β-cyano meso-unsubstituted porphyrins for intramolecular charge transfer

Article abstract of DOI:10.1016/S0040-4039(03)01458-8

β-Cyano meso-unsubstituted porphyrin 10, synthesized via a '3+1' condensation using a bromotripyrrane, shows an effective charge transfer interaction across the porphyrin between electron-acceptor and -donor groups in β-positions.

Full text of DOI:10.1016/S0040-4039(03)01458-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6103–6105
b-Cyano meso-unsubstituted porphyrins for intramolecular
charge transfer
Olivier Siria,b and Kevin M. Smitha,c,
*
aDepartment of Chemistry, University of California, Davis, CA 95616, USA
bLaboratoire de Chimie de Coordination, Universite´ Louis Pasteur, F-67070 Strasbourg, France
cDepartment of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
Received 9 May 2003; revised 6 June 2003; accepted 10 June 2003
Abstract—b-Cyano meso-unsubstituted porphyrin 10, synthesized via a ‘3+1’ condensation using a bromotripyrrane, shows an
effective charge transfer interaction across the porphyrin between electron-acceptor and -donor groups in b-positions.
© 2003 Elsevier Ltd. All rights reserved.
Porphyrin molecules functionalized with strong elec-
tron-donor and -acceptor groups connected by a large
conjugated p-system are good candidates as nonlinear
optical materials.1 First studies involved tetraphenyl-
porphyrins modified on the aryl groups2 and/or at a
b-pyrrolic position.3 However, efficient electronic cou-
pling between the donor and acceptor moieties across
the porphyrin is hindered due to the dihedral angle
between phenyl groups and the porphyrin ring. Existing
novel nonlinear optical porphyrins are based on 5,15-
diphenylporphyrins4 functionalized at the meso-posi-
tions and in which the electron donor, acceptor, and
linker are essentially coplanar. More recently, the
strength of aryl-porphyrin electronic coupling was eval-
uated with systems in which an aryl substituent is
linked to the porphyrin via a different connection.5 To
date, only a few attempts have been made to prepare
meso-free b-substituted porphyrins for intramolecular
charge transfer (CT). Previous studies have reported
that introduction of a single b-electron-attractor sub-
stituent has a significant influence on the p-orbital
energies in the case of tetraphenylporphyrin derivatives6
and meso-unsubstituted porphyrins.7 In this latter case,
the b-electron-attractor substituent was adjacent to a
b-electron-donor group so that no effective intramolec-
ular charge transfer could be expected. To the best of
our knowledge, only porphyrins bearing one b-nitro
group displayed effective CT between substituents in
b-positions.8
In the present work, we report the synthesis of meso-
unsubstituted 2,3,7,8,12,13-hexaalkylporphyrins, bear-
ing one or two b-cyano group on the same pyrrolic
subunit. Their electronic data are compared with those
obtained for tetraphenylporphyrin analogues9 and an
effective CT interaction across the porphyrin between
b-substituents is observed for 10.
b-Cyanoporphyrins have been prepared by transforma-
tion of the corresponding formylporphyrins into cyano-
substituted porphyrins10 (via the usual oxime
intermediate) but no porphyrin with adjacent formyl
groups has been reported to date. b-Cyanoporphyrins
could also be obtained by nucleophilic displacement of
bromide by cyanide.11 Bromination of porphyrins has
been extensively studied and Longo and co-workers12
showed that unsubstituted porphyrin was brominated
regioselectively at the meso-positions in the presence of
N-bromosuccinimide (NBS). 5,15-Diphenylporphyrin is
Keywords: charge transfer; b-cyanoporphyrins; porphyrins;
tripyrranes.
* Corresponding author. Tel.: +1-225-578-4422; fax: +1-225-578-5983;
e-mail: kmsmith@lsu.edu
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01458-8

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O. Siri, K. M. Smith / Tetrahedron Letters 44 (2003) 6103–6105
brominated exclusively at the meso-positions to afford
the meso-tetrasubstituted porphyrin.13 On the other
hand, deuteroporphyrin IX dimethyl ester, containing
at least one b-substituent adjacent to each meso-posi-
tion, reacts with NBS to produce only the b-substituted
bromo compounds (discrimination by steric effect of
the meso-substitution despite its higher reactivity).14
Because of this steric control, 8 could probably be
synthesized by direct bromination of 6. However,
acknowledging the limitation to prepare meso-unsubsti-
tuted bromoporphyrins, we report here a new method-
ology based on bromination of b-unsubstituted
tripyrrane and ‘3+1’ macrocyclization.
energies of the absorption in the Q-band region of 12
are essentially invariant with respect to the analogous
absorptions of Ni(TPP) 11, whereas the lowest energy
transition for 9 is slightly more intense relative to the
analogous transition for 6 (Fig. 1). Therefore, the cyano
substituent led to a small electronic effect only for 9
and this is accounted for by a weak CT between alkyl
groups and the cyano group. Considering the relative
intensities of the Q-bands, the addition of an adjacent
cyano group does not have a major influence for 13,
whereas the substitution for 10 led to a significant
change. As displayed in Figure 1, the electronic spec-
trum of 10 shows a slightly shouldered Soret band and
a Q-band dramatically more intense, the signature of an
effective intramolecular CT.4,8 This suggests that the
electronic excitation for 10 has more in common with a
CT transition than a proper Q band,4d and is accounted
for by an effective CT interaction between acceptor and
donor groups across the porphyrin ring via b-sub-
stituents. We also observed a red shift of the Q-band
which is consistent with such a CT phenomenon.21
This effectiveness of the CT interaction for 10 is due to
the presence of two adjacent cyano groups which
increase the electron-withdrawing effect, and to the
same direction of net dipole moment and charge trans-
Table 1. Absorption maxima and relative intensity of the
UV-visible spectra of nickel(II) porphyrins
Compound
u(nm) (rel. int.)
6
8
9
10
11
12
13
390 (1), 512 (0.10), 550 (0.16)
394 (1), 504 (0.08), 518 (0.05), 562 (0.14)
400 (1), 528 (0.09), 568 (0.22)
402 (br) (1), 588 (0.37)
412 (1), 526 (0.10)
421 (1), 535 (0.07), 570 (0.05)
434 (1), 518 (0.04), 554 (0.06), 598 (0.13)
The only preparation of bromoporphyrin via condensa-
tion has been reported by reaction of 4-bromo-2-
hydroxymethylpyrrole with 2-hydroxymethylpyrrole.12b
First, 3,4-dibromopyrrole was prepared15 and con-
densed with the appropriate substituted pyrrole leading
to 2. However, because of the instability of 3,4-dibro-
mopyrrole,15 216 could be better obtained after the
quantitative bromination reaction of the tripyrrane 117
with NBS at −78°C in anhydrous THF. Catalytic
debenzylation of 2 afforded the tripyrrane dicarboxylic
acid 4 in quantitative yield.
Free-base porphyrins 5 and 7 were synthesized by a
‘3+1’ approach,18 in which tripyrranes 3 and 4, respec-
tively, were reacted with a 2,5-bis-(N,N-dimethyl-
aminomethyl)pyrrole in refluxing methanol. Metala-
tions were carried out with nickel(II) acetylacetonate in
refluxing toluene, affording the Ni(II) porphyrins 6 and
8. Nucleophilic displacement of the bromine with cya-
nide in quinoline at 240°C led to the formation of 919
and 1020 in 35 and 32% yields, respectively; compound
9 is the hydrodebrominated compound.
The visible absorption spectra of the nickel(II) meso-
unsubstituted porphyrins (6, 8–10) are reported in
Table 1, as well as those obtained for tetra-
phenylporphyrin analogues 11–13.9 The absorption
spectra of the parent porphyrins 6 and 11 are typical,
displaying Soret band and electronically forbidden Q
transitions. In the case of the monocyanoporphyrin, the
Figure 1. Electronic absorption spectra of: 6 (—), 9 (---), 10
(···) in CH2Cl2.

O. Siri, K. M. Smith / Tetrahedron Letters 44 (2003) 6103–6105
6105
fer of the molecule (along the axis of the pyrrole N
atoms) which improves the CT process.2a In 10, polariz-
ability is enhanced by six donors at the b-positions of
three pyrrolic moieties and two strong acceptors of one
pyrrolic subunit. Therefore, 10 is the first example of
cyanoporphyrin for which the electronic transfer occurs
across the porphyrin exclusively between b-substituents.
7. Djerassi, C.; Lu, Y.; Waleh, A.; Shu, A. Y. L.; Goldbeck,
R. A.; Kehres, L. A.; Crandell, C. W.; Wee, A. G. H.;
Knierzinger, A.; Gaete-Holmes, R.; Loew, G. H.; Clezy,
P. S.; Bunnenberg, E. J. Am. Chem. Soc. 1984, 106, 4241.
8. (a) Siri, O.; Jaquinod, L.; Smith, K. M. Tetrahedron Lett.
2000, 41, 3583; (b) Wickramasinghe, A.; Jaquinod, L.;
Nurco, D. J.; Smith, K. M. Tetrahedron 2001, 57, 4261.
9. Jaquinod, L.; Khoury, R. G.; Shea, K. M.; Smith, K. M.
Tetrahedron 1999, 55, 13151.
Acknowledgements
10. Buchler, J. W.; Dreher, C.; Herget, G. Liebigs Ann.
Chem. 1988, 43.
11. (a) Callot, H. J. Tetrahedron Lett. 1973, 14, 4987; (b)
Callot, H. J. Bull. Soc. Chim. Fr. 1974, 1492.
12. (a) Nudy, L. R.; Coffey, J. C.; Longo, F. R. J. Hetero-
cycl. Chem. 1982, 19, 1589; (b) Nudy, L. R.; Hutchinson,
H. G.; Schieber, C.; Longo, F. R. Tetrahedron 1984, 40,
2359.
This work was supported by a grant from the National
Institutes of Health (HL-22252).
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1
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9.45 (s, 1H), 9.49–9.50 (each s, 1H), 9.63 (s, 1H). Anal.
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2H), 9.12 (s, 2H). Anal. calcd for C32H30N6Ni: C, 68.96;
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