Dinaphthoporphycenes

Article abstract of DOI:10.1021/ol101931y

Figure Presented. Naphthobipyrrole is a potentially useful building block for porphyrin and porphyrin analogue synthesis. Reported here is a simple, generalizable synthetic route to α-formylated, β-substituted naphthobipyrroles and their use in the preparation of binaphthoporphycenes. The resulting binaphthoporphycenes possess a planar geometry as determined via a single-crystal X-ray diffraction analysis; they also display absorption maxima that are bathochromically shifted compared to simple porphycenes.

Full text of DOI:10.1021/ol101931y

ORGANIC
LETTERS
2010
Vol. 12, No. 19
4424-4427
Dinaphthoporphycenes
Vladimir Roznyatovskiy,† Vincent Lynch,† and Jonathan L. Sessler*,†,‡
Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin,
1 UniVersity Station A5300, Austin, Texas, 78712, and Department of Chemistry,
Yonsei UniVersity, Seoul 120-749, South Korea
sessler@mail.utexas.edu
Received August 18, 2010
ABSTRACT
Naphthobipyrrole is a potentially useful building block for porphyrin and porphyrin analogue synthesis. Reported here is a simple, generalizable
synthetic route to r-formylated, ꢀ-substituted naphthobipyrroles and their use in the preparation of binaphthoporphycenes. The resulting
binaphthoporphycenes possess a planar geometry as determined via a single-crystal X-ray diffraction analysis; they also display absorption
maxima that are bathochromically shifted compared to simple porphycenes.
Systems with extended conjugation pathways have attracted
considerable attention in recent years due to their useful
optical properties. This is particularly true in the case of
porphyrin derivatives, where systems with larger π-electron
peripheries have been explicitly targeted for synthesis due
to their red-shifted light-absorbing properties. Such motiva-
tions have provided an incentive to develop so-called
expanded porphyrins (i.e., systems with larger cores than their
tetrapyrrolic “parents”).1 However, they have also led to the
study of a number of other analogues. Among these latter
are “fused” systems, wherein multiple direct links exist
between the aromatic subunits present in various porphyrin
and porphyrin analogues (e.g., pyrroles). Such systems
generally display lower energy electronic transitions in the
UV-vis spectral region, as well as enhanced nonlinear
optical behavior. For instance, the lowest energy λmax for a
linearly fused octaporphyrin was found to be 2800 nm.2
However, the preparation of fused porphyrin derivatives
generally requires a near-herculean synthetic effort and often
produces the target compounds in limited quantities.
An alternative approach to producing π-extended porphy-
rin-type frameworks involves annulation of additional aro-
matic rings onto the periphery of the macrocyclic core. To
date, linearly annulated tetrabenzo-, tetranaphtho-, and tet-
raanthroporphyrins in the form of their respective zinc
complexes have been studied in detail; these systems exhibit
bathochromic shifts with the λmax for the lowest-energy
† The University of Texas at Austin.
‡ Yonsei University.
(1) Sessler, J. L.; Weghorn, S. J. Expanded, Contracted & Isomeric
Porphyrins; Pergamon: Elmsford, NY, 1997.
(2) Tsuda, A.; Osuka, A. Science 2001, 293, 79.
10.1021/ol101931y  2010 American Chemical Society
Published on Web 09/01/2010

transition being moved to ca. 810 nm (Q-band) from the 602
nm seen for the lowest-energy Q-band of zinc tetraphe-
nylporphyrin.3 The palladium complexes of these annulated
systems were also found to behave as efficient sensitizers
for optical up-conversion via triplet-triplet annihilation when
mixed with appropriate dyes.4 Crossley and co-workers
recently reported even greater efficiency in triplet-triplet
annihilation up-conversion by using a palladium tetraqui-
noxaline porphyrin.5
Scheme 1. General Route to Disubstituted Naphthobipyrroles
Other annulated porphyrin analogues are known. These
include but are not limited to azuliporphyrin,6 phenanthroline
porphyrins,7 and benzosapphyrin.8 However, much less is
known about annulated porphycenes. In fact, to the best of
our knowledge, only the fused meso-benzoporphycenes,
dibenzoporphycene 1,9 and tetrabenzoporphycenes 210 have
been reported in the literature. On the other hand, the
interesting optical and coordination features of porphycenes11
provide an incentive to extend this limited class of com-
pounds to include other annulated porphycene derivatives.
With this goal in mind, we report here a simple and efficient
route to dinaphthoporphycenes 3a and b. We show that these
flat systems display optical features that are bathochromically
shifted compared to the parent porphycene.
The synthesis of the dinaphthoporphycenes 3a and b is
summarized in Scheme 1. It relies on the intermolecular
McMurry coupling of dialdehyde precursors pioneered by
Vogel,12 as do most other syntheses of porphycenes.13 The
problem is thus one of preparing the appropriate annulated
diformylated naphthobipyrroles. One potential precursor is
the unsubstituted 7d; this is a known compound.14 However,
this particular bipyrrole has been shown to react with various
electrophiles (diazonium salts, Vilsmeier-Haack formyla-
tion, aminomethylation reaction) predominantly at the so-
called ꢀ positions (i.e., at carbons 3 and 8);15 this makes it
less than ideal for our purposes. We therefore sought an
alternative approach that would give diformylated naphtho-
bipyrroles, such as 8a-c, wherein the reactive ꢀ positions
are blocked with alkyl groups. Here, an ancillary consider-
ation was that the judicious choice of ꢀ-substituents would
provide a means of modulating the solubility of these
intermediates, as well as that of the final porphycene targets.
(3) (a) Aleshchenkov, S. E.; Cheprakov, A. V.; Beletskaya, I. P. Dokl.
Chem. 2008, 422, 212. (b) Finikova, O. S.; Aleshchenkov, S. E.; Brinas,
R. P.; Cheprakov, A. V.; Carroll, P. J.; Vinogradov, S. A. J. Org. Chem.
2005, 70, 4617.
To create systems with high solubility in organic media,
branched alkyl chains are attractive as ꢀ-substituents. Such
branched species have been used as solubilizing groups in,
for example, polymer chemistry;16 further, one particular
branched system, 2-ethylhexane, functionalized at carbon 1
as both the halide and corresponding Grignard reagent, is
commercially available. Therefore, our initial efforts focused
on creating a bipyrrole containing this particular ꢀ-substitu-
ent. As a complement to this work, we also targeted the
synthesis of analogues bearing the smaller n-pentyl and
i-propyl substituents.
(4) Baluschev, S.; Yakutkin, V.; Miteva, T.; Avlasevich, Y.; Chernov,
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With the above considerations in mind, diethyl oxalate was
reacted with the requisite Grignard reagents at -78 °C in
accord with literature procedures.17 This gave the corre-
sponding ethyl R-oxocarboxylates 4a-c in good yields. Ethyl
pyruvate 4d is commercially available.
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131, 13538.
(12) Vogel, E.; Kocher, M.; Schmickler, H.; Lex, J. Angew. Chem., Int.
Ed. 1986, 25, 257.
(13) Sanchez-Garcia, D.; Sessler, J. L. Chem. Soc. ReV. 2008, 37, 215.
(14) Samsoniya, S. A.; Trapaidze, M. V.; Kuprashvili, N. A.; Kolesnikov,
A. M.; Suvorov, N. N. Khim. Geterotsikl. Soedin. 1985, 1222.
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Samsoniya, N. Sh.; Suvorov, N. N. Khim. Geterotsikl. Soedin. 1994, 1048.
Samsoniya, S. A.; Trapaidze, M. V.; Kuprashvili, N. A. Khim. Geterotsikl.
Soedin. 1998, 942. (b) Trapaidze, M. V.; Samsoniya, S. A.; Kuprashvili,
N. A.; Mamaladze, L. M.; Suvorov, N. N. Khim. Geterotsikl. Soedin. 1988,
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(16) (a) Li, X.; Xiao, Y.; Qian, X. Org. Lett. 2008, 10, 2885. (b) Surin,
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(17) (a) Rambaud, M.; Bakasse, M.; Duguay, G.; Villieras, J. Synthesis
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Org. Lett., Vol. 12, No. 19, 2010
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Subsequent reaction with 2,3-naphthalene bishydrazine18
in ethanolic media at rt led to the nearly quantitative
conversion to the corresponding bishydrazones 5a-d. Com-
pounds 5a-c are new compounds. However, the bishydra-
zone 5d was reported as an intermediate leading to the
synthesis of 7d (vide supra). However, full characterization
data were lacking, with only melting point and elemental
analysis results being provided.14
Given the solubility characteristics of 8c, only diformyl-
bipyrroles 8a and 8b were subject to McMurry coupling.21
Here, THF solutions of each aldehyde were added dropwise
to the preformed titanium reagent in THF at reflux. This was
done under an inert atmosphere over the course of 6 h by
means of a syringe pump. The reaction mixtures were then
maintained at reflux overnight. A basic workup then yielded
a yellow-green fluorescent solution that was assumed to be
dihydroporphycene (porphycenogen). This assignment was
made on the basis of what was seen in the case of 1; here,
it was found that the corresponding porphycenogen is rather
stable and can be isolated.22
Oxidation of the presumed porphycenogens was expected
to give the desired porphycenes. To facilitate this process, 1
equiv of DDQ was added. This caused the solution to turn
dark blue, as would be expected were the oxidation suc-
cessful. After chromatographic purification over silica gel,
porphycenes 3a and 3b were isolated in 52% and 15% yield,
respectively.
Interestingly, compounds 5a-d exist in two different
geometric isomeric forms. The individual species in
question can be isolated via column chromatography over
silica gel if desired. However, these compounds behave
identically in the following step, namely, the Fischer
indole synthesis. Thus, they were generally used without
further purification.19
Subjecting 5a-c to a 10-fold excess of p-toluenesulfonic
acid hydrate in ethanol under reflux gave the diethyl 3,8-
dialkyl-1,10-dihydro-benzo[e]pyrrolo[3,2-g]indole-2,9-dicar-
boxylate derivatives 6a-c in good to excellent yields after
purification via recrystallization or filteration through a short
silica gel column. We found these conditions20 to be superior
to the use of polyphosphoric ester in unspecified concentra-
tion as originally used for the preparation 6d.14
It is worth noting that diesters 6a-c are also formed as a
mixture of isomers, presumably different conformers arising
from the restricted rotation of the alkyl substituents. The
interconversion rate between the various isomers was such
that in some cases the 1H NMR signals were broadened, and
in others more than one species could be observed at rt. In
the case of the test system 6a, heating the sample to 100 °C
served to simplify the spectrum, producing one corresponding
to a time-averaged symmetric species.
As proved true for the intermediate bipyrrole derivatives
6a and 8a, porphycene 3a bearing branched alkyl substituents
1
showed poorly resolved H NMR spectral features at rt.
While not established unequivocally, this broadening is
ascribed to hindered rotation of the branched alkyl groups
and the existence of two or more tautomeric species at room
temperature. In the event, an increase in spectral resolution
could be achieved by heating the sample to 100 °C in
toluene-d8 (Figure 1).
Diesters 6a-c, which contain blocking ꢀ-substituents,
were saponified and decarboxylated to produce the R-free
species 7a-c. This was done using a standard one-pot
procedure that involves heating at reflux in ethylene glycol
under an inert atmosphere.21 This gave 7a-c in 83-90%
yield.
Vilsmeier-Haack formylation served to convert 7a-c to
8a-c in almost quantitative yield. As expected, 8a had the
highest solubility and proved soluble in most common
organic solvents. The isopropyl-substituted dialdehyde 8b
showed moderate solubility, which allowed for crystallization
from halogenated solvents, such as dichloroethane. On the
other hand, compound 8c, bearing linear n-pentyl substitu-
ents, proved minimally soluble in THF. In our estimation,
this made this particular derivative less attractive for use in
porphycene synthesis.22
Figure 1.
1H NMR spectra (low field portion) of porphycene 3a in
toluene-d8 at 27 °C (bottom) and 100 °C (top).
Both porphycenes 3a and 3b display NH proton reso-
nances at ca. 9.3 ppm (CDCl3, rt, 400 MHz). This is in
marked contrast to the original unsubstituted porphycene (NH
resonance at 3.2 ppm, CDCl3, 300 MHz).12 However, it is
in agreement with what is seen for the dibenzoporphycene
1 (NH signal at 10.6 ppm; CDCl3).23
(18) Franzen, H. Ber. Chem. 1905, 38, 266. Franzen, H. J. Prakt. Chem.
1908, 76, 205.
(19) For more details see Figure S1 in Supporting Information.
(20) Wagaw, S.; Yang, B. H.; Buchwald, S. L. J. Am. Chem. Soc. 1998,
120, 6621.
(21) Stepien, M.; Donnio, B.; Sessler, J. L. Chem.sEur. J. 2007, 13,
6853.
Both 3a,b showed identical features in their respective
UV-vis spectra (dichloromethane). In particular, Q bands
(22) Recently, we became aware that the group of Panda made 8c and
apparently carried it on to the corresponding porphycene. However, no
structural data were presented. (Panda, P. K.; Sarma, T. Abstracts of the
Sixth International Conference on Porphyrins and Phthalocyanines (ICPP6);
New Mexico, USA, 2010, p 191.)
(23) Pietrzak, M.; Shibl, M. F.; Broring, M.; Kuhn, O.; Limbach, H.-H.
J. Am. Chem. Soc. 2007, 129, 296.
4426
Org. Lett., Vol. 12, No. 19, 2010

were seen at 725, 671, and 556 nm, and a Soret band was
observed at 404 nm with shoulder at 389 nm (Figure 2).
structure revealed that this particular naphthoporphycene is
nearly planar (Figure 3). The average deviation for the
Figure 3. Three orthogonal views of the X-ray structure of 3b. All
hydrogen atoms bound to carbon atoms are omitted for clarity.
Thermal elipsoids were scaled to the 50% probability level.
Figure 2. UV-vis spectra for 3a and 3b recorded in dichlo-
romethane (0.11 mM).
Qualitatively, these spectral features are typical for por-
phycenes and are thought to reflect a lower symmetry than
typically found in porphyrins. In accord with design expecta-
tions, however, the visible portion of the spectrum is
somewhat red-shifted as compared to nonannulated por-
phycenes (e.g., 2,7,12,17-tetrapropylporphycene for which
Q bands at 634, 602, and 562 nm are seen).24 These
differences are attributed to the bipyrrole-fused naphthalene
groups and to the resulting extension of the π-electron
framework. Although further analyses are required, these
structural changes relative to nonannulated porphycenes may
also give rise to a possible intruder state,9b reflected in a
weak transition at ca. 505 nm. Compound 3b proved weakly
fluorescent in dichloromethane.
Due to its higher crystallinity (and corresponding ease in
handling) relative to 3a, porphycene 3b was subject to more
detailed analysis using cyclic voltammetry. This particular
annulated porphycene displays a reversible two-step oxida-
tion process, which is characterized by peak potentials at
0.89 and 1.24 V (cyclic voltammetry, 1.5 mM/0.1 M
TBAPF6/DCM vs SCE; 100 mV/s). The fact that oxidation
is more facile with respect to the tetrapropyl porphycene
parent is considered consistent with a larger π-electron
framework being present in 3.25 The observation of a
reversible, anodically shifted reduction wave at -0.78 V (vs
-1.04 V for the tetrapropyl system) provides further support
for this conclusion. However, in contrast to what is seen in
the case of the parent tetrapropyl porphycene, a second
cathodic wave could not be observed in the case of 3b.
This same, less soluble, isopropyl-substituted porphycene
(i.e., 3b) also yielded X-ray diffraction grade single crystals
upon recrystallization from chloroform/hexane. The resulting
nitrogen atoms from the mean plane (excluding isopropyl
groups) is (0.15 Å. On the basis of this structural parameter,
3b is slightly more distorted than tetrapropylporphycene for
which the corresponding deviation is (0.04 Å.12 The shortest
N-N distance in 3b is 2.49 Å, whereas the corresponding
separation in the parent, nonannulated porphycenes is typi-
cally 2.63 Å.12 These structural differences, together with
the NMR spectroscopic data discussed above, provide
support for the notion that the N-H-N hydrogen bonds in
the π-extended porphycenes reported here are stronger than
those present in the original, nonannulated porphycenes.
In conclusion, we have demonstrated a facile route to
disubstituted naphthobipyrroles 6-8a,c, chemistry that we
think provides new precursors for use in porphyrin analogue
synthesis. We have demonstrated the utility of these building
blocks via the rational preparation of annulated porphycenes
with extended π-frameworks. To the best of our knowledge,
one of these products, porphycene 3b, is the first annulated
porphycene to be subject to X-ray diffraction-based structural
analysis. Detailed physicochemical studies of it and other
new porphycene analogues are currently in progress.
Acknowledgment. This work was supported by the
National Institutes of Health (grant no. CA68682 to J.L.S)
and the National Science Foundation (grant no. 0741973 for
the X-ray diffractometer). Support under the WCU (World
Class University) program (R32-2008-000-10217-0) is also
acknowledged.
Supporting Information Available: Experimental pro-
cedures and characterization data of new compounds. X-ray
crystallographic data of compound 3b in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(24) Bernard, C.; Gisselbrecht, J. P.; Gross, M.; Jux, N.; Vogel, E. J.
Electroanal. Chem. 1995, 381, 159.
(25) Gisselbrecht, J. P.; Gross, M.; Koecher, M.; Lausmann, M.; Vogel,
E. J. Am. Chem. Soc. 1990, 112, 8618.
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