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 Pd-II 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 Pd-II catalysts show a negative dendritic effect, that is, the catalyst efficiency decreases as the dendrimer generation increases.