Pd-catalyzed reactions promoted by ultrasound and/or microwave irradiation

Palladium-catalyzed reactions are a cornerstone of modern synthetic chemistry. No other transition metal, has lately displayed a comparable efficiency and versatility in catalysis. Its compounds have revolutionized organic and organometallic synthesis, being systematically employed to catalyze a series of important and elegant carbon-carbon and carbon-heteroatom bond-forming reactions, whose applications range from natural products and pharmaceuticals to polymers and advanced materials. Many of them require prolonged heating and may be plagued by the formation of side- or unwanted products. The current drive toward cleaner chemistry and chemical engineering has spurred a search for more selective and energy-saving protocols, prompting a reconsideration of some metal-catalyzed processes that were once regarded as ideal syntheses. Two non-conventional activation tools, microwaves and ultrasound, have been shown to substantially reduce reaction times, increase product yields and enhance product purity by reducing or even eliminating side reactions. The present article provides an overview of this topic, showing how these energy sources have been successfully applied to some representative Pd-catalyzed reactions. Microwaves and ultrasound can be used alone, as well as combined in either sequential or simultaneous fashion; these variations can lead to significantly different results. These technical aspects are now becoming economically relevant, as Pd-catalyzed reactions are on the verge of being promoted out of the laboratory to the production scale by means of safe and reliable flow systems.