Neutrale und ionische Keto/Enoltautomere: Zum Ursprung der Stabilitätsumkehr und zu ungewöhnlichen Substituenten-Effekten / Neutral and Ionic Keto/Enol Tautomers: On the Origin of Stability Reversal and Unusual Substituent Effects

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Abstract
The reversal in stability for neutral keto/enol tautomers and their cation radicals is mainly due to the removal of an electron from an π-orbital of the enol, which has antibonding properties for the C-O-bond. The total electronic energy gained in the oxidation of enols is more than sufficient to compensate for the increase in nuclear/nuclear repulsion when comparing keto/enol tautomers; this, however, does not hold for the keto systems investigated. The marked methyl substituent effect found in O, C-1, and C-2 substituted enol cation radicals is not reflected in the charge distribution of the corresponding ions. Comparison of the MNDO calculated heats of formation of ions with that of their neutral counterparts indicates that the changes in bond dissociation energies account at least qualitatively for the experimentally observed effects

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