be important for the syntheses and the stabilization of
NCP. Here, we chose N-confused 5,20-diphenylporphyrin
(NCDPP, 3) as an initial target of outer R,â-free NCP with
less than four meso-substituents (Figure 1).
Scheme 1. Synthesis of 5,20-Diphenyl-NCP (NCDPP, 3)
R-proton of the confused ring (C3-H) and â-protons are
shifted slightly downfield (8.8-9.1 ppm) compared to
NCTPP (8.5-9.0 ppm). In addition, two new singlet signals
at 9.4 and 9.5 ppm assignable to meso-protons (C10-H and
C15-H) were observed.
Figure 1. Framework and atom numbering system of porphyrin
and N-confused porphyrin (NCP).
The UV/vis spectrum of 3 showed the absorption maxima
at 430 (Soret), 527, 566, and 705 nm (Q-bands) in CH2Cl2.
The spectrum was similar to that of NCTPP with respect to
the shape but showed hypsochromic shifts compared to
NCTPP.10 On the other hand, in DMF, the Soret band was
bathochromic shifted (434 nm) and the appearance of
characteristic Q-bands (579, 626, 679 nm) indicates the
formation of another tautomer of the inner-2H-type as
demonstrated in NCTPP (Figure 2).11
NCDPP (3) was synthesized according to the Scheme 1.
In the synthesis, we have used [3 + 1] condensation reac-
tion of 2,4-bis(phenylhydroxymethyl)pyrrole (1) and meso-
unsubstituted tripyrrane (2) referring to the successful
procedures used in the syntheses of hetero-NCP3 and
porphin.4At first, 2,4-dibenzoylpyrrole5 was reduced to the
corresponding dicarbinol (1) and the resulting diol was
reacted with tripyrrane (2)6 in the presence of an acid catalyst.
After the oxidation with o-chloranil, followed by column
chromatography on alumina and silica gel sequentially, the
target compound 3 was isolated in 7% yield and no other
NCP derivatives were obtained. Formation of meso-phenyl-
substituted porphyrins (<1%) was only detected in a trace
amount by NMR, UV/vis, and FABMS,7 which indicates that
the scrambling of phenyl groups was not significant in this
NCP formation reaction. When MSA (methanesulfonic acid),
a known effective acid for tetraaryl-NCP synthesis,8 was
applied instead of BF3‚MeOH, the yield of NCDPP decreased
to 3%.
In the 1H NMR spectrum of 3 in CDCl3, the inner C21-H
signal was observed at -5.9 ppm, which was shifted up-
field compared to that of N-confused tetraphenylporphyrin,
NCTPP, at -5.1 ppm. Such a substituent effect is known in
the normal porphyrin systems and is attributable to the
enhancement of the ring current as the result of the removal
of the electron-withdrawing meso-phenyl groups.9 The outer
Figure 2. Absorption spectra of 3 in CH2Cl2 (solid line) and in
DMF (broken line).
In a single crystal of 3, the two independent NCDPP
molecules form a pair with stacking to each other in the same
direction. The intermolecular distances between the porphyrin
rings are 3.55 Å in the pair (r1) and 3.69 Å between the
(3) (a) Heo, P.-Y.; Shin, K.; Lee, C.-H. Tetrahedron Lett. 1996, 37, 197.
(b) Heo, P.-Y.; Shin, K.; Lee, C.-H. Tetrahedron Lett. 1996, 37, 1521.
(4) Taniguchi, S.; Hasegawa, H.; Nishimura, M.; Takahashi, M. Synlett
1999, 73.
(5) Cadamuro, S.; Degani, I.; Dughera, S.; Fochi, R.; Gatti, A.; Piscopo,
L. J. Chem. Soc., Perkin Trans. 1 1993, 273.
(6) High-yield and stepwise synthesis of 2: Taniguchi, S.; Hasegawa,
H.; Yanagiya, S.; Tabeta, Y.; Nakano, Y.; Takahashi, M. Tetrahedron 2001,
57, 2103. Another one-pot synthesis: Ka, J.-W.; Lee, C.-H. Tetrahedron
Lett. 2000, 41, 4609.
(7) Formation of a trace amount of porphine, mono-, di-, and tri-
phenylporphyrins was detected by NMR and FABMS. meso-Diphenyl-â-
benzylporphyrin was also produced according to the quenching procedure
in the reduction step.
(9) Chemical shifts of the inner NH signal of TPP, 5,10-DPP, and porphin
were reported -2.76, -3.34, and -3.76 ppm, respectively. See, ref 17b
and: Medforth, C. J. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 5, Chapter
35.
(10) Such shifts were also observed in normal porphyrins. For example,
the Soret bands of TPP and 5,10-DPP were observed at 419 and 405 nm in
CH2Cl2, respectively.
(8) Geier, G. R., III; Haynes, D. M.; Lindsey, J. S. Org. Lett. 1999, 1,
1455.
(11) Furuta, H.; Ishizuka, T.; Osuka, A.; Dejima, H.; Nakagawa, H.;
Ishikawa, Y. J. Am. Chem. Soc. 2001, 123, 6207.
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Org. Lett., Vol. 5, No. 9, 2003