Diels-Alder approach to the synthesis of azulene-substituted benzene derivatives. Synthesis and redox behavior of 1,2-di(6-azulenyl)tetraphenylbenzenes

Article abstract of DOI:10.1016/S0040-4039(00)01445-3

1,2-Di(6-azulenyl)tetraphenylbenzenes and (6-azulenyl)pentaphenylbenzenes were synthesized by Diels-Alder reaction of di(6-azulenyl)acetylenes and 6-(phenylethynyl)azulenes with tetraphenylcyclopentadienone. Mono(6-azulenyl)benzenes exhibited a reduction

Full text of DOI:10.1016/S0040-4039(00)01445-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8343–8347
Diels–Alder approach to the synthesis of azulene-substituted
benzene derivatives. Synthesis and redox behavior of
1,2-di(6-azulenyl)tetraphenylbenzenes
Shunji Ito,a,* Haruki Inabe,a Testuo Okujima,a Noboru Morita,a Masataka Watanabeb and
Kimiaki Imafukuc
aDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
bInstitute for Chemical Reaction Science, Tohoku University, Sendai 980-8577, Japan
cDepartment of Chemistry, Faculty of Science, Kumamoto University, Kumamoto 860-8555, Japan
Received 11 July 2000; revised 18 August 2000; accepted 25 August 2000
Abstract
1,2-Di(6-azulenyl)tetraphenylbenzenes and (6-azulenyl)pentaphenylbenzenes were synthesized by Diels–
Alder reaction of di(6-azulenyl)acetylenes and 6-(phenylethynyl)azulenes with tetraphenylcyclo-
pentadienone. Mono(6-azulenyl)benzenes exhibited a reduction wave upon cyclic voltammetry (CV). In
contrast to the benzene derivatives, di(6-azulenyl)benzenes showed a two-step reduction wave at similar
potential region upon CV, which revealed the formation of dianions stabilized by 6-azulenyl substituents
under electrochemical reduction conditions. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: ethynylazulenes; Pd-cross-coupling reaction; Diels–Alder reaction; azulenylbenzenes.
Azulene (C10H8) is a theoretically interesting compound with many unusual properties.1
Especially, the azulene system has a tendency to stabilize cations, as well as anions owing to its
remarkable polarized properties, which are utilized to construct extremely stable methyl
cations.2 Recently, much attention has been focused on the aromatic compounds with extended
p-electron systems because of their special optical and electrical properties.3 However, to date,
molecules with potentially useful electronic properties constructed by non-benzenoid aromatic
compounds are fairly scarce. We have recently developed a simple route to unknown 1,3,5-tri(1-
azulenyl)benzenes by tetrachlorosilane-mediated cyclotrimerization of 1-acethylazulenes.4 Very
recently, Hafner et al. reported the ethynylation of azulenes in a five-membered ring utilizing
Pd-catalyzed cross-coupling reaction of bromo- and iodoazulenes with trimethylsiliylacetylene5
and the cyclooligomerization of the 1-ethynylazulenes catalyzed by CpCo(CO)2 to afford
1-azulenylbenzene derivatives and (cyclobutadiene)cobalt complexes in low yields.6
* Corresponding author. Tel/fax: +81-22-217-7714; e-mail: ito@funorg.chem.tohoku.ac.jp
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01445-3

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Herein we report an efficient preparation of azulene-substituted benzene derivatives by
Diels–Alder reaction of di(6-azulenyl)acetylenes (3a and 3b) and 6-(phenylethynyl)azulenes (4a
and 4b) with tetraphenylcyclopentadienone (5) to afford novel 6-azulenylbenzene derivatives 1a,
1b, 2a, and 2b, which are expected to have a different character from that of 1-azulenylbenzene
derivatives.7 According to the nature of the azulene systems, 1a and 1b would exhibit the
formation of stabilized closed shell dianions 6 by electrochemical reduction (Scheme 1).
Although there are many reports concerning carbocations stabilized by 1-azulenyl substituents,
little is known about carbanions stabilized by 6-azulenyl substituents because of the synthetic
difficulty of the functionalization of azulene in a seven-membered ring.1 Thus, we also report
here the redox behavior of these novel 6-azulenylbenzene derivatives (1a, 1b, 2a, and 2b) and
6-azulenylacetylenes (3a, 3b, 4a, and 4b) examined by cyclic voltammetry (CV).
Scheme 1. Redox system for the reduction of 1a, 1b and that for a general structure of violenes 7
Pd-catalyzed cross-coupling reaction of 6-bromoazulenes (8a and 8b)8 with trimethylsilyl-
acetylene under Sonogashira–Hagihara conditions9 at room temperature afforded 6-(trimethyl-
silylethynyl)azulenes (9a and 9b) in 86 and 84% yields, respectively. It is noteworthy that the
ethynylation of azulenes in the seven-membered ring readily proceeded using bromides at room
temperature. Treatment of 9a and 9b with potassium fluoride in DMF furnished 6-ethynylazu-
lenes (10a and 10b)10 in 78 and 93% yields, respectively. Cross-coupling reaction of 10a with 8a
using PdCl2(PPh3)2 as a catalyst afforded an inseparable mixture (1:2.2) of di(6-azu-
lenyl)acetylene (3a) and di(6-azulenyl)diacetylene (11), which was obtained from the oxidative
coupling of 10a by the Pd-catalyst.9,11 However, we found that the reaction of 10a with 8a
utilizing Pd(PPh3)4 as a catalyst exclusively gave the desired 3a in 98% yield. Similarly, 3b was
obtained by the reaction of 10b with 8b under similar conditions in 96% yield. Diels–Alder
reaction of 3a and 3b with a large excess of 5 in diphenyl ether at 160°C under an Ar
atmosphere for 1–2 days and subsequent chromatographic purification of the reaction mixture
on silica gel afforded 1a and 1b in 13 and 91% yields, respectively (Scheme 2).12

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Scheme 2. Synthesis of 1a and 1b. (a) TMSA, [PdCl2(PPh3)2], PPh3, CuI, Et3N, toluene, rt, 2.5 h (86%) for R=H, 1.5
h (84%) for R=COOEt; (b) KF, H2O, DMF, rt, 3 h (78%) for R=H, 3 h (93%) for R=COOEt; (c) 8a and 8b,
[Pd(PPh3)4], PPh3, CuI, Et3N, toluene, rt, 3 h (98%) for R=H, 1.5 h (96%) for R=COOEt; (d) 5, diphenyl ether,
160°C, 2 days (13%) for R=H, 1 day (91%) for R=COOEt
Synthesis of 2a and 2b as outlined in Scheme 3 also commenced with 8a and 8b, respectively.
6-(Phenylethynyl)azulenes (4a and 4b) were prepared by the Sonogashira reaction of 8a and 8b
with phenylacetylene in 90 and 87% yields, respectively. Diels–Alder reaction of 4a and 4b with
a large excess of 5 in diphenyl ether at 140 and 160°C, respectively, under an Ar atmosphere for
5 days afforded 2a and 2b in 32 and 93% yields, respectively. The spectral features of 1a, 1b, 2a,
and 2b are in agreement with the structure of these products. 1a, 1b, 2a, and 2b are remarkably
stable, showing no decomposition even after several weeks at room temperature.
Scheme 3. Synthesis of 2a and 2b. (a) Phenylacetylene, [PdCl2(PPh3)2], PPh3, CuI, Et3N, toluene, rt, 5 h (90%) for
R=H, 9 h (87%) for R=COOEt; (b) 5, diphenyl ether, 160°C, 1 day (32%) for R=H, 140°C, 5 days (93%) for
R=COOEt
1,2-Di(6-azulenyl)benzene 1b exhibited the voltammogram, as shown in Fig. 1(a). The
voltammogram was characterized by a barely separated two-step reduction wave at E1/2 −1.57
and −1.72 V. 1a also showed the transfer of two electrons at −2.00 and −2.20 V upon CV. Thus,
the redox system of 1a and 1b is depicted to be like a general structure of violenes 7, as shown

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Figure 1. Cyclic voltammograms of (a) 1b and (b) 3b in THF containing n-Bu4N+BF4 (0.1 M) as a supporting
electrolyte; V versus Ag/Ag+, Pt electrode, scan rate 100 mV s−1, and Fc/Fc+=0.10 V
−
in Scheme 1.13 The less negative reduction potentials of 1b compared with those of 1a are
attributable to the stabilization of anions by the ethoxycarbonyl groups substituted in the
five-membered ring. The first reduction potentials and even the second ones of 1a and 1b are almost
comparable with those of 2a (−2.06 V) and 2b (−1.63 V), respectively. Therefore, the two 6-azulenyl
substituents on benzene in a 1,2 relationship increased its electron-accepting properties because
of the closed-shell dianionic structure 6. Unfortunately, the reduction waves of 1a and 1b showed
poor reversibility, which exhibited the low stabilities of the expected radical anion and dianion
6 under the electrochemical reduction conditions. Thus, the measurement of the UV–vis
spectroscopy of 1b under electrochemical reduction conditions did not afford any evidence of the
formation of the radical anion and/or dianion 7 due to the instability of these species.
1,2-Di(6-azulenyl)acetylenes 3a and 3b are also an example of the two-electron redox system
stabilized by two 6-azulenyl substituents, as illustrated in Scheme 4. In contrast to the poor
reversibility of 1a and 1b, 3a and 3b exhibited a reversible two-step reduction wave (3a, E1/2 −1.41
and −1.61 V; 3b, E1/2 −0.96 and −1.15 V, respectively) upon CV (Fig. 1(b)). These reduction
potentials of 3a and 3b are less negative than those of 4a (−1.66 V) and 4b (−1.29 V), respectively.
Thus, the two 6-azulenyl substituents of 3a and 3b also increase the electron-accepting properties.
Scheme 4. Redox system for the reduction of 3a and 3b
We have demonstrated the Pd-catalyzed ethynylation of azulenes in a seven-membered ring
using bromoazulenes and the efficient preparation of novel 6-azulenylbenzene derivatives 1 and
2. Although the radical anion and dianion of 1 were not detected by UV–vis spectroscopy due
to the instability of these species, 1 was clarified to represent the presumed two-step redox system
upon CV. A cyclooligomerization approach to the synthesis of novel 6-azulenylbenzene derivatives
using (6-azulenyl)acetylenes by transition metal-catalyzed reaction will be published elsewhere.

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