Efficient preparation of 2-azulenylboronate and Miyaura-Suzuki cross-coupling reaction with aryl bromides for easy access to poly(2-azulenyl)benzenes

Article abstract of DOI:10.1016/j.tet.2004.04.057

This paper describes an efficient preparation of 2-azulenylboronate (6) starting from 2-iodoazulene by halogen-metal exchange reaction using n-BuLi and subsequent quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The boronate 6 has been found to undergo Pd-catalyzed Miyaura-Suzuki cross-coupling reaction with a range of aryl bromides including aromatic poly bromides utilizing Pd₂(dba)₃-P(t-Bu)₃ as a catalyst and establishes a strategy to produce novel poly(2-azulenyl)benzenes, some of which are found to be insoluble in common organic solvents, however. The redox behavior of 2-arylazulenes and poly(2-azulenyl)benzenes was examined by cyclic voltammetry (CV) and compared with those of 6-azulenylbenzene derivatives reported previously.

Full text of DOI:10.1016/j.tet.2004.04.057

Tetrahedron 60 (2004) 5357–5366
Efficient preparation of 2-azulenylboronate and Miyaura-Suzuki
cross-coupling reaction with aryl bromides for easy access to
poly(2-azulenyl)benzenes
b
b
b
b
Shunji Ito,a Tomomi Terazono, Takahiro Kubo, Tetsuo Okujima, Noboru Morita,
,
*
Toshihiro Murafuji,c Yoshikazu Sugihara,c Kunihide Fujimori,d Jun Kawakamia and Akio Tajiria
aDepartment of Materials Science and Technology, Faculty of Science and Technology, Hirosaki University, Bunkyocho 3,
Hirosaki 036-8561, Japan
bDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
cDepartment of Chemistry, Faculty of Science, Yamaguchi University, Yamaguchi 753-8512, Japan
dDepartment of Chemistry, Faculty of Science, Shinshu University, Matsumoto 390-8621, Japan
Received 9 February 2004; revised 22 April 2004; accepted 23 April 2004
Abstract—This paper describes an efficient preparation of 2-azulenylboronate (6) starting from 2-iodoazulene by halogen–metal exchange
reaction using n-BuLi and subsequent quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The boronate 6 has been found
to undergo Pd-catalyzed Miyaura-Suzuki cross-coupling reaction with a range of aryl bromides including aromatic poly bromides utilizing
Pd2(dba)3–P(t-Bu)3 as a catalyst and establishes a strategy to produce novel poly(2-azulenyl)benzenes, some of which are found to be
insoluble in common organic solvents, however. The redox behavior of 2-arylazulenes and poly(2-azulenyl)benzenes was examined by
cyclic voltammetry (CV) and compared with those of 6-azulenylbenzene derivatives reported previously.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
In recent years, there has been vital interest in transition-
metal catalyzed cross-coupling reaction that can be used for
carbon–carbon bond formation.1 Several applications of the
palladium-catalyzed reaction in the chemistry of azulene
have appeared in the literature, for example, palladium-
catalyzed vinylation,2 arylation,3 ethynylation,4 and alkyl-
Chart 1.
ation5 of azulenyl halides or triflate. We have recently
preparation of 2-(tri-n-butylstannyl)azulene (2) utilizing Pd-
developed the first versatile organometallic reagents of
catalyzed direct stannylation of 2-bromoazulene (3a).6b
azulenes, 6-(tri-n-butylstannyl)azulene (1a) and its 1,3-
diethoxycarbonyl derivative (1b), which have been sub-
Boronate reagents also represent an important class of
jected to the Pd(0)-catalyzed Stille cross-coupling reaction
synthetic intermediates for the transition-metal catalyzed
with aryl, acyl, and/or azulenyl halides (Chart 1).6 The study
reaction. Miyaura-Suzuki cross-coupling of organoborane
made up for the deficiency of the organometallic reagent for
compounds with a variety of organic electrophiles, cata-
the transition-metal catalyzed reaction of azulene itself.
lyzed by palladium, provides an efficient method for
Especially, application of the reagents is highly advan-
carbon–carbon bond formation.8 Synthesis of the boronate
tageous for multiple functionalization by azulenyl groups
reagents consists of conventional Pd-catalyzed cross-
because the method does not require the troublesome
coupling of aryl bromides, iodides, or triflates with either
preparation of a polymetallic species.7 However, extension
alkoxyboron derivatives such as bis(pinacolato)diboron (4)
of the methodology to the functionalization of azulenes at
or pinacolborane (5).9,10 The direct boronate formation of
the 2-position was so far hampered by the inefficiency of the
2-iodoazulene (3b) utilizing Pd-catalyst has been revealed
to be a convenient method for preparing 2-azulenylboronate
Keywords: Azulenylboronate; Palladium-catalyzed reaction; Miyaura-
(6) (Scheme 1).11 However, the yield of the boronate 6
Suzuki cross-coupling; Redox property; Violene-cyanine hybrid.
remains so far at most 42%. More recently, Ir-catalyzed
reaction of azulene (7) with bis(pinacolato)diboron (4) has
*
Corresponding author. Tel.: þ81-172-39-3568; fax: þ81-172-39-3541;
e-mail address: itsnj@cc.hirosaki-u.ac.jp
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.04.057

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S. Ito et al. / Tetrahedron 60 (2004) 5357–5366
Table 1. Pd-catalyzed syntheses of 2-azulenylboronate (6)a
Entry
X
Boron reagent
Catalyst
Base
Solvent
Yield
(%)b
Scheme 1.
6
3
improved the yield of the desired 2-azulenylboronate (6) to
70%, in spite of the formation of undesirable 1-azulenyl-
boronate (8) in a certain amount (10%) (Scheme 2).12
1
2
3
4
Br
I
Br
I
4
4
5
5
PdCl2(dppf) c KOAc DMSO
40
53
5
PdCl2(dppf)
PdCl2(dppf)
PdCl2(dppf)
KOAc DMSO
Et3N
Et3N
Dioxane
Dioxane 38
52
a
Reactions of 3a and 3b (1 mmol) with bis(pinacolato)diboron (4)
(1.1 mmol) were carried out at 80 8C for 5 h by using Pd-catalyst
(5 mol%) and KOAc (3 mmol) in DMSO (6 mL). Reactions of 3a and 3b
(1 mmol) with pinacolborane (5) (1.5 mmol) were carried out at 80 8C for
4 h by using Pd-catalyst (5 mol%) and triethylamine (3 mmol) in dioxane
(6 mL).
All yields are isolated yields.
PdCl2(dppf): [1,10-bis(diphenylphosphino)ferrocene]palladium(II)
dichloride.
b
c
3b) with pinacolborane (5) could not alter the situation
(entries 3 and 4).9,10
Scheme 2.
The aryl boronate reagents could be also prepared by
transmetalation between aryllithium or arylmagnesium
reagents and boron compounds which have a good leaving
group such as a halogen or an alkoxy group.16 Synthetic
inaccessibility of the metalated azulene due to the high
reactivity of azulenes with organolithium and magnesium
reagents to give dihydroazulene derivatives17 hampered
application to the transmetalation procedure. However,
recently, generation of 2-azulenyllithium and magnesium
reagents (9a and 9b) has been accomplished by the
transmetalation between 2-iodoazulene (3b) and n-BuLi or
(n-Bu)3MgLi.18 The 2-azulenylmagnesium reagent (9b)
has been revealed to exhibit the envisaged borylation with
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (10)
in 53% yield.19,20 We found the borylation was much
more effective by the use of 2-azulenyllithium reagent
(9a) prepared by halogen–metal exchange reaction of
2-iodoazulene (3b) using n-BuLi at low temperature.
Subsequent quenching of the reagent by 10 afforded the
Herein, we report an efficient preparation of 2-azulenyl-
boronate (6) and an efficient catalytic system for the
Miyaura-Suzuki cross-coupling reaction of 6 with aryl
bromides and the successful application to the Pd(0)-
catalyzed cross-coupling reaction of 6 with aromatic poly
bromides to afford poly(2-azulenyl)benzenes. We have
recently proposed that the poly(6-azulenyl)benzene deriva-
tives are considered to be a novel model compound of the
violene-cyanine hybrid recently reported by Hu¨nig et al.13
Depending on the number and position of the 6-azulenyl
substituents, the benzene derivatives provide a closed-shell
cyanine-type substructure by an overall two-electron
transfer.14 The poly(2-azulenyl)benzenes might also pro-
vide a closed-shell system as a cyanine dye by an overall
two-electron transfer, although the system does not provide
a formal cyclopentadienide substructure in the closed-shell
form. Herein, we also report the redox behavior of several
2-arylazulenes and poly(2-azulenyl)benzenes prepared by
the cross-coupling reaction of 6.
2. Results and discussion
2.1. Efficient synthesis of 2-azulenylboronate (6)
Employment of the reaction of 2-bromoazulene (3a)15 with
diboron 4 did not improve the yield of the desired
2-azulenylboronate (6) under the conditions originally
described by Miyaura et al. (entry 1) (Table 1).9 A slightly
larger amount of the Pd-catalyst (5 mol%) in the reaction of
2-iodoazulene (3b)15 with 4 was found to improve some-
what the yield of the desired 2-azulenylboronate (6) (entry
2), whereas a modified catalytic system of PdCl2(dppf) with
Et3N in dioxane in the reaction of 2-haloazulenes (3a and
Scheme 3.

S. Ito et al. / Tetrahedron 60 (2004) 5357–5366
5359
desired 2-azulenylboronate reagent (6) in 78% yield as a
single product (Scheme 3).
conversion ratio of the catalytic reaction (entry 9).24 Using
the chloride 11b or iodide 11c instead of the bromide 11a
decreased the yield of the desired cross-coupling product 12
(entries 10 and 11). The formation of 2,20-biazulene in
significant amounts in the case of the reaction with chloride
11b should be attributed to the homocoupling of 6 under the
reaction conditions.25
2.2. Miyaura-Suzuki cross-coupling reaction
To demonstrate the transformations utilizing 6, the reaction
with diethyl 2-amino-6-bromoazulene-1,3-dicarboxylate
has been conducted under the Miyaura-Suzuki cross-
coupling reaction conditions.11 However, in our initial
experiments in the cross-coupling reaction with aryl halides,
6 was found to be inefficient under similar conditions to
Miyaura-Suzuki’s (Table 2). Concretely, the reaction of 6
with 11a in the presence of PdCl2(PPh3)2 catalyst produced
the desired 2-(4-tolyl)azulene (12)21 in mediocre yield
(39%) together with undesired 2-phenylazulene (13)21,22
(entry1) (Chart 1). The choice of the catalytic system was
the key to the success of the reaction of 6 with aryl halides.
The use of Pd(PPh3)4 as a catalyst instead of PdCl2(PPh3)2
was also ineffective in this reaction (entry 2). Formation of
the by-product 13 could be rationalized by an aryl–aryl
exchange in the intermediate palladium(II) complex and
subsequent coupling with the 2-azulenylboronate (6).23
Indeed, substitution of the Pd(PPh3)4 catalyst with
Pd2(dba)3–P(t-Bu)3 in the catalytic protocol resulted in a
significant increase of the desired cross-coupling product 12
in 73% yield (entry 3). The Pd(OAc)2–P(t-Bu)3 catalytic
system was also effective in this reaction (entry 6).
However, the use of P(o-Tol)3 or PCy3 as a ligand did not
afford satisfactory results either with Pd2(dba)3 or Pd(OAc)2
as a Pd-catalyst (entries 4, 5, 7 and 8). The addition of KF as
a base in the catalytic protocol decreased significantly the
2.3. Generality
To examine the generality of the reaction conditions, the
cross-coupling reaction with several aryl bromides (15a–c)
was conducted under the Pd2(dba)3–P(t-Bu)3 reaction
conditions. The results of the cross-coupling reaction of 6
with the aryl bromides are summarized in Table 3. The
electron-deficient aryl bromide, 4-bromonitrobenzene
(15a), was efficiently reacted with 6 to afford the coupled
product 16a in high yield (entry 1). However, the reaction of
6 with 4-bromoacetophenone (15b) afforded the desired
coupled product 16b in moderate yield (entry 2). The
product 16b contains enolizable keto-group. The relatively
low yield of the product 16b should be attributed to the side
reaction arising from undesired aldol condensations. The
yield of 16b was slightly improved by the use of Pd(PPh3)4
as a catalyst (42%). In the case of the reaction of 6 with
electron-rich bromide, 4-bromoanisole (15c), the reaction
also proceeded smoothly to give the cross-coupling product
16c in good yield (entry 3). On the whole, 6 reacted with
several aryl bromides including an electron-rich one under
the Pd-catalyzed conditions and the isolated yields of the
cross-coupling product were generally high, except for 16b.
Table 2. Miyaura-Suzuki cross-coupling reaction of 6 with 4-tolyl halides 11a–ca
Entry
X
Catalyst
Ligand
Base
Solvent
Yield (%) b
12
13
14
7
6
1c
2
3
4
5
6
7
8
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
I
PdCl2(PPh3)2
Pd(PPh3)4
Pd2(dba)3
Ba(OH)2·8H2O
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KF
DME/H2O
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
39
27
73
54
25
72
27
42
7
21
19
d
P(t-Bu)3
P(o-Tol)3
PCy3
P(t-Bu)3
P(o-Tol)3
PCy3
P(t-Bu)3
P(t-Bu)3
P(t-Bu)3
Pd2(dba)3
Pd2(dba)3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
5
3
5
5
32
9
10
11
74
3
22
Cs2CO3
Cs2CO3
12
51
40
4
4
a
Reactions of 6 with 4-halotoluene (2 equiv.) were carried out at 80 8C for 24 h using Pd-catalyst (5 mol%), ligand (Pd:P¼1:2–3), and 1.5 equiv. of base in
solvent (6 mL/6 (1 mmol)).
All yields are isolated yields.
b
c
d
Pd-catalyst (10 mol%) and 2.0 equiv. of base in DME:H2O (50:1) (10 mL/6 (0.4 mmol)) were used.
Pd2(dba)3: tris(dibenzylideneacetone)dipalladium(0).

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S. Ito et al. / Tetrahedron 60 (2004) 5357–5366
Table 3. Cross-coupling reaction of 6 with aryl bromides 15a–ca
Entry
R
Catalyst
Ligand
Time (h)
Product (yield (%))b
1
2
3
NO2
COMe
OMe
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
P(t-Bu)3
P(t-Bu)3
P(t-Bu)3
6
24
19
16a (80)
16b (34)
16c (72)
a
Reaction conditions: aryl bromides (2 equiv.), Cs2CO3 (1.5 equiv.), Pd-catalyst (5 mol%), ligand (Pd:P¼1:3–4) in dioxane (3 mL/6 (0.4 mmol)) at 80 8C.
b
All yields are isolated yields.
2.4. Polysubstitution
adduct 25 in 13% yield (Scheme 8). The mass spectrum of
the insoluble material showed the correct Mþ ion peak at
m/z 582, which indicated the formation of the expected
tetrakis-adduct 26. The elimination of bromide was a side
reaction for the multi-functionalization of benzene with
2-azulenyl substituents.
Finally, we demonstrated the intended use of the 2-azulenyl-
boronate (6) in the synthesis of poly(2-azulenyl)benzenes.
The scope of this methodology for multiple substitution was
demonstrated utilizing the reaction of 6 with 1,2-di-, 1,4-di-,
1,3,5-tri-, 1,2,4-tri-, and 1,2,4,5-tetrabromobenzenes
(17–21). The reaction of 6 with 1,2-dibromobenzene (17)
afforded the desired coupling product, 1,2-bis(2-azulenyl)-
benzene (22) in 33% yield (Scheme 4). Likewise, the
reaction of 6 with 1,4-dibromobenzene (18) gave the
expected 1,4-bis(2-azulenyl)benzene (23) in 44% yield,
although the product 23 does not show any solubility in
common organic solvents (Scheme 5). Sublimation could be
used for the purification of the product 23 under reduced
pressure. The insoluble material exhibited an ion peak at m/z
330 upon mass spectrum, which corresponds to the correct
Mþ ion peak of 1,4-di(2-azulenyl)benzene (23).
In the case of the reaction of 6 with 1,3,5-tri- and 1,2,4-
tribromobenzenes (19 and 20), the desired tris-adducts 24
and 25 were obtained in 18 and 36% yields, respectively,
(Schemes 6 and 7). The reaction of 6 with 1,2,4,5-
tetrabromobenzenes (21) afforded an insoluble material
(13%) in a common organic solvent along with 1,2,4-tris-
Scheme 6.
Scheme 7.
Scheme 4.
Scheme 5.
Scheme 8.

S. Ito et al. / Tetrahedron 60 (2004) 5357–5366
5361
2.5. Redox behavior
The redox potentials (V vs Ag/Agþ) of 2-arylazulenes (12,
13, and 16a–c) and poly(2-azulenyl)benzenes (22, 24, and
25) measured by CV along with those of 6-azulenylbenzene
derivatives (27, 28, and 29)14 are summarized in Table 4
(Chart 2). Insolubility of the products 23 and 26 in common
organic solvents hampered the CV measurement of these
compounds.
Table 4. Redox potentialsa of 2-arylazulenes and poly(2- and 6-
azulenyl)benzenes
Sample Eo1x (V) Eo2x (V)
Er1ed (V)
Er2ed (V)
Er3ed (V) Ref.
12
13
16a
16b
16c
22
(þ1.07) (þ1.34) (21.85)
(þ1.07) (þ1.39) (21.82)
(þ1.05) (þ1.38) 21.33
(þ1.04) (þ1.33) 21.65
(þ0.76) (þ1.02) 21.87
(þ0.52) (þ0.89) 21.82
(þ0.69) (þ1.11) (21.72)b
(þ0.42) (þ1.10) 21.71
21.68
(22.17)
22.02
(21.85)b (22.01)b
24
25
21.87
(22.20)
22.11
27c
28c
29c
(þ0.81)
(22.00)
14
(þ0.73) (þ1.23) (21.75)b
(21.87)b (22.00)b 14
(þ0.78) (þ1.29) 21.74 (2e) 22.15
14
a
The redox potentials were measured by CV (0.1 M n-Bu4NBF4 in o-
dichlorobenzene, Pt electrode, scan¼100 mV s21, and Fcþ/Fc¼0.27 V).
In the case of irreversible waves, which are shown in parentheses, Eox and
Ered were calculated as Epa (anodic peak potential)20.03 and Epc
(cathodic peak potential)þ0.03 V, respectively.
b
c
The values are peak potentials measured by DPV.
The potentials have been measured in 0.1 M n-Bu4NBF4 tetrahydrofuran
solution (Fþc /Fc¼0.19 V).
Figure 1. Cyclic voltammograms of (a) 22, (b) 24, and (c) 25 in
o-dichlorobenzene containing n-Bu4NBF4 (0.1 M) as a supporting
Chart 2.
electrolyte; scan rate, 100 mV s21
.

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S. Ito et al. / Tetrahedron 60 (2004) 5357–5366
Scheme 9.
As seen from Table 4, 2-phenylazulene (13) showed an
irreversible one-electron transfer at 21.82 V upon CV. As
expected, the electron-donating group on the phenyl group,
that is, 4-methyl (12) and 4-methoxy (16c), slightly
decreases the electron affinity of the azulene ring. In
contrast to the one-electron transfer of 12, 13, and 16c,
compounds 16a and 16b showed two-stage, one-electron
reductions due to the redox reaction of the electron-
withdrawing substituents such as 4-nitro and 4-acetyl
groups. In addition to the two-stage reductions, compounds
16a and 16b showed improvement of the reversibility of the
CV waves due to the stabilization of the radical anionic
state.
2-azulenylboronate reagent (6) which has been found to
undergo Pd-catalyzed coupling with a range of aryl halides,
and herein established a strategy to produce novel poly(2-
azulenyl)benzene derivatives. The reaction of 6 with 1,2-di-,
1,4-di-, 1,3,5-tri-, 1,2,4-tri-, and 1,2,4,5-tetrabromo-
benzenes afforded 1,2-di-, 1,4-di-, 1,3,5-tri-, 1,2,4-tri-, and
1,2,4,5-tetra(2-azulenyl)benzene derivatives (22, 23, 24, 25
and 26). These results provide a straightforward method-
ology for the carbon–carbon bond formation at the
2-position of azulene. Although 2-azulenyl substituents do
not possess the formal cyclopentadienide substructure in the
electrochemically reduced form, the redox behaviors
examined by CV of these compounds clarified the presumed
multiple-electron transfer under the electrochemical
conditions.
Poly(2-azulenyl)benzenes (22, 24, and 25) revealed multi-
electron redox properties. 1,2-Di(2-azulenyl)benzene
(22) exhibited a well-resolved two-step reduction wave at
E1/2¼21.82 and 22.02 V, upon CV (Fig. 1(a)). The first
reduction potential and even the second one of 22 are almost
comparable with those of 12, 13, and 16c. Therefore, the
two 2-azulenyl substituents on a benzene ring exhibit
multiple electron affinity similarly to the reduction of
6-azulenyl derivative (27). Thus, the redox system of 22
could be considered to be that of violene as illustrated in
Scheme 9. 1,3,5-Tri(2-azulenyl)benzene (24) exhibited a
quasi-reversible three-step reduction wave at around
21.90 V, upon CV (Fig. 1(b)). The wave was identified as
the superimposition of three independent waves at 21.72,
21.85, and 22.01 V, by differential pulse voltammetry
(DPV) (Fig. 2) similarly to the reduction of 6-azulenyl
derivative (28). Therefore, the three 2-azulenyl substituents
on the benzene ring are concluded to result in an increase of
the multiplicity of electron affinity due to the reduction of
the respective azulene chromophore. The three-step
reduction exhibits the existence of the redox interaction
among the three 2-azulenyl groups. Similarly to the three-
step reduction of 24, 1,2,4-tri(2-azulenyl)benzene (25) also
showed a three-step reduction wave with excellent
reversibility upon CV (Fig. 1(c)), although the reduction
of 25 is expected to show a one-step, two-electron transfer
as observed in the reduction of 6-azulenyl derivative (29).
Consequently, the redox property of tri(2-azulenyl)-
benzenes (24 and 25) does not depend on the position of
the 2-azulenyl substituents on the benzene ring and the
redox system of 25 could be depicted in Scheme 10.
4. Experimental
4.1. General
Melting points were determined on a Yanagimoto micro
melting apparatus MP-S3 and are uncorrected. Mass spectra
were obtained with a JEOL HX-110, a Hitachi M-2500, or a
Bruker APEX II instrument, usually at 70 eV. IR and UV
spectra were measured on a Shimadzu FTIR-8100M and a
1
Hitachi U-3410 spectrophotometer, respectively. H NMR
spectra (13C NMR spectra) were recorded on a JEOL GSX
400 at 400 MHz (100 MHz), a JEOL JNM A500 at
500 MHz (125 MHz), or a Bruker AM 600 spectrometer
at 600 MHz (150 MHz). Gel permeation chromatography
(GPC) purification was performed on a TSKgel G2000H6.
Voltammetry measurements were carried out with a BAS
100B/W electrochemical workstation equipped with Pt
working and auxiliary electrodes, and a reference electrode
formed from Ag/AgNO3 (0.01 M, 1 M¼1 mol dm23) in a
tetrabutylammonium perchlorate (0.1 M) acetonitrile
solution. Elemental analyses were performed at the
Instrumental Analysis Center of Chemistry, Faculty of
Science, Tohoku University.
4.1.1. 2-(2-Azulenyl)-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane (6). To a solution of n-butyllithium (1.5 mL,
1.6 M solution in hexane, 2.4 mmol) in ether (10 mL) was
added dropwise at 2100 8C a solution of 2-iodoazulene (3b)
(256 mg, 1.01 mmol) in ether (20 mL). The mixture was
allowed to react at 280 8C for 30 min. A solution of
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (10)
(563 mg, 3.03 mmol) in ether (5 mL) was added dropwise to
the cooled mixture. The mixture was allowed to warm to
3. Conclusion
We have demonstrated an efficient preparation of

S. Ito et al. / Tetrahedron 60 (2004) 5357–5366
5363
room temperature to react for 1.5 h. The reaction was
quenched with water, and the organic layer was separated,
washed with brine, and dried with MgSO4. The solvent was
evaporated and the residue was purified by column
chromatography on silica gel with CH2Cl2 and GPC with
CHCl3 to afford 6 (201 mg, 78%). blue prisms; mp 97–
100 8C (sublimation) [lit.11 mp 99–101 8C].
4.1.2. General procedure for the Pd-catalyzed reaction
of 2-azulenylboronate (6). To a degassed solution of 6, aryl
halides, and base in solvent was added Pd-catalyst and
ligand. The resulting mixture was heated at 80 8C under an
Ar atmosphere. The reaction mixture was poured into water
and then extracted with toluene or CH2Cl2. The organic
layer was washed with water, dried with MgSO4, and
concentrated under reduced pressure. The products were
isolated by column chromatography on silica gel and/or gel
permeation chromatography (GPC) with CHCl3.
4.1.3. 2-Tolylazulene (12). The general procedure was
followed by using 2-azulenylboronate (6) (252 mg,
0.992 mmol), 4-bromotoluene (11a) (340 mg, 1.99 mmol),
Cs2CO3 (479 mg, 1.47 mmol), Pd2(dba)3 (45 mg,
0.049 mmol), and P(t-Bu)3 (57 mg, 0.28 mmol) in dioxane
(6 mL). Column chromatography on silica gel with 10%
ethyl acetate/hexane and GPC afforded 12 (157 mg, 73%).
blue plates; mp 216–217 8C [lit.21 mp 214 8C]; MS (70 eV)
m/z (relative intensity) 218 (Mþ, 100%); IR (KBr disk) nmax
1406, 806, 723 and 505 cm21; UV–vis (CH2Cl2) lmax, nm
(log 1) 246 (4.18), 275 sh (4.37), 288 sh (4.56), 301 (4.79),
312 (4.83), 345 sh (3.68), 361 (3.90), 378 (4.17), 397 (4.23),
435 (2.26), 534 sh (2.47), 573 (2.62), 613 (2.60), and 666 sh
1
(2.25); H NMR (400 MHz, CDCl3) d¼8.24 (d, J¼9.9 Hz,
2H, H-4,8), 7.85 (d, J¼8.3 Hz, 2H, H-20,60), 7.64 (s, 2H,
H-1,3), 7.47 (t, J¼9.9 Hz, 1H, H-6), 7.26 (d, J¼8.3 Hz, 2H,
H-30,50), 7.13 (dd, J¼9.9 Hz, 2H, H-5,7), and 2.40 (s, 3H,
40-Me).
4.1.4. 2-(4-Nitrophenyl)azulene (16a). The general
procedure was followed by using 2-azulenylboronate (6)
(100 mg, 0.393 mmol), 4-bromonitrobenzene (15a)
(161 mg, 0.797 mmol), Cs2CO3 (198 mg, 0.608 mmol),
Pd2(dba)3 (18 mg, 0.020 mmol), and P(t-Bu)3 (35 mg,
0.17 mmol) in dioxane (3 mL) at 80 8C for 6 h. Column
chromatography on silica gel with CH2Cl2 afforded 16a
(78 mg, 80%). green needles; mp 249–255 8C decomp.
(hexane wash) [lit.21 mp 248–249 8C]; MS (70 eV) m/z
(relative intensity) 249 (Mþ, 100%) and 202 (57); IR (KBr
disk) nmax 1514, 1509, 1347, 1331, 808 and 756 cm21
;
UV–vis (CH2Cl2) lmax, nm (log 1) 237 (4.27), 265 (4.16),
296 sh (4.50), 312 (4.58), 368 sh (4.26), 381 (4.45), 399
(4.46), 544 sh (2.54), 587 (2.70), 627 (2.70), and 683 sh
1
(2.39); H NMR (600 MHz, CDCl3) d¼8.36 (d, J¼9.9 Hz,
2H, H-4,8), 8.32 (d, J¼9.0 Hz, 2H, H-30,50), 8.08 (d,
J¼9.0 Hz, 2H, H-20,60), 7.71 (s, 2H, H-1,3), 7.61 (t,
J¼9.9 Hz, 1H, H-6), and 7.22 (dd, J¼9.9, 9.9 Hz, 2H,
H-5,7); 13C NMR (150 MHz, CDCl3) d¼147.2 (C-10), 146.5
(C-2), 143.0 (C-40), 141.4 (C-3a,8a), 138.2 (C-6), 0 137.7
(C-4,8), 128.0 (C-20,60), 124.4 (C-5,7), 124.3 (C-30,5 ), and
115.0 (C-1,3). Anal. Calcd for C16H11NO2: C, 77.10; H,
4.45; N, 5.62. Found: C, 76.83; H, 4.65; N, 5.55.
Figure 2. Differential pulse voltammograms of (a) 22, (b) 24, and (c) 25 in
o-dichlorobenzene containing n-Bu4NBF4 (0.1 M) as a supporting
4.1.5. 2-(4-Acetylphenyl)azulene (16b). The general
electrolyte; scan rate, 20 mV s21
.

5364
S. Ito et al. / Tetrahedron 60 (2004) 5357–5366
Scheme 10.
procedure was followed by using 2-azulenylboronate (6)
(110 mg, 0.433 mmol), 4-bromoacetophenone (15b)
(150 mg, 0.754 mmol), Cs2CO3 (195 mg, 0.598 mmol),
and Pd(PPh3)4 (27 mg, 0.023 mmol) in dioxane (3 mL) at
80 8C for 24 h. Column chromatography on silica gel with
CH2Cl2 afforded 16b (45 mg, 42%). blue plates; mp 247–
256 8C decomp. (ethyl acetate); MS (70 eV) m/z (relative
intensity) 246 (Mþ, 100%); IR (KBr disk) nmax 1647
(CvO), 1271, and 808 cm21; UV–vis (CH2Cl2) lmax, nm
(log 1) 242 (4.16), 303 sh (4.80), 314 (4.91), 360 sh (4.14),
376 (4.38), 396 (4.40), 542 sh (2.57), 582 (2.70), 623 (2.68),
and 676 sh (2.39); 1H NMR (600 MHz, CDCl3) d¼8.34 (d,
J¼9.90Hz, 2H, H-4,8), 8.05 (m, 2H, H-30,50), 8.05 (m, 2H,
H-20,6 ), 7.72 (s, 2H, H-1,3), 7.56 (t, J¼9.9 Hz, 1H, H-6),
7.20 (dd, J¼9.9, 9.9 Hz, 2H, H-5,7), and 2.66 (s, 3H,
400-COMe); 13C NMR (150 MHz, CDCl3) d¼197.7 (s,
4 -COMe), 148.0 (C-2), 141.3 (C-3a,8a), 141.0 (C-400),
137.4 (C-6), 136.9 (C-4,8), 136.3 (C-10), 129.0 (C-20,6 ),
127.6 (C-30,50), 124.1 (C-5,7), 114.9 (C-1,3), and 26.7 (q,
40-COMe). Anal. Calcd for C18H14O: C, 87.78; H, 5.73.
Found: C, 87.38; H, 5.92.
and 55.4 (40-OMe). Anal. Calcd for C17H14O: C, 87.15; H,
6.02. Found: C, 86.85; H, 6.17.
4.1.7. 1,2-Di(2-azulenyl)benzene (22). The general pro-
cedure was followed by using 2-azulenylboronate (6)
(203 mg, 0.799 mmol), 1,2-dibromobenzene (17) (95 mg,
0.40 mmol), Cs2CO3 (394 mg, 1.21 mmol), Pd2(dba)3
(42 mg, 0.046 mmol), P(t-Bu)3 (65 mg, 0.32 mmol), and
dioxane (5 mL) at 80 8C for 24 h. Chromatographic
purification on silica gel with CH2Cl2 and GPC afforded
22 (43 mg, 33%), 2-phenylazulene (13) (11 mg, 7%), and
azulene (7) (4 mg, 4%).
Compound 22. Blue crystals; mp 188–189 8C; MS (70 eV)
m/z (relative intensity) 330 (Mþ, 88%), 329 (53), 327 (69),
326 (Mþ24H, 100), 314 (49), and 313 (57); IR (KBr disk)
n
max 1456, 1401, 826, 762 and 731 cm21; UV–vis (CH2Cl2)
lmax, nm (log 1) 238 sh (4.44), 281 (4.93), 314 sh (4.60),
322 (4.62), 393 (4.14), 544 sh (2.65), 579 (2.88), 621 (2.75),
and 673 sh (2.42); 1H NMR (500 MHz, CDCl3) d¼8.07 (d,
J¼9.9 Hz, 4H, H-40,80), 7.71 (m, 2H, H-3,6), 7.44 (m,0 2H,
H-4,5), 7.43 (t, J¼9.9 Hz, 2H, H-60), 7.180 (s, 4H, H-1 ,30),
and 7.03 (dd, J¼9.9, 9.9 Hz, 4H, H-5 ,70); 13C NMR
(125 MHz, CDCl3) d¼151.3 (C-20), 140.2 (C-3a,8a), 136.9
(C-1,2), 136.3 (C-60), 135.9 (C-40,80), 131.8 (C-3,6),
127.8 (C-4,5), 123.0 (C-50,70), and 118.5 (C-10,30); HRMS
Calcd for C26H18—e 330.1403, found 330.1401. Anal.
Calcd for C26H18: C, 94.51; H, 5.49. Found: C, 94.23; H,
5.65.
4.1.6. 2-(4-Methoxyphenyl)azulene (16c). The general
procedure was followed by using 2-azulenylboronate (6)
(100 mg, 0.393 mmol), 4-bromoanisole (15c) (164 mg,
0.877 mmol), Cs2CO3 (193 mg, 0.592 mmol), Pd2(dba)3
(18 mg, 0.020 mmol), and P(t-Bu)3 (20 mg, 0.099 mmol) in
dioxane (3 mL) at 80 8C for 19 h. Column chromatography
on silica gel with CH2Cl2 and GPC afforded 16c (66 mg,
72%). blue plates; mp 229–234 8C decomp.; MS (70 eV)
m/z (relative intensity) 234 (Mþ, 100%); IR (KBr disk) nmax
1605, 1478, 1258, 1183, 1030 and 810 cm21; UV–vis
(CH2Cl2) lmax, nm (log 1) 248 sh (4.16), 275 (4.40), 307
(4.74), 317 (4.81), 369 sh (4.01), 388 (4.25), 405 (4.29), 533
4.1.8. 1,4-Di(2-azulenyl)benzene (23). Following the
general procedure, the reaction of 2-azulenylboronate (6)
(204 mg, 0.803 mmol) of 1,4-dibromobenzene (18) (93 mg,
0.39 mmol) in dioxane (5 mL) at 80 8C for 24 h in the
presence of Cs2CO3 (400 mg, 1.23 mmol), Pd2(dba)3
(40 mg, 0.044 mmol), and P(t-Bu)3 (47 mg, 0.23 mmol)
afforded an insoluble material in CH2Cl2. Mass spectrum of
the insoluble material showed a peak at m/z 330, which
corresponded to a correct Mþ ion peak of 23 (57 mg, 44%).
After the insoluble material was removed by filtration, the
organic layer was worked up. Column chromatography on
silica gel with CH2Cl2 and GPC with CHCl3 afforded
1
sh (2.53), 569 (2.64), 608 (2.60), and 658 sh (2.29); H
NMR (500 MHz, CDCl3) d¼8.25 (d, J¼9.8 Hz, 2H, H-4,8),
7.92 (d, J¼8.9 Hz, 2H, H-20,60), 7.62 (s, 2H, H-1,3), 7.47 (t,
J¼9.8 Hz, 1H, H-6), 7.15 (0dd0, J¼9.8, 9.8 Hz, 2H,0 H-5,7),
7.01 (d, J¼8.9 Hz, 2H, H-3 ,5 ), and 3.88 (s, 3H, 4 -OMe);
13C NMR (125 MHz, CDCl3) d¼160.0 (C-40), 149.8 (C-2),
141.4 (C-3a,8a), 135.7 (C-6), 135.2 (C-4,8), 129.2 (C-10),
128.9 (C-20,60), 123.7 (C-5,7), 114.4 (C-30,50), 113.8 (C-1,3),

S. Ito et al. / Tetrahedron 60 (2004) 5357–5366
5365
2-phenylazulene (13) (2 mg, 2%) and azulene (7) (2 mg,
2%).
C-400,800, or C-4000,8000), 135.9 (C-4), 132.5 (C-6), 131.2 (0C0 -3),
126.9 (C-5), 0123.8 (0C0 -5000,7000), 123.2 00(C-50,70 or C-5 ,7000),
123.1 (C-50,7 or C-500,0700), 118.6 (C-1 ,300), 118.4 (C-10,3 ),
and 114.6 (C-1000,3 ); HRMS Calcd for C36H24—e
456.1873, found 456.1874. Anal. Calcd for C36H24·
1/2H2O: C, 92.87; H, 5.41. Found: C, 92.78; H, 5.64.
Compound 23. Green crystals; mp .300 8C; MS (70 eV)
m/z (relative intensity) 330 (Mþ, 100%); IR (KBr disk) nmax
1410 and 808 cm21; HRMS Calcd for C26H18—e 330.1403,
found 330.1409. Anal. Calcd for C26H18·1/4H2O: C, 93.24;
H, 5.57. Found: C, 93.53; H, 5.63.
4.1.11. 1,2,4,5-Tetra(2-azulenyl)benzene (26). Following
the general procedure, the reaction of 2-azulenylboronate
(6) (417 mg, 1.64 mmol) with 1,2,4,5-tetrabromobenzene
(21) (147 mg, 0.374 mmol) in dioxane (10 mL) at 80 8C for
24 h in the presence of Cs2CO3 (802 mg, 2.46 mmol),
Pd2(dba)3 (74 mg, 0.081 mmol), and P(t-Bu)3 (96 mg,
0.47 mmol) afforded an insoluble material in CH2Cl2.
Mass spectrum of the insoluble material showed a peak at
m/z 582, which corresponded to a correct Mþ ion peak of 26
(29 mg, 13%). After the insoluble material was removed by
filtration, the organic layer was worked up. Column
chromatography on silica gel with CH2Cl2 and GPC
afforded 1,2,4-tri(2-azulenyl)benzene (25) (23 mg, 13%)
and azulene (7) (9 mg, 4%).
4.1.9. 1,3,5-Tri(2-azulenyl)benzene (24). The general
procedure was followed by using 2-azulenylboronate (6)
(301 mg, 1.18 mmol), 1,3,5-tribromobenzene (19) (128 mg,
0.407 mmol), Cs2CO3 (596 mg, 1.83 mmol), Pd2(dba)3
(66 mg, 0.072 mmol), P(t-Bu)3 (49 mg, 0.24 mmol),
dioxane (7 mL) at 80 8C for 24 h. Chromatographic
purification on silica gel with CH2Cl2 and GPC afforded
24 (32 mg, 18%), azulene (7) (46 mg, 30%), and the
recovered 6 (22 mg, 7%).
Compound 24. Greenish blue microneedles; mp .300 8C;
MS (70 eV) m/z (relative intensity) 456 (Mþ, 100%); IR
(KBr disk) nmax 1505, 1453, 1399 and 801 cm21; UV–vis
(CH2Cl2) lmax, nm (log 1) 245 (4.54), 301 (5.16), 314
(5.16), 381 (4.69), 399 (4.73), 541 sh (2.92), 575 (3.05), 615
(3.03), and 664 sh (2.67); 1H NMR (600 MHz, CDCl3)
d¼8.54 (s, 3H, H-2,4,6), 8.39 (d, J¼9.8 Hz, 6H, H-40,80),
7.89 (s, 6H, H-10,30), 7.57 (t, J¼90.8 Hz, 3H, H-60), and 7.23
(dd, J¼9.8, 9.8 Hz, 6H, H-50,7 ); 0 13C NMR (150 MHz,
CDCl3) d¼149.8 (C-20), 141.4 (C-3 a,80a), 137.8 (C-1,3,5),
136.06 (C-60), 136.2 (C-40,80), 126.9 (C-2,4,6), 123.9
(C-5 ,70), and 114.8 (C-10,3); HRMS Calcd for C36H24—e
456.1873, found 456.1874. Anal. Calcd for C36H24·1/2H2O:
C, 92.87; H, 5.41. Found: C, 92.73; H, 5.40.
Compound 26. Green microneedles; mp .300 8C; MS
(70 eV) m/z (relative intensity) 582 (Mþ, 66%), 565 (45),
490 (41), 489 (55), 466 (49), 465 (100), 454 (Mþ2C10H8,
69), 453 (47), 439 (41), 328 (Mþ22C10H7, 51), 291 (Mþ/2,
53), and 265 (78); IR (KBr disk) nmax 1574, 1453, 1399,
1383, 820 and 812 cm21; HRMS Calcd for C46H30—e
582.2342, found 582.2358. Anal. Calcd for C46H30·2/3H2O:
C, 92.90; H, 5.31. Found: C, 92.75; H, 5.36.
Acknowledgements
4.1.10. 1,2,4-Tri(2-azulenyl)benzene (25). The general
procedure was followed by using 2-azulenylboronate (6)
(254 mg, 1.00 mmol), 1,2,4-tribromobenzene (20) (105 mg,
0.334 mmol), Cs2CO3 (587 mg, 1.80 mmol), Pd2(dba)3
(59 mg, 0.064 mmol), P(t-Bu)3 (72 mg, 0.36 mmol), and
dioxane (7 mL) at 80 8C for 24 h. Chromatographic
purification on silica gel with CH2Cl2 and GPC afforded
25 (55 mg, 36%) and azulene (7) (3 mg, 2%).
The present work was supported by a Grant-in-Aid for
Scientific Research (No. 14540486 to S. I.) from the
Ministry of Education, Culture, Sports, Science and
Technology, Japan.
References and notes
Compound 25. Green crystals; mp 238.5–239 8C decomp.;
MS (70 eV) m/z (relative intensity) 456 (Mþ, 100%) and
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;
316 (5.04), 386 sh (4.53), 408 (4.61), 430 sh (4.53), 537 sh
1
(2.94), 579 (3.05), 618 (3.01), and 670 sh (2.68); H00N0 MR
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8.30 (0d0 , J¼1.9 Hz, 1H, H-3), 8.18 (d, J¼9.7 Hz, 2H,
H-400,8 ), 8.11 (d, J¼9.7 Hz, 2H, H-40,80), 8.06 (dd, J¼8.1,
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