Carbometalation Subject

Certain transition metal complexes can serve as templates for the synthesis of chelating NHC ligands. For example, 1-phenylphosphole complexes of pal-ladium(II) are attacked in a Diels-Alder reaction by 1-vinylimidazole. If 1,2-dichloroethane is used as the solvent the imidazole is alkylated in situ and then subjected to a spontaneous carbometallation reaction [Eq. (37)]. 

The mechanism of addition of aikylmagnesium halides to double bonds has been the subject of intensive investigation.6 According to the current interpretation, these reactions differ significantly in mechanism from reactions with organoaluminum species under similar reaction conditions. For this reason no general mechanism for can be lormulated carbometallation reactions. 

Carbometallation of multiple carbon-carbon bonds is an atom-economical approach to new organometallic species, which can be subjected to further transformations to complete a catalytic cycle. 

These organocopper intermediates were successfully trapped with electrophiles such as allyl bromide, acyl chlorides, and diphenylchlorophosphine, but not with benzaldehyde and methyl iodide. The resulting products were subjected to in situ oxidation with elemental sulfur to form stable alkenylphosphine sulfides 353 [103]. Owing to its synthetic potential, the regioselective carbometallation of substituted ynol ethers has recentiy witnessed a renaissance [91aj. Thus, the alkynyl ethers 354 were treated under different carbocupration conditions (Scheme 10.121). 

Insertion of alkynes can also be used to form a carbon-carbon bond at the same time as the carbon-zirconium bond. This process, carbometallation, is useful for the stereospecific construction of trisubstituted alkenes (Scheme 5.63). The active reagent is formed by mixing zirconocene dichloride and trimethylaluminium. Both metals are required for the reaction to proceed. The vinyl organometallic 5.221 produced may be subjected to protonolysis or halogenation. An application with iodination can be found in Scheme 8.121. 

---