Triphenylene-fused porphyrins

Article abstract of DOI:10.1021/ol200853g

Triphenylene has been successfully fused to the porphyrin periphery through a convenient oxidative ring-closure reaction. Bistriphenylene-fused porphyrins and a dibenzo[fg,op]tetracene-fused porphyrin have also been obtained using a similar approach. Thes

Full text of DOI:10.1021/ol200853g

ORGANIC
LETTERS
2011
Vol. 13, No. 12
3020–3023
Triphenylene-Fused Porphyrins
Lin Jiang,† James T. Engle,‡ Laura Sirk,† C. Scott Hartley,†
Christopher J. Ziegler,‡ and Hong Wang*,†
Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056,
United States, and Department of Chemistry, University of Akron, Akron, Ohio 44325,
United States
Wangh3@muohio.edu
Received April 1, 2011
ABSTRACT
Triphenylene has been successfully fused to the porphyrin periphery through a convenient oxidative ring-closure reaction. Bistriphenylene-fused
porphyrins and a dibenzo[fg,op]tetracene-fused porphyrin have also been obtained using a similar approach. These π-extended porphyrins
exhibited broadened and bathochromic shifted UVꢀvis absorptions.
Porphyrins fused with external aromatic rings show
promise in a broad range of applications in various areas,
including near-infrared dyes,1 molecular devices,2 organic
light-emitting diodes,3 and biosensors.4 In particular, por-
phyrins fused with polycyclic aromatic hydrocarbons
(PAHs) hold great potential in the fast growing areas of
molecular electronics and nanotechnology as heteroatom-
containing molecular graphene mimics.5 As such, the devel-
opment of new methodologies to fuse aromatic rings to the
porphyrin periphery has remained an intense area of interest.
The oxidative ring closure reaction has been proven to be
an effective approach to construct polycyclic aromatic
hydrocarbons.6 Recently, this approach has also been utilized
in porphyrins. Porphyrins fused with anthracene,7 pyrene,8
† Miami University.
‡ University of Akron.
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r
10.1021/ol200853g
Published on Web 05/23/2011
2011 American Chemical Society

azulene,9 and other porphyrins5c,10 have been reported. In
these π-expanded porphyrins, an aromatic fragment is de-
signed to match the geometry of the porphyrin, such that the
fusion occurs at the adjacent porphyrin meso- and β-posi-
tions. In this work, we use a different strategy. The polycyclic
aromatic hydrocarbons are introduced at the porphyrin β,β-
positions through a convenient oxidative coupling reaction.
We report the synthesis and characterization of the first
examples of triphenylene- and dibenzo[fg,op]tetracene-fused
porphyrins.
Scheme 2a
Despite recent progress,11 most of the syntheses of β,β-
fused extended porphyrins require harsh conditions and/or
are lengthy. Recently, we developed a concise method to
prepare functionalized benzoporphyrins (Scheme 1).12 In
this method, an alkene reacts with a β,β0 -dibromoporphyrin
through a three-step cascade reaction involving a vicinal
2-fold Heck reaction, 6-π electrocyclization, and subsequent
aromatization. This method made a new entry to access a
number of functionalized benzoporphyrins. We envisioned
that two vicinal aromatic rings could be introduced to the
fused benzene ring(s) via this approach, and more benzene
rings could then be fused to the porphyrin through the well-
established oxidative ring-closure reaction.
Scheme 1. Synthesis of opp-Dibenzoporphyrins via
Pd0-Catalyzed Three-Step Cascade Reaction12
a When R1 = NO2, 6 might be a mixture of two isomers.
nitrobenzoporphyrin 4aꢀd in 40ꢀ50% yields. Treatment
of 4aꢀd with excess of FeCl3 in a mixture of nitromethane
and dichloromethane (DCM) led to the formation of the
triphenylene fused porphyrin 5aꢀd in 35ꢀ40% yields.
Other isomers (6aꢀd) were also isolated in 7ꢀ12% yield.
When Zn(II) porphyrins (4b and 4d) underwent oxidative
coupling reactions, exclusive demetalation occurred, result-
ing in the formation of free base triphenylenoporphyrins
(5b, 5d, 6b, and 6d). The structures of 5aꢀd were deter-
1
mined by H and 13C NMR spectroscopy and MALDI-
In order to investigate the viability of this approach, we
prepared 2,3-dibromo-12-nitroporphyrins13 3aꢀd(Scheme 2).
Thus, 3aꢀd were reacted with 3-methoxystyrene in the
presence of a palladium catalyst to give aryl-substituted
TOF MS and were further confirmed by an X-ray crystal
structure of 5a (Figure 1). The crystal structure of 5a
reveals that the β,β-pyrrole-fused triphenylene moiety
retains its planarity, while the porphyrin core adopts a
slight saddle conformation.
(10) Kamo, M.; Tsuda, A.; Nakamura, Y.; Aratani, N.; Furukawa,
K.; Kato, T.; Osuka, A. Org. Lett. 2003, 5, 2079.
Initially, the identities of 6aꢀd could not be determined
because of their structural complexity. Compounds 5 and 6
are constitutional isomers, and their UVꢀvis absorptions
are also similar. However, the 1H NMR spectra of these two
compounds are readily distinguished. We speculated that
two types of oxidative couplings occurred to porphyrin
4aꢀd. To further investigate the nature of this oxidative
coupling reaction, we prepared a symmetrical benzopor-
(11) (a) Ito, S.; Murashima, T.; Uno, H.; Ono, N. Chem. Commun.
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E. H.; Fronczek, F. R.; Vicente, M. G. H.; Smith, K. M. Chem. Commun.
2006, 3900. (f) Silva, A. M. G.; de Oliveira, K. T.; Faustino, M. A. F.;
Neves, M.; Tome, A. C.; Silva, A. M. S.; Cavaleiro, J. A. S.; Brandao, P.;
Felix, V. Eur. J. Org. Chem. 2008, 704. (g) Lee, S. H.; Smith, K. M.
Tetrahedron Lett. 2005, 46, 2009. (h) Walter, A.; Cavaleiro, J. A. S.
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1
phyrin 4e from a simpler dibromoporphyrin 3e.13 The H
(12) Deshpande, R.; Jiang, L.; Schmidt, G.; Rakovan, J.; Wang, X.;
Wheeler, K.; Wang, H. Org. Lett. 2009, 11, 4251.
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Tetrahedron 1999, 55, 13151.
NMR spectrum of 6e, which is much simpler than those of
6aꢀd, suggests that 6e resulted from an unsymmetrical ring
closure of the two adjacent methoxyphenyl substituents
Org. Lett., Vol. 13, No. 12, 2011
3021

porphyrins fused with two triphenylenes to further extend
the porphyrin π-system, starting from tetrabromoporphyrin
713 (Scheme 3). Similar to the monofused systems, the
unsymmetrical ring-closure product (10) was also isolated in
14% yield. The symmetrical 9 was obtained in 48% yield. The
other two possible unsymmetrical ring-closure products may
exist but were not isolated because of the extremely low yields.
The geometry of triphenylene-fused porphyrins 5 pro-
vides an ideal template to conveniently fuse more aromatic
rings, if the proper substituents are installed on the fused
rings. Thus, we prepared monobenzoporphyrin 11 bearing
two methoxy groups on the phenyl substituents (Scheme 4).
Compound 11 was treated with 1,2-dimethoxybenzene in
the presence of FeCl3 as the oxidant, leading to the
formation of the desired dibenzo[fg,op]tetracene-fused
porphyrin 13 in 28% yield. Further investigation of this
reaction suggests that, as expected, this one-pot reaction
involves two steps: first, formation of the triphenylene-
fused 12; second, fusion of 1,2-dimethoxybenzene to the
triphenylene moiety of 12.
Figure 1. X-ray crystal structure of 5a: top, edge view; bottom,
top view.
(Scheme 2). A X-ray crystal structure of 6a further con-
firmed our speculation (see the Supporting Information).
This type of unsymmetrical oxidative ring closure is not
favored due to the expected steric hindrance generated by the
methoxy group in the bay region of the triphenylene and has
never been reported previously to our knowledge. We believe
that the observed unsymmetrical ring closure is caused by
the competition that arises from additional steric hindrance
introduced by the alkyl groups on the meso-phenyl rings and
the methoxy group on the fused benzene ring.
Scheme 4
Having established the feasibility of this approach
of fusing one triphenylene to porphyrins, we prepared
Scheme 3
UVꢀvis absorption spectra of 4e, 5e, 5c, 9, and 13 are
shown in Figure 2. The C2v symmetrical monobenzopor-
phyrin 4e showed an intense Soret band at 434 nm; upon
fusion of the triphenylene moiety (5e), the Soret band was
broadened and red-shifted to 447 nm; further extension of the
π-system (13) pushed the Soret band to 457 nm. When two
triphenylenes were fused (9), the UVꢀvis spectra displayed
an interesting shouldered Soret band (with a peak maximum
at 469 nm with a shoulder at 443 nm), a rare observation for
Ni(II) porphyrins with D2h symmetry. All the triphenylene-
fused porphyrins exhibited broadened, red-shifted, and more
intense Q bands, typical of π-extended porphyrins.
(14) Tsai, H.-H.; Simpson, M. C. J. Phys. Chem. A 2004, 108, 1224.
Org. Lett., Vol. 13, No. 12, 2011
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Figure 3. Optimized molecular structures of 5e, 5c, 9, and 13
(B3LYP/6-31G(d,p)).
Figure 2. UVꢀvis absorption spectra of 4e, 5e, 5c, 9, and 13 in
CH2Cl2.
two competing steric effects. These π-extended por-
phyrins displayed much broadened and bathochromic-
shifted UVꢀvis absorptions, as compared with their
unfused precursors. Structural optimization suggests
that the two planar triphenylene moieties of the bis-
triphenylene-fused porphyrin (9) do not lie in the same
plane due to significant distortion of the porphyrin
core, breaking the expected D2h symmetry. Currently,
we are engaged in generalizing this method to fuse
nonalkoxy-substituted triphenylene(s). The develop-
ment of methodologies to fuse more aromatic polyhy-
drocarbons is also underway.
Density functional calculations (B3LYP/6-31G(d,p))
were conducted for 5e0, 5c0, 130 and 90, simplified versions
of 5e, 5c, 13, and 9 with the isopropyl groups removed
(Figure 3). Similar to the crystal structure (5a), the β,β-
pyrrole-fused triphenylene moiety in 5c0 is essentially
planar in the calculated structure; the porphyrin core is,
however, a little more distorted from planarity, adopting a
conformation somewhere between a saddled and a ruffled
distortion. This exaggerated distortion of the porphyrin
core in the DFT-minimized geometries is a well-known
consequence of the shallow potential energy surfaces of
Ni(II) porphyrins.14 In the absence of a nitro group at the
porphyrin β- position, 5e0 shows a less distorted structure
with a ruffled conformation. In the more π-extended 130,
the fused polycyclic aromatic hydrocarbon slightly devi-
ates from planarity, likely due to the full occupation of the
methoxy groups at the bay positions. It should be noted
that, while both of the triphenylene moieties in the bis-
triphenylene-fused porphyrin 9 retain their planarity, the
porphyrin core displays a ruffled conformation with sig-
nificant distortion from planarity. As a result, the two
planar triphenylene moieties do not lie in the same plane.
This distortion causes the loss of its D2h symmetry.
Acknowledgment. H.W. thanks Miami University for
financial support. C.S.H. acknowledges support from the
U.S Air Force Office of Scientific Research (Award No.
FA9550-10-1-0377). C.J.Z. acknowledges the National
Science Foundation (CHE-0840446) for funds used to
purchase the diffractometer used in this work. The Na-
tional Science Fundation (CHE0839233) is acknowledged
for providing funds to purchase the MALDI TOF mass
spectrometer used in this work.
Supporting Information Available. Experimental pro-
cedures and characterization data for all new com-
pounds, including 1H and 13C NMR spectroscopy
and UVꢀvis absorption data. X-ray crystallography
data for compounds 5a and 6a (CIF).This material is
available free of charge via the Internet at http://pubs.
acs.org.
In summary, we have synthesized the first examples
of triphenylene-fused porphyrins through a Pd0-cata-
lyzed cascade reaction followed by oxidative ring clo-
sure. Further extension of the porphyrin π-system to
fuse one dibenzo[fg,op]tetracene has also been achieved
using a similar approach. Unusual unsymmetrical ring-
closure products were obtained, likely resulting from
Org. Lett., Vol. 13, No. 12, 2011
3023