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 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.
Glasnov
Toma N
TN
Christian Doppler Laboratory for Microwave Chemistry (CDLMC) and Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstrasse 28, 8010 Graz, Austria.
Findenig
Silvia
S
Kappe
C Oliver
CO