Simple pyrazoles as efficient organocatalysts for alkyne–CO2 carboxylation and one-pot construction of heterocycles†
Abstract
C–H carboxylation of alkynes with CO2 is a straightforward and atom-economical approach toward propiolic acids. Transition metal catalysis for this reaction has been intensively studied, but organocatalysis remains largely undeveloped. Here we disclose that simple pyrazoles are efficient bifunctional organocatalysts rivalling metal catalysts in promoting the carboxylation reaction. Organocatalytic carboxylation has been successfully utilized in orthogonal tandem catalysis for the one-pot construction of various heterocycles, including pyranones, quinazinones, pyridones, isoxazolo[3,2-a]isoindolones and imidazoles. In the synthesis of pyranones and quinazinones, we identified for the first time that the [3 + 3] cyclization between propiolates and acyl-activated methylene compounds involves C-to-C 1,3-acyl migration. The results illustrate new prospects for organocatalytic CO2 conversion and heterocycle synthesis.
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