J. Am. Chem. Soc. 2000, 122, 6773-6774
6773
Scheme 1a
meso-Unsubstituted Porphyrinogens and
Hexaphyrinogens: The First X-ray Characterization
Hidemitsu Uno,*,† Takashi Inoue,‡ Yumiko Fumoto,‡
Motoo Shiro,§ and Noboru Ono‡
AdVanced Instrumentation Center for Chemical Analysis and
Department of Chemistry, Faculty of Science
Ehime UniVersity, Matsuyama 790-8577, Japan
X-ray Research Laboratory, Rigaku Corporation
Akishima 196-8666, Japan
ReceiVed February 8, 2000
Recently, meso-octasubstituted porphyrinogens, namely calix-
[4]pyrroles,1 have been extensively studied due to their interesting
anion-binding ability1,2 and ability to π-donate to metal cations.3
The structures and reactivities of their neat and complexed forms
with anions,1,2 metals,3 and guest molecules4 have been fully
investigated by NMR and X-ray analyses. Porphyrinogens bearing
hydrogen atoms at their meso position are very important as key
intermediates in bio- and Rothemund syntheses of porphyrin
dyes.5 Their structures and isomerization6 are of particular interest
in connection not only with biological peripheral isomerism of
naturally occurring porphyrinoids,7 but also with N-C confusion
of pyrroles8 and formation of higher homologues9 of porphyrin
such as pentaphyrins and hexaphyrins. Porphyrinogens have been,
however, treated just as unstable intermediates and readily oxi-
dized to the targeted dyes without isolation in most cases. Conse-
quently, structural characterization of porphyrinogens has not yet
been done. In this contribution, we will show the first successful
a
a: R ) CO2Et; Ar ) Mesityl. b: R ) CO2Et; Ar ) 2,6-dichloro-
phenyl. c: R ) CO2Et; Ar ) Ph. d: R ) CF3; Ar ) Ph. e: R ) CF3;
Ar ) 2,6-dichlorophenyl.
X-ray structural analysis of the meso-unsubstituted porphyrinogen
3 and hexaphyrinogen 4 and some properties of these compounds.
Acid-catalyzed oligomerization of ethyl 2-(hydroxymethyl)-
3-mesitylpyrrole-4-carboxylate (2a), which was obtained by regio-
selective reduction of the corresponding pyrrole-2,4-dicarboxylate
1a,10 was carried out by treatment with p-toluenesulfonic acid in
dichloroethane (Scheme 1). After the oligomerization, the mixture
was oxidized with DDQ at room temperature to give a mixture,
from which white powdery crystals were obtained in 20% yield by
rinsing with chloroform. FAB mass and NMR analyses revealed
the crystals were a highly symmetric hexamer, which proved to
be hexaphyrinogen 4a. Silica gel chromatography of the residue
obtained from the mother liquor gave type I porphyrin 5a only in
3% yield and a small amount of the hexaphyrinogen (Table 1).
When the acid-treated mixture from the pyrrole 2a was directly
chromatographed on silica gel, the hexaphyrinogen 4a (16%) and
a small amount of an inseparable mixture of porphyrinogen 3a
and the porphyrin 5a (Table 1) were obtained.11 Separation of 3a
and 5a could not be achieved mainly due to the instability of 3a
toward oxidation. Although other cyclic oligomers such as
pentaphyrinogens were suggested to exist from the NMR and GPC
analyses of the reaction mixture, these oligophyrinogens could
† Advanced Instrumentation Center for Chemical Analysis.
‡ Department of Chemistry, Faculty of Science.
§ Rigaku Corporation; for X-ray analysis of 4a.
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1
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equivalence of the meso protons persisted in toluene-d8 solution
even at 100 °C. This fact and the stretching of NH (3319 cm-1
)
and CdO (1666 cm-1) in the IR spectrum strongly suggested
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(11) For the experimental procedure, see Supporting Information.
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10.1021/ja000482e CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/27/2000