A facile one-pot synthesis of meso-aryl-substituted [14]triphyrin(2.1.1)

Article abstract of DOI:10.1021/ja8068769

The synthesis and spectroscopic properties of a series of boron-free meso-aryl-substituted [14]triphyrin(2.1.1) compounds containing either peripheral bicyclo[2.2.2]octadiene (BCOD) (2a-c) or benzene rings (3a-c) (aryl = phenyl a, 4-fluorophenyl b, and 4-methylbenzoatephenyl c) are reported. These compounds represent the first examples of free-base contracted porphyrinoids with 14 π-electron aromatic systems containing only the standard pyrrole and isoindoline moieties of the porphyrins and tetrabenzoporphyrins. Copyright

Full text of DOI:10.1021/ja8068769

Published on Web 11/17/2008
A Facile One-Pot Synthesis of meso-Aryl-Substituted [14]Triphyrin(2.1.1)
Zhao-Li Xue,† Zhen Shen,*,† John Mack,‡ Daiki Kuzuhara,§ Hiroko Yamada,*,§ Tetsuo Okujima,§
Noboru Ono,§ Xiao-Zeng You,*,† and Nagao Kobayashi*,‡
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of
Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China, Department of Chemistry and
Biology, Graduate School of Science and Engineering, Ehime UniVersity, Bunkyo-cho 2-5, Matsuyama
790-8577, Japan, PRESTO, JST, Kawaguchi 332-0012, Japan, and Department of Chemistry, Graduate School of
Science, Tohoku UniVersity, Sendai 980-8578, Japan
Received September 9, 2008; E-mail: zshen@nju.edu.cn; yamada@chem.sci.ehime-u.ac.jp; youxz@nju.edu.cn;
nagaok@mail.tains.tohoku.ac.jp
Scheme 1. Synthesis of meso-Aryl-Substituted
There has been considerable interest in recent years in contracted
porphyrinoids such as subphthalocyanines,1 subporphyrazines,2 and
subporphyrins3 due to potential applications in a variety of high
technology fields.1d,c,3e In contrast with the porphyrins and phtha-
locyanines, the electronic structure of these molecules is based on
a 14 π-electron system, since there are three, rather than four,
pyrrole or isoindoline moieties. Although the subphthalocyanines
have been the focus of considerable interest for over two decades,
subporphyrins have only recently been characterized. In 2006,
Osuka and co-workers reported the synthesis of tribenzosubpor-
phyrins.3a Subsequently, Kobayashi and Osuka et al. independently
developed synthetic protocols for meso-aryl substituted subpor-
phyrins.3b-g With the exception of subpyriporphyrin,4 [18]triphy-
rin(6.1.1),5 [18]triphyrin(4.1.1),6 [15]triphyrin(1.1.3),7 [18]triphy-
rin(2.2.2), and [18]heterotriphyrin(2.2.2),8 the subporphyrins re-
ported to date have all been boron complexes with highly nonplanar
cone-shaped conformations.3 The development of new synthetic
pathways for the structural modification of contracted porphyrinoids
and the fine-tuning of their spectroscopic properties remains a
considerable challenge.
In this Communication, we report the facile synthesis of a series
of meso-aryl-substituted [14]triphyrin(2.1.1) and [14]benzotriphy-
rin(2.1.1) compounds. These compounds are unprecedented in
porphyrinoid research since they represent the first examples of
near planar metal -free contracted porphyrinoids with 14 π-electron
aromatic systems containing only the standard pyrrole and isoin-
doline moieties of the porphyrins and tetrabenzoporphyrins. The
insertion of a second sp2-hybridized carbon atom between one of
the three neighboring pairs of pyrroles eliminates the need for a
central boron atom.
[14]Triphyrins(2.1.1)a
a Reagents and conditions: (i) p-RC6H4CHO, BF3 ·Et2O, CH2Cl2, room
temp, 12 h; (ii) p-chloranil, 2 h; (iii) 220 °C, 2 mmHg, 20 min.
obtained by slow diffusion of hexane into the dichloromethane
solution. The triphyrin macrocycle is near planar, Figure 1. The
mean deviations of the ring atoms are 0.1077 Å for 2a and 0.1309
Å for 3a. In both 2a and 3a, the meso-phenyl rings are inclined
67.0°, 53.8°, 83.2°, and 86.5° with respect to the triphyrin mean
plane. These values are larger than the phenyl dihedral angles that
have been reported for the subporphyrins (ca. 50.3°),3d perhaps due
to the greater steric hindrance caused by the presence of fused
BCOD or benzene rings at the ꢀ-pyrrole positions. The average
bond length between the meso-carbon atoms and the phenyl rings
is 1.505 Å for 2a and 1.498 Å for 3a, Supporting Information,
Table S1. The pyrrole rings are tilted relative to the triphyrin mean
plane by 20.1°, 4.3°, and 6.7° in the case of 2a and 16.6°, 7.5°,
and 6.7° in the case of 3a. The peripheral bond distances of 2a
and 3a are very close to the values reported for subporphyrins with
the exception of the pyrrole(ꢀ)-pyrrole(ꢀ) distance in 3a (ca. 1.42
Å), which is significantly longer. The π-conjugation system is highly
delocalized. For example, the C18-C19 bond between the two
carbons bridging neighboring pyrroles (1.407 Å in 2a and 1.423 Å
in 3a) is similar in length to the C19-C20 (1.424 Å in 2a and
1.423 Å in 3a) and C17-C18 bonds (1.438 Å in 2a and 1.423 Å
We initially prepared meso-aryl-substituted [14]triphyrin(2.1.1)
serendipitously during a BF3•Et2O catalyzed Rothemund condensa-
tion of 4,7-dihydro-4,7-ethano-2H-isoindole (1) with aryl aldehyde
in dichloromethane, Scheme 1, which was followed by an oxidation
with p-chloranil. After silica gel column chromatography, 2a-c
(aryl ) phenyl 2a, 4-fluorophenyl 2b, and 4-methylbenzoatephenyl
2c) can be isolated in ca. 35% yield. The corresponding [14]ben-
zotriphyrin(2.1.1) compounds 3a-c can be formed quantitatively
based on a retro Diels-Alder reaction of the bicyclo[2.2.2]octadiene
(BCOD) rings by heating 2a-c at 220 °C under vacuum (2 mmHg)
for 20 min.
The triphyrin structures were determined definitively by X-ray
structural analysis of single-crystals of 2a and 3a, which were
Figure 1. The crystal structures of 2a (a) and 3a (b) with 50% probability
thermal ellipsoids (top, perspective view; below, side view with phenyl
groups omitted). The solvent molecules are omitted for clarity.
† Nanjing University.
‡ Tohoku University.
§ Ehime University.
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10.1021/ja8068769 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
exhibit marked red shifts relative to the spectra of 2a and 3a. Similar
trends have been observed previously in the spectra of porphyrin
dications.12
In conclusion, we have successfully synthesized a series of meso-
aryl substituted [14]triphyrins(2.1.1) in moderate yield from mild
Rothemund porphyrin condensation reactions and the corresponding
[14]benzotriphyrin(2.1.1) compounds based on retro Diels-Alder
reactions. These compounds represent the first examples of free-
base contracted porphyrinoids with 14 π-electron aromatic systems
containing only the pyrrole and isoindoline moieties of the por-
phyrins and tetrabenzoporphyrins. Like other contracted porphy-
rinoids, they have potential applications in a variety of high
technology fields.1c,d,3e
Figure 2. NH tautomers of [14]triphyrin(2.1.1).
Acknowledgment. This work was supported by Grant-in-Aids
for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan (No. 18550037 to H.Y. and
19655045 to N.K.), JGC-S Scholarship Foundation (T.O.), the
National Basic Research Program of China (No. 2006CB806104
and 2007CB925103), and NSFC (Nos. 20875043 and 20721002.
The authors thank Dr. Kimiko Hasegawa at Rigaku Corporation
for the X-ray single crystal measurements and Prof. Yi-Zhi Li at
Nanjing University for solving the X-ray structures.
Figure 3. MCD (top) and UV-visible absorption spectra (bottom) of 2a
and 3a in CHCl3 at 298 K.
in 3a). This, in turn, suggests that there is rapid exchange between
the two NH tautomers in the solid state, Figure 2 and that the
inner NH proton forms part of a three-centered hydrogen bond.
The average N-N distances (2.573 Å for 2a and 2.578 Å for
3a) lie in the range anticipated for a very strong hydrogen-bond
interaction.9
Supporting Information Available: Experimental procedures and
spectral data for 2a-c and 3a-c; bond lengths and angles for 2a and
3a. This material is available free of charge via the Internet at http://
pubs.acs.org.
The singlet NH resonance was determined to lie at 7.68 ppm
in the 1H NMR spectrum of 2c in CDCl3, since the peak
disappears upon addition of a drop of D2O, Figure S1b. With
the exception of three broad singlet peaks at 3.63, 3.35, and
2.47 ppm, which are associated with the bridge head of three
bicyclo[2.2.2]octadiene groups, the other signals of the peripheral
protons lie in the 7-9 ppm range, reflecting the 14 π-electron
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