Co-Catalyzed Radical Alkenylation of Alkyl Halides

Heck reaction - cobalt catalysis - radical addition

Significance

<P>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.</P>

Comment

<P>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.</P>