Umpolung of ketones via enol radical cations

Abstract

For several tautomeric systems (ketones/enols, imines/enamines and others) a distinct reversal of the stability order is observed when going from the neutral compounds to the radical cations, the first use of which in a new preparative α-Umpolung reaction has been documented for keto/enol systems. The present review provides a critical evaluation of the chemistry of enol radical cations in solution with a special emphasis on the Umpolung reaction and the intermediates thereof. Other enol type of radical cations are discussed with respect to their potential to provide α-carbonyl radical and α-carbonyl cation intermediates. Hence, this article does not constitute a comprehensive summary on all enol type of radical cation reactions. All potentials in this review are referenced versus SCE, unless noted otherwise. Potentials measured against the ferrocene/ferrocenium couple were converted to SCE by adding 0.334 V.