Arylpalladium intermediates diarylpalladium

The syntheses of trans and cis isomers of unsymmetrical diorganopalladium(II) complexes have been reported [104]. For example, the reaction of arylpalladium(II) halides, which have been prepared either by the oxidative addition of aryl halides to palladium(O) phosphine complexes or by the treatment of diarylpalladium(II) with HCl, with 1 equivalent of methyllithium provides /ra x-PdAr(Me)L2 complexes (eq (75)). On the other hand, treatment of arylpalladium(II) halides with excess methyllithium followed by methanolysis of the resulting dimethyl(aryl)palladate intermediates leads to c/.v-PdAr(Me)L2 complexes (eq (76)). 

Palladium salts and complexes are capable of coupling aryl halides under certain reaction conditions to yield biaryls. It is well known that palladium salts (as precatalysts) can be easily reduced to metallic palladium or palladium(O) complexes if the reduction was carried out in the presence of coordinating molecules (ligands, solvents). Once again, oxidative addition of aryl halide (I) to palladium(O) species occurs to form arylpalladium(II) halide (XIII). The latter subsequently reacts with another molecule of aryl halide to produce transient diarylpalladium(IV) intermediate (XIV). It undergoes rapid reductive elimination of a biaryl molecule (II) and forms the starting palladium(II) complex which is reduced by stoichiometric reductant - closing the catalytic cycle [17], Scheme 17. 

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