SUPERACID-CATALYZED ELECTROCYCLIZATION OF 1-PHENYL-2-PROPEN-1-ONES TO1-INDANONES - KINETIC AND THEORETICAL-STUDIES OF ELECTROCYCLIZATION OF OXONIUM-CARBENIUM DICATIONS

Strongly acidic conditions are required to induce the Nazarov-type cyc lization of aryl vinyl ketones, although chemical analogy with the Naz arov reaction would superficially imply a straightforward electrocycli zation reaction of the O-protonated monocation. In this paper we descr ibe the superacid-catalyzed prototype cyclization of 1-phenyl-2-propen -1-ones. The acidity dependence of these cyclization reactions as reve aled by kinetic measurements strongly suggests the involvement of the O,O-diprotonated dication rather than the O-protonated monocation. Tha t is, the cyclization of 1-phenyl-2-propen-1-ones represents an electr ocyclization of the oxonium-carbenium dication, We also describe the e ffect of substituents at the 2-position of 1-phenyl-2-propen-1-ones. A b initio calculations, based on the density functional theory, support the idea that electrocyclization of the dication is energetically mor e favarable than that of the monocation.