Synthesis and intramolecular pericyclization of 1-azulenyl thioketones

Article abstract of DOI:10.1021/jo702309b

Several azulene-substituted thioketones, 1-thiobenzoylazulene (1a) and di(1-azulenyl) thioketone (2a) and their derivatives (1b and 2b-d) with alkyl substituents on each azulene ring, were prepared and their intramolecular pericyclization reaction was exa

Full text of DOI:10.1021/jo702309b

Synthesis and Intramolecular Pericyclization of 1-Azulenyl
Thioketones
Shunji Ito,*,† Tetsuo Okujima,‡ Shigeru Kikuchi,‡ Taku Shoji,‡ Noboru Morita,‡
Toyonobu Asao,‡ Tadaaki Ikoma,§ Shozo Tero-Kubota, Jun Kawakami,† and Akio Tajiri†
Graduate School of Science and Technology, Hirosaki UniVersity, Hirosaki 036-8561, Japan, Department
of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan, Institute of Science
and Technology, Niigata UniVersity, Niigata 950-2181, Japan, PRESTO, Japan Science and Technology
Agency, Kawaguchi 332-0012, Japan, and Office of CooperatiVe Research and DeVelopment,
Tohoku UniVersity, Sendai 980-8579, Japan
itsnj@cc.hirosaki-u.ac.jp
ReceiVed October 25, 2007
Several azulene-substituted thioketones, 1-thiobenzoylazulene (1a) and di(1-azulenyl) thioketone (2a)
and their derivatives (1b and 2b-d) with alkyl substituents on each azulene ring, were prepared and
their intramolecular pericyclization reaction was examined. The thioketones with a 3-alkyl substituent
on each azulene ring exhibited the presumed pericyclization reaction under thermal and acid-catalyzed
conditions, although the cases of the 1-azulenyl thioketones without the 3-alkyl substituents afforded a
complex mixture under similar conditions. The intramolecular reaction following the intramolecular
hydrogen transfer afforded the products 13b, 14b, and 14c. The products 13b and 14b were converted
into the corresponding cations 18+ and 19+, which have structural similarity with that of the phenalenyl
cation. These cations exhibited the expected two-step reduction waves upon CV, although the ESR analysis
revealed that the neutral radical state did not have the presumed high stability.
Introduction
by Cox et al. under photochemical conditions to afford the
cyclized products after intermolecular hydrogen transfer.3
Azulene (C10H8) has attracted the interest of many research
groups due to its unusual properties as well as its beautiful blue
color.4 Cox et al. also have reported the photochemical reaction
of 1-thiobenzoylazulene (1a), which never produces the cyclized
product, but results in the recovery of the starting material under
their photochemical reaction conditions.3 They have concluded
that the photochemical pericyclization reaction proceeds through
the n,π* singlet state, which distinguishes the photochemical
reaction of aromatic thioketones from that of ketones, although
the other photophysical and photochemical properties might
affect the photochemical reaction. In the case of aromatic
Thiocarbonyl compounds have been extensively studied
because of their unique and interesting properties.1 Stabilized
carbothialdehydes are also now accessible and their quite
different properties from those of aldehydes are reported.2
Pericyclization reactions of aromatic thioketones are examined
* To whom correspondence should be addressed. Phone: +81-172-39-3568.
Fax: +81-172-39-3541.
† Hirosaki University.
‡ Department of Chemistry, Tohoku University.
§ Niigata University and Japan Science and Technology Agency.
Office of Cooperative Research and Development, Tohoku University.
(1) Duus, F. Thiocarbonyl Compounds. In ComprehensiVe Organic
Chemistry; Barton, D., Ollis, W. D., Jones, D. N., Eds.; Pergamon: Oxford,
UK, 1979; Vol. 3, pp 373-487.
(3) Cox, A.; Kemp, D. R.; Lapouyade, R.; De Mayo, P.; Joussot-Dubien,
J.; Bonneau, R. Can. J. Chem. 1975, 53, 2386-2393.
(4) Zeller, K.-P. Azulene. In Houben-Weyl: Methoden der Organischen
Chemie, 4th ed.; Kropf, H., Ed.; Georg Thieme: Stuttgart, Germany, 1985;
Vol. V, Part 2C, pp 127-418.
(2) (a) Ishii, A.; Ishida, T.; Kumon, N.; Fukuda, N.; Oyama, H.; Inamoto,
N.; Iwasaki, F.; Okazaki, R. Bull. Chem. Soc. Jpn. 1996, 69, 709-717. (b)
Watanabe, S.; Yamamoto, T.; Kawashima, T.; Inamoto, N.; Okazaki, R.
Bull. Chem. Soc. Jpn. 1996, 69, 719-724.
10.1021/jo702309b CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/16/2008
2256
J. Org. Chem. 2008, 73, 2256-2263

Pericyclization of 1-Azulenyl Thioketones
CHART 1
CHART 2
SCHEME 2
SCHEME 1
thioketones including the nonalternant azulene skeleton the π,π*
state should be very close to the n,π* state. Thus, the
pericyclization reaction under thermal conditions might be
possible in the case of azulene derivatives. However, little is
known about the azulene-substituted thioketones except for the
attempt for the photochemical pericyclization of 1-thiobenzoy-
lazulene (1a). The pericyclization reaction of azulene-substituted
thioketones under thermal conditions has never been examined.
We report herein the efficient preparation of 1-thiobenzoy-
lazulenes (1a and 1b) and di(1-azulenyl) thioketones (2a-d)
by the reaction of the corresponding carbonyl compounds with
sulfur-donating reagents (P2S5/Et3N and/or Lawesson’s reagent),
the pericyclization reaction of the azulene-substituted thioketones
under thermal and acid-catalyzed conditions, and the conversion
of the cyclized products, 3H-azulenothiophene derivatives, into
cationic species (Chart 1).
azulenyl) ketone (8a) along with di(1-azulenyl) diketone (9a).8
Oxalyl bromide (7) is used instead of oxalyl chloride in the
reaction of azulenes to afford 1-azulenecarbonyl bromides as
the reaction intermediate, which reacts with azulene derivatives
to give di(1-azulenyl) ketones including unsymmetrical deriva-
tives.9 Several di(1-azulenyl) ketone derivatives including the
parent ketone 8a are also obtained from the autoxidation
products of azulenes in trace yields.10 Recently, the ketone 8a
is obtained by the two-step sequence starting from 1-azulen-
ecarboxylic acid via ethyl 1-azuleneglyoxylate.11
We prepared the desired di(1-azulenyl) ketones (8a-d) by
the one-step reaction of azulenes (3a-d) with 7 in dichlo-
romethane (Scheme 2). The reaction of 4 molar amounts of
azulene (3a) with 7 in dichloromethane at room temperature
Results and Discussion
Preparation of 1-Azulenyl Ketones. 1-Benzoylazulene (5a)
was prepared by Vilsmeier-Haack reaction of azulene (3a),
using N,N-dimethylbenzamide (4) according to the literature.5
1-Benzoyl-3,6-di-tert-butylazulene (5b) was synthesized by the
procedure with use of 1,6-di-tert-butylazulene (3b)6 (83%). It
is noteworthy that the yield of the reaction was significantly
increased by the tert-butyl substituents on the azulene ring
(Scheme 1).
(7) Reid, D. H.; Stafford, W. H.; Stafford, W. L. J. Chem. Soc. 1958,
1118-1127.
(8) Asao, T. Pure Appl. Chem. 1990, 62, 507-512.
(9) (a) Saitoh, M.; Hashimoto, K.; Nakazawa, T.; Sugihara, Y. Tetra-
hedron Lett. 1993, 34, 3563-3566. (b) Saitoh, M.; Yano, J.; Nakazawa,
T.; Sugihara, Y.; Hashimoto, K. J. Electroanal. Chem. 1996, 418, 139-
145.
(10) (a) Matsubara, Y.; Takekuma, S.; Yokoi, K.; Yamamoto, H.; Nozoe,
T. Bull. Chem. Soc. Jpn. 1987, 60, 1415-1428. (b) Takekuma, S.;
Matsubara, Y.; Yamamoto, H.; Nozoe, T. Bull. Chem. Soc. Jpn. 1987, 60,
3721-3730. (c) Takekuma, S.; Matsubara, Y.; Yamamoto, H.; Nozoe, T.
Bull. Chem. Soc. Jpn. 1988, 61, 475-481. (d) Takekuma, S.; Matsubara,
Y.; Yamamoto, H.; Nozoe, T. Nippon Kagaku Kaishi 1988, 157-161. (e)
Takekuma, S.; Matsubara, Y.; Matsui, S.; Yamamoto, H.; Nozoe, T. Nippon
Kagaku Kaishi 1988, 923-932. (f) Takekuma, S.; Zhao, Z.; Matsubara,
Y.; Makihara, D.; Yamamoto, H.; Nozoe, T. Nippon Kagaku Kaishi 1993,
1270-1274.
Preparation of bis(5-isopropyl-3,8-dimethyl-1-azulenyl) ke-
tone (6) has been reported utilizing the reaction of 7-isopropyl-
1,4-dimethylazulene (guaiazulene) with oxalyl chloride in
dichloromethane in low yield (Chart 2).7 Similarly, the reaction
of azulene with oxalyl chloride affords the presumed di(1-
(5) Hafner, K.; Bernhard, C. Justus Liebigs Ann. Chem. 1959, 625, 108-
123.
(6) Ito, S.; Morita, N.; Asao, T. Bull. Chem. Soc. Jpn. 1995, 68, 1409-
1436.
(11) Kurotobi, K.; Takakura, K.; Murafuji, T.; Sugihara, Y. Synthesis
2001, 1346-1350.
J. Org. Chem, Vol. 73, No. 6, 2008 2257

Ito et al.
CHART 3
SCHEME 3
for several hours, following the treatment with ethanol, afforded
the desired ketone 8a (17%) along with diketone 9a (3%) and
ethyl azulene-1-carboxylate (10) (3%) (Chart 3).12 Addition of
sodium acetate to the reaction of 3a with 7 in dichloromethane
slightly improved the yields of the desired ketone 8a (24%)
with diketone 9a (15%). Lowering the reaction temperature did
not improve the yield of the desired products. When the reaction
was carried out at -78 °C, ethyl (1-azulenyl)glyoxylate (11)
was obtained as a major product in 76% yield. Changing the
base to triethylamine resulted into the formation of N,N-diethyl-
(1-azulenyl)glyoxylamide (12) as a major product (34%).
Bis(3,6-di-tert-butyl- and 3-methyl-1-azulenyl) ketones (8b13
and 8c10b) were obtained by the similar reaction of 3b and
1-methylazulene (3c)14 in 71% and 61% yields, respectively,
along with a small amount of diketones 9b (4%) and 9c (3%).
However, similar to the reaction of 3a with 7, the yields of
ketone 8d (17%) and diketone 9d (1%) by the reaction of 6-tert-
butylazulene (3d)6 with 7 were relatively low. Consequently,
the yield of the reaction of 3a-d with 7 is significantly affected
by the existence of the 1-alkyl substituent on the azulene ring,
similar to the results on the Vilsmeier-Haack reaction with N,N-
dimethylbenzamide (4).
Synthesis of 1-Azulenyl Thioketones. Thiocarbonyl com-
pounds are usually synthesized from the corresponding carbonyl
compounds by the reaction with sulfur-donating reagents such
as H2S,15 P2S5,16 and Lawesson’s reagent.17 Indeed, 1-thioben-
zoylazulene (1a) has been synthesized by the reaction of 5a
with HCl-H2S in relatively low yield.3 We applied the reaction
of 1-azulenyl ketones with P2S5/Et3N and Lawesson’s reagent
for the preparation of the corresponding 1-azulenyl thioketones
(Schemes 3 and 4). The results are summarized in Table 1. The
yield of 1-thiobenzoylazulene (1a) was significantly improved
by the use of these reagents. When 1-benzoylazulene (5a) was
treated with P2S5 in the presence of triethylamine, 1a was
obtained in 79% yield (entry 1). The reaction of 5a with
Lawesson’s reagent also afforded the desired thioketone 1a in
74% yield (entry 2).
SCHEME 4
TABLE 1. Syntheses of Azulene-Substituted Thioketones
yield
yield
1 or
13 or
14
entry ketone R1
R2
reagents
P2S5/Et3N
Lawesson’s reagent 1a 74 13a
2
%
%
1
2
3
4
5
6
7
8
9
5a
5a
5b
5b
8a
8a
8b
8b
8c
8c
8d
8d
H
H
H
H
1a 79 13a
0
0
0
t-Bu t-Bu P2S5/Et3N
1b 65 13b
t-Bu t-Bu Lawesson’s reagent 1b
2a 59 14a
Lawesson’s reagent 2a 56 14a
2b 82 14b
t-Bu t-Bu Lawesson’s reagent 2b
0
13b 89
H
H
H
H
P2S5/Et3N
0
0
0
t-Bu t-Bu P2S5/Et3N
0
14b 88
Me
Me
H
H
H
P2S5/Et3N
2c 80 14c
Lawesson’s reagent 2c 13 14c 67
2d 61 14d
t-Bu Lawesson’s reagent 2d 72 14d
0
10
11
12
t-Bu P2S5/Et3N
0
0
H
SCHEME 5
Thioketone 1a reacted with 2,3-dimethyl-1,3-butadiene (15)
to afford a [2 + 4] cycloadduct, 2-(1-azulenyl)-2-phenyl-4,5-
dimethyldihydrothiopyran (16), which indicated the CdS
structure in thioketone 1a (Scheme 5).1
The reaction of 5b with P2S5/Et3N also afforded the corre-
sponding thioketone, 3,6-di-tert-butyl-1-thiobenzoylazulene (1b),
in good yield (entry 3). In contrast to the reaction with P2S5/
Et3N, the reaction of ketone 5b with Lawesson’s reagent,
following chromatographic purification on Al2O3, afforded 3H-
azuleno[8,1-b,c]thiophene 13b as a sole product (entry 4).
Several azulenothiophene derivatives are prepared by the
reaction of azulene derivatives with sulfur,18 intramolecular
cyclization reactions,19 and the reaction of oxaazulanones with
enamines.20 This is the first example of the preparation of the
azulenothiophene derivative with 3H-azuleno[8,1-b,c]thiophene
skeleton by the pericyclization reaction of the 1-azulenyl
(12) (a) Nozoe, T.; Seto, S.; Matsumura, S.; Murase, Y. Bull. Chem.
Soc. Jpn. 1962, 35, 1179-1188. (b) Kawamoto, I.; Sugimura, Y.; Soma,
N.; Kishida, Y. Chem. Lett. 1972, 931-934.
(13) Ito, S.; Kubo, T.; Morita, N.; Ikoma, T.; Tero-Kubota, S.; Tajiri,
A. J. Org. Chem. 2003, 68, 9753-9762.
(14) Yasunami, M.; Miyoshi, S.; Kanegae, N.; Takase, K. Bull. Chem.
Soc. Jpn. 1993, 66, 892-899.
(15) (a) Paquer, D. Int. J. Sulfur Chem., B 1972, 7, 269-293. (b) Paquer,
D. Int. J. Sulfur Chem. 1973, 8, 173-194.
(16) (a) Scheeren, J. W.; Ooms, P. H. J.; Nivard, R. J. F. Synthesis 1973,
149-151. (b) Machiguchi, T.; Otani, H.; Ishii, Y.; Hasegawa, T. Tetrahe-
dron Lett. 1987, 28, 203-206.
(17) (a) Pedersen, B. S.; Scheibye, S.; Nilsson, N. H.; Lawesson, S.-O.
Bull. Soc. Chim. Belg. 1978, 87, 223-228. (b) Pedersen, B. S.; Scheibye,
S.; Clausen, K.; Lawesson, S.-O. Bull. Soc. Chim. Belg. 1978, 87, 293-
297.
(18) Hayashi, S.; Kurokawa, S.; Matsuura, T. Bull. Chem. Soc. Jpn. 1969,
42, 1404-1407.
2258 J. Org. Chem., Vol. 73, No. 6, 2008

Pericyclization of 1-Azulenyl Thioketones
TABLE 2. Pericyclization Reaction of Azulene-Substituted
Thioketones
CHART 4
thio-
entry ketone
condi-
tionsa
R1
R2
time
14 h
5 d
results
1
2
3
4
5
6
7
8
9
1a
1a
1b
1b
2a
2a
2b
2b
2c
2c
2d
2d
H
H
H
H
acidic
complex mixture
thermal complex mixture
acidic 13b (35%), 5b (18%)
thermal 13b (37%), 5b (24%)
acidic complex mixture
thermal complex mixture
14b (100%)
thermal 14b (76%)
acidic 14c (67%)
thermal 14c (35%)
t-Bu t-Bu 1 h
t-Bu t-Bu 1 d
H
H
H
H
1 h
3 d
t-Bu t-Bu 20 min acidic
t-Bu t-Bu 1 d
Me
Me
H
H
H
1 h
2 d
10
11
12
SCHEME 6. Proposed Reaction Mechanism for the
Acid-Catalyzed Pericyclization of 1-Azulenyl Thioketones
t-Bu 2 h
t-Bu 4 d
acidic
complex mixture
H
thermal complex mixture
a Acid-catalyzed cyclization was carried out in CHCl3 with concentrated
HCl. Thermal reaction was carried out in refluxing toluene.
thioketone. The methylene protons of 13b were observed at 3.45
ppm on the 1H NMR spectrum. The seven-membered-ring
protons of 13b were observed at the olefinic region [δ 6.45 (d,
1H, J ) 12.8 Hz, H5), 5.97 (s, 1H, H8), 5.66 (d, 1H, J ) 12.8
Hz, H6) ppm]. Therefore, 3H-azuleno[8,1-b,c]thiophene 13b has
the fused-ring structure between heptafulvene and thiophene
moieties.
The reaction of di(1-azulenyl) ketones 8a-d with P2S5 in
the presence of triethylamine afforded the corresponding thioke-
tones 2a-d in 59-82% yields (entries 5, 7, 9, and 11). In
contrast to the reaction with P2S5/Et3N, the reaction of 8b and
8c with Lawesson’s reagent afforded 14b and 14c, respectively,
as major products along with the presumed thioketones in a
small amount in the case of the reaction of 8c (entries 8 and
10). However, the reaction of 8a and 8d with Lawesson’s
reagent afforded thioketones 2a and 2d, exclusively (entries 6
and 12). Thus, 3H-azuleno[8,1-b,c]thiophene derivatives were
obtained by the reaction of the 1-azulenyl ketones with the
3-alkyl substituents with Lawesson’s reagent (Table 1).
The formation of the 3H-azuleno[8,1-b,c]thiophene deriva-
tives indicates that the expected intramolecular pericyclization
of the corresponding thioketones 1 and 2 proceeds under
preparatory conditions. The 1,5-hydorogen transfer of the
presumed pericyclization products results in the formation of
3H-azuleno[8,1-b,c]thiophene derivatives 13 and 14. To clarify
the formation of the azulenothiophene derivatives, the intramo-
lecular pericyclization of these thioketones under various
reaction conditions was examined.
Acid-Catalyzed Isomerization. We found that several
thioketones were transformed into the corresponding 3H-
azuleno[8,1-b,c]thiophene derivatives under acidic conditions
(Table 2). The treatment of thioketone 1b with concentrated
hydrochloric acid in chloroform at room temperature afforded
13b in 35% yield along with 5b in 18% yield, which should be
attributed to the acid-catalyzed hydrolysis of the thioketone 1b
(entry 3). However, the reaction of 1a afforded a complex
mixture instead of the 3H-azuleno[8,1-b,c]thiophene derivative
13a under similar acidic conditions (entry 1). In the cases of
di(1-azulenyl) thioketones, two thioketones 2b and 2c exhibited
the presumed pericyclization to afford 14b and 14c by a similar
acid-catalyzed reaction (entries 7 and 9). However, thioketones
2a and 2d did not afford the corresponding thiophene derivatives
14a and 14d under similar acidic conditions (entries 5 and 11).
Thus, the presence of alkyl substituent at the 3-position on each
azulene ring is important to the success of the pericyclization
reaction under acid-catalyzed conditions, as similar to the
observation of the reaction with Lawesson’s reagent. The
1-azulenyl thioketones without the 3-alkyl substituents afforded
a complex mixture instead of the corresponding 3H-azuleno-
[8,1-b,c]thiophene derivatives by the acid-catalyzed reaction
(Table 2).
1H and 13C NMR spectra were measured by using the deep
blue solution prepared by the addition of concentrated hydro-
chloric acid to a solution of thioketone 2b in CDCl3. The NMR
spectra, which are represented in the Supporting Information,
revealed the formation of cationic species 17+ as an intermediate
1
(Chart 4). H NMR chemical shifts of H5, H6, and H8 signals
in 17+ are apparently observed at downfield locations compared
with those of 14b, which exhibit the tropylium ion structure of
the intermediate. Neutralization of the deep blue solution with
an aqueous sodium hydrogencarbonate solution afforded 14b.
To demonstrate the hydrogen transfer during the formation
of the 3H-azuleno[8,1-b,c]thiophene derivatives, 1H NMR
measurement was examined utilizing DCl instead of HCl in
CDCl3, which led to the incorporation of only one deuteron at
the 4-position in the cationic intermediate 17+, essentially
quantitatively. Thus, a plausible reaction path could be drawn
as represented in Scheme 6. Pericyclization should be acceler-
ated by the protonation at the 3-position on the azulene ring.
Along the reaction path a hydrogen atom must be transferred.
This could be, in principle, a concerted 1,5-hydrogen transfer
or a two-step intermolecular process. We could conclude that
the isomerization involves the 1,5-hydrogen transfer process
(19) See e.g.: (a) Matsui, K.; Nozoe, T. Chem. Ind. 1960, 1302-1303.
(b) Replogle, L. L.; Katsumoto, K.; Ammon, H. L. J. Am. Chem. Soc. 1968,
90, 1086-1087. (c) Ammon, H. L.; Replogle, L. L.; Watts, P. H., Jr.;
Katsumoto, K.; Stewart, J. M. J. Am. Chem. Soc. 1971, 93, 2196-2202.
(d) Yamane, K.; Fujimori, K.; Takeuchi, T. Bull. Chem. Soc. Jpn. 1981,
54, 2537-2538.
(20) Fujimori, K.; Fujita, T.; Yamane, K.; Takase, K. Chem. Lett. 1983,
1721-1724.
J. Org. Chem, Vol. 73, No. 6, 2008 2259

Ito et al.
SCHEME 7. Proposed Reaction Mechanism for the
Pericyclization of 1-Azulenyl Thioketones under Thermal
Conditions
under acidic conditions because no deuterium exchange was
observed at the 3-position on the cationic species 17+ when
DCl was used.
Thermal Isomerization. The pericyclization reaction of these
thioketones was also examined under thermal conditions. The
thioketones, which gave the 3H-azuleno[8,1-b,c]thiophene
derivatives under acid-catalyzed reaction, also afforded the
pericyclized products under thermal conditions. However, the
thioketones, which gave a complex mixture under acidic
conditions, also did not give the pericyclized products (Table
2). When a solution of thioketone 1b in toluene was refluxed
for 1 day under an Ar atmosphere, the thiophene derivative 13b
was obtained in 37% yield along with 5b in 24% yield (entry
4). The formation of 5b under thermal conditions should be
ascribed to the hydrolysis of 1b during the prolonged heating
in toluene. Similar to the acid-catalyzed reaction, thioketone
1a afforded a complex mixture instead of 13a under thermal
conditions (entry 2). Thioketones 2b and 2c were also trans-
formed into the thiophene derivatives 14b and 14c under thermal
conditions (entries 8 and 10). As expected, thioketones 2a and
2d produced a complex mixture under thermal conditions
(entries 6 and 12). Therefore, under both thermal and acid-
catalyzed conditions the presence of an alkyl substituent at the
3-position on each azulene ring is important in the formation
of 3H-azuleno[8,1-b,c]thiophene derivatives.
The question arises as to why the pericyclization products
are not obained without 3-alkyl substituents. The azulene system
has high reactivity at 1,3-positions toward the electrophilic
reagents.4 It is well-known that the azulene derivatives react
with carbonyl compounds to give condensed products such as
di(1-azulenyl)methane derivatives under acidic conditions.6,21
3H-Azuleno[8,1-b,c]thiophenes 13b, 14b, and 14c have a fused-
ring structure between heptafulvene and thiophene moieties. The
heptafulvene is well-known as a highly reactive compound.22
In consideration of the results on the synthesis with Lawesson’s
reagent, the thioketones without 3-alkyl substituents are less
reactive toward the intramolecular pericyclization reaction,
compared with those with 3-alkyl substituents. Thus, the
explanation should be attributed to the smaller reactivity toward
the pericyclization reaction, which might accelerate the side
reaction to give a complex mixture due to the high reactivity
of the 3-position of the starting azulenyl thioketones or the
instability of the final thiophene derivatives under pericyclization
conditions.
SCHEME 8
of galvinoxyl as a radical scavenger, but the pericyclization
reaction was not affected by the radical scavenger. The reaction
of 2b in refluxing toluene afforded the corresponding cyclization
product 14b in 85% yield, even in the presence of the radical
scavenger.
To estimate the formation of the thiophene derivatives 13b,
14b, and 14c as the pericyclized products, we have also
performed the B3LYP/6-31G** density functional calculations24
of the nine possible azuleno[8,1-b,c]thiophene structures without
other substituents, although the reaction has not been proved
to proceed under thermodynamic control (see the Supporting
Information). Consideration of the relative stabilities of the
structural isomers based on the calculated total energy demon-
strates that the 3H-azuleno[8,1-b,c]thiophene structure is one
of the highly stable regioisomers among all possible structural
isomers. However, the presumed initial cyclization products
illustrated in Schemes 6 and 7 exhibited relatively high total
energies, which might be the reason for the formation of the
thiophene derivatives 13b, 14b, and 14c as the isolable products.
Preparation of Cationic Species. The pericyclized products
13b and 14b produced a stabilized cationic species 18+ and
19+ by hydride abstraction reaction. The reaction of 13b with
DDQ in dichloromethane at room temperature, followed by an
addition of a 60% aqueous HPF6 solution, yielded cation 18+
(96%) as a hexafluorophosphate (Scheme 8).6,13,25 The salt
19+‚PF6- was obtained by a similar reaction of 14b with DDQ
To obtain the theoretical aspect of 1-azulenyl thioketones,
the molecular orbital calculation of 1-azulenecarbothialdehyde
was performed by the B3LYP/6-31G** density functional
calculations (see the Supporting Information).23 The HOMO
includes the n orbital of the sulfur atom. On the other hand, the
HOMO-1, which is located at a similar energy level as that of
HOMO, and the LUMO are delocalized over the π system
including that of the sulfur atom. The examination of the
HOMOs and the LUMO suggests the possibility for the thermal
intramolecular pericyclization between the sulfur atom and the
8-position of the azulene ring by the 10π cyclization mode
instead of the photochemical reaction (Scheme 7). The thermal
pericyclization reaction of 2b was also examined in the presence
-
in 67% yield (Scheme 9). These new salts 18+‚PF6 and
-
19+‚PF6 are stable, deep-colored crystals, and are storable in
(23) The molecular orbital calculations were done with a Gaussian
revision E.2 program package, which was installed in an UNIX workstation
under a molecular design support system in IMRAM. Frisch, M. J.; Trucks,
G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.;
Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.;
Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.;
Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.;
Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.;
Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.;
Gonzalez, C.; Pople, J. A. Gaussian, revision E.2; Gaussian, Inc.: Pittsburgh,
PA, 1995.
(21) See e.g.: Franke, H.; Mu¨hlsta¨dt, M. J. Prakt. Chem. 1967, 35, 262-
270.
(22) (a) Doering, W. v. E.; Wiley, D. W. Tetrahedron 1960, 11, 183-
198. (b) Zimmerman, H. E.; Sousa, L. R. J. Am. Chem. Soc. 1972, 94,
834-842. (c) Neuenschwander, M.; Schenk, W. K. Chimia 1972, 26, 194-
197.
(24) The B3LYP/6-31G** density functional calculations were performed
by Spartan’04, Wavefunction, Inc.: Irvine, CA.
(25) Ito, S.; Kikuchi, S.; Morita, N.; Asao, T. J. Org. Chem. 1999, 64,
5815-5821.
2260 J. Org. Chem., Vol. 73, No. 6, 2008

Pericyclization of 1-Azulenyl Thioketones
SCHEME 9
TABLE 3. Redox Potentialsa of Compounds 18+ and 19+
FIGURE 1. UV-vis spectra of 18+‚PF6- (broken line) and 19+‚PF6
(solid line) in acetonitrile.
-
red
red
red
ox
ox
sample
E1
E2
E3
E1
E2
18+
19+
(-0.49)
-0.70
(-1.50)
-1.58
(+1.73)
+0.94
SCHEME 10
(-1.97)
(+1.43)
a The redox potentials were measured by CV (0.1 M Et4NClO4 in
acetonitrile, Pt electrode, scan rate 100 mV s-1, and Fc/Fc+ ) +0.07 V).
In the case of irreversible waves, which are shown in parentheses, Eox and
Ered were calculated as Epa (anodic peak potential) - 0.03 V and Epc
(cathodic peak potential) + 0.03 V, respectively.
the crystalline state. These cations 18+ and 19+ were fully
characterized by the spectral data as shown in the Supporting
Information. The UV-vis spectra of 18+ and 19+ in acetonitrile
are represented in Figure 1. These cations showed characteristic
absorption in the visible region [18+, 546 nm (log ꢀ 3.43); 19+,
639 nm (log ꢀ 4.56)]. The strong absorption of 19+ in the visible
region could be explained by the CT transition from the
substituted 3,6-di-tert-butyl-1-azulenyl group, as described by
the resonance structure in Scheme 9.
substituted 1-azulenyl group. The potential shift of the first
reduction wave of 19+ can be explained by the stabilization of
the cationic state owing to the substituted 1-azulenyl group. In
the case of reduction of the tri(1-azulenyl)methyl cation, the
second reduction wave is irreversible in all cases owing to the
destabilization of the anionic species arising from the destabi-
lization by the electron-donating 1-azulenyl substituents.6 The
reversibility upon CV of the second reduction wave of 19+ might
be attributed to the structural similarity with the phenalenyl
systems, which would stabilize the corresponding neutral radical
and the anionic species.
The oxidation of 18+ exhibited an irreversible wave at +1.73
V. In contrast to the one-step oxidation of 18+, cation 19+
exhibited a two-step oxidation wave. The reversible E1ox wave
of 19+ should be ascribed to the electron removal from the
1-azulenyl group forming a dicationic species.
Electrochromic Analysis. To obtain the aspect for the
reduced species, the electrochemical reduction of 19+ was
conducted by visible spectral monitoring and ESR measure-
ments. The visible spectral changes of 19+ are summarized in
the Supporting Information. The absorption bands of 19+ in
the visible region gradually disappeared along with increasing
the new absorption maxima at 448 nm during the electrochemi-
cal reduction. The color of the solution gradually changed from
deep blue to yellow during the electrochemical reduction. The
yellow solution turned reddish brown on further reduction. The
two-step color changes might correspond to the formation of a
neutral and an anionic species. The reverse oxidation of the
intermediary yellow solution did not regenerate the spectrum
of 19+ completely (regeneration 37%), even though good
reversibility was observed upon CV. Low reversibility for the
reduction of 19+ suggests that the neutral and anionic species
produced by the electrochemical reduction are unstable under
the conditions of the spectral measurements.
Redox Behavior. These cations 18+ and 19+ possess a
structural similarity with the phenalenyl systems26 in the point
of the peripheral 12π electrons, counting the sulfur atom as two
electrons like thiophene derivatives, with one central sp2 carbon
atom, although these cations are not symmetrically as distinct
from the phenalenyl systems. To clarify the electrochemical
property, the redox behavior of 18+ and 19+ was examined by
cyclic voltammetry (CV). Measurements were made by using
a standard three electrode configuration. Tetraethylammonium
perchlorate (0.1 M) in acetonitrile was used as a supporting
electrolyte with platinum wire auxiliary and working electrodes.
All measurements were carried out under an argon atmosphere
and potentials were related to the reference electrode formed
by Ag/AgNO3 and using as internal reference Fc/Fc+ that
discharges at +0.07 V.
Redox potentials (in volts vs Ag/AgNO3) of 18+ and 19+
are summarized in Table 3. The redox waves of 18+ and 19+
are shown in the Supporting Information. The reduction of 18+
exhibited a quasireversible two-step wave. The E1red wave might
be attributable to the one-electron injection to the cation forming
a neutral radical. The second reduction wave is considered as
the generation of an anionic species (Scheme 10).
Cation 19+ exhibited the three-step reduction, which was
characterized as two reversible processes at -0.70 and -1.58
V following an irreversible process at -1.97 V. The three-step
reduction may be considered as the formation of a neutral radical
and an anionic species in addition to the redox reaction of the
ESR Measurements. The ESR measurements were also
examined by using the cation 19+ under electrochemical
reduction conditions with use of a degassed dimethylformamide
solution prepared under a vacuum line containing tetrapropy-
lammonium perchlorate as a supporting electrolyte at room
temperature. The spectra are summarized in the Supporting
Information. The deep blue solution of 19+ did not indicate any
(26) (a) Goto, K.; Kubo, T.; Yamamoto, K.; Nakasuji, K.; Sato, K.;
Shiomi, D.; Takui, T.; Kubota, M.; Kobayashi, T.; Yakushi, K.; Ouyang,
J. J. Am. Chem. Soc. 1999, 121, 1619-1620. (b) Kubo, T.; Yamamoto, K.;
Nakasuji, K.; Takui, T.; Murata, I. Bull. Chem. Soc. Jpn. 1999, 74, 1999-
2009. (c) Nakasuji, K.; Kubo, T. Bull. Chem. Soc. Jpn. 2004, 77, 1791-
1804.
J. Org. Chem, Vol. 73, No. 6, 2008 2261

Ito et al.
days. After an addition of ethanol to the reaction mixture, the
mixture was poured into water and extracted with CH2Cl2. The
organic layer was washed with water, dried over MgSO4, and
concentrated under reduced pressure. The products were isolated
by column chromatography on silica gel with ethyl acetate/CH2-
Cl2 and GPC (gel permeation chromatography) with CHCl3 to afford
di(1-azulenyl) ketones 8a-d and di(1-azulenyl) diketones 9a-d.
Di(1-azulenyl) Ketone (8a). The general procedure was followed
by using azulene (3a) (263 mg, 2.05 mmol) and 7 (108 mg, 0.500
mmol) in CH2Cl2 (10 mL) at room temperature for 4 h. Chromato-
graphic purification on silica gel with ethyl acetate/CH2Cl2 and GPC
with CHCl3 afforded 8a (24 mg, 17%), di(1-azulenyl) diketone (9a)
(4 mg, 3%), and ethyl azulene-1-carboxylate (10) (3 mg, 3%). The
reaction of 3a (265 mg, 2.07 mmol) with 7 (108 mg, 0.500 mmol)
in the presence of sodium acetate (87 mg, 1.1 mmol) in CH2Cl2
(10 mL) at room temperature for 4 h afforded 8a (34 mg, 24%)
and 9a (24 mg, 15%).
ESR signals before the electrochemical reduction. The yellow
solution obtained by the electrochemical reduction of 19+ was
also ESR silent. This fact indicates that the yellow originates
from the closed shell species, probably due to the rapid coupling
of the unstable neutral radical to form ESR silent species during
the electrochemical reduction.
The ESR spectrum split into multiple lines due to hyperfine
interactions was obtained from the reddish brown solution
produced by further reduction of the yellow solution obtained
by the reduction of 19+. The reddish brown solution should
correspond to the two-electron reduction product of 19+. The
simulation with a g-value of 2.00235 and proton hyperfine
coupling (hfc) constants of 0.625 (2H), 0.360 (1H), and 0.170
mT (2H) well-reproduced the observed spectrum. The hfc
constants for the radical species are very similar to those of the
anion radical of azulene derivatives.27 Therefore, the observed
ESR spectrum could be explained by the hyperfine interactions
with 4,8-protons, 2-proton, and 5,7-protons, respectively, on the
anion radical of the 3,6-di-tert-butyl-1-azulenyl group. There-
fore, by combined electrochromic and ESR analyses it was
determined that the neutral radical species produced by the one-
electron reduction of 19+ is the reactive species that turned into
the ESR-silent yellow ones. The second electron injection, in
this case, could be caused by the redox reaction of the substituted
3,6-di-tert-butyl-1-azulenyl group to the azulenothiophene moi-
ety in contrast to the expectation. We also tried the reaction of
14b with potassium tert-butoxide in dry THF-d8 to obtain the
anionic species chemically, but the reaction did not afford any
evidence for the formation of the anionic species as suggested
by the results on the electrochromic and ESR measurements.
8a:10d,11 purple needles; mp 132.8-134.7 °C [lit.11 mp 129-
131 °C]; MS (70 eV) m/z 282 (M+, 100%), 281 (75), 254 (M+
-
CO, 20), 253 (40), 252 (48), 127 (M+ - COC10H7, 50), 126 (22);
IR (KBr disk) νmax 1586, 1489, 1460, 1429, 1416, 1393, 812, 766
cm-1; UV-vis (CH2Cl2) λmax (nm) (log ꢀ) 230 sh (4.58), 286 (4.63),
318 (4.56), 405 (4.45), 505 sh (2.98), 541 (3.05), 584 sh (2.91),
644 sh (2.33); 1H NMR (400 MHz, CDCl3) δ 9.67 (d, 2H, J ) 9.8
Hz, H8), 8.50 (d, 2H, J ) 9.8 Hz, H4), 8.21 (d, 2H, J ) 4.0 Hz,
H2), 7.80 (dd, 2H, J ) 9.9, 9.8 Hz, H6), 7.54 (dd, 2H, J ) 9.9, 9.8
Hz, H7), 7.43 (dd, 2H, J ) 9.8, 9.8 Hz, H5), 7.34 (d, 2H, J ) 4.0
Hz, H3); 13C NMR (100 MHz, CDCl3) δ 189.2 (CdO), 144.4 (C3a),
141.7 (C2), 140.7 (C8a), 139.2 (C6), 138.6 (C8), 138.3 (C4), 128.6
(C1), 127.8 (C7), 126.5 (C5), 117.4 (C3). Anal. Calcd for C21H14O:
C, 89.34; H, 5.00. Found: C, 89.27; H, 5.23.
9a: red crystals; mp 257.0-258.5 °C; MS (70 eV) m/z 310 (M+,
7%), 155 (M+ - COC10H7, 100); IR (KBr disk) νmax 1615 (CdO),
1495, 1408, 1393, 644 cm-1; UV-vis (CH2Cl2) λmax (nm) (log ꢀ)
235 sh (4.55), 269 (4.25), 300 sh (4.59), 317 (4.70), 383 sh (4.36),
403 (4.53), 493 sh (3.14), 520 (3.17), 561 sh (3.02), 616 sh (2.42);
1H NMR (500 MHz, CDCl3) δ 10.04 (d, 2H, J ) 9.9 Hz, H8), 8.52
(d, 2H, J ) 9.7 Hz, H4), 8.27 (d, 2H, J ) 4.2 Hz, H2), 7.91 (dd,
2H, J ) 9.8, 9.8 Hz, H6), 7.75 (dd, 2H, J ) 9.9, 9.8 Hz, H7), 7.59
(dd, 2H, J ) 9.8, 9.7 Hz, H5), 7.27 (d, 2H, J ) 4.2 Hz, H3); 13C
NMR (125 MHz, CDCl3) δ 191.7 (CdO), 146.6 (C3a), 143.0 (C2),
142.1 (C8a), 139.8 (C6), 139.6 (C8), 138.7 (C4), 130.3 (C7), 128.7
(C5), 121.6 (C1), 119.0 (C3). Anal. Calcd for C22H14O2: C, 85.14;
H, 4.55. Found: C, 85.06; H, 4.75.
General Procedure for the Thionation Reaction with P2S5/
Et3N. To a solution of azulene-substituted ketones 5a, 5b, and 8a-d
in CHCl3 was added P2S5 and triethylamine. After the resulting
mixture was stirred for 2-14 h, the reaction mixture was poured
into water and extracted with CH2Cl2. The organic layer was washed
with water, dried over MgSO4, and concentrated under reduced
pressure. The products were isolated by column chromatography
on silica gel or Al2O3 with CH2Cl2/hexane to afford the corre-
sponding thioketones 1a, 1b, and 2a-d.
General Procedure for the Thionation Reaction with Lawes-
son’s Reagent. To a solution of azulene-substituted ketones 5a,
5b, and 8a-d in toluene and/or benzene was added Lawesson’s
reagent. After the resulting mixture was stirred for 1.5-20 h, the
reaction mixture was poured into a NaHCO3 solution and extracted
with CH2Cl2. The organic layer was washed with water, dried over
MgSO4, and concentrated under reduced pressure. The products
were isolated by column chromatography on silica gel or Al2O3
with CH2Cl2/hexane to afford the thioketones 1a, 2a, 2c, and 2d
and/or the thiophene derivatives 13b, 14b, and 14c.
Conclusion
1-Azulenyl thioketones 1a, 1b, and 2a-d were synthesized
efficiently by the reaction of 5 and 8 with P2S5 in the presence
of triethylamine. The reactions of 5b, 8b, and 8c with Lawes-
son’s reagent afforded 13b, 14b, and 14c as major products.
Thermal and acid-catalyzed intramolecular pericyclization reac-
tions of the 1-azulenyl thioketones, which have 3-alkyl groups
on each azulene ring, resulted in the formation of 3H-azuleno-
[8,1-b,c]thiophene derivatives 13b, 14b, and 14c via 1,5-
hydorogen transfer from the initial cycloadducts. Cationic
intermediate 17+ under acidic conditions was detected by NMR
spectroscopy. Thiophene derivatives 13b and 14b produced the
stable cations 18+ and 19+, which could possess a structural
similarity with phenalenyl systems, by the reaction with DDQ.
However, the electrochromic and ESR analyses revealed that
the electrochemical reduction of 18+ and 19+ did not produce
the stabilized neutral radical with highly amphoteric redox
properties in contrast to the expectation.
Experimental Section
General. For general and electrochemical measurement details,
1
see the Supporting Information. The peak assignment of H and
13C NMR spectra reported was accomplished by decoupling, NOE,
CH COSY, COLOC, HMQC, and/or HMBC experiments.
General Procedure for the Synthesis of Di(1-azulenyl) ketones
(8a-d). Oxalyl bromide (7) was added to a solution of azulenes
3a-d in CH2Cl2. The resulting mixture was stirred for 1 h to 2
1-Thiobenzoylazulene (1a).3 The general procedure was fol-
lowed by using 1-benzoylazulene (5a) (2.37 g, 10.2 mmol), P2S5
(6.10 g, 27.4 mmol), and triethylamine (2.5 mL) in CHCl3 (100
mL) at 0 °C for 3 h. Chromatographic purification on silica gel
with CH2Cl2 afforded 1a3 (1.99 g, 79%). The reaction of 5a (152
mg, 0.654 mmol) with Lawesson’s reagent (226 mg, 0.658 mmol)
(27) (a) Bernal, I.; Rieger, P. H.; Frankel, G. K. J. Chem. Phys. 1962,
37, 1489-1495. (b) Bachmann, R.; Burda, C.; Gerson, F.; Scholz, M.;
Hansen, H.-J. HelV. Chim. Acta 1994, 77, 1458-1465.
2262 J. Org. Chem., Vol. 73, No. 6, 2008

Pericyclization of 1-Azulenyl Thioketones
in toluene (10 mL) and benzene (5 mL) at room temperature for
20 h and chromatographic purification on silica gel with CH2Cl2
also afforded 1a3 (121 mg, 74%). Green crystals; mp 73.8-
75.0 °C [lit.3 mp 68 °C]; MS (70 eV) m/z 248 (M+, 56%), 247
(M+ - H, 100); IR (KBr disk) νmax 1452, 1388, 1276, 1214, 1060,
786, 746, 690 cm-1; UV-vis (CH2Cl2) λmax (nm) (log ꢀ) 230 (4.50),
290 (4.41), 328 (4.17), 355 sh (4.03), 457 (4.26), 543 sh (3.16),
595 sh (3.03); 1H NMR (500 MHz, CDCl3) δ 9.61 (d, 1H, J ) 9.8
Hz, H8), 8.46 (d, 1H, J ) 9.5 Hz, H4), 8.03 (d, 1H, J ) 4.3 Hz,
H2), 7.81 (dd, 1H, J ) 9.8, 9.8 Hz, H6), 7.63 (dd, 2H, J ) 8.2, 1.2
Hz, H2′,6′), 7.60 (dd, 1H, J ) 9.8, 9.8 Hz, H7), 7.55 (dd, 1H, J )
9.8, 9.5 Hz, H5), 7.49 (tt, 1H, J ) 7.4, 1.2 Hz, H4′), 7.38 (t, 2H, J
) 8.2 Hz, H3′,5′), 7.23 (d, 1H, J ) 4.3 Hz, H3); 13C NMR (125
MHz, CDCl3) δ 225.6 (CdS), 150.9 (C1′), 148.1 (C3a), 142.1 (C2),
141.5 (C8a), 140.4 (C6), 139.9 (C8), 138.9 (C4), 138.0 (C1), 130.8
(C7), 130.3 (C4′), 129.3 (C2′,6′), 128.5 (C5), 127.7 (C3′,5′), 118.6 (C3).
HRMS calcd for C17H12S 248.0660, found 248.0665. Anal. Calcd
for C17H12S: C, 82.22; H, 4.87; S, 12.91. Found: C, 82.52; H,
5.01; S, 12.59.
Typical Example for the Acid-Catalyzed Isomerization into
Thiophene Derivatives. To a solution of 2b (93 mg, 0.18 mmol)
in CHCl3 (20 mL) was added hydrochloric acid (0.3 mL) at room
temperature. After the resulting mixture was stirred at the same
temperature for 20 min, the reaction mixture was poured into water
and extracted with CH2Cl2. The organic layer was washed with
5% NaHCO3 and water, dried over MgSO4, and concentrated under
reduced pressure. The residue was purified by column chromatog-
raphy on Al2O3 with 40% CH2Cl2/hexane to afford 14b (93 mg,
100%). The deep blue solution obtained by the addition of
hydrochloric acid to a solution of 2b in CDCl3 afforded the
following NMR signals assigned to 17+. 1H NMR (600 MHz,
CDCl3) δ 9.58 (s, 1H, H8), 9.10 (d, 1H, J ) 10.5 Hz, H8′), 8.81 (d,
1H, J ) 10.8 Hz, H4′), 8.37 (d, 1H, J ) 10.2 Hz, H6), 8.22 (d, 1H,
J ) 10.2 Hz, H5), 8.07 (s, 1H, H2′), 7.94 (dd, 1H, J ) 10.5, 1.3
Hz, H7′), 7.74 (dd, 1H, J ) 10.8, 1.3 Hz, H5′), 4.21 (dd, 1H, J )
7.8, 1.9 Hz, H4), 3.69 (dd, 1H, J ) 16.9, 7.8 Hz, H3), 3.37 (dd,
1H, J ) 16.9, 1.9 Hz, H3), 1.64 (s, 9H, 3′-t-Bu), 1.60 (s, 9H, 7-t-
Bu), 1.52 (s, 9H, 6′-t-Bu), 1.09 (s, 9H, 4-t-Bu); 13C NMR (150
MHz, CDCl3) δ 165.8 (C6′), 163.8 (C8b), 162.2 (C4a), 160.2 (C7),
151.5 (C8a), 150.9 (C2), 143.2 (C2b), 142.4 (C3′), 142.1 (C8), 141.2
(C6 and C3′a), 139.8 (C8′a), 137.1 (C4′), 136.6 (C5), 135.7 (C8′), 135.0
(C2′), 127.8 (C7′), 126.0 (C5′), 119.2 (C1′), 63.0 (C4), 40.0 (s, 7-t-
Bu), 39.0 (s, 6′-t-Bu), 36.8 (s, 4-t-Bu), 33.1 (s, 3′-t-Bu), 32.2 (C3),
31.6 (q, 7-t-Bu and 6′-t-Bu), 31.4 (q, 3′-t-Bu), 27.4 (q, 4-t-Bu).
Typical Example for the Thermal Isomerization into the
Thiophene Derivatives. A solution of 2b (90 mg, 0.17 mmol) in
toluene (60 mL) was refluxed for 24 h. After removing the solvent,
the residue was purified by column chromatography on Al2O3 with
40% CH2Cl2/hexane to afford 14b (68 mg, 76%).
Acknowledgment. S.I. thanks Kurata Foundation, Fund for
the Promotion of International Scientific Research, and The
Foundation for Japanese Chemical Research for financial
support. This work was partially supported by a Grant-in-Aid
for Scientific Research (Grant 18550026 to S.I.) from the
Ministry of Education, Culture, Sports, Science, and Technol-
ogy, Japan.
Supporting Information Available: General and experimental
-
details, cyclic voltammograms of 18+‚PF6 and 19+‚PF6-, spec-
-
troelectrograms of 19+‚PF6-, ESR measurements of 19+‚PF6
,
B3LYP/6-31G** density functional calculation of 1-azulenecar-
bothialdehyde, relative stability of azuleno[8,1-b,c]thiophene deriva-
tives, and copies of 1H and 13C NMR spectra of the reported
compounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
JO702309B
J. Org. Chem, Vol. 73, No. 6, 2008 2263