Cu(III)-CF₃ Complex Enabled Unusual ( Z)-Selective Hydro-trifluoromethylation of Terminal Alkynes

An efficient and selective hydro-trifluoromethylation of terminal alkynes is developed to enable the synthesis of 1,2-disubstituted trifluoromethylated Z-alkenes from terminal alkynes, a silane and a Cu(III)-CF₃ complex in DMF. The unusual Z-selectivity and the compatibility of various functional groups make this reaction complementary to previous vinyl-CF₃ bond-forming methods producing dominantly E-products or a mixture of E, Z-isomers. Extensive mechanistic studies indicate that this reaction may involve a key step of migratory insertion of initially formed Ar-C≡C-CF₃ into LCu(I)-H intermediate and subsequent hydrogenation of the vinyl-copper species to give finally the Z-trifluoromethylated styrenes. Deuterium labeling experiments show that DMF acts not merely as the solvent but also as the source of α-H in the Z-trifluoromethylated styrene products. Given the novel syn-difunctionalization of alkynes using Cu(III)-CF₃ complexes/a nucleophilic reagent we developed recently that are also in contrast to the previous anti-difunctionalization chemistry, Cu(III)-CF₃ complexes are demonstrated to show distinct reactivity properties compared to conventional CF₃ reagents (Togni's reagents and Umemoto-type reagents) and may thus enable the development of other interesting reactions.