Understanding palladium catalyzed reactions under mechanochemical conditions

Scientists have spent a considerable amount of time and effort designing a pre-catalyst that transforms into the active catalytic species upon entry into the reaction. Scientist typically consider stability and reactivity when designing a pre-catalyst. Often this harmony is difficult to ascertain and require processes that can be detrimental to both human health and the environment. Additionally, the cost of the catalyst is dictated not by the precious metal that conducts the chemistry but by the human labor it takes to create the precursor. We created an active catalyst that is generated in situ under mechanochemical conditions. Here in we report on forming carbon-carbon bonds without the use of solvent. Sonogashira coupling was used as the baseline for the genesis of this project. Previously, the solvent-free Sonogashira coupling of a variety of para-substituted aryl halides with trimethylsilylacetylene or phenylacetylene using a palladium catalyst and copper (0) co-catalyst under mechanochemical conditions was studied. In this current work we seek to use a combination of palladium and copper metal to successfully conduct the solvent-free Sonogashira reaction. We observed high yields when triphenylphosphine was added to the reaction mixture at elevated temperatures. Additionally, it is possible to chemoselectively react one halide over the other based on temperature.