Cathodic reductive couplings and hydrogenations of alkenes and alkynes catalyzed by the B12 model complex

Article abstract of DOI:10.1016/j.jorganchem.2017.02.002

The reductive coupling and hydrogenation of alkenes were catalyzed by the B₁₂ model complex, heptamethyl cobyrinate perchlorate (1), in the presence of acid during electrolysis at??0.7?V vs. Ag/AgCl in acetonitrile. Conjugated alkenes showed a good reactivity during electrolysis to form reduced products. The product distributions were dependent on the substituents at the C[dbnd]C bond of the alkenes. ESR spin-trapping experiments using 5,5-dimethylpyrroline N-oxide (DMPO) revealed that the cobalt-hydrogen complex (Co–H complex) should be formed during the electrolysis and it functioned as an intermediate for the alkene reduction. The electrolysis was also applied to an alkyne, such as phenylacetylene, to form 2,3-diphenylbutane (racemic and meso) and ethylbenzene via styrene as reductive coupling and hydrogenated products, respectively.

Full text of DOI:10.1016/j.jorganchem.2017.02.002

Journal of Organometallic Chemistry xxx (2017) 1e7
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Cathodic reductive couplings and hydrogenations of alkenes and
alkynes catalyzed by the B12 model complex
Hisashi Shimakoshi*, Zhongli Luo, Kazuya Tomita, Yoshio Hisaeda**
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The reductive coupling and hydrogenation of alkenes were catalyzed by the B12 model complex, hep-
tamethyl cobyrinate perchlorate (1), in the presence of acid during electrolysis at ꢀ0.7 V vs. Ag/AgCl in
acetonitrile. Conjugated alkenes showed a good reactivity during electrolysis to form reduced products.
The product distributions were dependent on the substituents at the C]C bond of the alkenes. ESR spin-
trapping experiments using 5,5-dimethylpyrroline N-oxide (DMPO) revealed that the cobalt-hydrogen
complex (CoeH complex) should be formed during the electrolysis and it functioned as an intermedi-
ate for the alkene reduction. The electrolysis was also applied to an alkyne, such as phenylacetylene, to
form 2,3-diphenylbutane (racemic and meso) and ethylbenzene via styrene as reductive coupling and
hydrogenated products, respectively.
Received 29 November 2016
Received in revised form
1 February 2017
Accepted 3 February 2017
Available online xxx
Keywords:
Electroorganic synthesis
Alkene reduction
B12
© 2017 Elsevier B.V. All rights reserved.
Co-H complex
Hydrogenation
1. Introduction
with the substrate to form an alkylated complex with the cobalt-
carbon
s-bond as the reaction intermediate. The alkylated com-
Electroorganic synthesis is currently expanding into various
interdisciplinary fields because of its versatile application to mo-
lecular transformations as well as its sustainable and economic
advantage [1,2]. Electroorganic synthesis is recognized as a green
process in synthetic organic chemistry since the redox process of a
substrate occurs due to an electric current without any chemical
reagent and waste after the reaction. Furthermore, indirect elec-
trolysis using a mediator facilitates the redox process of the sub-
strate to lower the applied potential resulting in an efficient
molecular transformation [3]. Various cobalt complexes were uti-
lized as the mediator for the electroorganic synthesis. Among the
cobalt complexes, heptamethyl cobyrinate perchlorate has been
used for the B12-dependent enzyme model complex and various
plex is a key intermediate for the B12 dependent enzymatic re-
actions [9e13]. Recently, a B12 inspired unique reaction for alkene
reduction was reported by us [14]. In the reaction, the photo-
chemically produced Co(I) species of the B12 derivative could
react with a proton to form the cobalt-hydrogen complex (CoeH
complex), then the CoeH complex was considered to catalyze the
alkene reduction. Similar alkene reductions catalyzed by the B12
derivatives were also reported by the A. Fischli and W. Donk groups
using Zn and Ti(III) citrate as the reducing reagent, respectively
[15,16]. We have now developed a new electroorganic synthesis
using the B12 derivative for alkene reduction in the presence of acid.
The reactive Co(I) species should be formed by a green electro-
chemical technique. Furthermore, the direct reduction of alkenes
requires an extremely negative potential over ꢀ2 V vs. SCE in an
organic solvent [17,18], while the alkene reduction may occur at a
more positive potential in the presence of the B12 complex since the
Co(I) species of the B12 complex can be formed at the potential and
reacts with a proton to form the CoeH complex as a reactive in-
termediate for the alkene reduction. In the present paper, we report
the electrochemical reductive coupling and hydrogenation of al-
kenes catalyzed by the B12 model complex in the presence of acid as
shown in Fig. 1. Two examples of the alkyne reduction are also
reported.
B
12-mimic reactions were achieved using this B12 model complex
during electrolysis [3,4]. The dechlorination of an organic halide
and the 1,2-migration reaction of a functional group were per-
formed as model reactions of the B12-dependent enzymes, reductive
dehalogenase and methylmalonyl-CoA mutase, respectively [5e8]. In
these reactions, electrochemically-formed Co(I) species reacted
* Corresponding author. Tel.: þ81 92 802 2828; fax: þ81 92 802 2828.
** Corresponding author.
E-mail addresses: shimakoshi@mail.cstm.kyushu-u.ac.jp (H. Shimakoshi),
yhisatcm@mail.cstm.kyushu-u.ac.jp (Y. Hisaeda).
http://dx.doi.org/10.1016/j.jorganchem.2017.02.002
0022-328X/© 2017 Elsevier B.V. All rights reserved.
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H. Shimakoshi et al. / Journal of Organometallic Chemistry xxx (2017) 1e7
around ꢀ0.7 V vs. Ag/AgCl, which implies that styrene may react
with the CoeH complex and the catalytic reduction of styrene
should occur around ꢀ0.7 V vs. Ag/AgCl in the presence of 1 and
acid.
2.2. Electrochemical reduction of alkenes
Based on the CV studies, the bulk electrolysis of styrene was
carried out in the presence of TFA (pKa ¼ 12.65 in CH3CN [21]) and a
catalytic amount of 1 using a divided cell equipped with a carbon
felt working electrode and zinc counter electrode. The results are
summarized in Table 1. 2,3-Diphenylbutanes (DB) (racemic and
meso) and ethylbenzene (EB) were formed from styrene as
reductive coupling and hydrogenated products, respectively (entry
1 in Table 1). Without the catalyst 1 or acid, the reaction did not
proceed (entries 5 and 6 in Table 1). The direct reduction of styrene
occurred at ꢀ2.8 V vs. Ag/AgNO3 in CH3CN on a platinum electrode
[22]. The B12 derivative 1 significantly accelerated the styrene
reduction at the positive applied potential. It should be noted that
the conventional hydrogenation of alkenes catalyzed by the cobalt
complexes was performed using H2 gas (3e6 atm) [23]. In contrast,
the electrochemical method reported here required no H2 gas for
the reaction and the reaction proceeded under normal atmosphere
at room temperature.
Fig. 1. Electrolysis of styrene catalyzed by B12 model complex (1) in the presence of
acid.
The imine/oxime type cobalt complex (2) (Fig. 7), which is a
well-known functional model of B12 [24], showed a very low
reactivity toward the styrene reduction (entry 7 in Table 1), but
vigorous hydrogen evolution was observed during the electrolysis.
The imine/oxime type cobalt complexes are known as good proton
reduction catalysts [25,26] and the intermediate CoeH complex
should be used for hydrogen production in the imine/oxime type
cobalt complex and not used for styrene reduction during the
electrolysis. The hydrogen evolution by the B12 derivative, cobyrinic
acid, combined the use with titanium oxide as a photosensitizer,
was recently reported by us and the turnover number of the cobalt
complex for hydrogen evolution was only 1 per hour, while
reduction of the arylalkenes efficiently proceeded with the 100
turnover number per hour based on the cobalt complex [14]. These
results imply that the CoeH complex of the B12 derivative has a low
reactivity for hydrogen evolution, but has a high reactivity for
alkene reduction.
2. Results and discussion
2.1. Cyclic voltammometry study
The cyclic voltammogram (CV) of 1 was obtained in acetonitrile
using a glassy carbon working electrode. The Co(II)/Co(I) redox
couple was observed at ꢀ0.61 V vs. Ag/AgCl as shown in Fig. 2. The
addition of a styrene did not change the voltammetric pattern,
which implies that the Co(I) species of 1 does not react with sty-
rene. In contrast, the voltammetric pattern dramatically changed
with the addition of trifluoroacetic acid (TFA) as shown in Fig. 2 (red
lines). Supernucleophilic Co(I) species may react with a proton to
form the cobalt-hydrogen complex, i.e., the CoeH complex [19,20].
The subsequent addition of styrene increased the cathodic current
The products yields were dependent on the type of acid. No
reaction occurred using the relatively weak acid, acetic acid
(pKa ¼ 22.3 in CH3CN [21]), under the same conditions (entry 2 in
Table 1). Using a strong acid, p-toluensulfonic acid (p-TsOH)
(pKa ¼ 8.7 in CH3CN [21]), also decreased the product yields (entry
4 in Table 1). To allow the reaction to occur using acetic acid as a
Table 1
Electrolysis of styrene catalyzed by B12 model complexes (1 and 2).a
Entry
Catalyst
Acid (pKa in CH3CN)
Products yieldb/%
DB (Racemic and meso)
EB
1
2
3c
4
5
6
7
1
1
1
1
TFA (12.65)
AcOH (22.3)
AcOH (22.3)
p-TsOH (8.7)
TFA (12.65)
None
34, 34
0, 0
34, 34
9, 9
0
0
15
0
16
3
0
0
None
1
2
TFA (12.65)
11, 11
Trace
a
Fig. 2. CVs of 1 (1 mM) in CH3CN containing of 0.1 M n-Bu4NClO4 under N2 (solid blue
line) and in the presence of 0.05 M styrene (dotted blue line), 0.5 M TFA (solid red line),
and 0.05 M styrene and 0.5 M TFA (dotted red line). (For interpretation of the refer-
ences to colour in this figure legend, the reader is referred to the web version of this
article.)
Conditions: [cobalt complex]
¼
5
ꢁ
10ꢀ4 M; [styrene]
¼
5
ꢁ
10ꢀ2 M;
[acid] ¼ 5 ꢁ 10ꢀ1 M; [n-Bu4NClO4] ¼ 1 ꢁ 10ꢀ1 M using a carbon felt cathode at room
temperature under N2. Applied potentials were ꢀ0.7 V vs. Ag/AgCl for 3 h.
b
Yield of products was based on the initial concentration of the substrate.
Applied potential was ꢀ1.2 V vs. Ag/AgCl for 4 h, [acetic acid] ¼ 1 M.
c
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H. Shimakoshi et al. / Journal of Organometallic Chemistry xxx (2017) 1e7
3
Table 2
proton source, a more negative applied potential, ꢀ1.2 V vs. Ag/AgCl
Electrolysis of styrene derivatives catalyzed by 1.a
with twice the acid concentration was required (entry 3 in Table 1).
The reaction was also applied to various styrene derivatives
(Table 2). The selectivity of the products was dependent on the
Entry
1
Substrate
Product
Yieldb/%
63
substituents on the C]C bond. a-Methyl styrene produced a 63%
yield of 2,3-dimethyl-2,3-diphenylbutane and 11% yield of 2-
propylbenzene at ꢀ0.7 V vs. Ag/AgCl as reductive coupling and
11
45
hydrogenated products, respectively (entry 1 in Table 2). a-Phenyl
styrene afforded only a hydrogenated product, 1,1-diphenylethane,
in 45% yield (entry 2 in Table 2) under the same reaction conditions.
2
The bulky phenyl group of
a-phenyl styrene may disturb the
coupling reaction. Direct electrolysis of
a-phenyl styrene required
more negative applied potential at ꢀ2.6 V vs. SCE in DMF to form
1,1-diphenylethane [18]. Therefore, it is clear that B12 derivative 1
significantly accelerated reduction of substrate at positive applied
3c
15/15 (racemic/meso)
potential. A substituent at the
alkene decreased the substrate reactivity and the reaction of
methylstyrene (E form) or -bromostyrene (E and Z mixtures) or E
b-position on the C]C bond of the
b
-
b
52
35
and Z stilbenes required a more negative applied potential of ꢀ1.2 V
vs. Ag/AgCl. At this potential, the product distributions were
changed for
b-substituted styrenes. A hydrogenated product, 1-
4c
5c
6c
propylbenzene, was obtained in 52% yield and the coupling prod-
ucts, 2,3-diphenylhexane (racemic and meso), were obtained in
only 30% yield from
and Z stilbenes, 1,2-diphenyl ethanes were obtained in 35% and 53%
yields, respectively (entries 4 and 5 in Table 2). In the case of
b-methylstyrene (entry 3 in Table 2). From E
53
b
-
bromostyrene, the debromination was coupled with the C]C bond
reduction and the corresponding hydrogenated product, ethyl-
benzene, was obtained in 58% yield and the coupling products, 2,3-
diphenylbutanes (racemic and meso), were obtained in 10% yield
(entry 6 in Table 2). The product selectivity was different from that
of the styrene reduction (entry 1 in Table 1). The difference in the
product distributions could be caused by the difference in the
applied potential from ꢀ0.7 V vs. Ag/AgCl for styrene to ꢀ1.2 V vs.
5/5 (racemic/meso)
58
(E and Z mixture)
Ag/AgCl for b-bromostyrene. The interesting reaction occurred in a-
7
28/28 (E/Z)
bromostyrene, and 2,2-diphenyl 2-butenes (E and Z mixtures) were
obtained in 55% yield as the main products (entry 7 in Table 2). A
similar reaction occurred using the B12eTiO2 hybrid photocatalytic
system [14]. Ethyl 2-phenylacrylate was reduced to the hydroge-
nated product, ethyl 2-phenylpropionate, in 47% yield (entry 8 in
Table 2).
The scope of substrate was examined using a variety of alkenes.
The C]C bond of the unsaturated esters, alkyl acrylates (3a-3d),
was reduced to form alkyl propionates (4a-4d) in moderate yields
as shown by Eqs. (1) and (2).
17
47
8
a
Conditions: [cobalt complex] ¼ 5 ꢁ 10ꢀ4 M; [styrene] ¼ 5 ꢁ 10ꢀ2 M; [n-
Bu4NClO4] ¼ 1 ꢁ 10ꢀ1 M using a carbon felt cathode at room temperature under N2.
Applied potentials were ꢀ0.7 V vs. Ag/AgCl for 3 h.
b
Yield of products were based on the initial concentration of the substrate.
Applied potential was ꢀ1.2 V vs. Ag/AgCl.
c
(1)
In contrast, the nonconjugated alkenes, such as allylbenzene
or vinylcyclohexane, did not react under standard conditions
(electrolysis at ꢀ0.7 V vs. Ag/AgCl, TFA ¼ 0.5 M) as shown in Eq.
(2)
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H. Shimakoshi et al. / Journal of Organometallic Chemistry xxx (2017) 1e7
(3), but a small amount of 1-propylbenzene was obtained from
the allylbenzene electrolysis at ꢀ1.2 V vs. Ag/AgCl as shown by
Eq. (4).
(3)
(4)
Fig. 3. Electrolysis progress at ꢀ1.2 V vs. Ag/AgCl showing the disappearance of phe-
nylacetylene (blue) by 1 in the presence of TFA and the appearance of reduction
products (2,3-diphenylbutenes: black, ethylbenzene: red, styrene: green);
[1] ¼ 5 ꢁ 10ꢀ4 M; [phenylacetylene] ¼ 5 ꢁ 10ꢀ2 M; [TFA] ¼ 5 ꢁ 10ꢀ1 M; [n-
Bu4NClO4] ¼ 1 ꢁ 10ꢀ1 M in CH3CN under N2. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
The reaction was also applied to the alkyne group. Electrolysis of
phenylacetylene at ꢀ0.7 V vs. Ag/AgCl afforded no reduced prod-
ucts in the presence of 1 and TFA, but afforded 54% of 2,3-
diphenylbutanes (racemic and meso), 39% of ethylbenzene, and
small amount of styrene at ꢀ1.2 V vs. Ag/AgCl as shown by Eq. (5).
From the time course of the electrolysis, phenylacetylene should be
first reduced to styrene and the further reduction of styrene could
produce the 2,3-diphenylbutanes (racemic and meso) and ethyl-
benzene as shown in Fig. 3. The difference in the product distri-
From diphenylacetylene,
E
and
Z
stilbenes and 1,2-
diphenylethane were obtained as shown by Eq. (6). It was noted
that all alkenes and alkynes used here required acid in the reaction
catalyzed by 1. The controlled reaction without acid provided no
reduced product in the all substrates.
(6)
bution from that of the reduction of styrene (entry 1 in Table 1,
Based on these results, we proposed the reaction mechanism
shown in Fig. 4. The Co(I) species of B12 may react with a proton to
form the cobalt-hydrogen intermediate, Co(III)eH. It is likely that
the Co(III)eH complex provides the hydrogen radical, and the
applied potential is ꢀ0.7 V vs. Ag/AgCl) could be caused by the
applied potential. The electrolysis at
a
more negative
potential, ꢀ1.2 V vs. Ag/AgCl, increased the yield of the hydroge-
nated product similar to the electrolysis of
b
-substituted styrenes
hydrogen radical could then attack the
b-position of the alkene
(entries 3e6 in Table 2).
avoiding steric hindrance of the benzene ring in the styrene de-
rivatives to form the
a-radical intermediate. If R is H or a methyl
(5)
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5
2.3. CoeH complex detection by spin-trapping experiment in ESR
The electrolysis of styrene was carried out in the presence of a
spin-trapping reagent, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO).
ESR signals of two DMPO trapping species were observed during
the electrolysis of styrene catalyzed by 1 at ꢀ0.7 V vs. Ag/AgCl in the
presence of TFA. The ESR signal was ascribed to the hydrogen
radical trapped DMPO, DMPO-H (g
¼
2.009, AN
¼
14.9 G,
AH ¼ 19.7 G), combined with the carbon-centered radical trapped
DMPO, DMPO-C (g ¼ 2.008, AN ¼ 15.0 G, AH ¼ 21.4 G) as shown in
Fig. 6(a). The obtained ESR parameters were similar to values
already reported [27]. The DMPO-H signal disappeared using
deuterated TFA (TFA-d1) instead of TFA as shown in Fig. 6(b). These
results suggested that the CoeH complex existed during the elec-
trolysis and provided a hydrogen radical. These ESR experimentvs
supported our proposal that the Co(III)eH complex of the B12 de-
rivative could have a hydrogen radical character and catalyze the
alkene and the alkyne reduction.
3. Conclusion
In summary, we have developed for the first time the electrol-
ysis of alkenes and alkynes by the B12 model complex in the pres-
ence of acid. The reductive coupling and hydrogenation of alkenes
proceeded at a positive applied potential compared to that of the
direct reduction of alkenes. The electrolysis was applied to a variety
of alkenes and the conjugated alkenes especially afforded a mod-
erate yield of products. The B12 derivative showed a good catalysis
for the reaction, and the CoeH species of the B12 complex was
proposed as an intermediate for the reaction. The bioinspired
electroorganic synthesis shown here can be applied to the sus-
tainable production of fine chemicals with the possible expense of
energy economy. Ongoing work in our laboratory is focused on the
application of this chemistry to other organic syntheses.
Fig. 4. Proposed mechanism for electrolysis of styrene catalyzed by 1 in the presence
of acid.
group, the radical coupling preferentially occurred to form a
coupling product. The hydrogenated product, ethylbenzene, could
be formed by three pathways from the substrate radical. Dispro-
portionation, hydrogen radical abstraction, and a second reaction
with the CoeH complex from the substrate radical are possible for
the ethylbenzene formation mechanism. If R is an electron-
withdrawing group, further reduction of the radical by the cath-
ode to form the carbanion intermediate is shown in Fig. 5. Pro-
tonation should afford a hydrogenated product when R is CO2Et.
When R is Br, the carbanion may lead to a carbene with the elim-
ination of the bromide ion [14], and the short-lived carbene could
react with the carbanion to form 2,3-diphynylbutane (E and Z).
4. Experimental section
4.1. Reagents and materials
The solvents and chemicals used in the syntheses were of re-
agent grade and used without further purification. The acetonitrile
solvent for the electrolysis was a special dry grade purchased from
Kanto Chemicals. Tetra-n-butylammonium perchlorate (n-Bu4N-
ClO4) was purchased from Nakalai Chemicals (special grade) and
dried at room temperature under vacuum before use. The prepar-
ative TLC (silica gel 60 F254) was purchased from Merck. Hepta-
methyl cobyrinate perchlorate (1) (Fig. 1) was synthesized by a
previously reported method [28,29]. The cobalt complex, [Co(III)
{(C2C3)(DO)(DOH)pn}Br2], (C2C3)(DO)(DOH)pn is 4,10-dipropyl-
5,9-diazatrideca-4,9-diene-3,10-dione dioxime), (2) (Fig. 1) was
prepared according to the literature [24].
4.2. Apparatus and characterization
The NMR spectra were recorded by a Bruker Avance 500 spec-
trometer at the Center of Advanced Instrumental Analysis of
Kyushu University. The UV-vis absorption spectra were measured
by a Hitachi U-3300 spectrophotometer at room temperature. The
MALDI-TOF mass spectra were obtained by a Bruker autoflex II
using 6-aza-2-thiothymine as the matrix. The GC-mass spectra
were obtained using a Shimadzu GC-QP5050A equipped with a
J&W Scientific DB-1 column (length 30 m; ID 0.25 mm, film
0.25
mm). Hydrogen gas was analyzed by a Shimadzu 14-B gas
chromatograph equipped with a SHINCARBON ST packed column
(Shimadzu GLC). The cyclic voltammograms (CV) were obtained
Fig. 5. Proposed mechanism for electrolysis of styrene derivatives with electron
withdrawing group at a-position catalyzed by 1 in the presence of acid.
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H. Shimakoshi et al. / Journal of Organometallic Chemistry xxx (2017) 1e7
Fig. 6. ESR spectra of 1 during the electrolysis of styrene at ꢀ0.7 V vs. Ag/AgCl in CH3CN under N2; [1] ¼ 5 ꢁ 10ꢀ4 M; [styrene] ¼ 5 ꢁ 10ꢀ2 M; [TFA] ¼ 5 ꢁ 10ꢀ1 M;
[DMPO] ¼ 5 ꢁ 10ꢀ1 M; [n-Bu4NClO4] ¼ 1 ꢁ 10ꢀ1 M in the presence of TFA (a) and in the presence of TFA-d1 (b).
J ¼ 7.2Hz, 6H, CH3), 1.28e1.44 (m, 4H, CH2), 2.60e2.62 (m, 2H, CH),
7.21e7.36 (m, 10H, Ph), 13C NMR (CDCl3,
d/ppm), 12.55, 27.80, 54.65,
126.37, 128.58, 128.83, 144.95.
4.3.2. DH (racemic)
Colorless liquid. GC-MS, Mþ ¼ 238; 1H NMR (CDCl3,
d/ppm), 0.77
(t, J ¼ 7.6Hz, 6H, CH3), 1.54e1.66 (m, 4H, CH2), 2.74e2.76 (m, 2H,
CH), 6.88e7.21 (m, 10H, Ph), 13C NMR (CDCl3,
d/ppm), 12.76, 26.24,
Fig. 7. Structure of the imine/oxime type cobalt complex, [Co(III){(C2C3)(DO)(DOH)pn}
Br2].
53.68, 126.00, 127.83, 129.46, 143.46.
4.3.3. 1,1-Diphenylethane
Colorless liquid. GC-MS, Mþ ¼ 182; 1H NMR (CDCl3,
d/ppm), 1.74
using a BAS CV 50W electrochemical analyzer. A three-electrode
cell equipped with a 1.6-mm diameter glassy carbon rod and
platinum wire as the working and counter electrodes, respectively,
was used. An Ag/AgCl (3.0 M NaCl) electrode served as the refer-
ence. The E1/2 value of the ferrocene-ferrocenium (Fc/Fcþ) was
0.40 V vs. Ag/AgCl with this setup.
(d, J ¼ 7.3Hz, 3H, CH3), 4.27 (q, J ¼ 7.3Hz, 1H, CH), 7.25e7.39 (m,10H,
Ph), 13C NMR (CDCl3,
146.92.
d/ppm), 22.44, 45.33, 126.58, 128.19, 128.92,
4.3.4. Butyl propionate (4a)
Colorless liquid. GC-MS, Mþ ¼ 130; 1H NMR (CDCl3,
d/ppm), 0.95
(t, J ¼ 7.3Hz, 3H, CH3), 1.16 (t, J ¼ 7.6Hz, 3H, CH3), 1.38e1.41 (m, 2H,
4.3. General bulk electrolysis
CH2), 1.60e1.66 (m, 2H, CH2), 2.35 (q, J ¼ 7.6Hz, 2H, CH2), 4.09 (t,
J ¼ 7.0Hz, 2H, CH2), 13C NMR (CDCl3,
d/ppm), 9.45, 13.98, 19.47,
The controlled-potential electrolysis of the alkene was carried
out in a divided cell equipped with a carbon felt cathode and a zinc
plate anode (1 ꢁ 3 cm2) at ꢀ0.7 V vs. Ag/AgCl in the presence of 1 at
room temperature in 0.1 M n-Bu4NClO4 containing acetonitrile. The
applied potential between the working and reference electrodes
during the electrolysis was maintained constant using a Hokuto
Denko HA BF-501A potentiostat, and the electrical quantity was
also recorded by it. The concentrations of the catalyst and substrate
were 5.0 ꢁ 10ꢀ4 M and 5.0 ꢁ 10ꢀ2 M, respectively. After the elec-
trolysis, the electrolyte solution was passed through silica gel with
the CHCl3 eluent, then analyzed by GC-MS. Authentic samples of
the products from the catalytic reactions (Tables 1 and 2) except for
those listed below were purchased from Aldrich or Tokyo Kasei
Kogyo (TCI). The 2,3-diphenylhexanes (racemic and meso) (DH)
were synthesized by reported methods [30], and the mixture of the
racemic and meso compound was separated by preparative TLC
using hexane as the eluent. 1,1-Diphenylethane, butyl propionate
(4a), and octyl propionate (4c) were synthesized by hydrogenation
of 1,1-diphenylethylene, butyl acrylate (3a), and octyl acrylate (3c)
using Pd/C under 1atm H2, respectively. Ethyl 2-phenylacrylate was
synthesized by the acid-catalyzed esterification of 2-phenylacrylic
acid in ethanol.
27.94, 31.07, 64.47, 174.86.
4.3.5. Octyl propionate (4c)
Colorless liquid. GC-MS, Mþ ¼ 186; 1H NMR (CDCl3,
d/ppm),
0.89e0.92 (t, J ¼ 7.3Hz, 3H, CH3), 1.15e1.18 (t, J ¼ 7.6Hz, 3H, CH3),
1.30e1.31 (m, 10H, CH2), 1.63e1.66 (m, 2H, CH2), 2.33e2.37 (q,
J ¼ 7.6Hz, 2H, CH2), 4.07e4.10 (t, J ¼ 6.9Hz, 2H, CH2), 13C NMR
(CDCl3, d/ppm), 9.54, 14.44, 23.01, 26.31, 28.02, 29.05, 29.55, 29.60,
32.16, 64.87, 174.97.
4.3.6. Ethyl 2-phenylacrylate
Colorless liquid. GC-MS, Mþ ¼ 176; 1H NMR (CDCl3,
d/ppm), 1.33
(t, J ¼ 7.2Hz, 3H, CH3), 4.3 (q, J ¼ 7.2Hz, 2H, CH2), 5.89 (s, 1H, CH),
6.35 (s, 1H, CH), 7.34e7.41 (m, 5H, Ph), 13C NMR (CDCl3,
d/ppm),
14.62, 61.51, 126.86, 128.48, 128.51, 128.69, 137.19, 141.99, 167.23.
4.4. Spin-trapping experiment by ESR
The ESR spectra were obtained using a Bruker EMX-Plus X-band
spectrometer at room temperature. The ESR spectra for the DMPO
spin-trapping products were observed during the electrolysis of
styrene (5.0 ꢁ 10ꢀ2 M) at ꢀ0.7 V vs. Ag/AgCl using a carbon felt
electrode in the presence of DMPO (5 ꢁ 10ꢀ1 M), 1 (5.0 ꢁ 10ꢀ4 M)
and TFA (5.0 ꢁ 10ꢀ1 M) in dry acetonitrile under nitrogen. The
settings for the ESR measurements were a frequency of 9.87 GHz,
4.3.1. DH (meso)
White solid. GC-MS, Mþ ¼ 238; 1H NMR (CDCl3,
d/ppm), 0.55 (t,
Please cite this article in press as: H. Shimakoshi, et al., Journal of Organometallic Chemistry (2017), http://dx.doi.org/10.1016/
j.jorganchem.2017.02.002

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