Ultrasound conditions

Base Ultrasound conditions Microwave conditions Classical heating... 

Fig. 5.35. Emulsion polymerisation in the presence and absence of inhibitor and in the presence and absence of ultrasound (Conditions 8% styrene (w/v), 0.8% surfactant (w/v), potassium persulphate, T= 70°C).

There are three distinct sets of ultrasound conditions based on frequency range and applications [5] ... 

Acetals can be depTotected ° to the corresponding carbonyl compounds using a catalytic amount of carbon tetrabromide (CBr4) in the acetonitrile-water mixture under thermal or ultrasound conditions. 

Drug Molecular weight (Da) Experimental system Ultrasound conditions Experimental conclusions Ref. 

The therapeutic ultrasound conditions correspond to a frequency in the range of 1-3 MHz and an intensity in the range of 0-2 W/cm. Therapeutic ultrasound has been attempted to enhance transdermal transport of more than... 

Diselanium Fulvenes from Phenylacetylene with Selenium and Base under PTC-Ultrasound Conditions, Synth. Commun., 26, 1617 19%). 

The application of ultrasound13 to the Strecker reaction was shown to improve the product outcome of this reaction. Treatment of aminoketone 14 using the classical Strecker reaction conditions afforded only the cyanohydrin. The desired product 15 could be produced after 12-13 days of stirring in acetic acid. Ultrasound conditions not only improved the yield of this reaction, but accelerated the reaction times by about 12-fold. 

The rather classic catalyst palladium on activated carbon has been applied by Sun and Sowa et al. [183] and Heidenreich et al. [158] without additional ligands. This simple system was able to convert (mainly activated) aryl chlorides in mixtures of water and an organic solvent (DMA and NMP, respectively). Lysen et al. were able to convert aryl chlorides in pure water and without addition of any ligand [184,185]. Suzuki reactions using Pd/C in an aqueous medium were also reported by Arcadi et al. (in the presence of surfactants) [186] and the group of Leadbeater, who applied microwave techniques [187]. Microwave (as weU as ultrasound) conditions were also employed by Cravotto and Palmisano et al. [188] The substrate scope of Suzuki-type reactions in the presence of Pd/C was extended to halopyri-dines and haloquinoUnes by Tagata and Nishida [189]. 

Figure 8.1.21 depicts the course of the butyl magnesium chloride concentration in the Grignard reaction with different ultrasound conditions. 

First of all, two examples of the change in selectivity of catalytic reactions performed in "silent" or ultrasound conditions should be recalled (for a discussion, see Ch. 4, p. 145). The classical paper by Ando 2 refers to the reaction between benzyl bromide and potassium cyanide on alumina in toluene. In the sonochemical reaction, the main product is benzyl cyanide, while in a silent condition it is the Friedel-Craft adduct. When the Strecker synthesis of a-amino nitriles from an aldehyde, potassium cyanide, and an amine in acetonitrile is performed with alumina and ultrasound, the main product (selectivity = 90%) is the a-amino nitrile. In the same conditions, except for the absence of ultrasoimd, the selectivity for the same product is only 64%. If ultrasound is used without alumina, the selectivity is 23%, and 6% if only stirring is used. 

Certain other reagents like samarium iodide in aqueous THF ", sodium dithionite in aqueous DMF , sodium sulfide in presence of polyethylene glycol " and metallic zinc along with nickel chloride. " Using the latter reagent (Zn/NiClj), a,P-unsaturated carbonyl compounds can be very readily reduced under ultrasound conditions (Scheme 105). 

Figure 1.38 (a) The PEPPSI-type-triazol-5-ylidene Pd catalyst reported by Huang et al. [121], (b) by Azua et al. under ultrasound conditions in glycerol [122], and (c) the 4-hydroxyacetophenone oxime-derived pal-ladacycle reported by Alacid and Najera [123]. 

Also obtained by an ultrasound assisted iodine catalyzed Friedel-Crafts acylation of phenol with pivaloyl chloride for 10 min at r.t. (84%) [8301]. The same pivaloylation, carried out at silent (non-ultrasound) conditions, gave only, after 6 h at r.t., a yield of 62%. 

Ruiz et al. [101] described a facile, efficient, and eco-friendly protocol for the synthesis of a wide variety of 6-chloro-5-formyl-l,4-dihydropyridines (127) under ultrasound condition compared with the conventional thermal method (Scheme 32). It is important to mention that 1,4-dihydropyridines are very attractive targets due to tiieir wide range of biological activities. These compounds are routinely used in the treatment of a variety of cardiovascular disorders such as hypertension, cardiac arrhythmias, or angina. 

Coehlo and coworkers synthesized the carboxylic acid precursor of (+)-efaroxan (118), an 0.2 adrenoreceptor antagonist, which is a treatment for neurodegenerative disease, migraine, and type II diabetes. The synthesis opened with an MBH reaction which afforded key moieties. Thus, 2-fluorobenzaldehyde 120 was converted to the MBH adduct 121 under ultrasound conditions in 90% yield. After acetylation of the hydroxyl group, the acetylated MBH adduct was treated with dimethyl cuprate to furnish the trisubstituted olefin 122 via an Sn2 reaction. The methyl ester of 122 was reduced to the allylic alcohol which then underwent a Sharpless asymmetric oxidation to furnish the epoxide 123. In several steps, the epoxy alcohol 123 was then converted to the carboxylic acid 124. Upon treatment with sodium hydride the a-hydroxy carboxylic acid, 124 cyclized to deliver the hydrobenzofuran 119 in 65% yield and hence the precursor of (+)-efaroxan (118). 

Complexes from Alkenediols. 2-Butene-1,4-diols react with Fc2(CO)9 in the presence of Lewis acids under ultrasound conditions to form Ti -allyliron complexes." ... 

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