Synthesis and characterization of cyclopentadienone-annelated hexadehydrodibenzo[12]annulene

Article abstract of DOI:10.1246/cl.2006.168

Hexadehydrodibenzo[12]annulene annelated by a cyclopentadienone moiety was synthesized. On the basis of the theoretical calculations and experimental NMR and IR spectra, it was shown to be weakly aromatic owing to the contribution of the diatropic [15]ann

Full text of DOI:10.1246/cl.2006.168

168
Chemistry Letters Vol.35, No.2 (2006)
Synthesis and Characterization of Cyclopentadienone-annelated
Hexadehydrodibenzo[12]annulene
Tsumoru Morimoto,1 Shuji Nagano,1 Daishi Yokoyama,1 Masutaka Shinmen,1 Kiyomi Kakiuchi,ꢀ1
Takashi Yoshimura,2 Motohiro Sonoda,2 and Yoshito Tobe2
1Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Takayama, Ikoma, Nara 630-0192
2Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University,
and CREST, Japan Science and Technology Agency (JST), Toyonaka, Osaka 560-8531
(Received September 20, 2005; CL-051197; E-mail: kakiuchi@ms.naist.jp)
Hexadehydrodibenzo[12]annulene annelated by a cyclopen-
tadienone moiety was synthesized. On the basis of the theoretical
calculations and experimental NMR and IR spectra, it was
shown to be weakly aromatic owing to the contribution of the
diatropic [15]annulenone resonance structure.
of 1 whereas it is 7.9 for the unannelated reference compound
3,10 indicating that the former is less antiaromatic than the latter.
The NICS value at the center of the [12]DBA ring of 1 is ꢁ1:4,
indicative of its weak diatropicity. In contrast, the NICS value
for the weakly antiaromatic hexadehydrotribenzo[12]annulene
was estimated to be 4.4.11 The theoretical chemical shifts of
the aromatic protons Ha, Hb, and He of 1 locate at the downfield
relative to the corrsponding protons of 3 and acetal 4 as shown
later (Figure 1),10 also indicating weak diatropicity of 1. Consis-
tent with the calculated magnetic property, the stretching
frequencies of the carbonyl groups of 1 and 3 are expected to
occur at 1772 and 1777 cmꢁ1, respectively. The calculated bond
distances and angles around the twelve-membered ring of 1,
however, are not much different from those of 3 and 4.
Our strategy for synthesis of 1 was based on the palla-
dium-catalyzed coupling reaction for all C–C bond formation,
from a commercially available 2,3,4,5-tetrachlorocyclopentadi-
enone dimethyl acetal (8) (Scheme 1). Namely, regioselective
Sonogashira coupling of 8 with ethynyl compounds and subse-
quent construction of the [12]DBA core using Sonogashira cou-
pling of the obtained bis-ethynyl derivative with an appropriate
dihalotolan followed by Suzuki–Miyaura reaction and deprotec-
tion would give the desired 1. This strategy is secured by
our previous findings that the Sonogashira ethynylation of 8
proceeds preferentially at the 3- and 4-positions compared to
the 2- and 5-positions.4c
Carbon-rich organic molecules have been attracting a great
deal of interest with regard to their novel structures and their po-
tential optoelectronic applications.1 Among them, perethynylat-
ed cyclic ꢀ-systems having a cyclobutadiene–metal complex,2 a
cyclopentadienylide–metal complex,3 and benzene4 as a nucleus
have been synthesized as scaffolds of two-dimensional networks
and three-dimensional cages as well as their substructures.5 Even
though some partially closed wheel-like structures consisting of
a central ꢀ-system and peripheral aromatic dehydrobenzo[14]-6
or [18]annulene7 moieties have been synthesized, little has been
known for those annelated by antiaromatic dehydrobenzo[12]-
annulenes ([12]DBAs), because of difficulties associated with
construction of the [12]DBA framework.8 In this context, we
disclosed here the first synthesis of a [12]DBA derivative 1
which is annelated by a cyclopentadienone moiety. The synthe-
sis of 1 represents the first step toward the synthesis of the
perfect wheel cyclopentadienylide 2, which may be called coran-
nulenyne after corannulene.5 Additionally, 1 is an interesting
molecule with regard to the contribution of the [15]annulenone
resonance structure.
The reaction of 8 with 2-methyl-3-butyn-2-ol in the pres-
ence of Pd(PPh3)4, CuI, and BuNH2 proceeded expectedly at
the 3- and 4-positions of 8 to give 3,4-disubstituted cyclopenta-
Indeed, the NICS9 value calculated by the GIAO-B3LYP/
6-31Gꢀ method is 3.3 at the center of the cyclopentadienone core
N3Et2
N3Et2
I
I
O
t-Bu
t-Bu
I
a, b
+
Hd
Hb
Ha
Hc
t-Bu
t-Bu
t-Bu
t-Bu
5
6
7
MeO OMe
MeO OMe
Cl Cl
Cl
Cl
t-Bu
t-Bu
He
MeO OMe
1
c
e, f
Cl
Cl
d
1
2
5
2
3 4
Cl
Cl
MeO OMe
O
t-Bu
t-Bu
t-Bu
t-Bu
HO
OH
8
t-Bu
t-Bu
9
10
Hd
Hb
Ha
Hd
Hb
Ha
Hc
Hc
Scheme 1. Reaction conditions: (a) Pd(PPh3)4, CuI, Et3N, THF,
55 ꢂC, 75%; (b) MeI, 125 ꢂC, sealed tube, 90%; (c) 2-methyl-3-
butyn-2-ol, Pd(PPh3)4, CuI, BuNH2, THF, 55 ꢂC, 55%; (d) 7, Pd-
(PPh3)4, CuI, PPh3, KOH, CH3(C8H17)3NCl, benzene/H2O (2:1
v/v), 85 ꢂC, 52%; (e) 4-tert-butylphenylboronic acid, Pd(PPh3)4,
K2CO3, THF, reflux, 92% of 4; (f) 50% TFA aq, CH2Cl2, rt, 92%.
t-Bu
t-Bu
He
t-Bu
t-Bu
3
4
Figure 1.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.2 (2006)
169
Table 1. Theoretical and experimental 1H NMR chemical shifts of
1, 3, and 4a
due to extension of conjugated ꢀ-electron system by linking
with a C–C triple bond between the benzene rings.
In conclusion, we synthesized cyclopentadienone-annelated
[12]DBA 1 by efficient cross-coupling reactions. Compound 1
showed weak diatropicity owing to the contribution of the
[15]annulenone resonance structure.
Ha
Hb
Hc
Hd
He
1
3
4
7.53
(7.35)
7.44
7.71
(7.73)
7.54
8.17
(8.38)
8.06
7.54
(7.34)
7.48
7.86
(7.66)
(7.28)
7.48
(7.08)
(7.40)
7.67
(7.27)
(8.16)
8.48
(7.85)
(7.24)
7.51
(7.32)
7.75
(7.37)
References and Notes
1
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Tykwinski, Wiley-VCH, Weinheim, 2005.
aTheoretical values are shown in parentheses.
dienone acetal 9. The diiodotolan 7, a coupling partner of 9, was
prepared as follows. (2-Iodophenyl)triazene (5) and (2-ethynyl-
phenyl)triazene (6), which were prepared by the method of
Haley,12 were coupled in the presence of Pd(PPh3)4 in Et3N at
55 ꢂC to give 2,20-bis(triazenyl)tolan in 75% yield, and then it
was converted by iodination with MeI to 7.7b
2
3
4
a) U. H. F. Bunz, V. Enkelmann, Angew. Chem., Int. Ed. Engl.
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a) U. H. F. Bunz, V. Enkelmann, J. Rader, Organometallics 1993,
¨
One-pot operation of deprotection of two acetone-protected
termini in 9 under the basic conditions and the succeeding dou-
ble Sonogashira coupling13 with 7 afforded 10 in 52% yield.
Since the 2,5-dihalo-3,4-disubstituted cyclopentadienone from
deprotection of 10 is highly unstable and unisolable, the aryl
groups were introduced to the 2- and 5-positions by Suzuki–
Miyaura coupling with tert-butylphenylboronic acid, leading
to 4. Finally, deprotection of 4 with aqueous trifluoroacetic acid
afforded the desired cyclopentadienone-annelated [12]DBA 1 in
92% yield.14
12, 4745. b) N. Jux, K. Holczer, Y. Rubin, Angew. Chem., Int. Ed.
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5
6
7
8
Theoretical10 and experimental H NMR data of 1 and the
1
reference compounds 315 and 4 are shown in Table 1. The theo-
retical chemical shifts are qualitatively in agreement with the ob-
served data except for Hc of 4.16 The resonances of Ha, Hb, and
He of 1 locate at the downfield compared with the corresponding
protons of 3 and 4, even though the observed chemical shift
difference is less pronounced than the calculated one. These
downfield shifts may be caused by deshielding effect of the dia-
magnetic ring current of the [15]annulenone circuit. Further-
more, in the FT-IR spectra, the C–O stretching vibration of the
carbonyl group in 1 (1697 cmꢁ1) appeared at the lower frequen-
cy region than that of 3 (1703 cmꢁ1), as expected from the the-
oretical study. These observations coupled with the theoretical
calculations strongly suggest that cyclopentadienone-annelated
[12]DBA 1 is diatropic owing to the contribution of the 14ꢀ
[15]annulenone resonance structure. However, attempts to pro-
tonate 1 with trifluoroacetic acid to form the corresponding hy-
droxy[15]annulenium ion17 failed because of its decomposition
under strongly acidic conditions.
a) J. M. Kehoe, J. H. Kiley, J. J. English, C. A. Johnson, R. C.
ˇ
´
Petersen, M. M. Haley, Org. Lett. 2000, 2, 969. b) O. S. Miljanic,
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9
a) P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. v. E.
´
Hommes, J. Am. Chem. Soc. 1996, 118, 6317. b) L. Nyulaszi,
P. v. R. Schleyer, J. Am. Chem. Soc. 1999, 121, 6872. Although
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dehydrobenzoannulenes and those which eliminated local anisotro-
py of triple bonds,11 the NICS values reported in the present paper
do not take the latter into account.
10 For theoretical study, tert-butyl groups of 1, 3, and 4 were omitted
for clarify.
11 A. J. Matzger, K. P. C. Vollhardt, Tetrahedron Lett. 1998, 39, 6791.
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As shown in Figure 2, the UV–vis absorption bands of 1
shift considerably to the longer wavelength than those of 3,
13 C. Huynh, G. Linstrumelle, Tetrahedron 1998, 44, 6337.
14 Spectroscopic data are listed in Supporting Information.
15 3 was prepared in 61% overall yield by Sonogashira coupling of 8
with 4-tert-butylphenylacetylene,4c followed by Suzuki–Miyaura
coupling with 4-tert-butylphenylboronic acid and deprotection of
its acetal moiety with aqueous TFA.14
6
1
16 While the theoretical chemical shift of Hc of 1 locates at downfield
relative to that of 4, experimentally the former appears at higher
field than the latter. The dihedral angles of the phenyl group relative
to the five-membered ring are 27.8 and 43.2ꢂ, respectively, indicat-
ing that Hc of 1 should be more susceptible to the anisotropic
deshielding effect in contrast to the observed chemical shifts. The
reason for this discrepancy is not understood.
17 a) R. T. Weavers, R. R. Jones, F. Sondheimer, Tetrahedron Lett.
1975, 16, 1043. b) H. Higuchi, N. Hiraiwa, T. Daimon, S. Kondo,
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221.
3
4
2
ε
0
250
350
450
550
wavelength / nm
Figure 2. UV–vis spectra of 1 (bold) and 3 (plain) in THF at rt.
Published on the web (Advance View) January 7, 2006; DOI 10.1246/cl.2006.168