Catalytic with hydrogen peroxide under phase

Fig. 1.26 Catalytic oxidations with hydrogen peroxide under phase transfer conditions.

A biphasic system consisting of the ionic liquid [BMIM]PF6 and water was used for the epoxidation reactions of a, 3-unsaturated carbonyl compounds with hydrogen peroxide as an oxidant at room temperature 202). This biphasic catalytic system compared favorably with the traditional phase transfer catalysts. For example, under similar conditions (15°C and a substrate/NaOH ratio of five), the [BMIM]PF6/H20 biphasic system showed a mesityl oxide conversion of 100% with 98% selectivity to oc, 3-epoxyketone, whereas the phase-transfer catalyst with tet-rabutylammonium bromide in a CH2CI2/H2O biphasic system gave a conversion of only 5% with 85% selectivity. 

Liquid phase catalytic oxidation of ethylbenzene with hydrogen peroxide over TS-1 molecular sieves is most appropriate for the production of 1-phenylethanol with high selectivity (up to 93 % of all the oxidation products in methanol) under the reaction conditions studied here. An additional increase of the 1-phenylethanol selectivity could be achieved with smaller amounts of the catalyst. The highest conversion to acetophenone is found over TS-2 zeolites but further oxidation easily takes place in this case. 

Asymmetric epoxidation of terminal alkenes with hydrogen peroxide was optimized with electron-poor chiral Pt(II) complexes bearing a pentafluorophenyl residue, as described in Section 23.3.1.6. The same catal3rtic system was made more sustainable by the employment of water as the solvent under micellar conditions. Surfactant optimization revealed the preferential use of neutral species like Triton-XIOO to solubihze both the catalyst and substrates. In several cases an increase of the asymmetric induction was observed (Scheme 23.43). The use of an aqueous phase and the strong affinity of the catalyst for the micelle allowed the recycling of the catalytic system by means of phase separation and extraction of the reaction products using an apolar solvent (hexane). The aqueous phase containing the catalyst was reused for up to three cycles with no loss of activity or selectivity. 

Formamidinesulfmic acid (obtained by the oxidation of thiourea with hydrogen peroxide) has been used to reduce disulfides to sulfides (see Eqs. 12.6 and 13.22) and N-tosylsulfilimines to sulfides (see Eqs. 12.7 and 13.23), under phase transfer catalytic conditions in the presence of hexadecyltributylphosphonium bromide [15]. Diphenyl, dibenzyl and dibutyldisulfides were reduced by this method to the corresponding sulfides in 72%, 62% and 90% yields respectively. Examples of the reduction of N-tosylsulfilimines are recorded in Table 12.4. 

Hydrogen peroxide in the presence of catalytic amounts of methyltrioxorhenium(VII), ReMeOs, is a convenient and efficient method for the a-hydroxylation of ketones. Particularly interesting is the HiOi/cetylpyridinium peroxotungstophosphate system which, under phase transfer conditions, provides a facile method for preparing aldehydes with one carbon atom less than the parent precursors. The ratio of the products changes with the experimental conditions. 

Calvert (2 ) has pointed out that gas-phase reactions of SO2 with ozone (O3), hydroxyl radical (OH ), and hydroperoxyl radical (HOp ) are too slow to account for the aforementioned rates of sulfate production. Consequently, the catalytic autoxidation of SO2 in deliquescent haze aerosol and hydrometeors has been proposed as a viable non-photolytic pathway for the rapid formation of sulfuric acid in humid atmospheres (30-35). In addition, hydrogen peroxide and ozone have been given serious consideration as important aqueous-phase oxidants of dissolved SO2 as discussed by Martin (35). Oxidation by H2O2 seems to be most favorable under low pH conditions (pH < 4) because of a rapid rate of reaction anc[ a negative pH-dependence that favors the facile conversion of HSO3 to sulfate. 

Method (a) uses nitric acid for the oxidation of the mixture of cyclohexanol/cyclohexanon available by the hydrogenation of phenol process (b) is based on hydroxycarbonylation of 1,3-butadien and process (c) on a catalytic green chemistry reaction with water as the only side product. In this process, cyclohexene is oxidized hy hydrogen peroxide in the presence of tungsten-hased catalyst under phase-transfer catalysis (PTC). 

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