Metal-free hydroarylation of the side chain carbon–carbon double bond of 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles in triflic acid

Abstract

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.

“One-Pot” Synthesis of γ-Pyrones from Aromatic Ketones/Heteroarenes and Carboxylic Acids

“…Unlike other tested carboxylic acids, the products of the reactions involving AdCH 2 COOH ( 2e ) or t -BuCH 2 COOH ( 2f ) were unsubstituted on their C-3 position ( 4r–s ). It might be caused by the ipso-substitution of the tert -butyl and adamantyl groups with protons under the superacidic condition …”

Synthesis of 5-arylacetylenyl-1,2,4-oxadiazoles and their transformations under superelectrophilic activation conditions

“…Thus, according our previous data on reactions of 5-styryl-1,2,4-oxadiazoles 1 [ 24 ] and results of the DFT calculations for protonation of 5-acetylenyl-1,2,4-oxadiazoles 3 ( Table 1 ), one would propose the following reaction pathways for compounds 3 in Brønsted superacids ( Scheme 2 ). Protonation of oxadiazole 3 affords dication B , which may react with counter anion of acid X − giving rise to vinyl derivatives 4 .…”

“…Then, electrophilic reactions of 5-acetylenyloxadiazoles 3a–d in different acids were studied. In our recent study on the electrophilic activation of 5-styryl-1,2,4-oxadiazoles 1 [ 24 ], it was shown by means of NMR spectroscopy and DFT calculation that the protonation of these oxadiazoles in Brønsted superacids TfOH and FSO 3 H gave reactive N,C-diprotonated species. The protonation of oxadiazoles 1 takes place at the nitrogen N4 and the α-carbon of the side chain C=C bond.…”

“…Then, electrophilic reactions of 5-acetylenyloxadiazoles 3a-d in different acids were studied. In our recent study on the electrophilic activation of 5-styryl-1,2,4-oxadiazoles 1 [24], it was shown by means of NMR spectroscopy and DFT calculation that the protonation of these oxadiazoles in Brønsted superacids species Aa-d is extremely energetically favorable. For the second protonation (reaction A→B) leading to dications Ba-d, the ΔG 298 values vary from −28.6 to 18.3 kJ/mol.…”

“…Based on our previous works on the chemistry of 1,2,4-oxadiazoles in superacids [ 24 – 25 ], we undertook this study on further investigation of the transformations of these heterocyclic compounds in electrophilic media. The main goals of this work were the synthesis of 5-arylacetylenyl-1,2,4-oxadiazoles and the study of their reactions with/without arenes under the conditions of superelectrophilic activation by the Brønsted superacid CF 3 SO 3 H (TfOH), the strong Lewis acids AlX 3 (X = Cl, Br), or the acidic zeolite CBV-720.…”