Cobalt- and iron-catalyzed reactions lead sometimes to very similar results. Thus, Grignard reagents and organozinc compounds efficiently react with alkenyl halides under cobalt catalysis.
However, some reactions are only possible with cobalt. As an example, 1,2-dichloroetylene gives an alkenyl chloride (monosubstitution) under cobalt catalysis and only the dialkylated product with iron. Our interest in cobalt-catalyzed reactions is directed to the discovery of such original reactivities.
We have shown that the coupling of various heteroaryl- or aryl chlorides with Grignard reagents or functionalized organocopper compounds takes place very efficiently in the presence of cobalt salts.
On the other hand, Grignard reagents can be coupled with both primary and secondary alkyl halides in satisfactory yields under cobalt-catalysis. It is the first report of a chemoselective coupling between an unactivated secondary w-bromoester and a Grignard reagent.
Aromatic Grignard reagents also react with primary and secondary alkyl halides in the presence of cobalt salts. This reaction can be performed under iron-catalysis but is then clearly less chemoselective.