Microwave-Assisted Cross-Coupling and Hydrogenation Chemistry by Using Heterogeneous Transition-Metal Catalysts: An Evaluation of the Role of Selective Catalyst Heating

• Published in: Chemistry : a European journal Vol. 15; no. 43; pp. 11608 - 11618
• Main Authors: Irfan, Muhammed, Fuchs, Michael, Glasnov, Toma N., Kappe, C. Oliver
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 02.11.2009; WILEY‐VCH Verlag
• Subjects: cross-coupling; heterogeneous catalysis; hydrogenations; mass transfer; microwave chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200902044

The concept of specific microwave effects in solid/liquid catalytic processes resulting from the selective heating of a microwave‐absorbing heterogeneous transition‐metal catalyst by using 2.45 GHz microwave irradiation was evaluated. As model transformations Ni/C‐, Cu/C‐, Pd/C‐, and Pd/Al2O3‐catalyzed carbon–carbon/carbon–heteroatom cross‐couplings and hydrogenation reactions were investigated. To probe the existence of specific microwave effects by means of selective catalyst heating in these transformations, control experiments comparing microwave dielectric heating and conventional thermal heating at the same reaction temperature were performed. Although the supported metal catalysts were experimentally found to be strongly microwave absorbing, for all chemistry examples investigated herein no differences in reaction rate or selectivity between microwave and conventional heating experiments under carefully controlled conditions were observed. This was true also for reactions that use low‐absorbing or microwave transparent solvents, and was independent of the microwave absorbtivity of the catalyst support material. In the case of hydrogenation reactions, the stirring speed was found to be a critical factor on the mass transfer between gas and liquid phase, influencing the rate of the hydrogenation in both microwave and conventionally heated experiments. Stir crazy! Selective heating of supported Ni, Cu, or Pd metal plays no role in microwave‐assisted transition‐metal‐catalyzed C–C/C–O cross‐coupling reactions and the hydrogenation of olefins (see scheme). However, the stirring speed has a major influence on the rate of hydrogenation as a result of improved mass transfer.