meso-porphyrinyl-substituted porphyrin and expanded porphyrins

Article abstract of DOI:10.1021/ol048953f

(Chemical Equation Presented) A series of meso-porphyrinyl-substituted expanded porphyrins were prepared from the 1:1 acid-catalyzed condensation reaction of 4-porphyrinyl-2,3,5,6-tetrafluorobenzaldehyde and pyrrole.

Full text of DOI:10.1021/ol048953f

ORGANIC
LETTERS
2004
Vol. 6, No. 21
3663-3666
meso-Porphyrinyl-Substituted Porphyrin
and Expanded Porphyrins
Yasuhide Inokuma and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, and
CREST (Core Research for EVolutional Science and Technology) of Japan Science and
Technology Agency (JST), Sakyo-ku, Kyoto 606-8502, Japan
osuka@kuchem.kyoto-u.ac.jp
Received June 5, 2004
ABSTRACT
A series of meso-porphyrinyl-substituted expanded porphyrins were prepared from the 1:1 acid-catalyzed condensation reaction of 4-porphyrinyl-
2,3,5,6-tetrafluorobenzaldehyde and pyrrole.
In recent years, expanded porphyrins that bear more than
five pyrrolic subunits have been attracting considerable
interest in light of their interesting optical, electrochemical,
and coordination properties.1,2 Among these, we reported that
a series of meso-aryl-substituted expanded porphyrins were
prepared in the Rothemund-Lindsey reaction of 2,6-disub-
stituted aryl aldehyde and pyrrole at a relatively high
concentration, ca. 67 mM each.3 In this paper, we report the
extension of this procedure to a formyl-substituted porphyrin
substrate that permits the synthesis of meso-porphyrinyl-
substituted porphyrin and expanded porphyrins. Porphyrin
subunits, covalently linked to an expanded porphyrin core,
would offer an opportunity of creating interesting electronic
interactions between two macrocycles both in the ground
and excited states, and such interactions may be tuned by
appropriate and selective metalation at porphyrin cores. In
addition, covalently linked porphyrin-expanded porphyrin
conjugates are expected to undergo distinct multi-redox
processes, which may be interesting in view of multibit
molecular-based information storage.4 Despite this rich
promise, little attention has been paid to covalently linked
porphyrin-expanded porphyrin so far.5
In our modified Rothemund-Lindsey protocol for the
synthesis of meso-aryl substituted expanded porphyrins,
aromatic aldehydes are limited to those bearing electron-
withdrawing substituents at both ortho-positions, and rather
high concentrations (ca. of 67 mM) of substrates are
necessary for achieving high yields of expanded porphyrins.3a
Thus, as a precursor of meso-porphyrinyl-substituted ex-
panded porphyrins, we designed formyl-substituted porphyrin
3a, which was prepared in 16% yield by the reaction6 of
4-(5,5-dimethyl-1,3-dioxan-2-yl)-2,3,5,6-tetrafluorobenzalde-
hyde (1),7 bis(3-ethyl-4-methyl-2-pyrrolyl)methane (2),8 and
(1) Sessler, J. L.; Gebauer, A.; Weghorn, S. J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: San Diego, 1999; Vol. 2, p 55.
(2) (a) Jasat, A.; Dolphin, D. Chem. ReV. 1997, 97, 2267. (b) Lash, T.
D. Angew. Chem., Int. Ed. 2000, 39, 1763. (c) Furuta, H.; Maeda, H.; Osuka,
A. Chem. Commun. 2002, 1795. (d) Sessler, J. L.; Seidel, D. Angew. Chem.,
Int. Ed. 2003, 42, 5134. (e) Chandrashekar T. K.; Venkatraman, S. Acc.
Chem. Res. 2003, 36, 676. (f) Ghosh, A. Angew. Chem., Int. Ed. 2004, 43,
1918.
(3) (a) Shin J.-Y.; Furuta H.; Yoza K.; Igarashi S.; Osuka A. J. Am.
Chem. Soc. 2001, 123, 7190. (b) Taniguchi, R.; Shimizu, S.; Suzuki, M.;
Shin, J.-Y.; Furuta, H.; Osuka, A. Tetrahedron Lett. 2003, 44, 2505. (c)
Shimizu, S.; Shin, J.-Y.; Furuta, H.; Rami, I.; Osuka, A. Angew. Chem.,
Int. Ed. 2003, 42, 78. (d) Suzuki, M.; Osuka, A. Org. Lett. 2003, 5, 3942.
(e) Lash, T. D.; Colby, D. A.; Ferrence, G. M. Eur. J. Org. Chem. 2003,
4533.
(4) Gryko, D. T.; Clausen, C.; Roth, K. M.; Dontha, N.; Bocian, D. F.;
Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7345.
(5) Noncovalently linked porphyrin-sapphyrins are known to undergo
intracomplex exciation energy transfer from porphyrin to sapphyrin. (a)
Kral, V.; Springs, S. L.; Sessler, J. L. J. Am. Chem. Soc. 1995, 117, 8881.
(b) Springs, S. L.; Gosztola, D.; Wasielewski, M. R.; Kral, V.; Andriensky,
A.; Sessler, J. L. J. Am. Chem. Soc. 1999, 121, 2281.
(6) Osuka, A.; Nagata, T.; Kobayashi, F.; Maruyama, K. J. Heterocycl.
Chem. 1990, 27, 1657.
(7) Krebs, F. C.; Jensen, T. J. Fluorine Chem. 2003, 120, 77.
10.1021/ol048953f CCC: $27.50
© 2004 American Chemical Society
Published on Web 09/18/2004

Scheme 1
benzaldehyde followed by acidic hydrolysis (Scheme 1).
afforded meso-porphyrinyl-substituted porphyrin and ex-
panded porphyrins (Figure 1). Because of much simpler 1H
NMR spectra of the products, we focused on the reaction of
3a with pyrrole. A solution of 3a and pyrrole (0.60 mM each)
was treated with BF3‚OEt2 for 2 h in the dark and was
subsequently oxidized with DDQ for overnight. Separation
by silica gel column gave pentakis(porphyrinyl)-N-fused
pentaphyrin 5 (8.2%, m/z ) 3885; calcd for C245H215N25F20
) 3887) and a fraction that contained many expanded
porphyrin products. Further separation of this fraction by
However the poor solubility of 3a hampered its dissolution
at 67 mM in CH2Cl2, and thus, a more soluble porphyrin
substrate 3b was prepared from the reaction of 1, 2, and 3,5-
dioctyolxybenzaldehyde in 29% yield. Initially, the porphyrin
3b was reacted with an equimolar amount of pyrrole at ca.
67 mM in the presence of BF3‚OEt2 (3 equiv) followed by
oxidation with DDQ. But this reaction gave only polymeric
intractable materials. Meanwhile, it was found that a similar
reaction at ca. 100-fold dilute concentration (ca. 0.60 mM)
Figure 1. meso-Porphyrinyl-substituted porphyrin 4, N-fused pentaphyrin 5, and hexaphyrin 6.
3664
Org. Lett., Vol. 6, No. 21, 2004

recycling GPC-HPLC chromatography gave tetrakis(por-
phyrinyl)porphyrin 4 (1.8%, m/z ) 3111; calcd for C196
-
Scheme 2
H174N20F16 ) 3111), hexakis(porphyrinyl)hexaphyrin 6 (11.4%,
m/z ) 4667; calcd for C294H262N30F24 ) 4668), and higher
homologues of meso-porphyrinyl-substituted expanded por-
phyrins.9 These products elute as a discrete band on GPC-
HPLC in the order of decreasing molecular size (Supporting
Information).
The absorption spectra of 4-6 are shown in Figure 2.
These spectra indicate predominant contribution of porphyrin
subtracting the absorbance of the porphyrin chromophores
from the spectra of 5 and 6 (Figure 4). It is evident that the
corrected absorption spectrum of 5 (λmax ) 562 nm) is similar
to that of oxidized form of N-fused [22]pentaphyrin 7-ox
(λmax ) 551 nm).11 Treatment of 5 with NaBH4 caused a
distinct color change from dark red to light orange (λmax
)
514 nm), which has been assigned to the formation of a
reduced form of meso-pentakis(porphyrinyl)-substituted N-
fused [24]pentaphyrin, 5-red, on the basis of the similarity
of its corrected absorption spectrum to that of 7-red. Upon
treatment with DDQ, 5-red was quantitatively oxidized to
5, as judged from a color change from light orange to dark
red (λmax ) 562 nm) (Scheme 2). Therefore, the product 5
Figure 2. UV-vis absorption spectra of 4-6 in CH2Cl2.
chromophores both in the Soret band (350-450 nm) and
Q-band (500-700 nm) regions but reveal distinct differences
in the latter region, particularly in the case of 6. While the
Soret band of 4 is split at 411 and 423 nm owing to the
exciton coupling between porphyrins bridged by a tetrafluo-
rophenylene spacer,10 those of 5 and 6 are observed at 408
nm, being nearly at the same position as that (408 nm) of
reference porphyrin 3c.
It is known that N-fused pentaphyrin 7 can take two stable
oxidation states, 7-ox (22π-electron, aromatic) and 7-red
(24π-electron, nonaromatic),11 and similarly hexaphyrin 8
can also take two stable oxidation states, 8-ox (26π-electron,
aromatic) and 8-red (28π-electron, nonaromatic) (Scheme
2).12 In both cases, these two stable oxidation states are
quantitatively interconvertible upon oxidation with DDQ and
reduction with NaBH4. These transformations can be moni-
tored by UV-vis spectroscopy as shown in Figure 3.
To determine the oxidation states of expanded porphyrin
cores in 5 and 6, the absorption spectra were corrected by
(8) Clezy, P. S.; Nichol, A. W. Aust. J. Chem. 1965, 11, 1835.
(9) meso-Heptakis(porphyrinyl)heptaphyrin (3.1%, m/z ) 5442; calcd
for C343H305N35F28 ) 5445) and meso-octakis(porphyrinyl)octaphyrin (1.6%,
m/z ) 6222; calcd for C392H348N40F32 ) 6223) were isolated but their full
characterizations have not been completed.
(10) (a) Wennerstrom, O.; Ericsson, H.; Raston, I.; Svensson, S.; Pimlott,
W. Tetrahedron Lett. 1989, 30, 1129. (b) Osuka, A.; Liu, B.; Maruyama,
K. Chem. Lett. 1993, 949.
(11) Shin J.-Y.; Furuta H.; Osuka A. Angew. Chem., Int. Ed. 2001, 40,
619.
Figure 3. Absorption spectra of meso-pentafluorophenyl expanded
porphyrins 7-ox, 7-red, 8-ox, and 8-red in CH2Cl2.
Org. Lett., Vol. 6, No. 21, 2004
3665

This may be ascribed to the substitution effects of meso-
attached tetrafluorophenylenylporphyrins. These results in-
dicate that the oxidation and reduction reactions of the
expanded porphyrin cores are possible in 5 and 6 with clear
color changes without any chemical changes in the peripheral
porphyrins.
The electrochemical properties of 3c, 5, 6, 7, and 8 were
comparatively studied by cyclic voltammetry in CH2Cl2
versus ferrocene/ferrocenium ion using tetrabutylammonium
hexafluorophosphate as an eletrolyte. Porphyrin 3c undergoes
one-electron reduction and oxidation at -1.78 and 0.48 V.
Besides these responses, there are additional electrochemical
waves associated with the interconversions of the expanded
porphyrin cores, -0.58 V for reduction of 5-ox to 5-red and
-0.13 V for oxidation of 5-red to 5-ox, and -0.67 V for
reduction of 6-ox to 6-red and 0.24 V for oxidation of 6-red
to 6-ox. These results are similar to the respective potentials
of the reference exapanded porphyrins: -0.70 V for reduction
of 7-ox to 7-red, 0.08 V for oxidation of 7-red to 7-ox,
-0.67 V for reduction of 8-ox to 8-red, and 0.24 V for
oxidation of 8-red to 8-ox, respectively. These results are
in line with the above chemical transformations and indicate
that the expanded porphyrin cores in 5 and 6 retain their
own electrochemical properties.
Last, the fluorescence from the porphyrin parts is signifi-
cantly quenched in 4, 5-ox, and 6-red and their quantum
yields are quite small, 1.6 × 10-4, 3.9 × 10-5, and 1.0 ×
10-4, respectively, in comparison to that of 3c (0.028).13
Although the mechanism of this strong fluorescence quench-
ing is not clear at the present stage, intramolecular excitation
energy transfer from the porphyrin units to the expanded
porphyrin cores or electron transfer involving easily acces-
sible oxidized or reduced state of expanded porphyrin cores
can be conceivable, thus suggesting strong electronic interac-
tions in the excited state.
Figure 4. Corrected absorption spectra of 5 (5-ox), 5-red, 6-ox,
and 6 (6-red) taken by subtracting the absorbance of the reference
porphyrin 3c from their absorption spectra.
isolated in the reaction of 3a and pyrrole has been assigned
to 5-ox. As is the same for 7-ox, the product 5 displays
characteristic low-field shifted six doublet signals due to the
outer peripheral â-protons (δ ) 9.68, 9.56, 9.14, 9.11, 9.04,
and 8.99 ppm) and high-field shifted two doublets (δ ) 2.88
and 2.36 ppm) and a singlet (δ ) -0.48), indicating the
presence of the aromatic ring current in the N-fused
pentaphyrin macrocycle. On the other hand, the 5-red
displays no such shifts and displays signals due to the outer
peripheral â-protons in 7.2-5.6 ppm.
The corrected absorption spectrum of 6 (λmax ) 603 nm)
is similar to that of 8-red (λmax ) 591 nm), and the treatment
of 6 with DDQ induced an immediate color change from
vivid green to dark purple (λmax ) 578 nm), indicating the
formation of an oxidized form, 6-ox, which was in turn
reduced with NaBH4 to recover 6. Thus, the product 6
isolated by recycling GPC-HPLC separation has been
assigned to be a reduced form, 6-red. As judged from the
color and eluting behavior, the hexaphyrin product isolated
from silica gel column is considered to be 6-ox, which seems
to be reduced during the separation by recycling GPC-
HPLC with CHCl3 containing a small amount of ethanol.
Similar facile reduction of [26]hexaphyrin to [28]hexaphyrin
has been often observed in solution containing aliphatic
alcohol.
In summary, simple one-pot acid-catalyzed condensation
reaction of 3a with pyrrole provided a series of meso-
porphyrinyl substituted porphyrin 4 and expanded porphyrins
5 and 6. Further studies on the property and chemical
reactivity of these molecules, particularly on the dynamic
interactions between porphyrin and expanded porphyrin are
currently under investigation and will be reported in due
course.
Acknowledgment. This work was partly supported by a
Grand-in-Aid for Scientific Research (B) (No. 15350022)
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
The corrected spectral shapes of 5-ox, 5-red, 6-ox, and
6-red are, respectively, similar to those of 7-ox, 7-red, 8-ox,
and 8-red, but the peak positions of 5 and 6 are ca. 10-nm
red-shifted compared with those of 7 and 8, respectively.
Supporting Information Available: Experimental pro-
cedures and characterization of products. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL048953F
(12) Neves, M. G. P. M. S.; Martins M.; Tome´, A. C.; Silvestre, A. J.
D.; Silva, A. M. S.; Fe´lix, V.; Drew, M. G. B.; Cavaleiro, J. A. S. Chem.
Commun. 1999, 385.
(13) Determined with reference to the value of ZnTPP (Φf ) 0.033) in
toluene. Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1.
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Org. Lett., Vol. 6, No. 21, 2004