Epoxidation, alkenes mechanism

The mechanism of alkene epoxidation is believed to be a concerted process mvolv mg a single bimolecular elementary step as shown m Figure 6 14... 

Sodium hexakis(formato)molybdate, 3, 1235 Sodium hypochlorite alkene epoxidation manganese catalysts, 6,378 Sodium ions biology, 6, 559 selective binding biology, 6, 551 Sodium molybdate, 3, 1230 Sodium peroxoborate, 3,101 Sodium/potassium ATPase, 6, 555 vanadate inhibition, 3, 567 Sodium pump, 6, 555 mechanism, 6, 556 Sodium pyroantimonate, 3, 265 Sodium salts... 

FIGURE 6.8 Mechanism of rate enhancement from methanol in alkene epoxidation.37 38... 

Fig. 10.3. Mechanism of the acid-catalyzed proton shift in the rearrangement of some alkene epoxides (10.10) to aldehydes (10.11)...

The kinetics of the catalytic oxidation of cyclopentene to glutaraldehyde by aqueous hydrogen peroxide and tungstic acid have been studied and a compatible mechanism was proposed, which proceeds via cyclopentene oxide and /3-hydroxycyclopentenyl hydroperoxide. " Monosubstituted heteropolytungstate-catalysed oxidation of alkenes by t-butyl hydroperoxide, iodosobenzene, and dioxygen have been studied a radical mechanism was proved for the reaction of alkenes with t-BuOOH and O2, but alkene epoxidation by iodosobenzene proceeds via oxidant coordination to the catalyst and has a heterolytic mechanism. ... 

While it is well established that HO—ONO can be involved in such two-electron processes as alkene epoxidation and the oxidation of amines, sulfides and phosphines, the controversy remains concerning the mechanism of HO-ONO oxidation of saturated hydrocarbons. Rank and coworkers advanced the hypothesis that the reactive species in hydrocarbon oxidations by peroxynitrous acid, and in lipid peroxidation in the presence of air, is the discrete hydroxyl radical formed in the homolysis of HO—ONO. The HO—ONO oxidation of methane (equation 7) on the restricted surface with the B3LYP and QCISD methods gave about the same activation energy (31 3 kcalmol" ) irrespective of basis set size . ... 

SCHEME 45. Hypothetical mechanism for epoxidation of alkenes by H2O2 in phenol... 

The metallacycle mechanism can also be considered a concerted mechanism. It is analogous to the one proposed for metal peroxo complexes and is based on the assumed formation of a cyclic intermediate that includes the peroxo group, the reactant molecule, and the metal ion (Mimoun, 1982, 1987 Huybrechts et al., 1992). For alkene epoxidation, the sequence of events would be represented as follows [Eq. (31)] ... 

A radical mechanism involving species coordinated at the same Ti schematically represented as follows for alkene epoxidation ... 

A) Nucleophilic attack of the alkene on the electrophilic oxygen atom covalently bound to the metal, which is reminiscent of Bartlett s butterfly mechanism for epoxidation of alkenes by percarboxylic acids.229... 

The reaction of (148) with norbomene produces a stable, well-characterized bright yellow netallacyclic compound (152) which, upon standing at 25 °C in toluene, slowly decomposes to pve exo-epoxynorbornene in almost quantitative yield (equation 199).210 The particular decomposition, which is related in some aspects to the mechanism of alkene epoxidation by d° metal-peroxo... 

There is currently significant debate about the mechanism of substrate oxidation by Q [62, 80, 81, 89]. Studies examining the MMO-catalyzed oxidation of nor-carane, of which the products derived from radical and cationic rearrangements clearly differ, indicated that both radical and cationic species are involved in product formation with a radical lifetime on the order of 20-150 ps [79, 90]. There is, furthermore, evidence suggesting that compound P may be able to effect alkene epoxidation directly [91]. Thus, in analogy with P450, multiple mechanisms and oxidants may be involved in the oxygenation of different substrates by MMO. 

Mechanism of alkene epoxidation by oxometalloporphyrins containing hypervalent Fe, Cr, and Mn atoms 92ACR314. 

The macrocyclic chemistry of tetradentate Schiff base complexes has been known for long time. However, the successful use of such a complex as an enantioselective catalyst in epoxidation reactions is a relatively recent finding. In these reactions complex 9.9 or an analogue is used. One of the possible routes for the synthesis of intermediate 9.2 of Table 9.1 involves the use of a similar catalyst. While complex 9.9 works well with unfunctionalized alkenes, for the epoxidation of allylic alcohols, dialkyl tartarates, 9.10, are the preferred ligands. As we shall see, the mechanisms of epoxidation in these two cases are different. Also for the tartarate-based system titanium is the metal of choice (see Section 9.3.3). 

In epoxidation reactions allyl alcohol can act as a prochiral alkene. Enantio-merically pure glycidol isomers (see Table 1.1) may be used to make S-propanolol 9.61, a drug for heart disease and hypertension. The mechanistic details of the epoxidation reaction with V5+ and Mo6+ complexes as catalysts were discussed in Section 8.6. The basic mechanism of epoxidation reaction, the transfer of an oxygen atom from f-butyl hydroperoxide to the alkene functionality, remains the same. 

The proposed mechanism for epoxidation with 9.38B as the catalyst is shown in Fig. 9.8. The oxidation state of manganese in 9.38A and 9.38B is three. The oxygen donor NaOCl or PhIO oxidizes Mn3+ to Mn5+, and an oxo complex such as 9.39 is produced. Reaction of 9.39 with the alkene produces the chiral epoxide and regenerates 9.38B. While there is enough evidence for the basic mechanism and the involvement of a catalytic intermediate such as 9.39, there is some controversy about the details of the oxygen atom transfer from 9.39 to the alkene. 

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. 

The work of Davis was, however, unable to distinguish which oxygen was attacked on the titanium peroxo complex when the alkene co-ordinates. Therefore, Neurock and Manzer conducted a theoretical study of the mechanism of alkene epoxidation using TS-1 with aqueous hydrogen peroxide.32 The workers concluded that their calculations to predict both the structure and relative bands in the IR spectra for TS-1 were in good agreement with experimental data. The calculations indicated that the oxygen closest to the titanium centre was the active site for alkene attack. The result was the direct formation of... 

Scheme 11. The mechanism of alkene epoxidation catalyzed by chromium-salen complexes...

Mechanism of epoxidation The oxygen transfer occurs by a two-step catalytic cycle (Scheme 1.14). In the first step oxygen is transferred to the Mn(III) by an oxidant. The oxygen coordinates to the metal. In the second step the activated oxygen is delivered to the alkene. 

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