Ultrasound-promoted Organic Synthesis - A Recent Update

Abstract

: Ultrasonication, nowadays, is well-regarded as an effective green tool in implementing a plethora of organic transformations. The last decade has seen quite useful applications of ultrasound irradiation in synthetic organic chemistry. Ultrasound has already come out as a unique technique in green chemistry practice, for its inherent properties of minimizing wastes and reducing energy and time, thereby increasing the product yields with higher purities under milder reaction conditions. The present review summarizes ultrasound-promoted useful organic transformations involving both carbon-carbon and carbon-heteroatom (N, O, S) bond-forming reactions in the absence or presence of varying catalytic systems, reported during the period 2016-2020.

Solid Lipid Nanoparticles (SLN) and Nanostructured Lipid Carriers (NLC) Prepared by Microwave and Ultrasound-Assisted Synthesis: Promising Green Strategies for the Nanoworld

“…In relation to the novel techniques for the elaboration of SLN, in addition to the MAS, the UAS stands out. This technique is based on the emission of acoustic waves with frequencies at different intensities that are characterized by exceeding the human hearing range (>16 kHz) [ 81 ]. These intensities can be classified as low and high, being inversely proportional to the frequency.…”

“…On the other hand, high-intensity ultrasound has frequencies between 16 to 100 kHz and powers between 10 to 1000 W/cm 2 . It is commonly used to alter the properties of materials, favoring their physical transformation, such as emulsion manufacturing, depolymerization, deflocculation, and particle size reduction [ 81 , 82 , 83 ].…”

“…This process causes the formation of bubbles that oscillate at a speed of 20,000 times per second and grow by tens of μm. These bubbles, when found at low ultrasound intensity, only oscillate in the same way, so they are not prone to implosion [ 81 , 82 ]. On the other hand, at high ultrasound intensities the oscillation is no longer stable, it changes in different directions and the bubbles begin to implode, rapidly releasing their internal energy.…”

“…Sonication has been found to be capable of breaking down particle aggregates, reducing sizes to nanometric scales, either alone or in conjunction with other nanoparticle formulation techniques. The characteristic random Brownian motion of colloidal suspensions tends to favor particle aggregation due to the intervention of attractive surface forces [ 81 , 82 , 84 ]. Changes in pressure, temperature, or volume can favor the rearrangement of particles in such a way that they self-assemble avoiding aggregation.…”

“…The most used equipment such as ultrasonic horns and ultrasonic cleaning baths operate at frequencies between 20 and 40 kHz respectively, however, the latter do not provide adjustable powers and complexity in terms of bath temperature control, which does not favor the generation of nanoparticles. Therefore, the probes in common use that do allow the selection and control of these parameters have been found to be the ones indicated for the formulation of nanocarriers [ 81 , 82 , 83 , 84 , 85 ].…”