Cyanoazulene-based multistage redox systems prepared from vinylcyclopropanecarbonitrile and cyclopentenone via divinylcyclopropane- rearrangement approach

Article abstract of DOI:10.1246/cl.131149

A series of 4-cyanoazulene derivatives 15 were synthesized from 4-cyanohexahydroazulen-1-one, which was efficiently obtained by the annulation of the seven-membered ring on the cyclopentenone skeleton via the divinylcyclopropane rearrangement. This synthetic protocol is also effective in preparing 1,1 bi(4-cyanoazulene) (6) and its p-phenylene-extended derivative 7, which undergo multistage redox reactions exhibiting electrochemical amphotericity.

Full text of DOI:10.1246/cl.131149

Received: December 9, 2013 | Accepted: January 7, 2014 | Web Released: January 11, 2014
CL-131149
Cyanoazulene-based Multistage Redox Systems Prepared from Vinylcyclopropanecarbonitrile
and Cyclopentenone via Divinylcyclopropane-rearrangement Approach
Keiji Tanino,* Takumasa Yamada, Fumihiko Yoshimura, and Takanori Suzuki*
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
(E-mail: tak@mail.sci.hokudai.ac.jp)
A series of 4-cyanoazulene derivatives 1-5 were synthesized
from 4-cyanohexahydroazulen-1-one, which was efficiently
obtained by the annulation of the seven-membered ring on the
cyclopentenone skeleton via the divinylcyclopropane rearrange-
ment. This synthetic protocol is also effective in preparing 1,1¤-
bi(4-cyanoazulene) (6) and its p-phenylene-extended derivative
7, which undergo multistage redox reactions exhibiting electro-
chemical amphotericity.
e−
e−
e−
e−
4
3
5
7
6
2
1
8
Azulene is a representative non-alternant and non-benzenoid
aromatic hydrocarbon with unique electronic properties.1 The
polar nature as well as the high electron-donating ability2 would
be the reasons for the recent attention on azulene derivatives
in terms of their potential use as NLO materials,3 conducting
polymers,4 redox-active liquid crystals,5 and dye-sensitized
solar cells.6 Besides the facile one-electron oxidation reaction,
azulenes could also undergo one-electron reduction (Scheme 1),
especially when attached with electron-withdrawing groups such
as a cyano group.7 In fact, 2,4,6,8-tetracyanoazulene has been
known as a strong electron acceptor, affording a highly
conducting CT complex and a stable anion-radical salt.8 Still,
there have been only a limited number of reports on the
properties of cyanoazulenes because of less accessibility9 of
these interesting electron acceptors.
Scheme 1. Redox scheme of azulene.
CN
CN
R
n
1
2
: R = SMe
: R = OSi(i-Pr)3
3 : R = OAc
4 : R = Me
5 : R = Ph
6 : n = 0
7 : n = 1
CN
Chart 1.
In this connection, we studied a new synthetic approach
toward a series of 4-cyanoazulenes 1-5 (Chart 1), where the key
step is the annulation of a seven-membered ring to a five-
membered ring10 via the divinylcyclopropane rearrangement to
furnish the hydroazulene framework. This versatile protocol
allowed us to synthesize 1,1¤-bi(4-cyanoazulene) (6) and its
p-phenylene-extended derivative 7, which undergo reversible
two-electron reduction to the corresponding dianions. Further-
more, electrochemical amphotericity was indicated by the low
oxidation potentials of 4-cyanoazulenes. The details are shown
herein.
Based on our recent success in the efficient syntheses of 2-
cyano-1,4-cycloheptadiene derivatives (Scheme 2),11 we envis-
aged that the hydroazulene skeleton would be constructed by
a thermal rearrangement of divinylcyclopropane having a
cyclopentene moiety. Thus, addition of a THF solution of
α-cyano carbanion generated from 2-vinylcyclopropanecarbo-
nitrile (A) and BuLi to 3-ethoxy-2-cyclopenten-1-one afforded
divinylcyclopropane B, which was then converted to 4-cyano-
1,2,3,5,8,8a-hexahydroazulen-1-one (C)12 upon heating at
150 °C in 1,2,4-trichlorobenzene (TCB) for 3 h (y. 89% over
two steps) (Scheme 3). By modifying the ketone functionality,
cyanohydroazulenone C could be converted not only to 4-cyano-
1-(methylsulfanyl)-2,3,5,8-tetrahydroazulene (H41) but also to
the corresponding silyl enol ether H42 and enol acetate H43.
These tetrahydrocyanoazulenes were transformed into 4-cyano-
CN
CN
CN
R1
R2
R1
R2
A
Scheme 2. Synthesis of 2-cyano-1,4-cycloheptadienes.
CN
CN
OEt
/ BuLi
THF
A
TCB
150 °C
O
–78 °C
O
O
B
C
(y. 89%, 2 steps)
Scheme 3. Preparation of 4-cyanohexahydroazulen-1-one.
azulenes 1-3,12 respectively, upon aromatization using p-
chloranil or DDQ (Scheme 4).13
Then, from azulenone C, another useful synthon D was
derived;12 D was transformed into 1-methyl-(4)12 and 1-phenyl-
4-cyanoazulene (5)12 over two steps, including the Negishi and
Suzuki-Miyaura coupling reactions, respectively, and succes-
sive aromatization13 (Scheme 5). Similarly, the p-phenylene-
connected azulene dyad 712 was obtained from triflate D by
Chem. Lett. 2014, 43, 607–609 | doi:10.1246/cl.131149
© 2014 The Chemical Society of Japan | 607

CN
Table 1. Redox potentials of 1, 6, and 7 measured by cyclic
voltammetry in CH2Cl2
a
b
Eox
Eox
Ered
Ered
2
Esum
2
1
1
C
Thioether
Dimer
Dyad
1
6
7
®
+1.15
¹0.98
¹0.97 ¹1.18 2.09
1.01d
®
2.13
O
+1.37c +1.12
®
+1.33c,d
®
2.34
TMSSMe
(y. 72%)
Ac2O
(y. 84%)
(i-Pr)3SiOTf
(y. 90%)
aE/V vs. SCE in CH2Cl2 containing 0.1 M Bu4NBF4 as a
supporting electrolyte. Pt electrode, scan rate 100 mV s
bEsum = Eox ¹ Ered
estimated as Epeak ¹ 0.03. Two-electron processes.
¹1
.
CN
CN
CN
.
1
cIrreversible waves. Eox values were
1
d
a)
CN
CN
CN
MeS
AcO
(i-Pr)3SiO
H41
H42
H43
p-chloranil
(y. 37%)
DDQ
(y. 43%)
p-chloranil
(y. 62%)
e−
e−
1
2
3
CN
CN
6•− (Form A)
e−
CN
6•−
Scheme 4. Preparation of 4-cyanoazulenes 1-3.
6
(Form B)
CN
e−
CN
CN
Comins' reagent
(y. 96%)
1,4-C6H4[B(OH)2]2/Pd(0)
C
D
CN
TfO
Me2Zn/Pd(0)
(y. 72%)
PhB(OH)2/Pd(0)
(y. 84%)
CN
62−
CN
62−
(Form A)
(Form B)
CN
CN
CN
b)
CN
CN
Me
Ph
H44
H45
DDQ
CN H87
DDQ
Pd/C
(y. 36%)
e−−
e−−
e−−
(y. 77%
2 steps)
(y. 86%)
4
5
7
e−−
Scheme 5. Preparation of 4-cyanoazulenes 4, 5, and dyad 7.
CN
72−
CN
72−
CN
7
(Form A)
(Form B)
CN
CN
(Bu3Sn)2Cu(CN)Li2
(y. 39%)
D /Pd(0), CuI
Scheme 7. Redox schemes of bi(4-cyanoazulene)-based ac-
D
ceptors.
(y. 52%)
E
Bu3Sn
Among the newly prepared compounds, azulene dimer 6
and dyad 7 are especially of interest from the viewpoints of
spectral and redox properties modified by electronic communi-
cation between the two azulene chromophores. The broadened
first band (S0 ¼ S1) of biazulene 6 in CH2Cl2 [-max: 693 nm
(log ¾ 2.76)] is red-shifted compared to that of 4-cyanoazulene
(8)9a [630 (2.93)] (Figure S1).16 The second band (S0 ¼ S2) of
6 [410 (4.00)] is also red-shifted [8: 361 (3.47)], indicating
effective conjugation between the two azulene chromophores
in 6. In the case of 7 with a p-phenylene spacer, interaction
between the two chromophores is less prominent. Although
the second band of 7 [397 (3.98)] is red-shifted compared to
1-phenyl derivative 5 [373 (3.74)], the first bands are similar to
each other [661 (2.55) for 5; 656 (2.69) for 7] (Figure S2).16
6
DDQ
(y. 13%)
H86
CN
Scheme 6. Preparation of 4-cyanoazulene dimer 6.
using diboronic acid in the cross-coupling step. To prepare 1,1¤-
bi(4-cyanoazulene) (6),14 the Stille coupling was adopted. Thus,
triflate D was first converted to organostannane E,12,15 which
was coupled with D (Scheme 6). Successful syntheses of 1-7
indicate the usefulness of the present synthetic protocol toward
4-cyanoazulene families.
608 | Chem. Lett. 2014, 43, 607–609 | doi:10.1246/cl.131149
© 2014 The Chemical Society of Japan

From the voltammetric analyses in CH2Cl2,17 we found
that 6 and 7 undergo reversible two-electron reduction to the
corresponding dianionic species (Table 1). Thus, biazulene 6
showed two pairs of well-separated waves, showing that the
spin and negative charge in 6•¹ are delocalized over the two
azulene moieties, as shown by Form B (Scheme 7a). The small
difference (0.21 V) between Ered and Ered indicates a reduced
Imafuku, J. Am. Chem. Soc. 2003, 125, 1669. b) S. Ito, M.
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Azulenoquinones are another class of related compounds
exhibiting reversible one-electron reduction: T. Nozoe, H.
Takeshita, Bull. Chem. Soc. Jpn. 1996, 69, 1149.
S. Schmitt, M. Baumgarten, J. Simon, K. Hafner, Angew.
Chem., Int. Ed. 1998, 37, 1077.
6
7
1
2
on-site Coulombic repulsion between negative charges in 62¹. In
contrast, p-phenylene-extended derivative 7 undergoes the one-
wave-two-electron-reduction process. This observation can be
rationalized by the smaller Coulombic repulsion in 72¹ and/or
less communication of two azulene parts in 7/7•¹/72¹. Thus,
a predominant contribution of Form A is suggested for dianion
72¹ because contribution from Form B with the p-quinoid
structure would be marginal (Scheme 7b).
8
9
a) S. Hünig, K. Hafner, B. Ort, M. Müller, Liebigs Ann.
Chem. 1986, 1222. b) M. Mąkosza, M. Kędziorek, S.
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Another interesting point is that 6 and 7 undergo one-
electron oxidation as well, thus demonstrating their electrochem-
10 Annulation of a five-membered ring to a seven-membered
ring is another approach to the azulene derivatives having a
cyano group on the five-membered ring: a) H. Tsuruta, T.
Sugiyama, T. Mukai, Chem. Lett. 1972, 185. b) S. Kuroda, T.
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12 Experimental detail and selected spectral data are given in
Supporting Information (ref 16).
13 Choice of oxidizing reagent is critical. DDQ with strong
oxidizing ability is effective but it occasionally causes over-
reaction-side-reaction especially for the derivatives 1, 2, and
4 with electron-donating groups at 1-position. In those cases,
p-chloranil oxidation or Pd/C-promoted dehydrogenation
was used instead.
ical amphotericity (Esum Ô Eox ¹ Ered1: 2.09 V for 6 and 2.34 V
1
for 7, respectively). Again, biazulene 6 without any spacer
showed two pairs of oxidation waves,18 thus confirming its
multistage redox behavior. Not only 6 and 7 but also other 4-
cyanoazulenes with an electron-donating group at 1-position
exhibit electrochemical amphotericity (Esum of thioether 1:
2.13 V). Thus, further variety of 4-cyanoazulene derivatives
would serve as promising candidates for seeking interesting
electronic nature;19 these materials would be available by the
present synthetic protocol. Studies in this vein are now in
progress.
This work was supported by Grant-in-Aid for Scientific
Research on Innovative Areas: “Organic Synthesis Based on
Reaction Integration” (No. 2105) from MEXT, Japan.
References and Notes
14 The low isolated yield of 6 is due to some difficulty in
separation from the partially dehydrogenated derivatives and
H2DDQ.
15 Usefulness of metalated azulenes in synthesizing substituted
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16 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
17 Cyclic voltammetry was conducted in CH2Cl2 containing
0.1 M Bu4NBF4 as a supporting electrolyte (E/V vs. SCE,
Pt electrode, scan rate: 100 mV s¹1). For the irreversible
oxidation wave, the oxidation potential was estimated as
Epeak ¹0.03 V. Ferrocene undergoes one-electron oxidation
at +0.53 V under the similar conditions.
1
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Chem. Lett. 2014, 43, 607–609 | doi:10.1246/cl.131149
© 2014 The Chemical Society of Japan | 609