Gold-Catalyzed Alkynylation: Acetylene-Transfer instead of Functionalization
Over the last fifteen years, gold has been raised from the status of an inert noble metal to one of the most-often-used catalysts in synthetic chemistry. The functionalization of the triple bond of alkynes has been especially successful in this respect. In contrast, gold-catalyzed alkynylation reactions only began to emerge in 2007. Since then, three different approaches have been successfully used for this transformation. 1) Gold nanoparticles have been shown to promote catalytic cycles based on the oxidative arylation of aryl halides to give a palladium-free Sonogashira reaction. 2) The use of benziodoxol(on)e hypervalent iodine compounds as oxidative alkynylation reagents has allowed the CH functionalization of electron-rich heterocycles under mild conditions with a very broad functional-group tolerance. 3) The use of iodobenzene acetate or Selectfluor as an external oxidant has led to the first alkynylation methods based on direct CH/CH coupling reactions. In only six years, gold-catalyzed alkynylation methods have grown from non-existent to useful synthetic procedures for the synthesis of structurally diverse alkynes. Considering that acetylenes are among the most-important building blocks for applications in synthetic chemistry, chemical biology, and materials science, there is tremendous potential for the further development of gold-catalyzed alkynylation reactions in the future.