Heterogeneous Versus Homogeneous Palladium Catalysts for Ligandless Mizoroki-Heck Reactions: A Comparison of Batch/Microwave and Continuous-Flow Processing

Mizoroki-Heck couplings of aryl iodides and bromides with butyl acrylate were investigated as model systems to perform transition-metal-catalyzed transformations in continuous-flow mode. As a Suitable ligandless catalyst system for the Mizoroki-Heck couplings both heterogeneous and homogeneous Pd catalysts (Pd/C and Pd acetate) were considered. In batch mode, full conversion with excellent selectivity for coupling was achieved applying high-temperature microwave conditions with I'd levels as low as 10(-3) mol%. In continuous-flow mode with Pd/C as it catalyst, significant Pd leaching from the heterogeneous catalyst was observed as these Mizoroki-Heck Couplings proceed by a homoge-neous mechanism involving soluble Pd colloids/nanoparticles. By applying low levels of Pd acetate as homogeneous Pd precatalyst, successful continuous-flow Mizoroki-Heck transformations were performed in a high-temperature/pressure flow reactor. For both aryl iodides and bromides. high isolated product yields of the cinnamic esters were obtained. Mechanistic issues involving the Pd-catalyzed Mizoroki-Heck reactions are discussed.