Reactions of alkyl 4-hydroxybut-2-ynoates with arenes under superelectrophilic activation with triflic acid or HUSY zeolite: Alternative propargylation or allenylation of arenes, and synthesis of furan-2-ones

Abstract

Reactions of alkyl 4-aryl(or 4,4-diaryl)-4-hydroxybut-2-ynoates [Ar(H or Ar')(OH)C4–C3≡C2–CO2Alk] with arenes under the action of triflic acid TfOH or HUSY zeolite result in the formation of two main compounds, aryl substituted furan-2-ones or products of propargylation of electron rich arenes. Key reactive intermediates in these transformations are the corresponding O,O-diprotonated forms of starting butynoates, Ar(H or Ar')(+OH2)C4–C3≡C2– C(=O+H)(OAlk), dehydration of which gives rise to mesomeric propargyl-allenyl cations Ar(H or Ar')(OH)4C+–C3≡C2–C(=O+H)(OAlk) ↔ Ar(H or Ar')(OH)4C = C3 = 2C+–C(=O+H)(OAlk), having two electrophilic centers on the carbons C4 and C2 respectively. Reactions of these species with arenes at C4 lead to products of arene propargylation, alternatively, reactions at C2 result in allenylation of arenes, followed by further transformation into furan-2-ones. Using quantum chemical calculations by the DFT method, it has been shown that the reactivity of such propargyl-allenyl cations is mainly explained by orbital factors. Plausible reaction mechanism is discussed.