Nickel catalysis vinylic substitution

Two other Ni(CO)4 substitutes, Ni(CO)3PPh3 and Ni(COD)2/dppe, prove to be appropriate for the catalysis of tandem metallo-ene/carbonylation reactions of allylic iodides (Scheme 7)399. This process features initial oxidative addition to the alkyl iodide, followed by a metallo-ene reaction with an appropriately substituted double or triple bond, affording an alkyl or vinyl nickel species. This organonickel species may then either alkoxycar-bonylate or carbonylate and undergo a second cyclization on the pendant alkene to give 51, which then alkoxycarbonylates. The choice of nickel catalyst and use of diene versus enyne influences whether mono- or biscyclization predominates (equations 200 and 201). 

Only a limited number of vinyl sulfones, e.g. phenyl ( )-2-phenylvinyl sulfone (14), undergo codimerization with MCR Homocyclodimerization of MCP is the most efficient side reaetion. Interestingly, yields and product distributions are solvent dependent. No reaction takes place with catalytic amounts of bis(t -cycloocta-l,5-diene)nickel(0)/triphenylphosphane. In this case the vinyl sulfones are strongly coordinated to the catalyst metal, thus preventing interaction with MCP. When the sulfones bear alkyl-substituted vinyl groups, isomerization to yield allyl sulfones usually proceeds faster than cycloaddition, at least in the case of palladium(O) catalysis. 

Due to sluggish reactivity of aryl and vinyl halides in nucleophiUc substitution reactions, the formation of sulfur-carbon(sp ) bonds is typically carried out using transition metal catalysis [22-27]. While the field is dominated by the use of palladium, copper, and nickel catalysts, considerable advances have been made using more abundant metal catalysts such as iron. Additionally, a number of transition metal-fiee approaches have been developed for the formation of sulfur-carbon(sp ) bonds. The following sections will highlight representative examples of C—S bond forming reactions. 

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