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

Authors

  • Toma N. Glasnov Dr.,

    1. Christian Doppler Laboratory for Microwave Chemistry (CDLMC) and Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstrasse 28, 8010 Graz (Austria), Fax: (+43) 316-380-9840
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  • Silvia Findenig,

    1. Institute of Chemistry, Analytical Chemistry, Karl-Franzens-University Graz, Universitätsplatz 1, 8010 Graz (Austria)
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  • C. Oliver Kappe Prof. Dr.

    1. Christian Doppler Laboratory for Microwave Chemistry (CDLMC) and Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstrasse 28, 8010 Graz (Austria), Fax: (+43) 316-380-9840
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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.

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