Research On Palladium-catalyzed Aerobic Difunctionalization Of C-C Triple Bond

Pd(0)-catalyzed processes is now recognized to be a useful tool for constructing complex organic compounds which has undergone great development during the past decades. However, special ligands and restricted starting materials usually are needed for respectful yields of the desired products and one or more molecular byproducts usually were losted during these transformations. Combining various factors of green chemistry with Pd-catalyzed coupling reactions, we developed several example of difunctionalization of alkynes in the system of Pd/O₂:1) Highly chemoselective palladium-catalyzed cross-trimerization between alkyne and alkenes leading to 1,3,5-trienes or 1,2,4,5-tetrasubstituted benzenes with dioxygen: A palladium-catalyzed 1:2 linear/cyclic cross-trimerization of alkyne with alkenes using molecular oxygen as the sole oxidant has been reported for the first time. Through changing the starting alkynes or additives, a series of 1,3,5-trienes or 1,2,4,5-tetrasubstituted benzenes are obtained with high chemoselectivity respectively and mechanisms of these reactions are proposed based on some controlled examination. In both transformations, water is the sole byproduct.2) Acetoxypalladation of unactivated alkynes and capture with alkenes to give 1-acetoxy-1,3-dienes taking dioxygen as terminal oxidant: A new and general protocol for the synthesis of 1-acetoxy-1,3-dienes by an acetoxypalladation/Heck cross-coupling/?-H elimination tandem process is described in which dioxygen is the terminal oxidant. Electron-rich and electron-deficient alkynes are both effective substrates in this system. Interesting, almost all the terminal alkenes including methyl methacrylate which is usually inert in other Heck cross-coupling are effective for the reaction. It is the first example of acetoxypalladation of diarylalkynes.3) An aerobic [2+2+2] cyclization via chloropalladation: From 1,6-diynes and acrylates to substituted aromatic carbocycles: A Pd-catalyzed aerobic [2+2+2] cyclization of 1,6-diynes and acrylates proceeding through chloropalladation process has been developed. Poly-substituted five-membered aromatic carbocycles/heterocycles were obtained in good to excellent yields. The key most absorbing point is the detection of intermediate C had been successfully isolated and characterized, and which is the first mechanistic evidence in [2+2+2] cyclization reactions via chloropalladation process. Results of mechanistic study are consistent with the proposed reaction mechanism.