Oxidation of alkenes to form epoxides

Attack of Br on a bromonium ion is a normai 5 2 substitution—the key orbitals involved are the HOMO of the bromide and the a of one of the two carbon-bromine bonds in the strained three-membered ring. As with all 5 2 reactions, the nucleophile maintains maximal overlap with the a by approaching in line with the leaving group but from the opposite side, resulting in inversion at the carbon that is attacked. The stereochemical outcome of more complicated reactions (discussed below) is important evidence for this overall reaction mechanism. 

You may wonder why the bromine attacks a carbon atom in the bromonium ion rather than the positively charged bromine atom. Well, in fact, it can do this as well, but the result is just regeneration of bromine and the alkene the first step of the reaction is reversible. 

You can think of the bromonium ion as a carbocation that has been stabiiized by interaction with a nearby bromine atom. You have seen a similar effect with oxygen—this oxonium ion was an intermediate, for example, in the 5, 1 substitution of MOM chloride on p. 338 of Chapter 15. 

The bromine is one atom further away but, with bromine being lower in the periodic table and having more diffuse lone pairs, it can have a similar stabilizing effect, despite the angle strain in a three-membered ring. 

The two types of stabilization are not eguivalent the cation and the bromonium ion are different molecules with different shapes, while the two representations of the oxonium ion are just that—they aren t different molecules. This stabilization of an adjacent cationic centre by a heteroatom with at least one lone pair to form a three-membered ring intermediate is not restricted to bromine or the other halogens, but is also an important aspect of the chemistry of compounds containing oxygen, sulfur, or selenium, as you will see in Chapter 27. 

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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. 

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. 

This thermally unstable compound reacts with triphenylphosphine to form the oxide, and with alkenes to form epoxides. Yields of up to 50% epoxide have been obtained using the alkenes norbornene and cyclohexene. Otsuka et al. have recently isolated the bromo analog compound PtBr(OOCOPh)-(PPh3)2 in a pure state and have shown that it will oxidize norbornene to ejco-norbornene epoxide in 40% yield. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

Diphenylphosphinic chloride reacts with the superoxide anion radical (Oi ) in CH3CN under mild conditions to form the diphenylphosphinic peroxy radical intermediate 77, which shows strong oxidizing abilities in the epoxidation of alkenes, oxidation of sulfoxides to sulfones, desulfurization of thioamides to amides and oxidation of triphenylphos-phines to triphenylphosphine oxide in good to excellent yields (equation 105). ... 

The radical mechanism has also been proposed as a general mechanism for oxidation of alkenes and aromatics, but several objections have been raised because of the absence of products typically associated with radical reactions. In classical radical reactions, alkenes should react also at the allylic position and give rise to allyl-substituted products, not exclusively epoxides methyl-substituted aromatics should react at the benzylic position. The products expected from such reactions are absent. Another argument was made against the radical mechanism based on the stereoselectivity of epoxidation. Radical intermediates are free to rotate around the C C bond, with the consequence that both cis- and /rani-epoxides are formed from a single alkene isomer, contrary to the evidence obtained with titanium silicates (Clerici et al., 1993). 

Epoxides can also be formed from the oxidation of alkenes by molecular oxygen via in situ generation of hydroperoxides by autoxidation.251,252 An interesting example is the direct stereoselective oxidation of cyclohexene by 02 to syn-l,2-epoxycyclohexan-3-ol catalyzed by CpV(CO)4 with a 65% yield and 99% stereoselectivity (equation 78).253... 

The workers proposed that alkyl hydroperoxides and aqueous hydrogen peroxide interact with TS-1 in a similar manner, forming titanium alkyl peroxo complexes and titanium peroxo complexes, respectively. However, the titanium alkyl peroxo complexes were not active because the substrate could not enter the void due to steric effects. Consequently, no activity was possible for either alkane hydroxylation or alkene epoxidation. Comparison with Ti02-Si02/alkyl hydroperoxide for alkane and alkene oxidation indicated that this material was active because the oxidation took place on the surface and not in the pores. Figures 4.4 and 4.5 show the possible mechanisms in operation for the oxidation of alkenes and alkanes with a TS-1/hydrogen peroxide system. 

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