Rate of epoxidations

Relative Rates of Epoxidation of Some Representative Alkenes with Peroxyacetic Acid... 

As shown m Table 6 4 electron releasing alkyl groups on the double bond increase the rate of epoxidation This suggests that the peroxy acid acts as an electrophilic reagent toward the alkene... 

A qualitatively similar behaviour was obtained during C3H6 epoxidation on Ag 43 Enhancement factor A values of the order of 150 were measured.43 Both the rates of epoxidation and oxidation to C02 increase with I>0 and decrease with I<0. The intrinsic selectivity to propylene oxide was very low, typically 0.03 and could be increased only up to 0.04 by using positive currents. This was again an exploratory study, as no reference electrode was used, thus T and UWr could not be measured 43... 

The latter case (Fig. 8.40b) is more interesting. Initially both rates decrease but at steady state the rate of epoxidation has decreased, while the rate of CO2 formation has increased. Thus epoxidation exhibits electrophobic behaviour but oxidation to C02 exhibits electrophilic behaviour.45... 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents and the reactivity of the peroxy acids is increased by EWG substituents.72 These structure-reactivity relationships demonstrate that the peroxyacid acts as an electrophile in the reaction. Decreased reactivity is exhibited by double bonds that are conjugated with strongly electron-attracting substituents, and more reactive peroxyacids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds.73 Electron-poor alkenes can also be epoxidized by alkaline solutions of... 

The yield of epoxide strongly depends on the substituent in benzene ring of styrene. The values of the ratio of the formation rates of epoxide and polyperoxide during styrene oxidation at 368 K are given below [110] ... 

Figure 7. Effect of current on the steady-state increase in the rates of epoxidation (rt) and deep oxidation (r2). Comparison with the rate of oxygen transport through the electrolyte G0z — i/4F. Intrinsic selectivity r10/r20 = 0.49. Conditions RC 2 at 400°C, Pet — 0.016 bar, P02 — 0.1 bar.

Figure 9. Effect of Ag catalyst-electrode surface area Q on the relative steady-state increase in the rates of epoxidation rt and deep oxidation r2 at constant imposed current i = 100 /jA, constant gas composition, 400°C, Po /Pet 7. Key O, r10/Ars and , ru/Ar%.

When oxygen is pumped to the catalyst the activity of oxygen on the silver catalyst-electrode increases considerably because of the applied voltage. It thus becomes possible to at least partly oxidize the silver catalyst electrode. In a previous communication it has been shown that the phenomenon involves surface rather than bulk oxidation of the silver crystallites (17). The present results establish the direct dependence of the change in the rates of epoxidation and combustion Ari and Ar2 on the cell overvoltage (Equations 2,3, and 5) which is directly related to the surface oxygen activity. 

The relative increase Ar /r Q in the rates of epoxidation (i=l) and combustion (i=2) is proportional to A/S, where A is the electrolyte surface area and S is the surface area of the silver catalyst electrode. Thus with a reactor having a low value of S (reactive oxygen uptake Q =.4 10 7 mol O2) a threefold increase in ethylene oxide yield was observed with a corresponding 20% increase in selectivity. 

The absolute increase Ar. in the rates of epoxidation and combustion is directly proportional to Pq2 and the cell overvoltage AV both at steady state and during galvanostatic transients. 

Many examples of the phase-transfer catalysed epoxidation of a,(3-unsaturated carbonyl compounds using sodium hypochlorite have been reported [e.g. 7-10]. The addition of transition metal complexes also aids the reaction [11], but advantages in reaction time or yields are relatively insignificant, whereas the use of hexaethyl-guanidinium chloride, instead of a tetra-alkylammonium salt, enhances the rate of epoxidation while retaining the high yields (>95%) [10]. Intermediate (3-haloalkanols are readily converted into the oxiranes under basic conditions in the presence of benzyltriethylammonium chloride [12]. 

The presence of the stereogenic centre at C(l) introduces an additional factor in the asymmetric epoxidation now, besides the enantiofacial selectivity, the diastereoselectivity must also be considered, and it is helpful to examine epoxidation of each enantiomer of the allylic alcohol separately. As shown in Fig. 10.2, epoxidation of an enantiomer proceeds normally (fast) and produces an erythro epoxy alcohol. Epoxidation of the other enantiomer proceeds at a reduced rate (slow) because the steric effects between the C(l) substituent and the catalyst. The rates of epoxidation are sufficiently significative to achieve the kinetic resolution and either the epoxy alcohol or the recovered allylic alcohol can be obtained with high enantiomeric purity [9]. 

The rate of epoxidation of cyclohexene with perbenzoic acid decreases with increasing solvent polarity. The epoxidation by poly(peracrylic acid) shows the opposite trend. A polar solvent causes the polar polymer to swell to a greater extent and the reaction rate is increased due to a higher local concentration of cyclohexene [Takagi, 1975]. 

From this series of calculations it is noted that the gas-phase reactivity of TFDO is substantially greater than that of DMDO. This rate difference has been ascribed largely to the inductive effect of the CF3 group. Fluoro-substituted dioxiranes have also played a unique role in the chiral epoxidation of alkenes. Flouk and coworkers have identified a novel stereoelectronic effect that increases the rate of epoxidation when the fiuorine substituent is anti to the oxygen of the developing C=0 group in the TS for epoxidation. 

These composite data strongly suggest that the presence of adventitious water or other hydrogen donors can markedly affect the observed rate of epoxidation. For example, Murray and Gu have reported AH = 5.0 kcalmol" for the DMDO epoxidation of cyclohexene in CDCI3 and 7.4 kcalmol" in acetone as solvent . Curci and coworkers also reported an a value of 9.3 kcalmol" for the DMDO epoxidation of isobutylene in acetone . These barriers are significantly lower than the 13-18 kcalmoD gas-phase barriers reported " at the B3LYP level of theory (Tables 3 and 4). Activation barriers of 12.6,... 

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