| Abstract
| - 22-Hydroxybenziporphyrin, a porphyrin analogue containing a phenol moiety, has been shown to existas an equilibrium mixture of two distinctly different tautomers. One of them actually contains the hydroxygroup and shows the local [6]annulene aromaticity in the phenol fragment. The other tautomer containsa keto group and exhibits a [20]annulenoid structure characterized by macrocyclic antiaromaticity. Thetautomerization process has been investigated in detail using variable-temperature 1H NMR spectroscopy.The process is very fast, with an estimated activation energy of ca. 30 kJ/mol. Further insight into theenergetics of the tautomerization is obtained from density functional (DFT) calculations. Surprisingly,the estimated energy of the antiaromatic keto species is 3−5 kcal/mol lower than the energy of thephenolic tautomer. The geometric and magnetic manifestations of aromaticity and antiaromaticity in thetwo tautomers are probed using a number of computational devices, including Wiberg bond indices,resonace weights derived from the harmonic oscillator model, and nucleus-independent chemical shifts.It is shown that mixing of phenolic and keto contributions in both tautomers is stronger than that inrelated tautomers of phenol. This effect is caused by extensive conjugation with the tripyrrolic unit of22-hydroxybenziporphyrin and, to a lesser extent, by intramolecular hydrogen bonding.
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