Superelectrophiles, Formed in Superacids.

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The Pictet-Spengler reaction, an acid-catalyzed intramolecular cyclization of intermediate imines of 2-arylethylamine to give 1,2,3,4-tetrahydroisoquinolines, has long been limited to active substrates which bear strongly electron-donating groups such as a methoxy or a hydroxy group on the cyclizing benzene ring. In this paper, we present superacid-catalyzed Pictet-Spengler reactions of imines of 2-phenethylamine, including the prototype Pictet-Spengler reaction of N-methylene-2-phenethylamine, to give the parent and 1-substituted 1,2,3,4-tetrahydroisoquinolines in moderate to high yields. The yields are dependent on the acidity of the media. A linear relationship was found between the rate of the cyclization and the acidity of the reaction media in kinetic studies of N-methylene-2-phenethylamine and related imines, strongly supporting the intervention of an additional protonative activation of the N-protonated imines, that is, the involvement of dicationic superelectrophiles, N,N-diprotonated imines (ammonium-carbenium dications).Strongly acidic conditions are required to induce the Nazarov-type cyclization of arylvinyl 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 electrocyclzation of the dication is energetically more favorable than that of the monocation.