Arylallyl cations and their stereospecific cyclization to bicyclic trienylic cations

Abstract

A series of arylallyl alcohols, where aryl = 2,6-dimethylphenyl, mesityl, duryl, or pentamethylphenyl, was prepared. On addition to strong acid, these generate arylallyl cations. In some cases these allyl cations are observable and were characterized as insitu solution species by nmr spectroscopy. In other cases, an immediate cyclization takes place, leading stereospecifically to a series of bicyclic trienyl cations, which were also thoroughly characterized. The stereochemistry of this cyclization reaction, which was initially expected to follow the "rules" for electrocyclic ring closures, was probed in some detail. It is tentatively concluded that the stereochemistry in the products is not that expected from a conrotatory ring closure. Two diarylmethyl cations were also prepared. However, these did not cyclize.