Alkynylpalladium intermediates

The addition of 1-alkynes to a,/3-unsaturated ketones in water is catalyzed by a palladium(n)/phosphine combination. Deuteration studies suggest that this reaction proceeds via carbometallation of the olefinic moiety by an alkynylpalladium intermediate (Equation (193)).1... 

Symmetrical and unsymmetrical 1,3-diynes have been prepared from 1,1-dibromo-l-alkenes. Alkynylpalladium intermediates undergo homocoupling when Cul is present in the reaction medium. On the other hand, the persistent intermediates can await addition of other 1-alkynes for their ultimate transformation. 

Oxidative carbonylation is another useful reaction. The following three types of oxidative carbonylation of alkynes are known. The first one is the synthesis of the acetylene carboxylates 299 via formation of the alkynylpalladium intermediate 298 from terminal alkynes (path a) [122]. 

On the contrary, monoalkoxycarbonylation of 1-alkynes to give 2-ynonate esters is a general reaction, which may occur either under basic conditions, favoring the formation of an alkynylpalladium species as intermediate (Scheme 16) [89-96] or under conditions allowing /3-H elimination from an alkoxycarbonylvinylpalladium species (Scheme 17) [83]. 

The shift towards the monoaminocarbonylation, obtained under these conditions, as compared with the dialkoxycarbonylation observed in alcoholic solvents, is due to the basicity of the amine, which favors the formation of an alkynylpalladium complex (Scheme 37). This species then inserts carbon monoxide with formation of an alkynoylpalladium intermediate, which undergoes nucleophilic displacement by R2NH to give the final 2-ynamide. 

Alkynylpalladium reagents can be useful intermediates in the synthesis of diynes by catalytic coupling reactions since the dialkynyl-palladium complex (PPh3)2Pd((3sCR)2 (9) has been shown to easily produce the corresponding diyne on reductive elimination. ... 

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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