The First Formation of (1Z)-1-Alkylidene-1H-isobenzofuranium Amides and 1H-Inden-1-ones: Acid-Promoted 5-exo Cyclization and Hydration/Aldol Condensation Reactions of o-Ethynylbenzophenones

(1Z)-1-(2,2-Dimethylpropylidene)-1H-isobenzofuranium bis(trifluoromethylsulfonyl)amides were synthesized through 5-exo cyclization reactions between sterically encumbered o-alkynylbenzophenones and bis(trifluoromethylsulfonyl)imide (Tf2NH). It was confirmed that the five-membered-ring isobenzofuranium amide isomerized to give the corresponding benzopyrylium amide, the six-membered-ring framework compound, in quantitative yield. Treatment of less encumbered o-alkynylbenzophenones with Tf2NH at ambient temperature resulted in acid-promoted hydration and sequential intramolecular aldol condensation reactions to afford 3-aryl-1H-inden-1-one derivatives in good yields. The proposed reaction mechanism was strongly supported by the reaction behaviour of 4-chloro- and 5-methoxy-2-ethynylbenzophenone derivatives as substrates with Tf2NH, leading to the formation of the corresponding 3-aryl-1H-inden-1-ones.