Sonochemical conditions

A moderate diastereoselectivity was observed in these reactions where a mixture of diastereomers could be generated.58 The reactivity of the halides followed the order of tertiary > secondary primary and iodide> bromide (chlorides did not react). The preferred solvent system was aqueous ethanol. The process was suggested to proceed by a free radical mechanism occurring on the metal surface under sonochemical conditions. Efforts to trap the intermediate [A] intramolecularly gave only a very low yield of the cyclization product (Scheme 10.4).59... 

Abstract The behaviour of many metal ions which are stable in more than one oxidation states in their aqueous solutions has been studied under sonochemical reaction conditions. Fe(II) is oxidized to Fe(III) and Fe(III) is reduced to Fe(II) with equal ease under sonochemical conditions. Besides, the oxidizing power of Cr2072 is found to be less than the MnO ions, therefore, in a system containing both species, Cr3+ is susceptible to oxidation to Cr61 and the MnO to reduction to Mn2+. 

Tab 5.22 -Elimination from bromoacetals in a monomode reactor (75 W) comparison with sonochemical conditions and classical heating. 

Nitrogen alkylation occurs easily under sonochemical conditions [128], Indole and carbazole undergo N-alkylation with increased rates and yields under sonication in the presence of polyethylene glycol methyl ether as PTC. No reaction occurs in the absence of the PTC (Eq. 3.28). 

Use of an organozinc reagent for amino acid synthesis under sonochemical conditions... 

Sonochemical homopolymerization of dichlorosilanes in the presence of sodium is successful at ambient temperatures in nonpolar aromatic solvents (toluene or xylenes) only for monomers with a-aryl substituents. Dialky 1-dichlorosilanes do not react with dispersed sodium under these conditions, but they can be copolymerized with phenylmethyldichlorosilane. Copolymers with a 30-45% content of dialkylsilanes were formed from equimolar mixtures of the corresponding comonomers. Copolymerization might indicate anionic intermediates. A chloroterminated chain end in the polymerization of phenylmethyldichlorosilane can participate in a two-electron-transfer process with sodium (or rather two subsequent steps separated by a low-energy barrier). The resulting silyl anion can react with both dichlorosilanes. The presence of a phenyl group in either a or P position in chloroterminated polysilane allows reductive coupling, in contrast to peralkyl species, which do not allow the reaction. Therefore, dialkyl monomers can copolymerize, but they cannot homopolymerize under sonochemical conditions. 

Reduction of esters. Primary alcohols are obtained in good yields by reduction with the NaBHa-polyethylene glycol-400 system under sonochemical conditions. 

Wittig reactions. Vinylic chalcogenides are formed under sonochemical conditions using K2CO3 as base. 

Sinislcrra, JV Fuenies, A Marinas, JM. Barium hydroxide as catalyst in organic reactions. 17. Interfacial solid-liquid Wiuig-Homcr reaction under sonochemical conditions. Journo of Oryiank Chemisny, 1987 52, 3875-3879. 

Aromatization. Rapid dehydrogenation of Hantzsch 1,4-dihydropyridines occurs on exposure to manganese dioxide under sonochemical conditions. 

An early account of the role of Pd nanoparticles in the Heck reaction was published by Srinivasan and coworkers in 2001 [283]. They carried out Heck reactions of dif ferent aryl iodides with acrylates and styrene at ambient temperature and observed a significant rate enhancement by combining ultrasonic irradiation with the use of the ionic liquid 1,3-dibutylimidazolium tetrafiuoroborate as the reaction medium. Under the sonochemical conditions the formation of Pd-biscarbene complexes was observed and these later transformed into Pd nanoparticles. 

Direct contact of the body with liquids or solids subjected to high-intensity ultrasound of the sort used to promote chemical reactions should be avoided. (In contrast, ultrasound used for medical diagnostic imaging is relatively benign.) Under sonochemical conditions, cavitation is created in liquids, and it can induce high-energy chemistry in liquids and tissues. Cell death from... 

Excellent results are obtained in the absence of an initiator, even at temperatures unusually low for this type of reaction. Without sonication, the conversion is poor. The reaction can be run with a probe generator, but it remains effective when a bath is used. Sonochemical conditions greatly enhance the stereoselectivity, probably because of the temperature factor. For example, hexyne gives the triphenylstannyl olefin in >93% cis selectivity. By photolysis, a 1 1 cis trans mixture is obtained. 

In the case of less congested compounds engaged in an intramolecular addition (Eq. 2), no significant differences exist between the spontaneous thermal and sonochemical processes carried out under various sonochemical conditions. ... 

The sonochemical effect is particularly high since, under simple reflux, no product is detected. With an aromatic bromide the reaction proceeds under sonication without PTC. Similarly, Corral et al prepared aryl thienyl ethers under PTC or sonochemical conditions. The advantages of the sonochemical reaction, a yield double that of the silent process, remain, however, modest. 

The sonochemical conditions minimize p-elimination.274 Perfluoroalkylation of arene tricarbonyl-chromium aldehydes occurs without decomposition of the transition metal group used to induce stereoselectivity in the addition.275 l,l/l-Trifluoro-2-bromo-2-propene adds to an aldehyde function in the presence of zinc and copper chloride (Eq. 62).276 The adduct was used for the s3mthesis of steroid precursors via a ring opening of the benzocyclobutene to an ortho quino-dimethane, followed by an intramolecular Diels-Alder reaction. 

The reaction was also tried in pure acetonitrile (10 mL) as well as 10% and 50% mixtures of acetonitrile in dichloromethane (10 mL). In these solvents cholesterol remains insoluble and the reactions become less efficient, either under silent or sonochemical conditions. 

Sonochemistry could provide a handle for experimentally modifying the concentration gradients at the vicinity of the surface or, equivalently, the thickness of the diffusion layer. It would seem that a series of previously published works could be profitably reinterpreted using this gradient formulation, ith expected consequences for a deeper imderstanding of the reaction events taking place at the solid-liquid interface, under normal and sonochemical conditions as well. 

Propargyloxycarbonyl (Poc) protected amines react with 1 at room temperature and the corresponding amines are obtained in quantitative yields. Deprotection of Poc in N-Poc-S-benzylcysteine ferf-butyl ester by 1 has resulted in a selective deprotection of Poc in the presence of methyl, allyl, and ferf-butyl esters (eq 20). Reaction of 1 with A(-Poc-S-benzylcysteine ferf-butyl ester is very facile under sonochemical conditions and provides the dipeptide [H2N-(Bzl)Cys-Cys(Bzl)-Ofiu] in 1 h at room temperature. Other protecting groups such as 5-benzyl and ferf-butyl groups are unaffected under these reaction conditions. Poc group is stable to acids and bases and deprotection of ferf-butyl and methyl esters is acconplished conveniently in the presence of W Poc protected amino acid derivatives. 

The second pathway is analogous to the reaction of an activated metal, similar to those generated using Riecke s methodology, with alkyl halides. However, Luche reports that pre-activation of the zinc gave poor yields of organozinc under sonochemical conditions. 

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