22-hydroxybenziporphyrin: switching of antiaromaticity by phenol-keto tautomerization.
22-hydroxybenziporphyrin, a porphyrin analogue containing a phenol moiety, has been shown to exist as an equilibrium mixture of two distinctly different tautomers. One of them actually contains the hydroxy group and shows the local annulene aromaticity in the phenol fragment. The other tautomer contains a keto group and exhibits a annulenoid structure characterized by macrocyclic antiaromaticity. The tautomerization 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 the energetics 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 the phenolic tautomer. The geometric and magnetic manifestations of aromaticity and antiaromaticity in the two 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 in related tautomers of phenol. This effect is caused by extensive conjugation with the tripyrrolic unit of 22-hydroxybenziporphyrin and, to a lesser extent, by intramolecular hydrogen bonding.