Superacid-Catalyzed Electrocyclization of 1-Phenyl-2-propen-1-ones to 1-Indanones. Kinetic and Theoretical Studies of Electrocyclization of Oxonium−Carbenium Dications

Strongly acidic conditions are required to induce the Nazarov-type cyclization of aryl vinyl ketones, although chemical analogy with the Nazarov reaction would superficially imply a straightforward electrocyclization reaction of the O-protonated monocation. In this paper we describe the superacid-catalyzed prototype cyclization of 1-phenyl-2-propen-1-ones. The acidity dependence of these cyclization reactions as revealed by kinetic measurements strongly suggests the involvement of the O,O-diprotonated dication rather than the O-protonated monocation. That is, the cyclization of 1-phenyl-2-propen-1-ones represents an electrocyclization of the oxonium−carbenium dication. We also describe the effect of substituents at the 2-position of 1-phenyl-2-propen-1-ones. Ab initio calculations, based on the density functional theory, support the idea that electrocyclization of the dication is energetically more favarable than that of the monocation.