Epoxidation relative rate

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

The speed of autoxidation was compared for different carotenoids in an aqueous model system in which the carotenoids were adsorbed onto a C-18 solid phase and exposed to a continnons flow of water saturated with oxygen at 30°C. Major products of P-carotene were identified as (Z)-isomers, 13-(Z), 9-(Z), and a di-(Z) isomer cleavage prodncts were P-apo-13-carotenone and p-apo-14 -carotenal, and also P-carotene 5,8-epoxide and P-carotene 5,8-endoperoxide. The degradation of all the carotenoids followed zero-order reaction kinetics with the following relative rates lycopene > P-cryptoxanthin > (E)-P-carotene > 9-(Z)-p-carotene. 

Dimethylsulfonium methylide is both more reactive and less stable than dimethylsulfoxonium methylide, so it is generated and used at a lower temperature. A sharp distinction between the two ylides emerges in their reactions with a, ( -unsaturated carbonyl compounds. Dimethylsulfonium methylide yields epoxides, whereas dimethylsulfoxonium methylide reacts by conjugate addition and gives cyclopropanes (compare Entries 5 and 6 in Scheme 2.21). It appears that the reason for the difference lies in the relative rates of the two reactions available to the betaine intermediate (a) reversal to starting materials, or (b) intramolecular nucleophilic displacement.284 Presumably both reagents react most rapidly at the carbonyl group. In the case of dimethylsulfonium methylide the intramolecular displacement step is faster than the reverse of the addition, and epoxide formation takes place. 

Figure 8. Overvoltage effect on the rates of ethylene epoxidation (i = 1, 0) and deep oxidation (i = 2, 9). At constant overvoltage the relative rate increases are proportional to Pqi/Pet at 400°C.

Limonene (10.128) is an analogue of 4-vinylcyclohexene, and, like the latter, it undergoes epoxidation of both the C(1)=C(2) and C(8)=C(9) bonds. Like in the dioxide 10.127, the two epoxide groups are hydrated at different rates by EH. Indeed, incubations in rat liver microsomes showed that hydrolysis of limonene 1,2-epoxide was 70 times slower than that of the 8,9-epoxide, a much larger difference than that observed for the dioxide 10.127 [192], Comparison of EH-catalyzed hydration of the four epoxy groups in 4-vinyl-cyclohexene and limonene confirmed that the relative rates decreased with increasing steric hindrance at these groups. 

In order to assess whether intramolecular cooperativity could occur within the dendrimeric [Co(salen)]catalyst the HKR of racemic l-cyclohexyl-l,2-ethenoxide was studied at low catalyst concentrations (2xl0 " M). Under these conditions the monomeric [Co(salen)] complex showed no conversion at all, while the dendritic [G2]-[Co(salen)]catalyst gave an impressive enantiomeric excess of 98% ee of the epoxide at 50% conversion. Further catalytic studies for the HKR with 1,2-hexen-oxide revealed that the dendritic catalysts are significantly more active than a dimeric model compound. However, the [Gl]-complex represents already the maximum (100%) in relative rate per Go-salen unit, which was lower for higher generations [G2] (66%) and [G3] (45%). 

Two extreme epoxidation modes, spiro and planar, are shown in Fig. 9 [33, 34, 53, 54, 76-85]. Baumstark and coworkers had observed that the epoxidation of cis-hexene of dimethyldioxirane was seven to nine times faster than the corresponding epoxidation of tran.y-hexene [79, 80]. The relative rates of the epoxidation of cisitrans olefins suggest that spiro transition state is favored over planar. In spiro transition states, the steric interaction for cw-olefm is smaller than the steric interaction for fran -olefm. In planar transition states, similar steric interactions would be expected for both cis- and trans-olefms. Computational studies also showed that the spiro transition state is the optimal transition state for oxygen atom transfer from dimethyldioxirane to ethylene, presumably due to the stabilizing interactions... 

The arene oxide valence tautomer of oxepins in principle should undergo nucleophilic substitution reactions (Sn2) which are characteristic of simple epoxides. In reality oxepin-benzene oxide (7) is resistant to attack by hard nucleophiles such as OH-, H20, NH2- and RNH2. Attempts to obtain quantitative data on the relative rates of attack of nucleophiles on (7) in aqueous solution hqye been thwarted by competition from the dominant aromatization reaction. 

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

Swain, C. G., and C. B. Scott, Quantitative correlation of relative rates. Comparison of hydroxide ion with other nucleophilic reagents toward alkyl halides, esters, epoxides, and acyl halides , J. Am. Chem. Soc., 75, 141-147 (1953). 

The Sharpless epoxidation is sensitive to preexisting chirality in selected substrate positions, so epoxidation in the absence or presence of molecular sieves allows easy kinetic resolution of open-chain, flexible allylic alcohols (Scheme 26) (52, 61). The relative rates, kf/ks, range from 16 to 700. The lower side-chain units of prostaglandins can be prepared in high ee and in reasonable yields (62). A doubly allylic alcohol with a meso structure can be converted to highly enantiomerically pure monoepoxy alcohol by using double asymmetric induction in the kinetic resolution (Scheme 26) (63). A mathematical model has been proposed to estimate the degree of the selectivity enhancement. 

Evidence of variables that influence the relative rates of reaction of olefins and alcohols was obtained from experiments with compounds that have both olefinic and alcoholic functions and by the competitive oxidation of mixtures of olefins and alcohols. The data of Table VI show that when the double bond has no substituents, as in allyl alcohol, but-3-en-l-ol, or 2-methylbut-3-en-l-ol, only the epoxide is formed but when the double bond has substituents, the epoxida-tion rate is decreased and ketone and aldehyde products from the oxidation of the OH group are formed. This effect is more pronounced with a greater degree of substitution. Since the double bond and the OH group are part of the same molecule, the difference must arise from the different abilities of the reactants to coordinate and react at the titanium center restricted transition-state shape selectivity is a possibility. The terminal double bond, sterically less hindered, interacts strongly with titanium, preventing coordination of the competing OH... 

Both undergo oxidation with epoxide and aldehyde formation, but the former gives an epoxidation/alcohol oxidation ratio of 3, whereas the latter gives an epoxidation/alcohol oxidation ratio of only 0.2. That this difference is due to the steric requirements inside the pores of the catalyst is demonstrated by the fact that the relative rates of epoxidation and alcohol oxidation of the same molecules on a large-pore Ti02/Si02 catalyst are almost the same (Tatsumi et al., 1993). 

The ratio of the rates of epoxidation of the two enantiomers, las/ s]ow, has been defined as the relative rate ( rel) and is related to the percent conversion of allylic alcohol to epoxy alcohol and the enantiomeric purity of the remaining allylic alcohol. A mathematical relationship between these variables exists and can be represented graphically as shown in Figure 6A.5 [14]. 

If values are known for two of the three variables, the third can be predicted by use of this graph. Inspection of the graph reveals that relative rates of 25 or more are very effective for achieving kinetic resolution of 1-substituted allylic alcohols. With a relative rate of 25, the epoxidation need be carried to <60% conversion to achieve essentially 100% ee for the unreacted alcohol. A convenient method for limiting the extent of epoxidation to 60% is simply by controlling the amount of oxidant used in the reaction. However, for some substrates (see Table 6A.8, entries 1, 9, or 10) even fcfast is extremely slow and the epoxidation takes several days [2,13,104-106]. To shorten the time needed for such reactions, an alternate practice is to use an... 

Figure 6AJ5. Dependence of enantiomeric excess on relative rate in the epoxidation of 1 -substituted allylic alcohols.

Relative rate data for the kinetic resolution/epoxidation of 1-substituted allylic alcohols of varying structure are summarized in Table 6A.8. The j values at -20°C for all entries in Table 6A.8 were determined using DIPT as the chiral ligand. Additionally, for several entries (1-3, 10, 11) the dependence of rel on temperature, 0 versus -20°C, and on steric bulk of the tartrate ester (DIPT vs. DET vs. DMT) has been measured. Lower reaction temperature and larger tartrate ester groups are factors that clearly increase the magnitude of kre] and, therefore, improve the efficiency of the kinetic resolution process. Although the results summarized in Table 6A.8... 

Table 2. Relative rate constants for the reaction of epoxides with ethanol at 50 °C. 471 (Reproduced by courtesy of Marcel Dekker, Inc.)...

The relative rates of epoxidation of ethyl 4-substituted-(E)-cinnamates by 200 gave a Hammett p value of —1.53, indicating an electrophilic oxygen-atom transfer316 the reaction rate is increased by protic solvents317. 

Scheme3.6. Oxidation of a strained alkene by air [20] and relative rates of epoxidation of various cyclic alkenes [21].

The relative rates and stereochemistry of epoxidation reactions of 5-substituted-adamantan-2-ones with two sulfur ylids (methylenedimethylsulfurane and its oxy-sulfurane analogue) have been studied in DMSO and in benzene.318... 

The rate constants for oxidation of a series of cycloalkenes with ozone have been determined using a relative rate method. The effect of methyl substitution on the oxidation of cycloalkenes and formation of secondary organic aerosols has been analysed.155 Butadiene, styrene, cyclohexene, allyl acetate, methyl methacrylate, and allyl alcohol were epoxidized in a gas-phase reaction with ozone in the absence of a catalyst. With the exception of allyl alcohol, the yield of the corresponding epoxide ranged from 88 to 97%.156 Kinetic control of distereoselection in ozonolytic lactonization has been (g) reported in the reaction of prochiral alkenes.157... 

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