Electron transfer-induced four-membered cyclic intermediate formation: Olefin cross-coupling vs. olefin cross-metathesis

Article abstract of DOI:10.1016/j.electacta.2010.10.042

An electron transfer-induced four-membered cyclic intermediate, formed between a radical cation of an enol ether and an unactivated olefin, played a key role in the pathway toward either cross-coupling or cross-metathesis. The presence of an alkoxy group on the phenyl ring of the olefin entirely determined the synthetic outcome of the reaction, which mirrored the efficiency of the intramolecular electron transfer.

Full text of DOI:10.1016/j.electacta.2010.10.042

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Electrochimica Acta 56 (2011) 1037–1042
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electron transfer-induced four-membered cyclic intermediate formation:
Olefin cross-coupling vs. olefin cross-metathesis
Yohei Okada, Kazuhiro Chiba∗
Department of Applied Life 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 16 August 2010
Received in revised form 8 October 2010
Accepted 15 October 2010
Available online 21 October 2010
An electron transfer-induced four-membered cyclic intermediate, formed between a radical cation of an
enol ether and an unactivated olefin, played a key role in the pathway toward either cross-coupling or
cross-metathesis. The presence of an alkoxy group on the phenyl ring of the olefin entirely determined the
synthetic outcome of the reaction, which mirrored the efficiency of the intramolecular electron transfer.
© 2010 Elsevier Ltd. All rights reserved.
Keywords:
Electron transfer-induced
Four-membered cyclic intermediate
Radical cation
Olefin cross-coupling
Olefin cross-metathesis
1. Introduction
fer from the alkoxyphenyl group was essential for the formation
of the cyclobutane ring. The alkoxyphenyl group functioned as an
Since an effective catalyst for olefin cross-metathesis was first
developed by Grubbs, this reaction has been employed extensively
to construct a wide variety of carbogenic skeletons in synthetic
chemistry [1–4]. The investigation and modification of novel cat-
alysts for olefin cross-metatheses still remain important areas of
inquiry [5–8]. However, the theoretical study of the exchange of
substituents between different olefins involving a key intermedi-
ate consisting of a four-membered ring coordinated to a transition
metal has also been of interest [9,10].
To date, electrochemical processes have proven to be practi-
cal methods for triggering reactions based on olefins—intra- and
intermolecular—because of their ability to reverse the polarity of
the olefins. Several radical ion-based olefin cross-couplings have
also been reported [11–14]. Furthermore, the mechanistic details of
olefin cross-couplings have been described [15–22]. In this context,
we previously developed an electron transfer-induced, intermolec-
ular, olefin cross-coupling reaction (Scheme 1) [23–27]. Enol ethers
were anodically oxidized to generate the corresponding radical
cations, which were trapped by the unactivated olefins to form
cyclobutane ring frameworks. We also demonstrated that olefin
cross-metathesis between enol ethers and unactivated olefins
could be induced through an electrode process (Scheme 2) [28].
For a cross-coupling to occur, an intramolecular electron trans-
electron-donating group, and the cyclobutyl moiety tethered by
a methylene unit acted as an electron-accepting group. Consid-
ering the olefin cross-metathesis, a lack of this alkoxy group was
required.
Four-membered cyclic intermediates are thought to be involved
in these reactions; however, the mechanistic details are ambiguous.
In particular, the presence of radical cations, which would offer
clear-cut evidence for the formation of the four-membered cyclic
intermediate, has yet to be detected (Scheme 3). With these results
and theories in hand, we sought to study the mechanistic aspects
of electron transfer-induced four-membered cyclic intermediates.
2. Experimental
2.1. General information
1H and 13C NMR spectra were recorded in CDCl3 with TMS as
the internal standard on JEOL alpha 600 (600 MHz). The follow-
ing abbreviations were used to explain multiplicities: s, singlet;
d, doublet; t, triplet; q, quartet; quint, quintet; sext, sextet; sept,
septet; and m, multiplet. TLC analysis was performed with Merk
Silica Gel 60 F254 plates, and detection was achieved by UV absorp-
tion (254 nm) and by charring the plates after spraying them with
12 molybdo(VI) phosphoric acid n-hydrate in 95% ethanol. Column
chromatography was performed with silica gel (0.04–0.063 mm).
All reagents and solvents were purchased from Kanto Chemical,
Tokyo Kasei Kogyo, Aldrich, and Wako and used as received. Redox
∗
Corresponding author. Tel.: +81 42 367 5667; fax: +81 42 360 7167.
E-mail address: chiba@cc.tuat.ac.jp (K. Chiba).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.10.042

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Y. Okada, K. Chiba / Electrochimica Acta 56 (2011) 1037–1042
Scheme 1. Electron transfer-induced olefin cross-coupling.
Scheme 2. Electron transfer-induced olefin cross-metathesis.
potentials were measured by cyclic voltammetry using a glassy
carbon-working electrode, Pt wire counter electrode, and Ag/AgCl
reference electrode.
3. Spectra information
3.1. 1-Methoxy-4-((2-methoxy-3-octylcyclobutyl)methyl)
benzene 3a (all trans, major)
1H NMR (CDCl3, 600 MHz) ı: 7.07 (2H, d, J = 8.1 Hz), 6.81 (2H,
d, J = 8.1 Hz), 3.78 (3H, s), 3.23 (3H, s), 3.15 (1H, t, J = 6.6 Hz),
2.86 (1H, dd, J = 13.9, 5.9 Hz), 2.57 (1H, dd, J = 13.9, 8.8 Hz),
2.27–2.16 (1H, m), 1.99–1.89 (2H, m), 1.37–1.12 (14H, m), 0.88
(3H, t, J = 6.6 Hz), 0.83–0.74 (1H, m); (13DCl3, 150 MHz) ı: 157.7,
132.7, 129.4, 113.6, 85.9, 56.5, 55.2, 41.9, 40.6, 40.3, 35.2, 31.9,
29.7 29.6, 29.3, 27.3, 25.4, 22.7, 14.1; IR (NaCl, cm−1): 2924,
2853, 1612, 1513, 1464, 1247, 1040, 821; MS (rel. int., m/z):
318(M+, 1), 178(68), 163(7), 148(100), 121(27), 71(33); HRMS
(m/z) calc. for C21H34O2: 318.2559 (M−CH3O: 287.2445); found:
287.2438.
2.2. Olefin cross-coupling reactions
Olefins possessing an alkoxyphenyl group (4.0 mmol)
and aliphatic enol ethers (0.20 mmol) were added to 1.0 M
LiClO4/CH3NO2 (20 mL). The reaction cells were capped with
septa equipped with carbon felt anodes (20 mm × 20 mm), carbon
felt cathodes (20 mm × 20 mm), and Ag/AgCl reference elec-
trodes. Electrolysis was performed at 1.2 V (vs. Ag/AgCl). Once
the reactions were complete, the reaction mixtures were poured
into EtOAc, and the EtOAc solutions were successively washed
with brine and dried over anhydrous MgSO4. After filtration and
evaporation under reduced pressure, the residues were purified
by silica gel column chromatography using n-hexane and EtOAc
(isocratic, 1–5% EtOAc in n-hexane) to give the cycloadducts. The
product yields and diastereomer ratios were determined by NMR
and GC–MS (JEOL JMS-K9).
3.2. 1-Methoxy-4-(2-(2-methoxy-3-octylcyclobutyl)ethyl)
benzene 21a (all trans, major)
1H NMR (CDCl3, 600 MHz) ı: 7.09 (2H, d, J = 8.1 Hz), 6.82 (2H,
d, J = 8.1 Hz), 3.79 (3H, s), 3.30 (3H, s), 3.10 (1H, t, J = 7.3 Hz),
2.58–2.44 (2H, m), 2.02 (1H, quint, J = 8.8 Hz), 1.99–1.92 (2H, m),
1.92–1.82 (1H, m), 1.65–1.55 (1H, m), 1.38–1.13 (14H, m), 0.88
(3H, t, J = 7.3 Hz), 0.72 (1H, q, J = 8.8 Hz); 13NMR (CDCl3, 150 MHz)
ı: 157.6, 134.6, 129.2, 113.7, 86.5, 56.4, 55.2, 40.6, 40.0, 37.4,
35.3, 32.8, 31.9, 29.7, 29.6, 29.3, 27.4, 25.4, 22.7, 14.1; IR (NaCl,
cm−1): 2925, 2852, 1613, 1513, 1463, 1246, 1039, 824; MS (rel. int.,
m/z): 332(M+, 1), 192(12), 162(10), 147(1), 121(100), 71(25); HRMS
(m/z) calc. for C22H36O2: 332.2715 (M−CH3O: 301.2602); found:
301.2619.
2.3. Olefin cross-metathesis reactions
The olefins (4.0 mmol) and aliphatic enol ethers (0.20 mmol)
were added to 1.0 M LiClO4/CH3NO2 (20 mL). The reaction cells
were capped with septa equipped with carbon felt anodes
(20 mm × 20 mm), carbon felt cathodes (20 mm × 20 mm), and
Ag/AgCl reference electrodes. Electrolysis was performed at 1.2 V
(vs. Ag/AgCl). After the reactions were completed, the product
yields were determined by GC–MS.
CH2
1.2 V (vs. Ag/AgCl)
1.1 F/mol
OMe
CH2
+
R
CH2
1.0 M LPC/NM
(+)CF-CF(-)
R
MeO
R
MeO
?
Scheme 3. Possible reaction mechanism for the electron transfer-induced olefin cross-metathesis.

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Y. Okada, K. Chiba / Electrochimica Acta 56 (2011) 1037–1042
1039
MeO
C8H17
OMe
MeO
1.2 V (vs. Ag/AgCl)
OMe
0.5 F/mol
C8H17
+
1.0 M LPC/NM
(+)CF-CF(-)
2
3a-c
1a-b
20 mol equiv.
79%
dr 2:1:1
Scheme 4. Electron transfer-induced olefin cross-coupling between 4-allylanisole (2) and 1-methoxydec-1-ene (1a-b).
Scheme 6. Electron transfer-induced olefin cross-metathesis between allylcyclo-
hexane (6) and 1-methoxydec-1-ene (1a-b).
Scheme 5. Electron transfer-induced olefin cross-metathesis between allylbenzene
(4) and 1-methoxydec-1-ene (1a-b).
mediates and the formation of subsequent olefin cross-metathesis
products.
3.3. 1-Methoxy-4-(3-(2-methoxy-3-octylcyclobutyl)propyl)
benzene 23a (all trans, major)
Next, we became interested in investigating the role of the
phenyl ring in this system. The radical cation of 1a-b was anodically
generated in the presence of an excess amount of 4-phenyl-1-
butene (16), where the phenyl group was expanded one alkyl
group from the four-membered cyclic intermediate compared
to 4 and gave 5 in good yield (Scheme 8). Notably, the pre-
sumed thermodynamic effect of the phenyl group was completely
quenched when the tether to the olefin was only two methylenes
long.
1H NMR (CDCl3, 600 MHz) ı: 7.08 (2H, d, J = 8.1 Hz), 6.82 (2H,
d, J = 8.1 Hz), 3.79 (3H, s), 3.29 (3H, s), 3.05 (1H, t, J = 6.6 Hz),
2.58–2.47 (2H, m), 2.00 (1H, quint, J = 8.8 Hz), 1.98–1.88 (2H, m),
1.64–1.46 (4H, m), 1.39–1.14 (14H, m), 0.88 (3H, t, J = 6.6 Hz), 0.67
(1H, q, J = 9.5 Hz); (13Cl3, 150 MHz) ı: 157.6, 134.8, 129.2, 113.6,
86.5, 56.4, 55.2, 40.6, 40.3, 35.2, 35.0, 34.7, 31.9, 29.7, 29.6, 29.5,
29.3, 27.4, 25.4, 22.7, 14.1; IR (NaCl, cm−1): 2925, 2852, 1614,
1514, 1462, 1247, 1040, 827; MS (rel. int., m/z): 206(53), 174(56),
147(18), 134(100), 121(69), 71(68); HRMS (m/z) calc. for C23H38O2:
346.2872 (M−CH3O: 315.2758); found: 315.2756.
A four-membered cyclic intermediate is thought to play a role in
olefin cross-metathesis; however, little evidence for the formation
of a counterpart radical cation 17• has been detected (Scheme 9).
+
We, thus, turned our attention to the mechanistic aspects of elec-
tron transfer-induced olefin cross-metatheses. For this purpose,
(4-methoxybut-3-en-1-yl)benzene (17a-b), which was expected to
form from a one-electron reduction of 17•+, was prepared to com-
pare with the products from the oxidation of 1a-b [28]. With 17a-b
independently synthesized, we found the anodic oxidation of 1a-b
in the presence of excess 16 gave 5 in good yield, while also form-
ing 17a-b. This result clearly suggests that a four-membered cyclic
intermediate plays a key role in the reaction. However, it should be
4. Results and discussion
The present work began with the preparation of 1-methoxydec-
1-ene (1a-b) [28]. The radical cation of 1a-b was effectively trapped
by an excess of 4-allylanisole (2) to afford the corresponding
[2 + 2] cycloadduct 3a-c in good yield as a diastereomeric mix-
ture (Scheme 4). In contrast, when an excess of allylbenzene (4)
was used, 1a-b was anodically oxidized to give dec-1-ene (5),
an olefin cross-metathesis product, in moderate yield (several
other unidentifiable products were also formed; Scheme 5). The
oxidation potential of 1a-b (Epox = 1.33 V vs. Ag/AgCl) was lower
than 2 (Epox = 1.51 V vs. Ag/AgCl) and 4 (Epox = 2.19 V vs. Ag/AgCl),
enabling the selective anodic oxidation of 1a-b in the presence
of excess 2 or 4. In the reaction with 2, the presence of the
electron-donating alkoxy on the phenyl group provided a ther-
modynamic effect by increasing the electron donor character of
2 and rendering an effective intramolecular electron transfer. For
4, the phenyl group’s electron donor ability was hindered and
led to a complicated reaction. The olefin cross-metathesis became
the dominant process when an anodic oxidation of 1a-b was
attempted in the presence of excess allylcyclohexane (6), which
is incapable of forming a phenyl radical cation; 5 was obtained in
a good yield with high chemoselectivity (Scheme 6). The use of
excess non-1-ene (7) instead of 6 also gave 5 in good yield, sug-
gesting steric hindrance is not a concern when a terminal olefin
is used (Scheme 7). On the other hand, the yields of the olefin
cross-metatheses with varying internal olefins were significantly
lower (Table 1). These results suggest that the substitutions on
the olefins disfavor the formation of four-membered cyclic inter-
noted that 17• could partially be trapped by 16, probably repro-
+
ducing 16 and 17• with some degree of decomposition during the
+
reaction.
In the absence of an olefin nucleophile, the oxidative cleav-
age of 17a-b resulted in the formation of 3-phenylpropanal (18)
Scheme 7. Electron transfer-induced olefin cross-metathesis between non-1-ene
(7) and 1-methoxydec-1-ene (1a-b).
Scheme 8. Electron transfer-induced olefin cross-metathesis between 4-phenyl-1-
butene (16) and 1-methoxydec-1-ene (1a-b).

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Y. Okada, K. Chiba / Electrochimica Acta 56 (2011) 1037–1042
Table 1
Electron transfer-induced olefin cross-metathesis between olefins 8–12 and 1-methoxydec-1-ene (1a-b).
.
Entry
1
Olefin
Producta
2
3
4
5
a
Determined by GC–MS.
Scheme 9. Possible reaction mechanism of the electron transfer-induced olefin cross-metathesis between 4-phenyl-1-butene (16) and 1-methoxydec-1-ene (1a-b).
through electrolysis (Scheme 10). A possible reaction mechanism
for the formation of 18 is depicted below (Scheme 11) [29,30]. Also
of note, a small amount of nonanal (19), the oxidative cleavage
product of 1a-b, was detected in the product mixtures of all elec-
tron transfer-induced olefin cross-metatheses (Scheme 12). When
monitoring the application of electricity, the yields of 5, 17a-b,
Scheme 10. Oxidative cleavage of (4-methoxybut-3-en-1-yl)benzene (17a-b).
18, and 19 were clearly detected for each stage (Fig. 1). Based on
these results, there looked to be two pathways for 17•+, gener-
Scheme 11. Possible reaction mechanism for the oxidative cleavage of (4-methoxybut-3-en-1-yl)benzene (17a-b).