1,3-Diarylpropynones ArC≡CCOAr' in superacids with H 0 ranging from -20 to -14 undergo protonation at the carbonyl oxygen atoms to give stable ArC≡CC(O + H)Ar' ions or at the acetylenic C 2 atom with formation of reactive ArC + =CHCOAr' species. The effects of the Ar and Ar' substituents and reaction conditions on the intramolecular cyclization of ArC + =CHCOAr' to 3-arylinden-1-ones are discussed.

According to the 1 H and 13 C NMR data, 3-arylpropynoic acids and their esters XC 6 H n -C C-CO 2 R (R = H, Me, Et) having electron-withdrawing substituents in the benzene ring (X = NO 2 , CN, COMe, CO 2 Me) exist in HSO 3 F at -80 to 0°C as XC 6 H n -C C-C + (OH)OR ions. Derivatives with other substituents (X = H, F, Me, MeO) in HSO 3 F or CF 3 SO 3 H above -40°C undergo protonation at the acetylenic carbon atom neighboring to the acid group to give unstable vinyl-type XC 6 H n -C + =CH-CO 2 R cations which are then transformed into mixtures of stereoisomeric (Z and E) fluorosulfonates or triflluoromethanesulfonates XC 6 H n -CY=CH-CO 2 R (Y = OSO 2 F, OSO 2 CF 3 ), the E isomer prevailing.

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In superacids with H0=-14 to -20, it has been found that 1,3-diarylpropynones ArC triple bond CCOAr' are either protonated on oxygen of carbonyl groups with the formation of stable ions ArC triple bond CC(O+H)Ar' or undergo further transformations when the highly conjugated system is electron-rich enough. In the latter case, 3-arylindenones are produced very rapidly and with high efficiency (up to 95% yield in less than 30 min). The influence of the substituents Ar, Ar' and of the reaction conditions on the behavior of 1,3-diarylpropynones and on the intramolecular cyclisation have been studied. From the collected data, a mechanism has been proposed involving vinyl cations ArC+=CHCOAr' and/or dications ArC+=CHC(O+H)Ar'.