Copper-Free Monomeric and Dendritic Palladium Catalysts for the Sonogashira Reaction: Substituent Effects, Synthetic Applications, and the Recovery and Re-Use of the Catalysts

A series of bis(tert‐butylphosphine)‐ and bis(cyclohexylphosphine)‐functionalized PdII monomers and polyamino (DAB) dendritic catalysts were synthesized and investigated for Sonogashira carbon–carbon coupling reactions in a copper‐free procedure. The influence of phosphine substituents (tBu versus Cy) on the reaction kinetics was investigated by a GPC technique to monitor the reactions. The dendritic catalysts containing the cyclohexylphosphine ligands could be recovered and reused without loss of efficiency until the fifth cycle. The dendritic PdII catalysts show a negative dendritic effect, that is, the catalyst efficiency decreases as the dendrimer generation increases. The copper‐free Sonogashira reaction was catalyzed by monomeric and recoverable dendritic PdII complexes containing a bidentate phosphine (the benzyl group in the monomers models the dendritic branch; see scheme: Cat=DAB‐dendr‐[N(CH2PR2)2Pd(OAc)2]x, x=4, 8, 16; R=Cy, tBu). The kinetics, which were investigated by means of GPC, confirmed the dramatic influence of the diphosphine substituents (tBu versus Cy). The catalytic efficiency decreases as the dendrimer generation increases.