Co-Catalyzed Radical Alkenylation of Alkyl Halides

Key words

Heck reaction - cobalt catalysis - radical addition

Significance

This work describes a novel method of intermolecular coupling of alkenes with alkyl halides in the presence of trimethylsilylmethylmagnesium chloride and a cobalt catalyst CoCl₂-1,6-bis(diphenylphosphino)hexane. In many cases it is an alternative to the Heck reaction because it does not require expensive palladium catalysts. This method shows good functional-group compatibility. The reaction proceeds via a radical pathway. An intramolecular version of this process allows to create complex carbon frameworks. Additionally, a Co-catalyzed cross-coupling of iodoalkenes with Me₃SiCH₂MgCl leading to allyltrimethylsilanes is described.

Comment

This original method is especially convenient for the synthesis of polysubstituted styrenes, including those bearing functional groups. Mechanistically, the key steps of the process are the formation of a radical from the alkyl halide, its addition to the double bond and the trapping of the product by the Co(l) complex. Since the process is of radical nature, either a styrene has to be used to ensure the selective radical attack, or the reaction has to be performed intramolecularly, affording cyclization products. Interestingly, trimethyl­silylmethyl magnesium halide is a unique Grignard reagent, allowing to perform such transformations. The reaction has obviously a high synthetic potential.