The metal-free reaction of 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles with arenes in neat triflic acid (TfOH, CF3SO3H), both under thermal and microwave conditions, leads to 5-(2,2-diarylethyl)-3-aryl-1,2,4-oxadiazoles. The products are formed through the regioselective hydroarylation of the side chain carbon–carbon double bond of the starting oxadiazoles in yields up to 97%. According to NMR data and DFT calculations, N4,C-diprotonated forms of oxadiazoles are the electrophilic intermediates in this reaction.

No abstract

Acetylene derivatives of 1,2,4-oxadiazoles, i.e., 5-(2-arylethynyl)-3-aryl-1,2,4-oxadiazoles, have been obtained, for the first time reported, from 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles by their bromination at the carbon–carbon double bond followed by di-dehydrobromination with NaNH2 in liquid NH3. The reaction of the acetylenyl-1,2,4-oxadiazoles with arenes in neat triflic acid TfOH (CF3SO3H) at room temperature for 1 h resulted in the formation of E/Z-5-(2,2-diarylethenyl)-3-aryl-1,2,4-oxadiazoles as products of regioselective hydroarylation of the acetylene bond. The addition of TfOH to the acetylene bond of these oxadiazoles quantitatively resulted in E/Z-vinyl triflates. The reactions of the cationic intermediates have been studied by DFT calculations and the reaction mechanisms are discussed.

Linear‐conjugated enynones,1,5‐diarylpent‐4‐en‐2‐yn‐1‐ones [Ar2−CH=CH−C≡C−C(=O)Ar1], have been cyclized into 2,6‐diaryl‐2,3‐dihydropyran‐4‐ones in triflic acid TfOH (CF3SO3H). Reactions of these enynones with arenes Ar3H in TfOH have afforded, at first stage, products of hydroarylation of the acetylene bond, 1,3,5‐triarylpent‐2,4‐dien‐1‐ones [Ar2−CH=CH−C(Ar3)=CH−C(=O)Ar1], which have been further cyclized into alkylidene indanes. Plausible reaction mechanisms have been proposed. According to quantum chemical calculations by DFT method, initial key reactive intermediates are vinyl type dications [Ar2−CH=CH−C+=CH−C(=O+H)Ar1], generated under the protonation of carbonyl oxygen and acetylene bond of starting enynones in TfOH.