Dihydrogen/triple bond addition

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

The addition of dihydrogen to triple bonds, which can be readily achieved either using ionic reagents such as LiAlH4 to give tnms double bonds or catalytically affording cis-alkenes, has found important applications in the synthesis of cyclic alkenes with interesting it-parameters. 

The addition of dihydrogen to double and triple bonds by palladium-catalysts in general does not involve palladium-carbon bonds thus, this is not subject of this paragraph. We included just one illustrative example that works on either small or large... 

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