O(P) reactions with small alkenes adsorbed on Rh, Au, and ice

Using molecular beams, we studied the reaction of O(³P) with 1- and 2-butene on the surface of Rh(1 1 1) and ice, and propene on the surface of Au(1 1 1), in vacuum at cryogenic temperatures. Unlike in the gas phase, and similar to reactions in the condensed phase, only the oxygen addition products were observed. The surface serves as a sink for the excess energy of this highly exoergic process, stabilizing the adduct product channels.     

The effect of ultrasound on photochemical reactions.

Sonochemistry and photochemistry are based on very different physical phenomena, which a priori seems to exclude any possibility to observe more than a simple superposition of their respective effects. Literature data are examined, first the few studies devoted to sonochemical and photochemical reactions in cases where a direct comparison is possible. The second part is devoted to an analysis of experiments in which both irradiations are effected simultaneously. Cases exist from which it can be concluded, at least provisionally, that sonication is able to quench long-lived excited states, because of the intense streaming which enhances strongly the probability of collisional deactivations.     

Surface-catalyzed photochemical reactions of physisorbed molecules

The rate of laser photochemical reactions on surfaces can be enhanced over their gas-phase values by the interplay of a number of surface phenomena. These effects include simple increases in molecular density due to physisorption, spectral shifts in the adsorbed phase, and enhancement in the optical electric field due to surface contouring.     

Ultrasound effect on Suzuki reactions. 1. Synthesis of unsymmetrical biaryls

Ultrasound effect on the Pd(0) catalysed reaction of arylboronic acid with halobenzenes was investigated. The effect of catalyst, base as well as solvent was tested. Heterogenous reaction of iodoarenes with different arylboronic acids, catalysed by Pd/C and KF as the base in methanol:water mixture resulted in good yields of cross-coupling products. Reaction time of sonochemical reaction was 1 h, while 4 h of reflux was necessary to achieve comparable results. Bromobenzenes gave best results using aqueous solution of PdCl2 as the catalyst, potassium carbonate as the base in toluene:water two phase system using TEBA (benzyltriethylammonium chloride) as PT catalyst. Chlorobenzenes gave just feeble yields of cross-coupling products.     

Sonochemical and photochemical reactions of bromotrichloromethane in the presence and absence of 1-alkene

Comparison between sonolysis and photolysis of bromotrichloromethane in the presence and absence of 1-alkenes was performed to elucidate the characteristics of homogeneous sonochemistry. Results indicate that the photolysis of BrCCl3 generates radicals in a homogeneous and dispersed state, while sonolysis generates them in a heterogeneous and localized state. In the photochemical reaction the BrCCl3-adduct was obtained over a wide range of mole fractions of BrCCl3, whereas in the sonochemical reaction the yield of the BrCCl3-adduct showed a sharp maximum in a range of high mole fractions of BrCCl3. This may be attributed to the large variation in surface tension.     

Ultrasound and ionic liquid: an efficient combination to tune the mechanism of alkenes epoxidation

In this proof of concept study, the advantageous properties of both H₂O₂/NaHCO₃/imidazole/Mn(TPP)OAc oxidation system and MOPyrroNTf₂ ionic liquid have been combined under ultrasonic irradiation to give an exceptionally favorable environment for Mn(TPP)OAc catalyzed olefin oxidations. The results reveal the crucial role played by the ultrasonic irradiations that influence drastically the oxidation process. In MOPyrroNTf₂ and under ultrasonic irradiation, the mechanism probably involves an oxo-manganyl intermediate at the expense of the classical bicarbonate-activated peroxide route.     

EPR of C60 thermal/photochemical reactions with polystyrene and polymethyl methacrylate

Thermal and photochemical free radical reaction products of C₆₀ with polymethyl methacrylate (PMMA) and polystyrene (PS) in orthodichlorobenzene solution were detected by EPR (electron paramagnetic resonance). Thermal radicals (<100°C) of C₆₀/PMMA and C₆₀/PS samples gave single line first-derivative EPR spectra withg=2.0029. Ultraviolet photolysis of a C₆₀/PMMA solid phase sample gave two radical species; whereas, photolysis of a C_60/PS solid phase sample gave only one free radical. EPR signals were also recorded for UV and thermal C₆₀ reaction with free radical initiator, azobis(isobutyronitrile).     

The allylation reactions of aromatic aldehydes and ketones with tin dichloride in water

The allylation reactions of aromatic aldehydes and ketones were carried out in 31–86% yield using SnCl₂–H₂O system under ultrasound irradiation at r.t. for 5 h. The reactions in the same system gave homoallyl alcohols in 21–84% yield with stirring at r.t. for 24 h. Compared with traditional stirring methods, ultrasonic irradiation is more convenient and efficient.     

Ultrasound in synthesis: natural products and supersonic reactions?

Sonochemical syntheses of a number of useful reagents such as samarium diiodide, sodium phenylselenide, tertiary alkoxides and the reducing agent [(PPh₃P)CuH]₆ are discussed. The rates of these sonochemical reactions can be increased by the addition of electron-transfer agents, such as benzophenone ketyls. These results would not be expected on the basis of mechanical effects alone and lend support to proposals that ultrasound acts preferentially on single electron-transfer processes. Ultrasound also allows the rapid and efficient generation of tetracarbonyliron, which can be trapped by simple organic compounds, such as conjugated dienes, vinyl epoxides and allylic alcohols. In the latter cases, this allows the facile preparation of π-allyltricarbonyliron lactone and lactam complexes that can be converted to give a range of biologically active β-, γ- and δ-lactones, including a number of alkaloids and antibiotics. In addition, the δ-lactone products have been incorporated into several complex multi-step syntheses of macrocyclic natural products. The synthesis of key fragments of the potent antiparasitic agent avermectin B1a demonstrates that the sonochemical reaction conditions are fully compatible with the synthesis of highly functionalized molecules.     

Mechanistic aspects of reactions of triphenylphosphine with electron‐deficient alkenes in acetic acid solution

Kinetic data provide evidence that proton transfer to the carbanion centre of a phosphonium zwitterion is the rate‐determining step in quarternization reactions of triphenylphosphine with electron‐deficient alkenes in acetic acid solution. This conclusion is based on the third‐order rate equation, the reactivity of the alkenes, and solvent isotope effects in deuteroacetic acid. Copyright © 2013 John Wiley & Sons, Ltd.     

A quantum-chemical study of intermediates of the 1O2 photogeneration sensitized by buckminsterfullerene and accompanying photochemical reactions

The intermediates of hypothetical photochemical reactions that accompany the quenching of the ³C ₆₀ ^* triplet state by triplet oxygen are studied by the (U)PBE0 quantum-chemical method. The diradical C₆₀-O-O formed from ³O₂ and photoexcited buckminsterfullerene ³C ₆₀ ^* is characterized by a negative binding energy ?1.11 eV (with respect to C₆₀ and ³O₂), the singlet-triplet splitting ΔE _ST of 0.07 eV, and the dipole moment of 3.2 D at the equilibrium internuclear separations 1.522 Å (CO) and 1.294 Å (OO). Its decay produces ¹O₂. The formation of a dioxetane circle lowers the energy by 0.8 eV. The ground-state energy of diketone C₅₈(C=O)₂ is 2.0 eV lower than the energy of C₆₀-O-O. The metastable centrosymmetric diradical C₆₀-C₆₀, formed upon ineffective light absorption by clusters (C₆₀)_N, has a single interpolyhedral C-C bond (1.657 Å). Its triplet state T ₁ lies 0.16 eV higher than the S ₁ singlet. The S ₁ → S ₀ relaxation leads to the formation of a stable C₆₀-C₆₀ dimer with a shorter (1.584 Å) bis-single exothermic (+0.24 eV) bond of polyhedra. The photoexcited C₆₀-C₆₀ dimer is able to form isomeric metastable diradicals C₆₀-C₆₀-O-O.     

Ultrasound effect on the aromatic nucleophilic substitution reactions on some haloarenes

The sonochemical nucleophilic aromatic substitutions on haloarenes with different amines have been studied. A beneficial ultrasound effect was observed and high yields of the products were obtained after 15-30 min sonication. The reaction course of the nucleophilic aromatic substitution was found to be strongly dependent on nucleophilicity, bulkiness, and boiling point of amines as well as on the electron-withdrawing property of the substituents on the haloarenes.