This is an author version of the contribution published on:On the mechanochemical activation by ultrasound Cravotto G.; Calcio Gaudino E. ; Cintas P Chem. Soc. Rev., 2013,42, 7521-7534 The definitive version is available at:On the mechanochemical activation by ultrasound Cravotto G.; Calcio Gaudino E. ; Cintas P. AbstractChemists have discovered, and recently actively exploited, the fact that subjecting certain molecules to ultrasound waves can bring about transformations that give insight into the correlation between classical tribological processes and the mechanical action caused by collapsing microbubbles when sonic waves propagate through a liquid medium. Chemical transformations induced by ultrasound take place in solution via mechanisms that are markedly different from those associated with molecular activation in the solid state. Both fields, however, share some striking similarities and numerous sonochemical reactions can be rationalized in purely mechanical terms. This tutorial review examines the tribochemical interpretation of sonochemical reactivity and how the multifaceted action of cavitational phenomena determines molecular evolution. A series of case studies involving solids, crystals, and polymers illustrate the mechanical properties of sound waves.

In recent years, chemistry in flowing systems has become more prominent as a method of carrying out chemical transformations, ranging in scale from microchemistry up to kilogram-scale processes. Compared to classic batch ultrasound reactors, flow reactors stand out for their greater efficiency and flexibility as well as lower energy consumption. This paper presents a new ultrasonic flow reactor developed in our laboratory, a pilot system well suited for reaction scale up. This was applied to the transesterification of soybean oil with methanol for biodiesel production. This reaction is mass-transfer-limited initially because the two reactants are immiscible with each other, then because the glycerol phase separates together with most of the catalyst (Na or K methoxide). In our reactor a mixture of oil (1.6 L), methanol and sodium methoxide 30% in methanol (wt/wt ratio 80:19.5:0.5, respectively) was fully transesterified at about 45 degrees C in 1h (21.5 kHz, 600 W, flow rate 55 mL/min). The same result could be achieved together with a considerable reduction in energy consumption, by a two-step procedure: first a conventional heating under mechanical stirring (30 min at 45 degrees C), followed by ultrasound irradiation at the same temperature (35 min, 600 W, flow rate 55 mL/min). Our studies confirmed that high-throughput ultrasound applications definitively require flow reactors.

Bio-waste valorisation has been one of the hottest research topics worldwide over the last decade.

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The massive increase in glycerol production from the transesterification of vegetable oils has stimulated a large effort to find novel uses for this compound. Hence, the use of glycerol as a solvent for organic synthesis has drawn particular interest. Drawbacks of this green and renewable solvent are a low solubility of highly hydrophobic molecules and a high viscosity, which often requires the use of a fluidifying co-solvent. These limitations can be easily overcome by performing reactions under high-intensity ultrasound and microwaves in a standalone or combined manner. These non-conventional techniques facilitate and widen the use of glycerol as a solvent in organic synthesis. Glycerol allows excellent acoustic cavitation even at high temperatures (70-100 °C), which is otherwise negligible in water.Herein, we describe three different types of applications: 1) the catalytic transfer hydrogenation of benzaldehyde to benzyl alcohol in which glycerol plays the dual role of the solvent and hydrogen donor ; 2) the palladium-catalyzed Suzuki cross-coupling; and (3) the Barbier reaction. In all cases glycerol proved to be a greener, less expensive, and safer alter-native to the classic volatile organic solvents.

Glycerol has the potential to be both an excellent renewable solvent in modern chemical processes and a versatile building block in biorefineries. Both of these potential applications may be made easier and more convenient by microwave and/or ultrasound ir radiation. As glycerol is a nontoxic, biodegradable compound, it will provide important environmental benefits to new platform products. Furthermore, significant markets in polymers, polyethers, fuel additives, nanoparticles and other valuable compounds may well be opened up by cutting down the high purification cost of glycerol. The aim of this review is to highlight the best literature examples of glycerol being used, either as solvent or as a reagent, to give interesting results under microwave or ultrasound irradiation.

The tosyl group (Ts) on 6 I-O-(p-toluenesulfonyl)--cyclodextrin has been substituted with halogenides, nitrogen and sulfur nucleophiles under mechanochemical conditions and the reaction has been investigated in this work. The preparation of mono-substituted cyclodextrin (CD) derivatives, such as azido-, thioureido-, iodo-and thioethers, is shown to be more advantageous in a planetary ball mill (BM) than classic solution methods. All BM reactions

The development of sustainable protocols for the reductive amination is a highly desirable pursuit in the domain of green synthesis. Magnetic nanocatalysts have found a unique niche in chemical synthesis in recent years as the recovery of expensive and/or toxic catalysts after their use are some of the salient features of these greener processes. Herein, we report the application of a recyclable nickel silica eggshell iron-based magnetic nanoparticles (Fe3O4@SiO2-Ni) for the expeditious microwave-assisted reductive amination of aryl aldehydes and ketones in aqueous ammonia; several desired primary amines were produced in good-to-excellent conversions. Extensive characterization of both, fresh and recycled Fe3O4@SiO2-Ni catalysts, showed that the Ni nanoparticles are highly dispersed on the silica shell and that the metal active phase is highly stable as the core–shell morphology is maintained after reaction, indeed the catalyst is recyclable up to six runs without deactivating. A synergic effect between the Ni nanoparticles and the silica support has been hypothesized wherein the Fe3O4@SiO2-Ni system worked as a bifunctional catalyst; support facilitates the activation of the substrate, and the metal nanoparticles promote the subsequent imine hydrogenation.