Alkenes oxidation, catalytic selectivity

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

The overall reaction catalyzed by epoxide hydrolases is the addition of a H20 molecule to an epoxide. Alkene oxides, thus, yield diols (Fig. 10.5), whereas arene oxides yield dihydrodiols (cf. Fig. 10.8). In earlier studies, it had been postulated that epoxide hydrolases act by enhancing the nucleo-philicity of a H20 molecule and directing it to attack an epoxide, as pictured in Fig. 10.5, a [59] [60], Further evidence such as the lack of incorporation of 180 from H2180 into the substrate, the isolation of an ester intermediate, and the effects of group-selective reagents and carefully designed inhibitors led to a more-elaborate model [59][61 - 67]. As pictured in Fig. 10.5,b, nucleophilic attack of the substrate is mediated by a carboxylate group in the catalytic site to form an ester intermediate. In a second step, an activated H20... 

Just as aromatic rings are generally inert to oxidation, they re also inert to catalytic hydrogenation under conditions that reduce typical alkene double bonds. As a result, it s possible to reduce an alkene double bond selectively in the presence of an aromatic ring. I or example, 4-phenyl-3-buten-2-one is reduced to 4-phenyl-2-butanone at room temperature and atmospheric pressure using a palladium cataly st. Neither the benzene ring nor the ketone carbonyl group is affected. 

Aqueous sodium hypochlorite is another low-priced oxidant. Very efficient oxidative systems were developed which contain a meso-tetraarylporphyrinato-Mn(III) complex salt as the metal catalyst and a QX as the carrier of hypochlorite from the water phase to the organic environment. These reactions are of interest also as cytochrome P-450 models. Early experiments were concerned with epoxidations of alkenes, oxidations of benzyl alcohol and benzyl ether to benzaldehyde, and chlorination of cyclohexane at room temperature or 0°C. A certain difficulty arose from the fact that the porphyrins were not really stable under the reaction conditions. Several research groups published extensively on optimization, factors governing catalytic efficiency, and stability of the catalysts. Most importantly, axial ligands on the Mn porphyrin (e.g., substituted imidazoles, 4-substituted pyridines and their N-oxides), 2 increases rates and selectivities. This can be demonstrated most impressively with pyridine ligands directly tethered to the porphyrin [72]. Secondly, 2,4- and 2,4,6-trihalo- or 3,5-di-tert-butyl-substituted tetraarylporphyrins are more... 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

MPR)TPP)C0 epoxidized a variety of alkenes with Cl2pyNO as oxidant, the chloro system giving higher yields (56-98%) with Ru (8.6 wt.%.) oxidant substrate = 1 14(X) KXX), in benzene, atr. t. for 24 h. Cw-stilbene and norbomene yield exclusively cM-epoxides. In contrast, with the M-41(m) system, the Ru((4-Cl)i 4-MPR)TPP)CO polymer oxidized frani-stilbene and rra j-P-methylstyrene to the corresponding trans epoxides in 90 and 86 % yields, respectively. Several alkenes were oxidized catalytically for the first time with high selectivity 3,4-dihydronaphthalene yielded 62% epoxide. 

Ga203 activated above about 800 K behaves like AI2O3 for olefin isomerization a ff-allyl intermediate is formed on basic sites. However, when it is activated at 573 K, it shows a broad IR band at 2940 cm assigned to OH stretching of 0(-GaO(OH) and a band at 3650 cm due to surface OH, and exhibits peculiar catalytic activity for isotopic exchange between D2 and hydrocarbon, which cannot be explained by mechanisms such as alkyl reversal and T-allyl intermediate mechanisms. For example, direct cis-trans isomerization of n-alkenes was totally selective below 433 K. Novel mechanisms involving o-bonded alkyls and vinyls adsorbed via bonding to oxide ions on the surface have been proposed. 

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