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Keywords:

  • cross-coupling;
  • flow chemistry;
  • heterogeneous catalysis;
  • microwave-assisted reactions;
  • nanoparticles;
  • reaction mechanisms

Abstract

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 Pd levels as low as 10−3 mol %. In continuous-flow mode with Pd/C as a catalyst, significant Pd leaching from the heterogeneous catalyst was observed as these Mizoroki–Heck couplings proceed by a homogeneous 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.