Synthesis of new halogenated pentiptycene building blocks

Article abstract of DOI:10.1021/ol900127m

The synthesis of central-ring halogenated pentiptycene phenols 12-14 and dihalogenated pentiptycenes 15-17 is reported. Their utilities as building blocks for preparing new pentiptycene-derived π-conjugated systems through the Sonogashira, Heck, and Suzuk

Full text of DOI:10.1021/ol900127m

ORGANIC
LETTERS
2009
Vol. 11, No. 6
1429-1432
Synthesis of New Halogenated
Pentiptycene Building Blocks
Jye-Shane Yang,*,† Jyu-Lun Yan,‡ Ying-Xue Jin,*,†,§ Wei-Ting Sun,† and
Ming-Che Yang†
Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan, 10617,
Department of Chemistry, National Central UniVersity, Chungli, Taiwan, 32001, and
College of Chemistry and Engineering, Haerbin Normal UniVersity,
Harbin, China 150025
jsyang@ntu.edu.tw; tgh.123@163.com
Received January 20, 2009
ABSTRACT
The synthesis of central-ring halogenated pentiptycene phenols 12-14 and dihalogenated pentiptycenes 15-17 is reported. Their utilities as
building blocks for preparing new pentiptycene-derived π-conjugated systems through the Sonogashira, Heck, and Suzuki reactions are also
demonstrated.
Pentiptycene (1) is a member of the iptycene family and
possesses a rigid, aromatic, and H-shaped scaffold.1 Such a
structural feature has been applied to the formation of
functional molecules such as fluorescent chemosensors,2
molecular machines,3 and low dielectric constant materials;4
to the creation of intriguing supramolecular structures;5 and
to the understanding of structure-property relationships.6
Interestingly, all of these applications are associated with
functionalization of the “sterically shielded” central phe-
nylene ring. As such, the availability of the central-ring
prefunctionalized pentiptycene building blocks is crucial in
the development of new pentiptycene-based materials.
It has been shown that the readily prepared7,8 pentiptycene
quinone 2 is an excellent precursor for a variety of central-
ring functionalized pentiptycene building blocks.1 Specifi-
cally, in addition to the symmetrically disubstituted pentip-
tycene hydroquinone (3) and pentiptycene diacetylene (4),7
a series of unsymmetrically substituted pentiptycenes such
as compounds 5-11 have recently been derived from 2
(Scheme 1).9 However, previous attempts to prepare the
halogenated pentiptycene phenols 12-14 and pentiptycene
dihalides 15-17 were unsuccessful.9,10 We report herein the
† National Taiwan University.
‡ National Central University.
§ Haerbin Normal University.
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10.1021/ol900127m CCC: $40.75
Published on Web 02/20/2009
2009 American Chemical Society

Doyle et al.,11 where tert-butyl nitrite is the diazotizing agent,
CuBr2 or CuCl2 is the source of halogens, acetonitrile
(MeCN) is the solvent, and the reaction temperature is 65
°C. An extension of this method by replacing the cupric
halides with KI affords the desired pentiptycene iodide 14.
It should be noted that the same reaction carried out in THF
led to other undesired products, including compounds 2, 3,
6, and 11.9 Cadogan and Molina have suggested that THF
can promote the decomposition of arenediazonium ions to
form the corresponding aryl radicals.12 Indeed, generation
of the pentiptycene radical 19 followed by reactions with
molecular oxygen, THF, and tert-butyl nitrite can nicely
account for the products 3 (and the oxidized form 2), 6, and
11, respectively.9 In other words, in THF the desired
reactions between the diazonium ion and cupric halides or
KI cannot compete with the THF-promoted pentiptycene
radical formation and/or the subsequent reactions. The THF
effect on promoting the decomposition of diazonium ions is
further evidenced by the fact that the conversion 5 f 11
shown in Scheme 1 is nearly terminated at the stage of the
diazonium compound 18 when the solvent is replaced by
MeCN (Scheme 2). As a result, the diazonium salt 18 can
be isolated as a yellowish solid powder. Interestingly, the
powder of 18 displays high thermal stability, as no noticeable
decomposition was observed for 18 upon heating up to 300
°C (mp >300 °C) or keeping at 4 °C for as long as 60 days.
We believe that, in addition to the hydroxyl substituent,13
both the steric and electronic factors of the pentiptycene
scaffold play a role in its stability, namely, the steric shielding
effect due to structural rigidity and bulkiness and the
homoconjugative interactions14 among the peripheral and the
central phenylene rings. The conversion of 18 f 12-14 can
be accomplished simply by stirring acetonitrile solutions of
18 and the corresponding halides (i.e., CuCl2, CuBr2, and
KI) at room temperature for 24 h or at 65 °C for 30 min.
Scheme 1
synthesis of these halogenated pentiptycene building blocks
and their potential utility toward the development of new
pentiptycene-incorporated π-conjugated systems.
The halogenated pentiptycene phenols 12-14 can be
prepared from the deaminative halogenation of 5 either in a
one-pot reaction or by a two-step method through the
diazonium salt 18 (Scheme 2). The one-pot reaction condition
for the synthesis of 12 and 13 is based on the method of
The synthesis of pentiptycene dihalides 15-17 is outlined
in Scheme 3. The key step is the Pd-catalyzed reduction of
the triflate group in 20 with triethylsilane, which generates
the nitropentiptycene 21. The reduction reaction is based
on the method of Kotsuki et al.,15 but in our case it requires
the more reactive Pd catalyst Pd(PPh3)4 to obtain a reasonable
Scheme 2
(11) Doyle, M. P.; Siegfried, B.; Dellaria, J. F., Jr. J. Org. Chem. 1977,
42, 2426–2431.
(12) Cadogan, J. I. G.; Molina, G. A. J. Chem. Soc., Perkin I 1973,
541–542.
(13) Bra¨se, S.; Dahmen, S.; Popescu, C.; Schroen, M.; Wortmann, F.-J.
Chem. Eur. J. 2004, 10, 5285–5296.
(14) The homoconjugative interactions of π groups through the
bicycle[2.2.2]octane framework has previously been discussed and termed
as “hyperconjugative” interactions. See: Kim, Y.; Zhu, Z.; Swager, T. M.
J. Am. Chem. Soc. 2004, 126, 452–453.
(15) Kotsuki, H.; Datta, P. K.; Hayakawa, H.; Suenaga, H. Synthesis
1995, 1348–1350.
1430
Org. Lett., Vol. 11, No. 6, 2009

deaminative iodination of 24 under the same condition for
the conversion of 23 f 16.
Scheme 3
As represented by the building blocks 13 and 15 and
the Heck, Suzuki, and Sonogashira reactions,21 a variety
of pentiptycene-incorporated π-conjugated systems
(25-30) can be constructed (Schemes 4 and 5). It should
Scheme 4
be noted that pentiptycene mono- or ditriflates display little
or no such reactivity,1,2c although many other aryl triflates
have been shown to be alternative substrates for these Pd-
yield (45%). This reaction appears to be more efficient for
aryl triflates containing electron-withdrawing substituents,
because the corresponding triflate derived from 5 (i.e., an
aryl triflate containing an electron-donating amino group)
shows no such reactivity. The subsequent reduction of the
nitro group provides the pentiptycene aniline 22. Under the
deaminative bromination conditions described for the reaction
5 f 13, aniline 22 can be readily converted to the desired
pentiptycene dibromide 15. The occurrence of oxidative
bromination prior to deaminative bromination for anilines
with CuBr2 and tert-butyl nitrite in MeCN has previously
been observed.16 With the knowledge of the reaction
mechanism,16 the brominated aniline 23 can be obtained in
the absence of tert-butyl nitrite, and the subsequent deami-
native iodination affords the pentiptycene dihalide 16. The
aniline 23 can also be prepared by reacting 22 with
N-bromosuccinimide (NBS) in DMF;17 however, the yield
is much lower (40%). Nonetheless, this result inspired us to
use N-iodosuccinimide (NIS) for the synthesis of 24 from
Scheme 5
19
22.18 It should be noted that the use of reagents I2/Ag2SO4
catalyzed coupling reactions.22 In this context, the haloge-
nated pentiptycenes are the critical building blocks toward
new pentiptycene-derived π-conjugated systems. In particu-
lar, the pentiptycene dihalides 15-17 allow one to construct
pentiptycene-incorporated π-conjugated polymers of a variety
of backbones. Currently, the known pentiptycene-based
conjugated polymers are limited to only the alternating arene-
pentiptycene poly(p-phenyleneethynylene)s (PPEs), prepared
from the Sonogashira coupling of the pentiptycene diacety-
lene 4 and dihalogenated arenes.2b,7
and benzyltriethylammonium dichloroiodate (ICl2 )20 for the
-
same purpose led to a mixture of unseparable compounds
and unreacted starting material, respectively. The pentip-
tycene diiodide 17 can then be prepared through the
(16) Doyle, M. P.; Van Lente, M. A.; Mowat, R.; Fobare, W. F. J. Org.
Chem. 1980, 45, 2570–2575.
(17) Mitchell, R. H.; Lai, Y.-H.; Williams, R. V. J. Org. Chem. 1979,
44, 4733–4735.
(18) Castanet, A.-S.; Colobert, F.; Broutin, P.-E. Tetrahedron Lett. 2002,
43, 5047–5048.
(19) Sy, W.-W. Synth. Commun. 1992, 22, 3215–3219.
(20) Kajigaeshi, S.; Kakinami, T.; Yamasaki, H.; Fujisaki, S.; Okamoto,
T. Bull. Chem. Soc. Jpn. 1988, 61, 600–602.
In summary, the synthesis of several new pentiptycene
halides and dihalides is reported, and their potential utilities
Org. Lett., Vol. 11, No. 6, 2009
1431

as building blocks for the construction of new π-conjugated
systems are demonstrated. In addition to the halogenated
pentiptycenes 12-17, the central-ring functionalized pen-
tiptycene intermediates 18 and 20-24 (Schemes 2 and 3)
might also find particular utilities in constructing pentip-
tycene-based materials.
Supporting Information Available: Experimental pro-
cedures and characterization data for 12-18 and 20-30 and
1H and 13C NMR spectra for new compounds 12-18, 20-28,
and 30 This material is available free of charge via the
Internet at http://pubs.acs.org.
OL900127M
Acknowledgment. We thank the National Science Council
of Taiwan, Academia Sinica, and National Taiwan University
for financial support.
(21) Hegedus, L. S. In Organometallics in Synthesis, Schlosser, M., Ed.;
John Wiley & Sons: New York, 1994; Chapter 5.
(22) Yin, L.; Liebscher, J. Chem. ReV. 2007, 107, 133–173.
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Org. Lett., Vol. 11, No. 6, 2009