Metal Catalysis with Microwaves in Organic Synthesis: a Personal Account

In the last 25 years, the influence of microwave (MW) irradiation in chemical synthesis has intrigued chemists for the exceptional results obtained in shortening reaction times. Inspired by recent papers that demonstrate the existence of a non-thermal effect in some MW assisted reactions, we examine herein a series of reactions catalysed by homogeneous and heterogeneous metals occurring under MW irradiation. The review contains examples of hydroformylation, hydrogenation (including enantioselective versions with 98 % ee), hydrogenolysis, hydrogen transfer and borrowing hydrogen reactions carried out under MWs and metal catalysis. An increase in the influence of MW irradiation was often observed in reactions performed with heterogeneous catalysts such as Pd/C. In these cases, a “microwave catalysis” can be possible. The increase in the (apparent) reaction rate requires the contribution of MW irradiation that modifies the catalyst surface, driving the reaction progress as occurring in other kinds of catalysis.