Hydroarylation of enamides enabled by HFIP via a hexafluoroisopropyl ether as iminium reservoir

Abstract

Here we describe that HFIP greatly expands the scope with respect to both reaction partners of the Brønsted acid-catalyzed hydroarylation of enamides. The reaction is fast and practical and can be performed on the gram scale. A hexafluoroisopropyl ether intermediate was isolated from the reaction mixture and was shown to convert to the product when resubmitted to the reaction conditions. Extensive kinetic studies and computations reveal that the hexafluoroisopropyl ether is formed rapidly and serves as a slow-release reservoir for the key cationic intermediate, preventing the oligomerization of the substrate under the reaction conditions. Given the relatively low electrophilicity of the cationic intermediates in the present study, it seems likely that HFIP also actively participates in other reactions involving more electrophilic carbocations.