Power ultrasound in metal-assisted synthesis: From classical Barbier-like reactions to click chemistry

The search for more efficient and greener synthetic procedures to obtain highly functionalized chemical structures has always found in metal-assisted reactions a noteworthy strategy. All these reactions fall in the main domain of sonochemistry; in fact few techniques can compete with power ultrasound in so efficiently activating a metal surface, thus enhancing and accelerating its subsequent reaction with an organic substrate. Young researchers will certainly benefit from the rich literature and past experience of several pioneers who have, since the early eighties, laid the foundations of modern sonochemical synthetic protocols. Herein we provide a concise overview that describes how ultrasound acts in such a way as to make it a fundamental tool in improving the classical one-step coupling promoted by zero-valent metal species, usually referred to as Barbier-like reactions. From early hallmarks to recent accomplishments, especially the latest Cu-catalyzed alkyne-azide reaction (the so-called Click reaction), intended to be a universal ligation in chemistry and biology; we highlight the role and crucial effects of sonication on these processes.

Research highlights
► Key sonochemical transformations promoted by metals.
► Ultrasound allows the use of non-activated metals, crude reagents and aqueous media.
► Green and sustainable synthetic procedures.
► Metallic copper is the simplest solid catalyst for sonochemical click reactions.