Complementary and synergic effects of microwaves and ultrasound in metal-assisted synthesis
While synthetic protocols involving metal dispersions have long been a favourite domain in sonochemistry, the irradiation of metal particles with microwaves has been a forbidden practice because of the risk of arcing and explosion. Modern reactors and suitable procedures under inert atmosphere, make metal-assisted reactions under combined ultrasound-microwave irradiation feasible and safe. The interaction of acoustic and electromagnetic waves with a heterogeneous reaction mixture containing metals usually strongly accelerates the process. Both the hot spots generated by acoustic cavitation and dielectric heating activate the metal surface enhancing its reactivity even with relatively low metal content.
The specific advantages of microwaves (MW) and ultrasound (US) in organic synthesis have been widely described (1, 2). Their ability to enhance reaction rates, yields and selectivity may become additive when they are used in combination (3), either in sequential or simultaneous fashion as inferred from recent examples from the literature (4). Synthetic chemists are increasingly paying attention to combinations of enabling technologies with an eye to achieving the double goal of obtaining high efficiency and meeting the green criteria of energy savings and the absence of dangerous or harsh reagents. In fact, the European scenario is being profoundly changed by the directive “REACH” (5) that will dramatically influence the design of industrial synthetic protocols. Meanwhile IPPC (Integrated Pollution Prevention Control) is aimed at reducing the contribution of industry to non-sustainable development (6).
In recent years, several research groups have concentrated on developing a hybrid combination of MW and US irradiation. This has now become technically feasible and can lead to additive or synergic effects. The combination represents a valuable strategy in process inte ...