Reactions of Dimethyl Ester of Acetylene Dicarboxylic Acid (DMAD) with Arenes Under the Superelectrophilic Activation Conditions

Abstract

Protonation of DMAD MeO₂C−C≡C−CO₂Me in the Brønsted superacid CF₃SO₃H (triflic acid, TfOH) gives rise to stable O,O-diprotonated species MeO(⁺HO═)C−C≡C−C(═OH⁺)OMe according to NMR data and DFT calculation. This cation reacts with arenes ArH at room temperature for 0.3-1 h with a stereoselective formation of the corresponding trans-dimethyl arylfumarates E-MeO₂C−C(Ar)═CH−CO₂Me, as products of hydroarylation of the DMAD acetylene bond. Increasing the reaction time leads to further, deeper inter- and intramolecular transformations of dimethyl arylfumarates with arenes, resulting in various compounds, indenes, indenones, and indanones, that depend on the nucleiphilicity of arenes and reaction time. Plausible mechanisms of multistep cationic transformations of DMAD with arenes in TfOH are discussed.