Transmetallation 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. 

Although catalytic amounts of Ag20 have been found to promote the palladium-catalyzed coupling of aryl boronic acids and terminal alkynes, the authors in this case do not attribute the desired reactivity to the formation, and subsequent transmetalla-tion, of a silver acetylide. Rather, it is proposed that the Ag20 activates the alkynylpalladium complex to allow transmetallation from the boronic acid, and that any competing formation of the silver acetylide results in a homocoupling of the alkynes (Scheme 1.55).123... 

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." " ... 

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 most frequently encountered and generally robust methods for the synthesis of alkynylpalladium(ii) complexes are copper(i)-catalyzed dehydrohalogenation and transmetallation involving isolated organotin(iv) compounds or well-defined alkynylcopper(i) species (Scheme 2). 

---