Ring size selective synthesis of meso-aryl expanded porphyrins

Article abstract of DOI:10.1016/S0040-4039(03)00328-9

meso-Aryl expanded porphyrins were prepared in a ring size selective manner from methanesulfonic acid-catalyzed reaction of dipyrromethane and tripyrromethane with aryl aldehydes.

Full text of DOI:10.1016/S0040-4039(03)00328-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2505–2507
Ring size selective synthesis of meso-aryl expanded porphyrins
Ryuichiro Taniguchi, Soji Shimizu, Masaaki Suzuki, Ji-Young Shin, Hiroyuki Furuta and
Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
Received 6 January 2003; revised 29 January 2003; accepted 31 January 2003
Abstract—meso-Aryl expanded porphyrins were prepared in a ring size selective manner from methanesulfonic acid-catalyzed
reaction of dipyrromethane and tripyrromethane with aryl aldehydes. © 2003 Elsevier Science Ltd. All rights reserved.
Expanded porphyrins having a macrocycle larger than
porphyrins have attracted considerable attention in
light of their unique and fascinating optical, electro-
chemical, and coordination properties.1 As a result, a
variety of expanded porphyrins have been prepared
differing in ring size, ring connectivity, and hetero atom
replacement.2 However, the synthesis of meso-aryl sub-
stituted expanded porphyrins, which may be regarded
as the ‘legitimated’ members of meso-aryl-substituted
expanded porphyrin in respect of the alternate arrange-
ment of a pyrrole and a meso-methine carbon, has still
remained very limited. Recently we found that a series
of meso-aryl substituted expanded porphyrins were
formed in the reaction of 2,6-disubstituted aryl alde-
hyde and pyrrole under modified Rothemund–Lindsey
conditions.3,4 Although this method is convenient and
effective, the simultaneous formation of every size of
expanded porphyrins at least up to dodecamer
inevitably causes serious separation difficulties. Thus,
ring size selective synthesis of meso-aryl expanded por-
phyrins is strongly desirable, since it will lead to facile
product isolation. Another favorable improvement may
be more production of larger meso-aryl expanded por-
phyrins, since our original method tend to provide
smaller ones as major products. Typically, our original
synthesis of the meso-aryl expanded porphyrins by the
reaction of pyrrole and pentafluorobenzaldehyde (67
mM each) in dry CH2Cl2 with BF3·OEt2 followed by
the oxidation with DDQ led to isolation of porphyrin
4
(11ꢀ12%), N-fused pentaphyrin NFP5 (14ꢀ15%),
hexaphyrin5 (16ꢀ20%), heptaphyrin (4ꢀ5%), octa-
phyrin (5ꢀ6%), and nonaphyrin (2ꢀ3%). Among these
products, octaphyrin and nonaphyrin are expected to
exhibit novel reactivities due to their unique symmetric
structures,3 but separation of heptaphyrin, octaphyrin,
and nonaphyrin is usually very difficult due to their
close Rf values on silica gel and alumina columns. In
this paper we report ring-size selective synthesis of
expanded porphyrins using dipyrromethane 1 and
tripyrromethane 2 as the starting materials in place of
pyrrole.
Dipyrromethane 1 and tripyrromethane 2 were pre-
pared by TFA-catalyzed reaction of pentafluoro-benz-
aldehyde with excess pyrrole following the method
reported by Lee (Scheme 1).6
A solution of 1 and pentafluorobenzaldehyde (33 mM
each) in CH2Cl2 was treated with acid for 2 h followed
by oxidation with DDQ (Scheme 2, Table 1). Use of
BF3·OEt2 as an acid catalyst led to scrambling, as
indicated by the formation of expanded porphyrins
Scheme 1.
Keywords: porphyrinoids; expanded porphyrin; size selective synthesis.
* Corresponding author. Tel.: +81-75-753-4008; fax: +81-75-753-3970; e-mail: osuka@kuchem.kyoto-u.ac.jp
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00328-9

2506
R. Taniguchi et al. / Tetrahedron Letters 44 (2003) 2505–2507
with odd number of the pyrrole subunits at room
temperature (entry 1). Lowering the reaction tempera-
ture to 0°C did not suppress scrambling (entry 2).
Methanesulfonic acid (MSA) was found more suitable
for ring size selective synthesis of meso-aryl expanded
porphyrins when used at 0°C (entry 4), which allowed
the formation of meso-aryl expanded porphyrins
including porphyrin (9%), hexaphyrin (19%), octa-
phyrin (38%) and decaphyrin (15%), while the scram-
bling was observed at room temperature (entry 3). High
yields of octaphyrin and decaphyrin should be noted,
since the original method did not achieve such yields.
Isolation of octaphyrin and decaphyrin from this reac-
tion mixture now became much easier. Similar ring size
selective synthesis of meso-aryl expanded porphyrins
was also effected with trichloroacetic acid. MSA gave
expanded porphyrins in the reaction of 5-(2,6-
difluorophenyl)dipyrromethane and 5-(2,6-dichloro-
phenyl)dipyrromethane, also in a ring-size selective
manner.
In the next step, we examined the similar reaction of
5,10-bis(pentafluorophenyl)tripyrromethane
2
with
aldehyde (Scheme 3 and Table 2). As was the case for
1, the use of BF3·OEt2 also led to entire scrambling
both at room temperature and 0°C (entries 1 and 2),
Scheme 2.
Table 1. Yields of expanded porphyrins in the reaction of 1
Entry
Ar
Acid
Temperature
Cyclization time (h)
Productsa
1b
2
C6F5
C6F5
C6F5
C6F5
BF3·OEt2
BF3·OEt2
MSA
MSA
TCA
MSA
MSA
MSA
rt
0°C
rt
0°C
rt
rt
2
2
2
2
2
2
2
2
4, 5, 6, 7, 8, 9
4 (8%), 5 (11%), 6 (24%), 7–9 (16%, not isolated)
4, 5, 6, 7, 8, 9
4 (9%), 6 (19%), 8 (38%), 10 (15%)
4 (6%), 6 (14%), 8 (5%), 10 (trace)
4, 5, 6
3b
4
5
C6F5
6b
7
C6F2H3
C6F2H3
C6Cl2H3
0°C
rt
4 (4%), 6 (12%)
4 (18%), 6 (17%), 8 (12%), 10 (trace)
8
a 4: porphyrin, 5: N-fused pentaphyrin (NFP5), 6: hexaphyrin, 7: heptaphyrin, 8: octaphyrin, 9: nonaphyrin, 10: decaphyrin.
b Not isolated. Yields reported refer to the isolated yields.
Scheme 3.

R. Taniguchi et al. / Tetrahedron Letters 44 (2003) 2505–2507
Table 2. Yields of expanded porphyrins in the reaction of 2
2507
Entry
Ar
acid
Temperature
Cyclization time (h)
Productsa
1b
2b
3b
4
C6F5
C6F5
C6F5
C6F5
BF3·OEt2
BF3·OEt2
MSA
MSA
MSA
rt
0°C
rt
0°C
0°C
2
2
2
2
2
4 (3%), 5 (10%), 6 (16%), 7–9 (16%)
4 (8%), 5 (11%), 6 (24%), 7–9 (16%)
4 (3%), 5 (6%), 6 (23%), 7–9 (6%)
6 (23%), 7 (3%), 8 (4%), 9 (32%)
4 (12%), 6 (21%), 7 (9%), 9 (22%)
5
C6Cl2H3
a 4: porphyrin, 5: N-fused pentaphyrin (NFP5), 6: hexaphyrin, 7: heptaphyrin, 8: octaphyrin, 9: nonaphyrin.
b Not isolated. Yields reported refer to the isolated yields.
though hexaphyrin was a major product. Use of
MSA at 0°C gave rise to improved results, giving
hexaphyrin (23%) and nonaphyrin (32%) as the major
products in addition to the scrambling products, hep-
taphyrin (3%) and octaphyrin (4%). The reaction of
5,10-bis(2,6-dichlorophenyl)tripyrromethane also gave
hexaphyrin (21%) and nonaphyrin (22%) as the major
products (entry 5).
Technology) of Japan Science and Technology Corpo-
ration (JST).
References
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, pp. 55–124.
2. (a) Lash, T. D. Angew.Chem., Int. Ed. 2000, 39, 1763–1767
and references cited therein; (b) Sessler, J. L.; Seidel, D.;
Lynch, V. J. Am. Chem. Soc. 1999, 121, 11257–11258; (c)
Setsune, J.; Maeda, S. J. Am. Chem. Soc. 2000, 122,
12405–12406; (d) Furuta, H.; Maeda, H.; Osuka, A. Chem.
Commun. 2002, 1795–1804.
3. Shin, J.-Y.; Furuta, H.; Yoza, K.; Igarashi, S.; Osuka, A.
J. Am. Chem. Soc. 2001, 123, 7190–7191; Lindsey method:
Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguettaz, A. M. J. Org. Chem. 1987, 52, 827–836.
4. Shin, J.-Y.; Furuta, H.; Osuka, A. Angew. Chem., Int. Ed.
2001, 40, 619–621.
In summary, we have developed the ring-size selective
synthesis of meso-aryl expanded porphyrins by using
dipyrromethane 1 and tripyrromethane 2 as the start-
ing substrates. This synthetic method is clearly better
than the original one in view of much easier product
isolation, particularly providing an easy synthetic
access to octaphyrin and nonaphyrin.
Acknowledgements
This work was supported by Grant-in Aids for Scien-
tific Research (No. 111233205 and 12440196) from
the Ministry of Education, Science, Sports and Cul-
ture of Japan and by CREST (Core Research for
Evolutional Science and Technology) and PRESTO
(Precursory Research for Embryonic Science and
5. Neves, M. G. P. M. S.; Martins, R. M.; Tome, A. C.;
Silvestre, A. J. D.; Silva, A. M. S.; Felix, V.; Drew, M. G.
B.; Cavaleiro, J. A. S. Chem. Commun. 1999, 385–386.
6. Ka, J.-W.; Lee, C.-H. Tetrahedron Lett. 2000, 41, 4609–
4613.