Catalytic σ-activation of carbon-carbon triple bonds: reactions of propargylic alcohols and alkynes.

The majority reactions of alkynes in the literature are reported to proceed via either structural σ-activation or catalytic π-activation of C≡C bonds. We skillfully designed novel methods for the catalytic σ-activation of C≡C bonds of alkynyl compounds. For terminal alkynyl compounds, σ-activation was achieved by silver(i)-catalyzed C-H functionalization. Whereas σ-activation of internal alkynes was accomplished by the generation of propargylic cations from propargylic alcohols under Lewis-acid catalysis. These σ-activated species have been successfully used for new C-C and C-heteroatom bond formation reactions. Plausible reaction pathways were proposed based on typical control experiments to help the readers to gain insights into reactions and for further discovery of new reactions based on this concept of catalytic σ-activation of C≡C bonds.