Dual Catalysis with Copper and Rhenium for Trifluoromethylation of Propargylic Alcohols: Efficient Synthesis of α-Trifluoromethylated Enones

Trifluoromethylation of propargylic alcohols to provide (Z)‐α‐trifluoromethylated enones and β‐unsubstituted α‐trifluoromethylated enones proceeded with high yield and selectivity in the presence of CuI/Re2O7. The Z isomer was formed under kinetic control, though it is less stable than the E isomer in terms of steric repulsion. The right combination: Trifluoromethylation of propargylic alcohols to (Z)‐ or terminal α‐trifluoromethylated enones proceeded with high yield and selectivity in the presence of CuI and Re2O7. The Z isomer was formed under kinetic control, and isomerization of Z to E isomer occurred under the reaction conditions. Trifluoromethylation of a silyl allenyl ether suggests that the main reaction pathway does not involve an allenol intermediate; a novel reaction mechanism is proposed.