Co(I)- versus Ru(II)-Catalyzed [2+2+2] cycloadditions involving alkynyl halides

Alkynyl chlorides, bromides, and iodides have been tested as [2 + 2 + 2] cycloaddition partners using CpCo(CO)(dimethylfumarate) and Cp*Ru(cod)Cl as precatalysts. A series of cocyclizations between diynyl dihalides and alkynes, as well as intramolecular cycloadditions of triynyl dihalides, has been carried out. While this study confirmed the versatility of the ruthenium complex with all kinds of halides, the cheap air-stable cobalt complex proved nonetheless efficient with alkynyl bromides.