Oxygen Interactions and reactions

Oxygen Interactions and Reactions on Palladium(lOO) Coadsorption Studies with C2H4, H2O, and CH3OH... 

The mechanism of the reaction of tributyl [(trimethylsilyl)methylene]phosphorane with benzaldehyde and its para-substituted analogues has been examined. It has been found that the electronic nature of the para-substituents strongly influences the stereochemical and kinetic outcome of the Peterson oleflnation, whereas temperature substantially affects their Hammett correlation. This indicates that the Peterson oleflnation is a multistep reaction involving the formation of at least an oxyan-ion/betaine and a carbanion as intermediates. The E selectivity seems to result from the silicon-oxygen interaction and interactions of steric substituents in competing erythro-and tereo-betaines. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

The chiral catalyst 142 achieves selectivities through a double effect of intramolecular hydrogen binding interaction and attractive tt-tt donor-acceptor interactions in the transition state by a hydroxy aromatic group [88]. The exceptional results of some Diels-Alder reactions of cyclopentadiene with substituted acroleins catalyzed by (R)-142 are reported in Table 4.21. High enantio- and exo selectivity were always obtained. The coordination of a proton to the 2-hydroxyphenyl group with an oxygen of the adjacent B-0 bond in the nonhelical transition state should play an important role both in the exo-endo approach and in the si-re face differentiation of dienophile. 

The reactions that have been described above have indicated that sulphones interact poorly with electrophiles. However, in 1970, Whiting and coworkers announced the synthesis of aryloxysulphoxonium salts , by the reaction of sulphones with the potent electrophile produced in the thermolysis of aryldiazonium tetrafluoroborates or hexaflu-orophosphates. Fluorobenzenes are by-products of the reaction. In a subsequent paper , Whiting described the reactions of the aryloxysulphoxonium salts, 5, with oxygen and nitrogen nucleophiles. The fundamentals of these are outlined in equations (34) (oxygen nucleophile) and (35) (nitrogen nucleophile). 

Other authors also determined by FTIR that organic nitrocompounds are formed as primary products of the NO CH4-SCR reaction on ZSM-5-based catalysts [121-124], They preadsorbed nitromethane on the sample placed in the IR cell and followed by IR its transformation into other intermediates under 02 and NO versus time at different temperatures. For Cu- and Co-ZSM-5, it was shown that around 300°C adsorbed nitromethane is easily converted into isocyanates and then melamine via polymerization of the former species. Both species easily interact with molecular oxygen, while no reaction with NO is observed and the reactivity depends on the temperature and the nature of the transition metal cation. 

It is rather difficult to study function 2 separately, as the catalyst generally presents, simultaneously, functions 1 and/or function 3. Nevertheless, the mild interaction of HC with N02 can be approached through the direct N02/HC reaction, even in the absence of dioxygen. It has to be compared to the total oxidation of HC in the presence of oxygen, as it is a competitive reaction for the HC consumption. In contrast, it will be very interesting to compare the two catalytic pathways of elementary steps, and reaction intermediates, in both oxidation reactions (mild and total oxidation of reductant). 

Optically active sulfonium and selenonium salts are well known and the stereochemistry of the isomers has been studied.1 3 Optically active cyclic diaryl(alkoxy)-sulfonium salts 14, 15, and 16, stabilized by intramolecular sulfur-oxygen interaction, were synthesized in 2000 by reacting optically active spirosulfuranes with trimethyloxonium tetrafluoroborate.29 The absolute configurations were assigned on the basis of the reaction mechanism. The sulfonium salts were hydrolyzed in KHC03aq. to yield optically active sulfoxides in over 86% ee (Scheme 7). 

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