Gold α-oxo carbenoids in catalysis: catalytic oxygen-atom transfer to alkynes.

An overview of reactive gold α-oxo carbenoid intermediates in the gold-catalyzed functionalization of alkynes is presented. Such intermediates can be generated from inter- and intramolecular oxidation of alkynes by nucleophilic oxygen-atom donor groups, such as amine N-oxides, pyridine N-oxides, nitrones, nitro compounds, sulfoxides, and epoxides. These O-atom transfer processes occur by gold-mediated addition-elimination reactions. In catalytic systems, α-oxo carbenoids can undergo nucleophilic attack by imine, arene, and migrating hydride as well as alkyl groups, leading to cascade reactions and the construction of new skeletons. The facile construction of C-E (E=C, N, S, or O) bonds makes it an attractive step-economic approach to value-added molecules from readily available starting materials. The scope, mechanisms, and reactivity of such α-oxo carbenoid species are discussed. The remarkable diversity of structures accessible is demonstrated with various recent examples.