Power ultrasound in organic synthesis: moving cavitational chemistry from academia to innovative and large-scale applications

Ultrasound promoted protection of alcohols: an efficient solvent-free pathway for the preparation of silyl ethers in the presence of no additive

Various types of alcohols and phenols were rapidly protected by hexamethyldisilazane in good to excellent yields at room temperature using high-intensity ultrasound irradiation in the presence of no solvent or any additive. Good to excellent chemoselectivity was observed for competitive protection of hydroxyl groups in favor of sterically less hindered alcohols. Exclusive protection of phenols in the presence of aromatic amines was also demonstrated successfully.

The combined use of microwaves and ultrasound: improved tools in process chemistry and organic synthesis

Microwave heating and ultrasonic waves are among the most simple, inexpensive, and valuable tools in applied chemistry. Besides saving energy, these green techniques promote faster and more selective transformations. Could they be combined to enhance their effects still further? As they are of a basically different nature (quantum and non-quantum fields), each must be fine-tuned by its specific parameters; a combined device will often be subject to additional hazard limitations. However, recent developments evidence that such a combination is certainly possible and safe, ranging from simple modifications to flow systems that are well suited for automation and scaling-up. By using selected examples, this concept article gives an overview of apparatus currently available for simultaneous or tandem irradiation and explains how it can be utilized in organic synthesis and analysis.

Impact of ultrasound on hydrophobic interactions in solutions: ultrasonic retardation of benzoin condensation

Kinetics of the benzoin condensation of benzaldehyde in presence of KCN as the catalyst in water and in ethanol-water binary solutions were investigated without sonication and under ultrasound at 22 kHz. A statistically significant 20% decrease of the rate was observed in water. The retardation effect of ultrasound gradually decreases up to 45 wt% ethanol content. We report an evidence of ultrasonic retardation of reactions and thereby a direct evidence for sonochemical processes in the bulk solution. Ultrasound can disturb solvation of the species in the solution. If breaking down the stabilization of the encounter complexes between the reagents, sonication hinders the reaction while perturbation of the solvent-stabilization of the reagents accelerates the reaction.

Ultrasonic Evidence of Hydrophobic Interactions. Effect of Ultrasound on Benzoin Condensation and Some Other Reactions in Aqueous Ethanol

The kinetics of KCN-catalyzed benzoin condensation of benzaldehyde in water and ethanol-water binary mixtures was investigated both under ultrasound at 22 kHz and without sonication. Thermodynamic activation parameters were calculated from kinetic data obtained at 35, 50, and 65 degrees C. Evidence that ultrasound can retard reactions is reported and hence a direct proof that sonochemical processes occur in the bulk solution. Former results and literature data for ester hydrolyses and tert-butyl chloride solvolysis are involved in the discussion. A quantitative relationship between sonication effects and the hydrophobicity of reagents is presented for the first time. Ultrasound affects hydrophobic interactions with the solvent, which are not manifested in conventional kinetics. When it suppresses the stabilization of the encounter complexes between reagents, sonication hinders the reaction but accelerates it when it perturbs the hydrophobic stabilization of the ground state of a reagent.

The combination of oxalic acid with power ultrasound fully degrades chrysotile asbestos fibres

The simultaneous action of power ultrasound and oxalic acid, as a chelating agent, rapidly converts chrysotile asbestos into water soluble material and a non-asbestos debris, not classifiable as hazardous under worldwide safety regulations.

A Speedy One-Pot Synthesis of Second-Generation Ionic Liquids Under Ultrasound and/or Microwave Irradiation

The present work describes an efficient one-pot synthesis of second-generation ionic liquids (ILs), combining in one step the Menshutkin reaction and anion metathesis. Working in a closed vessel under microwaves, or better still under simultaneous ultrasound and/or microwave irradiation, in a few minutes a series of ILs with 1-methylimidazole or pyridine cores were obtained in high yields (80–97% isolated). Under conventional heating, ILs could not be prepared in one pot in acceptable times and yields, whereas our protocol, carried out with commercially available equipment, was highly effective and reproducible. Moreover, 1H NMR analysis and ion-exchange chromatography showed that the present solventless procedure afforded satisfyingly pure ILs.

Effect of Ultrasound on the Base-Catalyzed Hydrolysis of 4-Nitrophenyl Acetate in Aqueous Ethanol

The kinetics of base-catalyzed hydrolysis of 4-nitrophenyl acetate was studied in water and water-ethanol mixtures under ultrasound (21.1 kHz) using a probe equipped with a quartz horn and, by comparison, without sonication. The ethanol concentration was varied from 0 to 50 wt % and pH from 7.5 to 9. The increase of reaction rate was inversely proportional to the strength of substrate interactions with the solvent structure. It was found that the effect of ultrasound could be related to the perturbation of solute-solvent interactions. This conclusion seems to apply to both acid-catalyzed and base-catalyzed hydrolysis reactions. Parallel experiments with a titanium horn revealed the catalytic action of undetermined chemical species formed from dispersed titanium metal.

Improved syntheses of bis(beta-cyclodextrin) derivatives, new carriers for gadolinium complexes

In the last decade a number of reports have been published on the synthesis and characterization of bridged cyclodextrin dimers (bis-CDs) connected with linkers of different lengths and structures. These dimers, having two hydrophobic cavities in close proximity, display much higher binding affinities and molecular selectivities than parent CDs, forming stable supramolecular adducts. We describe new synthetic protocols for the preparation of bis(beta-CDs) bearing 2-2', 3-3' and 6-6' bridges. Some of the critical steps were carried out either under high-intensity ultrasound (US) or microwave (MW) irradiation. Bis(beta-CDs) containing 6-6' ureido- and thioureido-bridges were prepared in high yields by a MW-promoted aza-Wittig reaction using polymer-bound triphenylphosphine, while those containing 2,2' and 3,3' bridges were prepared from mono-alkenyl beta-CDs by the cross-metathesis reaction (homodimerization) in the presence of 2(nd)-generation Grubbs catalyst under sonochemical conditions. By these improved protocols CD dimers could be obtained in gram amounts to prepare stable adducts of bis-CDs with contrast agents (CAs) containing gadolinium(iii) chelates. In the case of Gd(iii) chleate "G-1" the inclusion complexes were found to be 2 to 3 orders of magnitude more stable than that formed by beta-CD (K(ass) = 4.3 x 10(4) M(-1)vs 8.0 x 10(2) M(-1)). Relaxivity increased as well by factors of 3 and 4, viz. from 9.1 mM(-1) s(-1) (beta-CD) to 27.7 and 35 mM(-1) s(-1).