Reactions of linear conjugated dienone structures with arenes under superelectrophilic activation conditions. An experimental and theoretical study of intermediate multicentered electrophilic species

Abstract

Reactions of linear conjugated dienone structures ArCHCHCHCHC(O)X, 1,5-diarylpenta-2,4-dien-1-ones (X = Ar′), 5-phenylpenta-2,4-dienoic acid (Ar = Ph, X = OH) and its methyl ester (Ar = Ph, X = OMe), with arenes under superelectrophilic activation conditions by Brønsted superacids (CF₃SO₃H and FSO₃H) or strong Lewis acid (AlCl₃) result in the formation of various compounds, such as conjugated enones, indanes, and carbocyclic derivatives. The formation of the reaction products depends on the structures of starting compounds (dienone and arene) and on the reaction conditions (temperature, time, and medium acidity). In these transformations, starting dienones are precursors of di- and tri-centered electrophilic synthons leading to target products. A NMR study and DFT calculations have shown that the most probable reactive intermediates should be O,C-diprotonated species Ar⁺CHCH₂CHCHC(O⁺H)X derived from the protonation of starting dienones. Plausible mechanisms of electrophilic transformations are discussed.