Percarbonates, epoxidation with

The formation of peracids as the effective oxidizing species has often been proposed for oxidations with sodium percarbonate in the presence of organic acids or acid anhydrides30-32. It was observed that at room temperature and in dichloromethane as solvent, the addition of acetic anhydride induced the epoxidation by sodium perborate of mono-, di- and trisubstituted alkenes, including a,/i-unsaturated ketones in a slightly exothermic reaction33 (equation 6). 

Chiral pyrrolidine derivatives, proline, and amino acid-derived imidazolidinones mediate the asymmetric epoxidation of ,/i-unsalurated aldehydes. Protected a,a-diphenyl-2-prolinol catalyses the asymmetric formation of 2-epoxyaldehydes, with hydrogen peroxide or sodium percarbonate as the oxygen sources, with 81-95% conversion with up to 96 4 dr and 98% ee.204... 

The epoxidation of alkenes by means of either sodium perborate or sodium percarbonate and acetic anhydride can be enhanced by sonication [46], When sodium perborate was used with sonication, the reaction time could be reduced significantly, but the yield was not significantly improved. With sodium percarbonate both a higher yield and a shorter reaction time were obtained. The mechanistic aspects of these reactions were discussed. 

Several oxidants were tested in an epoxidation reaction in the presence of iminium salt catalysts to determine which offers the best profile in the absence of water [42]. These reactions were carried out at 0 °C with 1-phenylcyclohexene as substrate and (17) and /or (24) as catalysts (5-20 mol%), in dichloromethane as solvent. Most of the systems examined showed either high levels of background epoxidation (alkaline hydrogen peroxide, peracids, persulphates) or very low rates of reaction, even in the presence of 20 mol% of the catalysts (perselenates, percarbonates, perborates and iodosobenzene diacetate). Tetra-N-butylammo-nium Oxone, reported by Trost [43], was also unsuccessful as oxidant. 

Several methods for the epoxidation of a,(3-unsaturated carbonyl compounds have been reported. The use of amino acid derivatives or peptides as chiral ligands for epoxidation continues to be an active area of investigation. The use of silica bound poly-L-leucine, 21, with sodium percarbonate appears to be an excellent route to enantiomerically pure keto... 

Oxidations. Alcohols are oxidized in dichloroethane with catalytic amounts of a dichromate salt using sodium percarbonate as a recycling agent and PTC. - Epoxidations of enones by sodium perborate or NaOCP under PTC conditions give high yields. However, with NaOCl and hexaethylguanidinium chloride, cyclohexenones give 6,6-dichloro-2,3-epoxycyclohexanones. 

Two advantages of CA[Mn] are a broader substrate range and the lack of aldehyde side products. CA[Mn] catalyzed epoxidation of fran.y-P-niethyl styrene (46% ee), but this substrate was not oxidized by CPO [57], We did not detect any aldehyde side products in the CA[Mn] catalyzed epoxidations. In contrast, the CPO and CiP-catalyzed epoxidation of styrene formed 24% [57] and 52% [55] benzaldehyde side product. These aldehyde side products form simultaneously with epoxide during styrene epoxidation catalyzed by heme peroxidases [55]. Althongh the mechanism of aldehyde formation is nnknown, this simultaneous formation of aldehyde suggests that it forms from a reaction intermediate of epoxidation. Since the mechanism of epoxidation for the CA[Mn] percarbonate reaction differs from that for the heme peroxidases, this different mechanism may account for the lack of aldehyde side products. 

An interesting extension of this concept is to use a CO2/H2/O2/H2O mixture containing a catalyst for H2O2 fomiation in the CO2 phase directly for the epoxidation of alkenes [44], e.g., in a direct synthesis of propene oxide (Scheme 2) [45]. It has been shown that H2O2 will react with aqueous bicarbonate to form percarbonate... 

Inexpensive sodium percarbonate can be used, both as oxidant and base [135], for the asymmetric epoxidation of a,p-unsaturated ketones using silica-grafted PLL 93 [136]. Tang and co-workers [136] have described the best results with a silica-grafted PLL with = 45. Under these conditions, the epoxidized products were obtained with 70-93% ee and 40-94% yield (Scheme 12.19). The catalyst can be recovered and reused with no erosion in the observed ee s. 

It should be noted that the syndiotactic 1,2-PB epoxidation by the sodium hypochlorite and percarbonic acid salts carried out in the alkaline enables to prevent the disclosure reactions of the epoxy groups and a gelation process of the reaction mass observed at polydiene epoxidation by aliphatic peracids [31, 32], The functionalization degree of 1,2-PB in the reactions with the stated epoxidizing agents (a <16%, Table 3.1) is determined by the content of inner double bonds in the polymer. 

Other major industrial applications for hydrogen peroxide include the manufacture of sodium percarbonate and sodium perborate, used as mild bleaches in laundry detergents. It is used in the production of certain organic peroxides such as dibenzoyl peroxide, used in polymerisations and other chemical processes. Hydrogen peroxide is also used in the production of epoxides such as propylene oxide. Reaction with carboxylic acids produces a corresponding peroxy acid. Peracetic acid and meta-chloroperoxybenzoic acid (commonly abbreviated mCPBA) are prepared from acetic acid and /weto-chlorobenzoic acid, respectively. The latter is commonly reacted with alkenes to give the corresponding epoxide. 

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