Benziporphyrins constitute a class of porphyrin analogs in which one of the pyrrole rings is replaced with a benzene ring [1]. The aromaticity of these molecules depends on their topology [2, 3], tautomerism [4, 5], and coordination chemistry [5]. Here we will describe 22-hydroxybenziporphyrin 1, which has been shown to exist as an equilibrium mixture of the 6-π electron phenolic tautomer 1 and the 20-π electron keto form 2. Aromaticity and tautomerism of these two forms has been probed with proton NMR spectroscopy and DFT calculations.

[1] M. Stępień, L. Latos-Grażyński, Acc. Chem. Res. 2005, 38, 88-98.
[2] a) K. Berlin, E. Breitmaier, Angew. Chem. Int. Ed. Engl. 1994, 33, 1246-1247. b) M. Stępień, L. Latos-Grażyński, Chem. Eur. J. 2001, 7, 5113-5117.
[3] M. Stępień, L. Latos-Grażyński, J. Am. Chem. Soc. 2002, 124, 3838-3839.
[4] T. D. Lash, Angew. Chem. Int. Ed. Engl. 1995, 34, 2533-2535.
[5] M. Stępień, L. Latos-Grażyński, T. D. Lash, L. Szterenberg, Inorg. Chem. 2001, 40, 6892-6900.

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