Expanding the porphyrin π-system by fusion with anthracene

Article abstract of DOI:10.1021/ol801500b

(Chemical Equation Presented) Synthesis of β,meso,β-anthracene triply fused and β,meso-anthracene doubly fused porphyrins has been achieved via oxidative intramolecular ring closure of meso-(9-anthryl)porphyrins and meso-(1-anthryl)porphyrins, respectivel

Full text of DOI:10.1021/ol801500b

ORGANIC
LETTERS
2008
Vol. 10, No. 18
3945-3947
Expanding the Porphyrin π-System by
Fusion with Anthracene
Nicola K. S. Davis, Miłosz Pawlicki, and Harry L. Anderson*
Department of Chemistry, Chemistry Research Laboratory, UniVersity of Oxford,
12 Mansfield Road, Oxford OX1 3TA, U.K.
harry.anderson@chem.ox.ac.uk
Received July 2, 2008
ABSTRACT
Synthesis of ꢀ,meso,ꢀ-anthracene triply fused and ꢀ,meso-anthracene doubly fused porphyrins has been achieved via oxidative intramolecular
ring closure of meso-(9-anthryl)porphyrins and meso-(1-anthryl)porphyrins, respectively. Fusion was only possible when the anthracene carried
electron-donating alkoxy substituents. The fused porphyrins exhibit strongly red-shifted UV-vis absorption spectra and reduced electrochemical
HOMO-LUMO gaps (relative to the unfused tetraaryl porphyrin precursor).
The synthesis of very large flat π-conjugated chromophores
and “molecular-graphenes” is an important recent develop-
ment in materials science, stimulated by the remarkable
electronicpropertiesemanatingfromthesmallHOMO-LUMO
gaps of these molecules. Examples include the hexabenzo-
coronenes developed by Mu¨llen and co-workers1 and the
edge-fused porphyrin tapes from Osuka’s group, which have
electronic absorption bands stretching far into the infrared.2
A variety of π-extended porphyrins have been synthesized
by fusing benzene,3 naphthalene,4 pyrene,5 and azulene6
across the meso- and ꢀ-positions of porphyrins. Anthracene-
fused porphyrins represent a notable absence from this family
of compounds, which is surprising in the light of the good
geometrical match between the ꢀ,meso,ꢀ-edge of a porphyrin
and the 1,9,8-edge of an anthracene.7 Here, we report the
first synthesis of triply fused (ꢀ,meso,ꢀ) and doubly fused
(ꢀ,meso) anthracene-porphyrin conjugates 1c and 2, re-
spectively. Fusion to an anthracene unit strongly perturbs
the electronic structure of the porphyrin, shifting the absorp-
tion into the near-infrared and reducing the electrochemical
HOMO-LUMO gap by making the macrocycles easier to
oxidize.
(1) (a) Wu, J.; Pisula, W.; Mu¨llen, K. Chem. ReV. 2007, 107, 718–747.
(b) Mu¨llen, K.; Rabe, J. P. Acc. Chem. Res. 2008, 41, 511–520.
(2) (a) Tsuda, A.; Furuta, H.; Osuka, A. Angew. Chem., Int. Ed. 2000,
39, 2549–2552. (b) Tsuda, A.; Osuka, A. Science 2001, 293, 79–82.
(3) (a) Barloy, L.; Dolphin, D.; Dupre´, D.; Wijesekera, T. P. J. Org.
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M.; Hayashi, S.; Eu, S.; Umeyama, T.; Matano, Y.; Imahori, H. Chem.
Commun. 2007, 2069–2071.
Anthracene-fused porphyrins 1c and 2 were synthesized
as shown in Scheme 1. Initially, we attempted to prepare
the triply fused compound with an unsubstituted anthracene
1a. Suzuki coupling of porphyrin boronic ester 35,8 with
9-bromoanthracene 4a in the presence of Buchwald’s SPhos
(5) Yamane, O.; Sugiura, K.; Miyasaka, H.; Nakamura, K.; Fujimoto,
T.; Nakamura, K.; Kaneda, T.; Sakata, Y.; Yamashita, M. Chem. Lett. 2004,
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10.1021/ol801500b CCC: $40.75
Published on Web 08/23/2008
2008 American Chemical Society

Scheme 1. Synthesis of Anthracene-Fused Porphyrins
ligand (2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl)9
and potassium hydroxide gave the meso-(9-anthryl)porphyrin
5a in 44% yield (whereas under standard Suzuki coupling
conditions protodeboronation predominates). Oxidative fu-
of acetoxy substituents results in exclusive bromination at
the 10-position of 1,8-diacetoxyanthracene 6d to give 4d
(45% yield). The acetoxy groups were then hydrolyzed. Octyl
chains were introduced at this stage, to give 4c, in order to
increase the solubility of the final fused porphyrin. The
Suzuki coupling of 4c with 3 gave 5c with 36% yield.
Oxidative fusion of 5c was carried out using Sc(OTf)3/DDQ
in dichloromethane.2a After workup, no starting material was
detected, and the triply fused porphyrin 1c was isolated in
82% yield.
2b
sion of 5a to 1a was attempted both using DDQ/Sc(OTf)3
and using tris(4-bromophenyl)aminium hexachloroanti-
monate (BAHA),2a but in both cases ring-fused products were
not detected and unreacted 5a was reisolated. Attempts at
the oxidative fusion of other meso-(9-anthryl)porphyrins10
also failed, so we decided to test whether electron-rich
alkoxyanthracenes would be more amenable to this reaction,
as found with pyrene.5
Synthesis of 10-bromo-1,8-dimethoxyanthracene 4b was
first attempted by bromination of 1,8-dimethoxyanthracene
6b with NBS; however, this gave an inseparable 1:2 mixture
of 4b and 7b (Scheme 2, see the Supporting Information).
Suzuki coupling of this mixture of bromoanthracenes with
3 yielded a 1:4 mixture of porphyrins 5b and 8, which also
proved to be inseparable. Oxidative fusion of this mixture
of isomers with DDQ/Sc(OTf)3 gave the doubly fused
anthracene-porphyrin 2 as the only isolable product.11
Regioselective synthesis of a 10-bromo-1,8-dialkoxyan-
thracene was achieved as shown in Scheme 2. The presence
The expansion of the π-system in porphyrins 1c and 2
results in dramatic changes to their UV-vis absorption
spectra (Figure 1). Porphyrin 2 displays a similar spectrum
to naphthalene-fused porphyrins.4 Striking differences are
be observed between the spectra of the unfused and fused
porphyrins 5c and 1c. The fused anthracene-porphyrins 1c
and 2 display significant bathochromic shifts with longest
wavelength absorption maxima at 855 and 725 nm respec-
tively.
Cyclic and square wave voltammetry were carried out on
5c, 2, and 1c so as to determine their redox potentials (in
THF with 0.1 M Bu4NPF6, relative to internal ferrocene/
ferrocinium). The three compounds are reduced at about the
same potential (-1.21, -1.23, and -1.19 V vs Fc/Fc+,
respectively), whereas their first oxidation potentials (0.39,
0.02, and 0.00 V vs Fc/Fc+, respectively) show that fusion
to the anthracene π-system reduces the HOMO-LUMO gap
and makes the porphyrin easier to oxidize.
Scheme 2. Synthesis of Anthracene Derivatives
In conclusion, doubly and triply fused anthracene-
porphyrins have been synthesized. Under the reaction condi-
tions used in this work, ring fusion between the anthracene
and the porphyrin does not occur unless the anthracene has
alkoxy substituents. The red-shifted absorption spectra of
anthracene-fused porphyrins point to possible applications
3946
Org. Lett., Vol. 10, No. 18, 2008

Figure 1
. Absorption spectra in chloroform. meso-(9-anthryl)porphyrin 5c (a) has a typical porphyrin spectrum, whereas the spectra of the
ꢀ,meso doubly fused porphyrin 2 (b) and the ꢀ,meso,ꢀ triply fused porphyrin 1c (c) are distorted and red-shifted.
in areas such as photovoltaics,12 photodynamic therapy,13
and nonlinear optics.14
Supporting Information Available: Experimental details,
1
characterization data, H, 13C NMR and mass spectra, and
electrochemical data. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the EPSRC and the Euro-
pean Comission (Intra-European Fellowship to M.P. MEIF-
CT-2006-041629) for support and the EPSRC Mass Spec-
trometry Service (Swansea) for mass spectra.
OL801500B
(11) The triply-fused porphyrin 1b is probably formed during the DDQ/
Sc(OTf)3 oxidation of the 1:4 mixture of 5b and 8 and no unreacted 5b is
detected in the product mixture, but if 1b is formed in this reaction it appears
to be so insoluble that it is lost during chromatography; 1b was not isolated.
(12) Rio, Y.; Rodr´ıguez-Morgade, M.; Torres, T. Org. Biomol. Chem.
2008, 6, 1877–1894.
(7) The idea of synthesizing anthracene-fused porphyrins was suggested
by: Yen T. F. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.;
Ann Arbor Scientific: Ann Arbor, 1975; pp 1-30. We thank a reviewer
for drawing this article to our attention.
(8) Hyslop, A. G.; Kellett, M. A.; Iovine, P. M.; Therien, M. J. J. Am.
Chem. Soc. 1998, 120, 12676–12677.
(13) MacDonald, I. J.; Dougherty, T. J. J. Porphyrins Phthalocyanines
2001, 5, 105–129.
(9) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am.
Chem. Soc. 2005, 127, 4685–4696.
(14) Senge, M. O.; Fazekas, M.; Notaras, E. G. A.; Blau, W. J.;
Zawadzka, M.; Locos, O. B.; Mhuircheartaigh, E. M. N. AdV. Mater. 2007,
19, 2737–2774.
(10) Krivokapic, A.; Anderson, H. L. Org. Biomol. Chem. 2003, 1, 3639–
3641.
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