀ
after cooling to 25 C (Scheme 2).
their hydrophobic natures could be estimated to be at the same
level as cyclooctane (4). The ring-flip of cyclohexane was suppos-
edly responsible for the thermomorphic properties.
Table 3
Phase switching temperatures of cycloalkane-based themomorphic systems using
an electrolyte solution composed of potassium hydroxide and 1:1:2 (v/v/v) mixtures
of pyridine, methanol, and acetonitrile
Phase switching temperature ( C)a
ꢀ
Cycloalkane
b
Cyclopentane (1)
Cyclohexane (2)
Cycloheptane (3)
Cyclooctane (4)
d
48
55
63
Scheme 1. Structures of cycloalkanes used in this study.
a
Biphasic conditions composed of 1:1 (v/v) mixtures of several cycloalkanes
1e4) and electrolyte solution (20 mL) were gradually heated with stirring to form
(
monophasic conditions at the temperature.
The thermomorphic properties of cycloalkanes (1e4) were also
studied using methanol as a typical polar organic solvent. 1:1 (v/v)
mixtures of several cycloalkanes (1e4) and methanol were pre-
pared. In this case, cyclopentane (1) was miscible with methanol to
form monophasic conditions even at 25 C, while cyclooctane (4)
was immiscible with methanol to form biphasic conditions even
around the boiling point of the solvent (Table 2). When cyclohexane
b
ꢀ
Miscible even at 25 C.
With these results in hand, we then turned our attention to the
ꢀ
application of cycloalkane-based thermomorphic systems as less-
polar electrolyte solutions for practical electrochemical trans-
formations. In this context, the oxidative dimerization of carbox-
ylate anions through decarboxylation, known as Kolbe-coupling,
was chosen as a model, which is still under active investigation in
(2) or cycloheptane (3) was utilized, thermal changes at a moderate
temperature range were sufficient to regulate their mixing and

Y. Okada et al. / Tetrahedron 68 (2012) 5857e5862
5859
Scheme 2. Phase switching of cycloalkane-based thermomorphic systems.
(
8) was selectively dissolved into the less-polar cyclohexane (2);
OH
R
Kolbe-coupling
thus, the dimer could be immediately removed from the electrolyte
solutions by only cooling to form the biphasic solutions, enabling
consecutive recycling of electrolyte solutions (Scheme 5).
R
R
2
O
Kolbe-dimer
On the other hand, when 3-phenylpropanoic acid (9) was used
as a substrate, the less-polar electrolyte solutions were critical for
the reaction. The anodic oxidation of 3-phenylpropanoic acid (9) in
the 1:1:2 (v/v/v) mixture of pyridine, methanol, and acetonitrile
using potassium hydroxide (saturated) as a supporting electrolyte
under constant current gave the Kolbe-dimer (10) in low yield
(Scheme 6). In this case, the formation of an insoluble polymeric
film was observed at the surface of the electrodes, decreasing the
yield of the Kolbe-dimer (10). The addition of an equal volume of
cyclohexane (2) significantly improved the reaction efficiency. The
reaction mixture was heated by electrical resistance to construct
monophasic conditions as less-polar electrolyte solutions during
application of the current, which inhibited the formation of the
insoluble polymeric film at the surface of the electrodes, affording
the Kolbe-dimer (10) in good yield. The same reaction conditions
could also be applied to the Kolbe-coupling of several carboxylic
acids (11e13) to give the corresponding Kolbe-dimers (14e16) in
good yields (Scheme 7). It should be noted that the monophasic
conditions were formed by heating using electrical resistance to
realize less-polar electrolyte solutions during the application of the
-
H+
O-
-
e-, -CO2
R
2
2
R
O
Scheme 3. Reaction mechanism of Kolbe-coupling.
both the academic and industrial fields (Scheme 3).11 Initially, the
anodic oxidation of octanoic acid (7) was carried out in a 1:1:2 (v/v/
v) mixture of pyridine, methanol, and acetonitrile using potassium
hydroxide (saturated) as a supporting electrolyte under constant
12
current to give the Kolbe-dimer (8) in excellent yield (Scheme 4).
Even in the presence of an equal volume of cyclohexane (2), the
Kolbe-dimer (8) was obtained in excellent yield, also realizing
gram-scale production. In this case, the reaction mixture formed
ꢀ
biphasic conditions at 25 C, which was heated by electrical re-
sistance to construct monophasic conditions, realizing a less-polar
electrolyte solution, during the application of the current. The ad-
dition of octanoic acid (7) did not affect the thermomorphic
properties of the system. Because of the polarity, the Kolbe-dimer
ꢀ
current, while cooling to 25 C was sufficient to reform the biphasic
conditions, enabling the rapid separation of the Kolbe-dimers
(14e16) from the electrolyte solutions.

5860
Y. Okada et al. / Tetrahedron 68 (2012) 5857e5862
OH
2
2
constant current
2
2
O
(+) Pt - Pt (-)
8
7
a
electrolyte solution
current density
yield (%)
pyridine:MeOH:MeCN = 1:1:2
without c-Hex
b
5 mA/cm2
93
2
pyridine:MeOH:MeCN = 1:1:2
with c-Hex
5 mA/cm2
b
c
2
95 (91)
a
b
c
2
Determined by NMR
2
.50 mmol of octanoic acid (7) was used
5.0 mmol of octanoic acid (7) was used
Scheme 4. Kolbe-coupling of octanoic acid (7).
Heating
Cooling
Biphasic condition
=
Monophasic condition
Biphasic condition
"
Less-polar electrolyte solution"
OH
2
2
2
=
O
8
7
Scheme 5. Schematic illustration of the Kolbe-coupling of octanoic acid (7) using cyclohexane-based thermomorphic systems.
Scheme 6. Kolbe-coupling of 3-phenylpropanoic acid (9).

Y. Okada et al. / Tetrahedron 68 (2012) 5857e5862
5861
Scheme 7. Kolbe-coupling of several carboxylic acids (11e13).
3
. Conclusion
4.2. Kolbe-couplings without cyclohexane
In conclusion, we found that several cycloalkanes (1e6) showed
Carboxylic acids (2.5 mmol) were added to electrolyte solutions
(20 mL) using potassium hydroxide as a supporting electrolyte
(saturated). The undivided reaction cell was capped with a septum
equipped with platinum electrodes (10 mmꢁ20 mm). The elec-
trolysis was then performed at constant current. After completion
of the reaction, the reaction mixture was poured into EtOAc, and
the EtOAc solution was successively washed with brine. The organic
unique thermomorphic properties in combination with several
typical polar organic solvents, including acetonitrile, methanol, and
pyridine, leading to the construction of less-polar electrolyte so-
lutions. Their mixing and separation can be regulated reversibly at
a
moderate temperature range. We also demonstrated that
cycloalkane-based thermomorphic systems as less-polar electro-
lyte solutions are efficient for practical electrochemical trans-
formations using Kolbe-coupling as a model. Thus, the reaction
mixtures were heated by electrical resistance to construct mono-
phasic conditions, which inhibited electrode passivations, and the
hydrophobic products can be immediately removed from the
electrolyte solutions by only cooling to reform the biphasic condi-
tions. Less-polar electrolyte solutions assisted by cycloalkane-based
thermomorphic systems would be a significant aid for expanding
the possibility of electrochemical transformations.
layer was dried over anhydrous MgSO
4
. After filtration and evap-
oration under reduced pressure, the residue was purified by silica
gel column chromatography using n-hexaneeEtOAc to give the
products. The products yields were determined by NMR.
4.2.1. Tetradecane (8, oil). 1H NMR (600 MHz, CDCl
)
d
1.33e1.22
32.0,
3
13
(24H, m), 0.88 (6H, t, J¼6.8 Hz) C NMR (150 MHz, CDCl
3
) d
ꢂ1
29.7, 29.4, 22.7, 14.1 IR (NaCl, cm ) 2902, 2733, 2677, 1464, 1381,
þ
1302, 895, 719 MS (rel int.) m/z 198 (M , 2), 155 (1), 141 (1), 127 (1),
1
13 (2), 99 (5), 85 (34), 71 (65), 57 (100) HRMS calcd for C14
H
30
4
4
. Experimental section
198.2347 (MꢂH 197.2269), found 197.2280.
.1. Kolbe-couplings with cyclohexane
Carboxylic acids (2.5 mmol) were added to electrolyte solutions
4.2.2. 1,4-Diphenylbutane (10, solid). 1H NMR (600 MHz, CDCl
)
3
1
3
d
7.30e7.11 (10H, m), 2.63 (4H, t, J¼6.8 Hz), 1.71e1.62 (4H, m)
C
NMR (150 MHz, CDCl
3
)
d
142.5, 142.5, 128.4, 128.4, 128.3, 128.3,
ꢂ1
(
(
20 mL) using potassium hydroxide as a supporting electrolyte
saturated) in the presence of the cycloalkanes. The undivided re-
128.2, 128.2, 128.2, 125.7, 125.6, 35.8, 31.1 IR (KBr, cm ) 3099, 3057,
3026, 2929, 2856, 1954, 1865, 1805, 1603, 1495, 1452, 1348, 1119,
1065, 1024, 906, 742, 696 MS (rel int.) m/z 210 (M , 85), 195 (1), 181
þ
action cell was capped with a septum equipped with platinum
electrodes (10 mmꢁ20 mm). The electrolysis was then performed
at constant current. After completion of the reaction, the cyclo-
alkane phase was concentrated under reduced pressure and the
residue was purified by silica gel column chromatography using n-
hexaneeEtOAc to give products. The products’ yields were de-
termined by NMR.
(2), 167 (4), 130 (3), 117 (11), 104 (10), 91 (100), 77 (11), 65 (23), 51
(4) HRMS calcd for C16
H18 210.1409, found 210.1434.
4.2.3. 5,10-Dimethyltetradecane (14, oil). 1H NMR (600 MHz, CDCl
)
3
13
d
1.35e1.05 (22H, m), 0.90e0.83 (12H, m) C NMR (150 MHz,
CDCl 37.2, 36.8, 36.8, 32.8, 29.4, 27.5, 27.4, 27.4, 23.1, 23.1, 19.7,
3
) d

5
862
Y. Okada et al. / Tetrahedron 68 (2012) 5857e5862
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ꢂ1
1
9
1
(
9.7, 14.2, 14.1 IR (NaCl, cm ) 2958, 2927, 2852, 1468, 1373, 1153,
12, 731 MS (rel int.) m/z 211 (2), 197 (1), 169 (11), 141 (21), 127 (4),
13 (7), 99 (14), 85 (100), 71 (69), 57 (95) HRMS calcd for C16
MꢂC 169.1956), found 169.1948.
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d
.2.4. 1,20-Dibromoicosane (15, solid). 1H NMR (600 MHz, CDCl
3.41 (4H, t, J¼6.8 Hz), 1.85 (4H, quint, J¼6.8 Hz), 1.42e1.26 (32H,
3
)
9
S. Chem. Commun. 2011, 10211e10223.
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m) C NMR (150 MHz, CDCl
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d
34.1, 32.8, 29.7, 29.7, 29.6, 29.5,
9.4, 28.8, 28.2 IR (KBr, cm ) 2922, 2845, 1471, 1331, 1298, 1267,
236, 1205, 717, 642 MS (rel int.) m/z 359 (2), 233 (2), 219 (3), 207
4), 191 (4), 177 (5), 163 (5), 151 (8), 137 (25), 127 (4), 111 (6), 97 (20),
3 (44), 71 (87), 57 (100) HRMS calcd for C20 438.1497
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2
1
(
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9
(
Mꢂ Br 359.2313), found 359.2295.
.2.5. Dimethyl tetradecanedioate (16, solid). 1H NMR (600 MHz,
4
CDCl
1
2
2
7
3
)
d
3.67 (6H, s), 2.30 (4H, t, J¼7.6 Hz), 1.62 (4H, quint, J¼7.2 Hz),
13
.29e1.26 (16H, m) C NMR (150 MHz, CDCl
3
)
d
174.3, 51.4, 34.1, 34.1,
ꢂ1
9.5, 29.5, 29.4, 29.4, 29.2, 29.1, 29.1, 24.9 IR (KBr, cm ) 2983, 2917,
850, 1743, 1475, 1464, 1435, 1387, 1309, 1254, 1203, 1169, 991, 881,
29 MS (rel int.) m/z 255 (35), 222 (7), 213 (37), 194 (6), 181 (26), 163
104, 2531e2556; (c) Studer, A.; Hadida, S.; Ferritto, R.; Kim, S.-Y.; Jeger, P.; Wipf,
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(
23),140 (8),121 (7),112 (25), 98 (96), 84 (62), 74 (100), 55 (77) HRMS
calcd for C16
30 4
H O
286.2144 (MþNa 309.2042), found 309.2062.
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Supplementary data
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. (a) Kim, S.; Ikuhisa, N.; Chiba, K. Chem. Lett. 2011, 40, 1077e1078; (b) Kim, S.;
Tsuruyama, A.; Ohmori, A.; Chiba, K. Chem. Commun. 2008, 1816e1818; (c) Kim,
S.; Yamamoto, K.; Hayashi, K.; Chiba, K. Tetrahedron 2008, 64, 2855e2863; (d)
Hayashi, K.; Kim, S.; Kono, Y.; Tamura, M.; Chiba, K. Tetrahedron Lett. 2006, 47,
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Additional table and scheme, general information, experimental
1
13
details, characterization data, and copies of H NMR and C NMR
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.tet.2012.04.112.
1
0. (a) Okada, Y.; Yamaguchi, Y.; Chiba, K. Eur. J. Org. Chem. 2012, 243e246; (b)
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