Oxidation of alkenes to epoxides

Thallium triacetate, TI(0C0CH3)3 1.5H20 (mp 182 C), like the monoacetate, is used for the stereoselective acetoxylation of alkenes [411] and for oxidations of alkenes to epoxides [412]. 

Prilezhaev reaction Oxidation of alkenes to epoxides using peroxycarboxylic acids. 362... 

The supported reagent is capable of catalysing oxidation reactions including the oxidation of alkylaromatics using air as the oxidant, and the selective oxidation of alkenes to epoxides (Table 1) in the presence of air and an aldehyde. 

POM-pillared LDHs have also been used for the oxidation of alkenes to epoxides with H2O2 alone or more efficiently assisted by bromide. As previously discussed, a,(3-unsaturated ketones can be readily epoxidized with the help of normal LDH catalysts. So far, however, the epoxidation of nonactivated olefins requires the use of POM-LDH catalysts. For example, cyclohexene is selectively epoxidized using Ni2Al-SiWii04o" as catalyst, with the combination of oxygen and aldehyde as oxidant (487,488). The epoxidation of 2-hexene and 3-methylstyrene was also investigated by the same authors. 

Abstract This chapter covers one of the most important areas of Ru-catalysed oxidative chemistry. First, alkene oxidations are covered in which the double bond is not cleaved (3.1) epoxidation, cis-dihydroxylation, ketohydroxylation and miscellaneous non-cleavage reactions follow. The second section (3.2) concerns reactions in which C=C bond cleavage does occur (oxidation of alkenes to aldehydes, ketones or carboxylic acids), followed by a short survey of other alkene cleavage oxidations. Section 3.3 covers arene oxidations, and finally, in section 3.4, the corresponding topics for aUcyne oxidations are considered, most being cleavage reactions. 

Conversion of alkenes to epoxides The simplest epoxide, ethylene dioxide, is prepared by catalytic oxidation of ethylene, and alkenes are also oxidized to other epoxides by peracid or peroxy acid (see Section 5.7.2). 

The nature of the arsonium substituents also influences the course of the reaction. Gosney et al. (33) studied the reaction between arsonium salts of type 50 and benzaldehyde in THF using -butyllithium to generate the ylide. The results, summarized in Table IX, clearly show that electron-donating substituents at the arsenic atom promote the formation of al-kenes. At one extreme, the reaction of 0) gives a notable yield of stilbene almost to the exclusion of stilbene oxide, whereas at the other, the reaction of (a) yields predominantly stilbene oxide. Table IX shows that electron-donating substituents at arsenic increase the ratio of alkene to epoxide. 

Titanosilicalite (TS-1)[165,166], a highly siliceous MFI type zeolite in which 0.1 to 2.5% of the Si atoms are replaced by Ti, is the most successful example for the use of isomorphously substitited zeolites. As a consequence of the high Si/Al ratio of TS-1 the material contains only a negligible concentration of strong Bronsted acid sites. In fact, the presence of acid sites is detrimental to the selectivity of the catalysts, as discussed below. TS-1 has been found to be a selective oxidation catalyst for a wide variety of reactions such as the conversion of alkenes to epoxides [167], alcohols to aldehydes [168], alkanes to secondary alcohols and ketones [169,170], phenol to hydroquinone and catechol [171] and amines to hydroxylamines [ 172]. A schematic representation of the chemistry is given in Fig. 7 which is adapted from ref [17]. 

Oxidations with chromic oxide encompass hydroxylation of methylene [544] and methine [544, 545, 546] groups conversion of methyl groups into formyl groups [539, 547, 548, 549] or carboxylic groups [550, 55i] and of methylene groups into carbonyls [275, 552, 553, 554, 555] oxidation of aromatic hydrocarbons [556, 557, 555] and phenols [559] to quinones, of primary halides to aldehydes [540], and of secondary halides to ketones [560, 561] epoxidation of alkenes [562, 563,564, and oxidation of alkenes to ketones [565, 566] and to carboxylic acids [567, 565, 569]. 

Although high-valent rhenium-oxo complexes such as perrhenate, Rc207, and ReOjX (X = F, Cl, Br) are widely known, their use as oxidants is rather rare. The stoichiometric oxidation of alkenes to a mixture of ketone and epoxide by Rc207 has been reported in the patent literature. For example, 2-butene is transformed to 2,3-epoxybutane and 2-butanone by Rc207 at 100 °C. A catalytic oxidation was observed in the presence of excess hydrogen peroxide but precise data on this reaction are lacking. 

One of the most exciting developments in asymmetric catalysis over the past 25 years has been the discovery of transition metal complexes that catalyze the oxidation of alkenes to chiral epoxides and 1,2-diols. Equations 12.16, 12.17, and 12.18 show examples of epoxidation and 1,2-dihydroxylation. 

The preparation and use of supported polyoxometal complexes with H2O2 for the oxidation of sulphur containing compounds to sulphoxides, sulphonic acids, and epoxidation of alkenes to epoxides. 

Oxidation of Alkenes to Oxirans by Peroxy-acids. An improved procedure for epoxidation using aromatic peroxy-acids has been reported. After a normal epoxidation with 3-chloroperoxybenzoic acid (mCPBA) in CH2CI2, activated KF is added to the crude mixture, and this results in the precipitation of both wCPBA and the aromatic acid by-product, leaving an acid-free reaction mixture for normal work-up. As an alternative, the insoluble mCPBA-KF complex itself may be used for the epoxidation of alkenes overnight at room temperature. After filtration and treatment of the CH2CI2 solution with more KF (to ensure removal of any residual peroxy-acid), normal work-up leads to yields in excess of 95% for cyclohexene and styrene oxides. 

Oxidation of Alkenes to Oxirans, using Peroxides. The peroxide (15 R = OOH) is a useful oxidant for a number of alkenes, giving epoxides in good to moderate yields and generating (15 R = OH). The reactivity of this peroxide is two orders of magnitude lower than that of peroxyacetic acid but at least one order of magnitude greater than that of a-peroxy-esters and -nitriles. Its selectivity relative to the structure of the alkene is similar to that for peroxyacetic acid. 

Oxidation of Alkenes to Oxirans by Peroxy-acids. The mechanism of the reaction of m-chloroperbenzoic acid with double bonds has been investigated through a study of the epoxidation of a series of cycloalkenes (of ring sizes 5,6,7,8, and 12) and substituted cyclohexenes.The second-order rate constants were determined in CHCI3 at 0—30°C, and the data support a 1,3-dipolar cycloaddition reaction. 

Oxidation of Alkenes to Oxirans, using Peroxides. Two reviews in this area have been published, one dealing with new methods for the catalytic epoxidation of alkenes using hydrogen peroxide and the other with selective oxidation of alkenes and alkynes with t-butyl hydroperoxide. ... 

Attack of the oxygen atom of NO2 anion at Pd-coordinated alkene ligand afforded metallacycle compounds (Scheme 8.32) [53]. The X-ray structure determination of the product from dicylopentadiene complex of Pd(II) established the cis oxypalladation. The metallacycle thus formed can be regarded as an intermediate in Pd-catalyzed oxidation of alkenes to ketones or epoxides with the use of NO2 ligand as a mediator and O2 as an oxidant. 

Halohydrins are easily prepared and dehydrohalogenation occurs readily at low temperatures. Another way epoxides can be formed is by catalytic vapor-phase oxidation of alkenes to form oxiranes. 

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