Visible-Light Photocatalytic Decarboxylation of α,β-Unsaturated Carboxylic Acids: Facile Access to Stereoselective Difluoromethylated Styrenes in Batch and Flow.

The development of synthetic methodologies which provide access to both stereoisomers of α,β-disubstituted olefins is a challenging undertaking. Herein, we describe the development of an operationally simple and stereoselective synthesis of difluoromethylated styrenes via a visible-light photocatalytic decarboxylation strategy using fac -Ir(ppy)3 as the photocatalyst. Meta- and para-substituted cinnamic acids provide the expected E -isomer. In contrast, ortho -substituted cinnamic acids yield selectively the less stable Z -product, whereas the E -isomer can be obtained via continuous-flow processing through accurate control of the reaction time. Furthermore, our protocol is amenable to the decarboxylative difluoromethylation of aryl propiolic acids.