and synthetic accessibility. Herein, we report the straight-
forward synthesis of a metalloporphyrin 1 bearing two UPy
moieties at the 10- and 15-positions (Scheme 1) and its
selective and spontaneous cyclization to a tetramer, which
is stable over a wide concentration range. Simultaneously,
we show the possibility that the present cyclic tetramer can
be a new functional unit toward higher-ordered supramo-
lecular porphyrin arrays.
Scheme 1
The porphyrin 1(2H) was synthesized by the reaction
between the activated isocytosine10 and 5,10-bis(4-aminophe-
nyl)-15,20-diphenylporphyrin, which can be readily prepared
from a commercially available free-base meso-tetraphe-
nylporphyrin (TPP).11 The activated isocytosine substituted
by an aromatic ring at the 6-position of the pyrimidinon ring
was not as stable as the one with an aliphatic chain. The
activated isocytosine was partially deactivated by water
during a workup procedure. However, by the simultaneous
and immediate addition of aminoporphyrin and magnesium
sulfate to an organic layer after quenching an excess
carbonyldiimidazole (CDI) by water, deactivation could be
minimized; hence, the porphyrin 1(2H) could be synthesized
in good yield.
synthetic accessibility. As the pioneers pointed out, establish-
ing an efficient synthetic method for a porphyrin possessing
multivalent hydrogen-bonding units is a key issue.5b How-
ever, it is inevitable that one would encounter a simple
dilemma when one tries to fabricate a hydrogen-bonded
porphyrin array of significant stability; for the stronger
hydrogen-bonding unit, harder synthetic efforts would be
required. As an antidote against such a perplexing situation,
applying a strong hydrogen-bonding unit,7,8 2-ureido-4[1H]-
pyrimidinone (UPy), is a favorable approach because it has
both the ability to form a strong duplex via a self-
complementary quadruple hydrogen bond (Kass > 107 M-1)9
Figure 1. Calibration curve of molecular weight vs the diffusion
coefficient of a polystyrene (PDI < 1.05, at 7.2 mg/mL) and
determination of the molecular weight of porphyrin 1(2H) in
chloroform.
(6) Hydrogen bonds have also been applied for fabricating a hemoprotein
mimic. For example: (a) Arai, S.; Ohkawa, H.; Ishihara, S.; Shibue, T.;
Takeoka, S.; Nishide, H. Bull. Chem. Soc. Jpn. 2005, 78, 2007. (b) Ohkawa,
H.; Arai, S.; Takeoka, S.; Nishide, H. Chem. Lett. 2003, 32, 1052. (c)
Hayashi, T.; Asai, T.; Hokazono, H.; Ogoshi, H. J. Am. Chem. Soc. 1993,
115, 12210.
(7) Recent reviews about a multiple hydrogen-bonding unit: (a) Sijbesma,
R. P.; Meijer, E. W. Chem. Commun. 2003, 5. (b) Zimmerman, S. C.;
Corbin, P. S. Struct. Bonding 2000, 96, 63.
To estimate the molecular weight of the porphyrin 1(2H)
in solution, DOSY (diffusion ordered spectroscopy) was
performed.12,13 The molecular weight could be successfully
calibrated by the normalized diffusion coefficient Dnorm of a
polystyrene with regulated molecular weights (Figure 1),
which was defined as the ratio to the diffusion coefficient
(8) Recent examples about a supramolecular architecture using multiple
hydrogen-bonding units: (a) Ligthart, G. B. W. L.; Ohkawa, H.; Sijbesma,
R. P.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 810. (b) Park, T.; Todd,
E. M.; Nakashima, S.; Zimmerman, S. C. J. Am. Chem. Soc. 2005, 127,
18133. (c) Meyer, M. F.; Nakashima, S.; Zimmerman, S. C. Org. Lett. 2005,
7, 3005. (d) Zhao, X.; Wang, X.-Z.; Jiang, X.-K.; Chen, Y.-Q.; Li, Z.-T.;
Chen, G.-J. J. Am. Chem. Soc. 2003, 125, 15128.
(9) (a) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Hirschberg, J. H.
K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278,
1601. (b) So¨ntjens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.;
Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 7487. (c) Beijer, F. H.;
Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer, E. W. J. Am. Chem.
Soc. 1998, 120, 6761.
(10) Keizer, H. M.; Sijbesma, R. P.; Meijer, E. W. Eur. J. Org. Chem.
2004, 2553.
(11) Lugaaya, R.; Jaquinood, L.; Fronczek, F. R.; Vicente, M. G. H.;
Smith, K. M. Tetrahedron 2004, 60, 2757.
(12) Mass spectrometry (MALDI-TOF, CSI-TOF) was unsuccessful to
detect the molecular ion signal of the assembly of porphyrin 1(2H).
(13) The evaluation of molecular weight by DOSY has also been reported
in the literature: (a) Viel, S.; Capitani, D.; Mannina, L.; Segre, A.
Biomacromolecules 2003, 4, 1843. (b) Zhao, T.; Beckham, H. W.
Macromolecules 2003, 36, 9859. (c) Zhao, T.; Beckham, H. W.; Gibson,
H. W. Macromolecules 2003, 36, 4833. (d) Michelsen, U.; Hunter, C. A.
Angew. Chem., Int. Ed. 2000, 39, 764.
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Org. Lett., Vol. 8, No. 11, 2006