Metal groups aryl/vinyl halide reactions

The transition metal catalyzed carbon-carbon bond formation between organomagnesium reagents and aryl (vinyl) halides has been one of the pioneering entries into cross-coupling chemistry. The reaction has been widely utilized since than in azine chemistry,22 with the limitation that the functional group tolerance of Grignard reagents is only moderate. Here only some of the more recent developments will be mentioned. 

Four oxidation states of palladium are encountered in organometallic chemistry see Palladium Inorganic Coordination Chemistry) In order of importance, they are Pd , Pd , Pd, and Pd . With the reduction of palladium from Pd to Pd , the metal changes its reactivity from electrophile to nucleophile. However, unlike main group nucleophiles such as thiolates or cyanide, Pd complexes react with both alkyl halides and aryl or vinyl halides. Reactions of Pd complexes with these latter sp halides generate new Pd aryl or vinyl bonds through the process of oxidative addition. 

The mechanism for the Heck reaction is shown in Scheme 4.63. The first step involves the oxidative addition of an aryl or vinyl halide, R -X, to a palladium(O) species. This species normally contains an auxiliary donor, L, where L is often a phosphine. This may be preceded by a reduction of the metal if a palladium(II) salt is employed initially. Thereafter, two different pathways are possible depending on which group dissociates to provide a vacant coordination site for the incoming alkene. If a neutral ligand (such as a phosphine) detaches and the halide is retained, the active species immediately prior to the C-C coupling step is the neutral complex III-90. Conversely, if the anionic ligand (such as a halide) dissociates, the active species is the cationic complex III-91. 

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