Oxidative addition alkynylpalladium complexes

Oxidative addition of alkyl halides to Pd(0) is slow. Furthermore, alkyl-Pd complexes, formed by the oxidative addition of alkyl halides, undergo facile elimination of /3-hydrogen and the reaction stops at this stage without undergoing insertion or transmetallation. Although not many examples are available, alkynyl iodides react with Pd(0) to form alkynylpalladium complexes. 

Aryl-, alkenyl-, and alkynylpalladium complexes (Table 9) are generated in manners similar to the preparation of alkyl- and benzylpalladium derivatives. However, oxidative addition is much more widely applicable and satisfactory than in the preparation of alkyl-palladium derivatives, since aryl, alkenyl, and alkynyl halides are not only much more reactive toward Pd(0) complexes than their alkyl counterparts but are also uncomplicated by j8-elimination. 

The synthesis of alkynylpalladium(ii) complexes usually proceeds through oxidative addition of a halo-alkyne or of a terminal alkyne to palladium(O) compounds, or by transmetallation employing main group alkynyl compounds and palladium(ii) halide precursors. Especially, the dialkynylpalladium complexes are of much interest as molecular inorganic materials because of their remarkable phase behavior and nonlinear optical properties. Several accounts and... 

The mechanism of the palladium-catalyzed non-oxidative heterocoupling of alkynes was examined recently." NMR studies revealed oxidative addition of haloalkynes to [Pd L2] via a [Pd(r7 -XG=GPh)L2] complex 23. The observed drastic acceleration of the oxidative addition by added GuX and the m-configuration of the resulting [PdX(G=GPh)L2] 24 led to the suggestion of intermediate heterobimetallic complexes 25. Intermediates underway to a dialkynylpalladium species from 24 were not supported by NMR it could proceed through transmetallation by alkynylcopper(i) species, or via complex 26." An alternative mechanism involves transmetallation between two alkynylpalladium complexes m-[PdH(G=GR )L2] and m-[PdX(G=GR )L2], to provide the dialkynylpalladium complex [Pd(G=GR )(G=GR )L2] from which reductive elimination takes place." " ... 

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