Cobalt catalysis reductive coupling

Reduction of complexes [(NHsjsCoCIIIjL], where L is a reducible ligand by Eu " and is subject to catalysis by the L/L° couple when the ligand is released from the cobalt(II) product.These catalysts have a variety of fates. Addition of excess Eu " or suppresses catalysis " by oxidation of the active species L and allows examination of the uncatalyzed reaction. For the 4-carboxamide derivative (5) a positive hydrogen dependence in the rate law is... 

Other metals can catalyze Heck-type reactions, although none thus far match the versatility of palladium. Copper salts have been shown to mediate the arylation of olefins, however this reaction most probably differs from the Heck mechanistically. Likewise, complexes of platinum(II), cobalt(I), rhodium(I) and iridium(I) have all been employed in analogous arylation chemistry, although often with disappointing results. Perhaps the most useful alternative is the application of nickel catalysis. Unfortunately, due to the persistence of the nickel(II) hydride complex in the catalytic cycle, the employment of a stoichiometric reductant, such as zinc dust is necessary, however the nickel-catalyzed Heck reaction does offer one distinct advantage. Unlike its palladium counterpart, it is possible to use aliphatic halides. For example, cyclohexyl bromide (108) was coupled to styrene to yield product 110. 

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