A strategy for the stepwise ring annulation of all four pyrrolic rings of a porphyrin

Article abstract of DOI:10.1039/b714612d

The repeated introduction of an α-dione unit and its reaction with an arene-1,2-diamine allows the stepwise annulation of all four pyrrolic rings of a porphyrin, as is demonstrated by the synthesis of a trisquinoxalinoporphyrin, a tetrakisquinoxalinoporphyrin and the more elaborated bisporphyrin. The Royal Society of Chemistry.

Full text of DOI:10.1039/b714612d

View Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
A strategy for the stepwise ring annulation of all four pyrrolic rings of a
porphyrin{
Tony Khoury and Maxwell J. Crossley*
Received (in Cambridge, UK) 21st September 2007, Accepted 29th October 2007
First published as an Advance Article on the web 5th November 2007
DOI: 10.1039/b714612d
which are extended in three and four directions, respectively, and
the bis-porphyrin 3, in which the possibilities for constructing
extended porphyrin arrays are demonstrated. Systems 1 and 2,
which are laterally-extended porphyrins with an enlarged
p-electronic framework, are interesting compounds in their own
right.
The repeated introduction of an a-dione unit and its reaction
with an arene-1,2-diamine allows the stepwise annulation of all
four pyrrolic rings of a porphyrin, as is demonstrated by the
synthesis of a trisquinoxalinoporphyrin, a tetrakisquinoxalino-
porphyrin and the more elaborated bisporphyrin.
Porphyrins with b,b9-pyrrolic bonds that are fused with aromatic
rings have applications in a variety of fields from molecular
electronics to biomedicine.1–3 The syntheses of the simplest tetra-
annulated representatives, the tetrabenzoporphyrins4,5 and tetra-
naphthaloporphyrins,6 have been reported. The methodology for
their construction, however, is not amenable for making more
elaborate arrays or for stepwise building-up of square grid
networks of porphyrins (Fig. 1), which might find use in molecular
circuitry and other molecular electronics applications. We now
outline a strategy that allows the stepwise ring annulation of each
of the four pyrrolic rings of a porphyrin. The key to this approach
is the successive introduction into each ring of a functionality that
can be elaborated readily. We have chosen to expand an approach
to ring annulation outlined in the synthesis of much simpler
extended porphyrin systems, in which the ring fusion was
created by the reaction of a porphyrin-a-dione with an arene-1,2
-diamine.7
Our approach is illustrated by the synthesis of end-capped
trisquinoxalinoporphyrin 1 and tetrakisquinoxalinoporphyrin 2,
In previous work, we have developed the syntheses of
porphyrin-2,3-diones from 2-aminoporphyrins and 2-hydroxypor-
phyrins, and porphyrin-2,3,7,8-tetraones and porphyrin-2,3,12,13-
tetraones from diaminoporphyrins.7,8 These have been used to
create extended porphyrin systems for a variety of uses.7–11 Our
synthesis of the tetraones7 involved initial dinitration of the
porphyrin periphery, which led to a mixture of isomers that were
carried forward to common products. Porphyrins with two
functionalised pyrrolic rings are also accessible in a stepwise
manner by the nitration of porphyrin-2,3-diones and further
elaboration, and by similar sequences on quinoxalino[2,3-b]por-
phyrins.12 The latter compounds serve as models of more elaborate
Fig. 1 Stepwise functionalisation of porphyrin pyrrolic rings allows
controlled mono-, syn-bis-, anti-bis-, tris- and tetrakis-annulation.
School of Chemistry, The University of Sydney, Building F11,
NSW 2006, Australia. E-mail: m.crossley@chem.usyd.edu.au;
Fax: +61 2 9351 3329; Tel: +61 2 9351 2751
{ Electronic supplementary information (ESI) available: Selected spectra
and characterisation data. See DOI: 10.1039/b714612d
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4851–4853 | 4851

View Online
systems and allow the introduction of two different annulated ring
systems, for example one appending a C60 and the other a
ferrocene.11 The route to compounds 1 and 2 is outlined in
Scheme 1. Corner zinc(II) porphyrin-tetraone7 4 was functionalised
further at one or both b,b9-pyrrolic faces for the synthesis of
trisquinoxalinoporphyrin 1 and tetrakisquinoxalinoporphyrin 2.{
Trisquinoxalinoporphyrin 1 was prepared by first reacting 4
with ortho-phenylenediamine in CH2Cl2 solution to afford the
corner zinc(II) bisquinoxalinoporphyrin 5 in 90% yield. Nitration
of 5 afforded an inseparable mixture of corner zinc(II) 12- and 13-
nitro-bisquinoxalinoporphyrins 6 in 83% yield, but both gave the
same dione subsequently. Demetallation of 6 was achieved using
HCl, affording corner 12- and 13-nitro-bisquinoxalinoporphyrins
7 in 89% yield. The tin(II)-mediated reduction of 7 gave the
corresponding corner 12- and 13-amino-bisquinoxalinoporphyrins
8, which were photo-oxidised in the presence of ortho-phenylene-
diamine for 5 h to give trisquinoxalinoporphyrin 1 in 64% yield.
The intermediate corner bisquinoxalinoporphyrin-12,13-dione 9
was formed as an intermediate. The overall yield from 4 of 1 was
47%.
Corner zinc(II) porphyrin-tetraone 4 also served as a precursor
to tetrakisquinoxalinoporphyrin 2. Nitration of 4 in gave an
isomeric mixture of corner zinc(II) 12,17-, 12,18- and 13,17-dinitro-
porphyrin-tetraones 10 in 59% yield. Reaction of 10 with ortho-
phenylenediamine gave the corner zinc(II) 12,17-, 12,18- and
13,17-dinitro-bisquinoxalinoporphyrins 11 in 66% yield.
Scheme 1 Reagents and conditions: (i) ortho-phenylenediamine in CH2Cl2, 5 min; (ii) NO2 in light petroleum, CH2Cl2; (iii) HCl–CH2Cl2; (iv) SnCl2?2H2O
in HCl–Et2O; (v) O2 and ortho-phenylenediamine in CH2Cl2, hn, 5 h; (vi) O2 and ortho-phenylenediamine in CH2Cl2, hn, 45 min; (vii) Zn(OAc)2?2H2O in
CHCl3–CH3OH, heat, 5 min; (viii) O2 and ortho-phenylenediamine in CH2Cl2 hn, 20 min.
4852 | Chem. Commun., 2007, 4851–4853
This journal is ß The Royal Society of Chemistry 2007

View Online
Fig. 2 Electronic absorption spectra in CHCl3 of trisquinoxalinopor-
phyrin 1 (blue), tetrakisquinoxalinoporphyrin 2 (red) and bis-porphyrin 3
(green).
photo-oxidation of 15, afforded the elaborated bis-porphyrin 3
in 62% yield.
Increasing the number of quinoxaline units fused to the
porphyrin macrocycle results in a widening of the light absorption
window and a red-shift in electronic spectra compared to simple
porphyrins due to the increase in the p-electronic network. For
example, 2 and 3 exhibit a wide absorption window, with Q band
maxima up to 697 and 735 nm, respectively (Fig. 2). Such broad
absorption properties might find use in artificial light harvesting
systems and light detectors.
Scheme 2 Reagents and conditions: (i) O2 in CH2Cl2, hn, 80 min; (ii)
HCl–CH2Cl2, 2 h, followed by SnCl2?2H2O in HCl–Et2O, 10 min; (iii)
toluene, reflux, 36 h.
Treatment of 11 with a HCl–CH2Cl2 solution afforded free base
corner 12,17-, 12,18- and 13,17-dinitro-bisquinoxalinoporphyrins
12 in 83% yield. The isomeric mixture 12 was reduced using
SnCl2?2H2O in HCl–Et2O to afford the diamino analogues 13,
which were then photo-oxidised in the presence of ortho-
phenylenediamine to afford amino-trisquinoxalinoporphyrin 15
in 83% yield by way of dione 14. The amino-trisquinoxalinopor-
phyrin 15 was converted into its zinc(II) derivative 16. Photo-
oxidation of 16 gave zinc(II) trisquinoxalinoporphyrin-dione 17,
which was reacted in situ with ortho-phenylenediamine to afford
zinc(II) tetrakisquinoxalinoporphyrin 18 in 84% yield.
Demetallation of 18 by treatment with HCl in CH2Cl2 gave
tetrakisquinoxalinoporphyrin 2 in 93% yield. The overall yield of 2
from 4 was 22%.
In this work, a strategy for the step-wise annulation of all four
pyrrolic rings of a porphyrin has been demonstrated, and a
synthetic protocol has been established that will enable the
generation of more complex systems that may be of value in
applications from molecular electronics to artificial light harvesting
systems.
Notes and references
{ All new compounds have been fully characterised, including elemental
analysis and/or high resolution ESI-FT/ICR.
1 T. D. Lash, in The Porphyrin Handbook, ed. K. M. Kadish, K. M.
Smith and R. Guilard, Academic Press, New York, 2000, vol. 2, pp. 125.
2 O. S. Finikova, A. V. Cheprakov and S. A. Vinogradov, J. Org. Chem.,
2005, 70, 9563.
3 V. Gottumukkala, O. Ongayi, D. G. Baker, L. G. Lomax and
M. G. H. Vincente, Bioorg. Med. Chem., 2006, 14, 1871.
4 N. Kobayashi, W. A. Nevin, S. Mizunuma, H. Awaji and
M. Yamaguchi, Chem. Phys. Lett., 1993, 205, 51.
5 S. Ito, T. Murahima, H. Uno and N. Ono, Chem. Commun., 1998, 1661.
6 O. S. Finikova, A. V. Cheprakov, P. J. Carroll and S. A. Vinogradov,
J. Org. Chem., 2003, 7517.
The complete demetallation of zinc(II) tetrakisquinoxalinopor-
phyrin 18 by HCl treatment required 5 h, in contrast to simple
zinc(II) porphyrins, which are demetalled in a matter of seconds
with this reagent. This difference is an indication that the
quinoxalino substituents are able to exert a strong electron-
withdrawing effect on the porphyrin ring, especially under acidic
conditions.
In order to illustrate how the synthetic strategy outlined
here can be utilised for the preparation of more elaborated
systems, bis-porphyrin 3, in which all pyrrolic rings are annulated,
was synthesised. The photo-oxidation of 17 in the presence of
1,2,4-triamino-5-nitro-benzene afforded zinc(II) amino-nitro-
tetrakisquinoxalinoporphyrin 19 in 65% yield (Scheme 2). The
conversion of 19 into the diamino-tetrakisquinoxalinoporphyrin
20 in 78% yield was achieved by acidic demetallation followed
by reduction using SnCl2?2H2O. The condensation of diamine
20 and trisquinoxalinoporphyrin-dione 21, obtained by the
7 M. J. Crossley, L. J. Govenlock and J. K. Prashar, J. Chem. Soc., Chem.
Commun., 1995, 2379.
8 M. J. Crossley and L. G. King, J. Chem. Soc., Chem. Commun., 1984,
3523.
9 M. J. Crossley and P. L. Burn, J. Chem. Soc., Chem. Commun., 1991,
1569.
10 M. J. Crossley, P. J. Sintic, J. A. Hutchison and K. P. Ghiggino,
Org. Biomol. Chem., 2005, 3, 852.
11 D. Curiel, K. Ohkubo, J. R. Reimers, S. Fukuzumi and M. J. Crossley,
Phys. Chem. Chem. Phys., 2007, 9, 5260.
12 M. J. Crossley, C. S. Sheehan, T. Khoury, J. R. Reimers and P. J. Sintic,
New J. Chem., 2007, DOI: 10.1039/b712643c.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4851–4853 | 4853