Effective synthesis of diiodinated picene and dibenzo[a,h]anthracene by AuCl-catalyzed double cyclization

Article abstract of DOI:10.1016/j.tetlet.2012.01.071

6,13-Diiododibenzo[a,h]anthracene and 5,8-diiodopicene were synthesized by AuCl-catalyzed double cyclization. The highly selective reaction yielded a new class of peri-halogenated fused aromatics.

Full text of DOI:10.1016/j.tetlet.2012.01.071

Tetrahedron Letters 53 (2012) 1617–1619
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Tetrahedron Letters
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Effective synthesis of diiodinated picene and dibenzo[a,h]anthracene by
AuCl-catalyzed double cyclization
Takahiro Nakae a, , Ryuji Ohnishi a, Yoshiharu Kitahata b, Tatsuya Soukawa b, Hisako Sato a, Shigeki Mori c,
⇑
Tetsuo Okujima a, Hidemitsu Uno a, Hiroshi Sakaguchi d,
⇑
a Department of Chemistry and Biology, Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan
b Department of Chemistry, Faculty of Science, Ehime University, Matsuyama, Ehime 790-8577, Japan
c Department of Molecular Science, Integrated Center for Sciences, Ehime University, Matsuyama, Ehime 790-8577, Japan
d Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
6,13-Diiododibenzo[a,h]anthracene and 5,8-diiodopicene were synthesized by AuCl-catalyzed double
cyclization. The highly selective reaction yielded a new class of peri-halogenated fused aromatics.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 9 December 2011
Revised 16 January 2012
Accepted 18 January 2012
Available online 28 January 2012
Keywords:
Gold catalysis
Cyclization
Fused-ring system
Polyaromatics
Rearrangement
Fused aromatic compounds have received much attention because
of their applications in organic electronics.1 For example, pentacene2
and rubrene3 crystals show high electron mobility, and metal-doped
picene4 demonstrates superconductivity. The rigid structure of fused
aromatics makes them useful building blocks for molecular nanoma-
chines. Newly developed methodologies for preparing fused aromatics
provide a new class of compounds bearing various functionalities.
Fused aromatics bearing halogen atoms are valuable starting materi-
als. Their functions can be extended by the introduction of substitu-
ents. It is difficult to synthesize fused aromatic compounds by direct
and selective halogenation, and it is hard to avoid multi-addition
during the reaction. Selective benzannulation from oligo-phenylenes5
with halogen atoms is a promising method for obtaining peri-haloge-
nated polyaromatics in high yield and selectivity.
Herein, we demonstrated the double cyclization of dihaloethy-
nylterphenyl compounds 1 and 3 to synthesize dihalogenated
dibenzo[a,h]anthracene 26–8 and picene 49 in high yields (Schemes
1 and 2). Fürstner and co-workers reported AuCl-catalyzed cycliza-
tion with 1,2-halogen migration from haloethynylbiphenyl to
halophenanthrene.10 They reported that 2-bromoethynylbiphenyl
gave 9-bromophenanthrene in a 77% yield, while iodoethynyl
compound gave 9-iodophenanthrene in a 76% yield. Preparations
of 6,13-diiododibenzo[a,h]anthracenes from bis(arylethynyl)
terphenyls are reported in the literature.11,12 To the best of our
knowledge, there has been no example of 5,12-free 6,13-dihalodi
benzo[a,h]anthracene.
We first prepared brominated terphenyl 1a and applied the
reaction conditions (AuCl 20 mol %, 80 °C, 24 h) reported in Ref.
10 (Table 1, entry 3). The reaction mixture was analyzed by 1H
NMR using an internal standard. Only 35% of the desired 2a was
formed. Increasing the amount of the AuCl catalyst or reaction time
was not effective in improving the product yield. Although the
reaction mixture was initially heterogeneous because of the insol-
ubility of AuCl in toluene, it subsequently became a clear solution,
and then turned red (the characteristic color of Au nanoparticles)
after several hours. The presence of gold(0) was finally observed
in the reaction mixture. These observations indicated that the AuCl
catalyst might have been disproportionate to the inactive gold(0).
To avoid undesirable inactivation, we optimized the reaction
conditions. The highest yield of 2a was achieved at 60 °C (entry
2), while lower and higher reaction temperatures were ineffective
(entries 1 and 4).
Iodoethynyl 1b drastically improved the reaction yield and
selectivity, and the compound was converted to the corresponding
fused aromatic 2b in a quantitative yield (entry 5). The alkyl group
at the 4,400-position of terphenyl 1 did not affect the reaction (entry
6). The scope and limitations of functional group tolerance are
under investigation.
⇑
Corresponding authors.
E-mail address: nakae@ehime-u.ac.jp (T. Nakae).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.071

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T. Nakae et al. / Tetrahedron Letters 53 (2012) 1617–1619
Table 1
I
I
I
Optimization of reaction condition of gold(I)-catalyzed double cyclization of
AuCl cat.
(20 mol%)
dihaloethynyl terphenyl 1a
R
R
R
R
toluene
60 °C
24 h
97% (NMRa)
X
4
X
I
I
3
Au cat.
R = t-Bu
R
R
R
R
R
R
toluene
temp.
time
X
I
3% (NMRa)
X
5
Scheme 1. Preparation of 5,8-diiodopicene 4 by AuCl-catalyzed double cyclization
of haloethynyl terphenyl 3. aNMR yield was determined by 1H NMR analysis using
an internal standard.
Entry Substrate
Catalyst
(mol %)
Temp
(°C)
Time
(h)
Yield (%),
NMRb
Yieldc
(%)
1
1a(R = C6H13
X = Br)
1a
1a
1a
1b(R = C6H13
X = I)
1c(R = t-Bu,
X = I)
,
AuCl, 20
rt
24
6
I
2
3
4
5
AuCl, 20
AuCl, 20
AuCl, 20
AuCl, 20
60
80
Reflux
60
24
24
24
24
53
35
32
99
49
AuCl cat.
,
84
92
I
6
AuCl, 20
60
24
92
1c
I
1) n-BuLi
2) H2O
a
Conditions: dihaloethynyterphenyl 1 (10 mg) and gold(I) catalyst in toluene
(1.0 mL) were stirred under N2 atmosphere.
b
NMR yield was determined by NMR analysis using an internal standard.
Isolated yield.
c
I
only 6
2c
I
AuCl cat.
3
I
I
I
I
1) n-BuLi
2) H2O
4
7
major
+
Figure 1. X-ray crystallographic structure of 4.
I
I
Au C
5
6
trace
(a)
(b)
I
I
-0.274
-0.181
7
Scheme 2. Preparation of hydrogenated compounds 6 and 7 from 2c and 4.
9
The regioselectivity of double cyclization was investigated
Scheme 3. (a) Proposed mechanism for achieving double cyclization. (b) Mulliken
electron density of model compound calculated by Gaussian09W15 on B3LYP
density functional at the 6-31G(d) level for C and H, and at the LANL2DZ level for I.
using compound 3, which might give picene
4
or
dibenzo[a,h]anthracene 5 (Scheme 1). 1H NMR analysis of the reac-
tion mixture from compound 3 showed high selectivity (major/
minor = 97:3, Fig. S1). A single crystal, which was obtained from
the crude product of 3, was subjected to X-ray crystallographic
analysis. The structure was determined to be a type of picene, as
shown in Figure 1. There is a possibility that the minor component
crystallized preferentially. In order to manipulate the major
component to obtain the picene-type compound 4, cyclized
compounds from 1c and 3 were treated with n-BuLi, followed by
water, to obtain hydrogenated dibenzo[a,h]anthracene and picene
(Scheme 2). A comparison of the 1H NMR spectra clearly shows
that picene 4 is the major component (Fig. S2).
gives a phenyphenanthrene intermediate, which yields either
picene or dibenzo[a,h]anthracene by reacting at the 7- or 9-posi-
tion, respectively. The steric effect of the gold–vinylidene interme-
diate might be favorable in yielding the dibenzo[a,h]anthracene
structure. However, its effect should be small to yield 5. On the
other hand, the electronic effects estimated from the DFT calcula-
tions of a model compound (Scheme 3b) support the experimental
results. The Mulliken charge density at the 7-position is higher
than that at the 9-position. Therefore, the selective formation of
picene 4 occurs under electronic control.
The reaction mechanism could be considered through
a
gold–vinylidene intermediate,13,14 where the iodine atom migrates
to the neighboring carbon atom, followed by electrophilic aromatic
substitution (Scheme 3a). The initial cyclization from compound 3
In conclusion, we successfully synthesized diiodinated picene
and
dibenzo[a,h]anthracene
from
diiodoethynylterphenyl
compounds through AuCl-catalyzed double cyclization. AuCl

T. Nakae et al. / Tetrahedron Letters 53 (2012) 1617–1619
1619
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Acknowledgments
This work was supported by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan, through a Grant-in-
Aid for Scientific Research on Innovative Areas ‘Emergence in
Chemistry’ (No. 20111006 to H.S. and T.N.) and for Young Scientists
(B) (No. 22750130 to T.N.), as well as by the Joint Research Program
on Zero-Emission Energy Research, Institute of Advanced Energy,
Kyoto University (B-14 to T.N.), and by the TEPCO Research Founda-
tion (T.N.). T.N. thanks Professor Naoto Chatani (Osaka University)
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Business Laboratory of Ehime University for its help with TOF-MS
spectroscopy. The authors also thank the reviewers for their
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.01.071.
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