Reactivity in epoxidation

The rate also decreases with an increase in the chain length of the alkene molecule (hex-l-ene > oct-1-ene > dodec-l-ene). Although the latter phenomenon is attributed mainly to diffusion constraints for longer molecules in the MFI pores, the former (enhanced reactivity of terminal alkenes) is interesting, especially because the reactivity in epoxidations by organometallic complexes in solution is usually determined by the electron density at the double bond, which increases with alkyl substitution. On this basis, hex-3-ene and hex-2-ene would be expected to be more reactive than the terminal alkene hex-l-ene. The reverse sequence shown in Table XIV is a consequence of the steric hindrance in the neighborhood of the double bond, which hinders adsorption on the electrophilic oxo-titanium species on the surface. This observation highlights the fact that in reactions catalyzed by solids, adsorption constraints are superimposed on the inherent reactivity features of the chemical reaction as well as the diffiisional constraints. 

From the DSC experiment, it is evident that the iron cation is less reactive in epoxide polymerization then Brensted acids obtained from, e.g. sulfonium salts (see Fig. 11). Therefore, a heat treatment after the illumination step is necessary. The narrow enthalpy peak observed could refer to a very uniform pathway. 

The complex 27 [Mn20(0Ac)2tptn] is able to catalyze the oxidation of various al-kenes including styrene, cyclohexene and trans-2-octene to the corresponding epoxides in good yields and turnovers of up to 870 (Scheme 10.12). In sharp contrast, complex 28 (Scheme 10.12) based on tpen, featuring a two-carbon spacer between the three N-donor sets in the ligand, was not reactive in epoxidation reactions [85]. 

The most commonly used peracids are weta-chloroperbenzoic acid, MCPBA, 11.18, and peracetic acid, MeCOjH (used as a 40 % solution in acetic acid). MCPBA was long considered to be the most convenient peracid, as it is a solid and reasonably stable. Recently, magnesium monoper-oxyphthalate (MMPP), 11.19, has come to be preferred over MCPBA reactivity in epoxidation is comparable, MMPP is a crystalline solid, and it is more stable than MCPBA. Peracetic acid has to be stored at low temperature, and the peracid content is usually titrated before use, as this decays over time. It is, however, a more powerful oxidant than the aromatic peracids. Pertrifluoroacetic acid, CF3CO3H, is a still more powerful oxidant. 

Sulfonate Esters. Sucrose sulfonates are valuable intermediates for the synthesis of epoxides and derivatives containing halogens, nitrogen, and sulfur. In addition, the sulfonation reaction has been used to determine the relative reactivity of the hydroxyl groups in sucrose. The general order of reactivity in sucrose toward the esterification reaction is OH-6 OH-6 > OH-1 > HO-2. 

This kind of chemical reactivity of epoxides is rather general. Nucleophiles other than Grignard reagents react with epoxides, and epoxides more elaborate than ethylene oxide may be used. All these features of epoxide chemistry will be discussed in Sections 16.11-16.13. 

Desymmetrization of meso-bis-allylic alcohols is an effective method for the preparation of chiral functionalized intermediates from meso-substrates. Schreiber et al has shown that divinyl carbonyl 58 is epoxidized in good enantioselectivity. However, because the product epoxy alcohols 59 and 60 also contain a reactive allylic alcohol that are diastereomeric in nature, a second epoxidation would occur at different rates and thus affect the observed ee for the first AE reaction and the overall de. Indeed, the major diastereomeric product epoxide 59 resulting from the first AE is less reactive in the second epoxidation. Thus, high de is easily obtainable since the second epoxidation removes the minor diastereomer. 

In general sulfur ylide-mediated epoxidation cannot be used to form an epoxide with an adjacent anion-stabilizing group such as an ester, as the requisite ylide is too stable and does not react with aldehydes [23], With the less strongly electron-withdrawing amide group, however, the sulfur ylide possesses sufficient reactivity for epoxidation. The first example of an asymmetric version of this reaction was by... 

Example Special tiases DBN (23) and DBU (24) arc exceptionally reactive in climination react ions under mild conditions. DBK allowed elimination of HBr from (25) even in the presence of the epoxide so that mono-epoxy-naphthalene (26) could be made for the first time. "... 

Payne rearrangement. The Payne rearrangement2 of a primary cts-2,3-epoxy alcohol to a secondary 1,2-epoxy alcohol usually requires a basic aqueous medium, but it can be effected with BuLi in THF, particularly when catalyzed by lithium salts. As a consequence, the rearrangement becomes a useful extension of the Sharpless epoxidation, with both epoxides available for nucleophilic substitutions. Thus the more reactive rearranged epoxide can be trapped in situ by various organometallic nucleophiles. Cuprates of the type RCu(CN)Li are particularly effective for this purpose, and provide syn-diols (3).3... 

Guengerich, F.P. (2003) Cytochrome P450 oxidations in the generation of reactive electrophiles epoxidation and related reactions. Archives of Biochemistry and Biophysics, 409, 59-71. 

Fig. 10.14. Reactivity ofdiol epoxides (Nu = H20, HCT, or another nucleophile), a) Hydrolytic reaction of diol epoxides to tetrols. b) Internal H-bonding in diol epoxides with syw-config-uration and rendering the distal C-atom more electrophilic (modified from [104]). c) General representation of proton-catalyzed (A-H = H+), general acid catalyzed (A-H = acid), or intra-molecularly catalyzed (A-H = syn-OW group) activation of the distal C-atom toward... 

The data in Table 10.1 suggest that the reactivity of epoxide hydrolase toward alkene oxides is highly variable and appears to depend, among other things, on the size of the substrate (compare epoxybutane to epoxyoctane), steric features (compare epoxyoctane to cycloalkene oxides), and electronic factors (see the chlorinated epoxides). In fact, comprehensive structure-metabolism relationships have not been reported for substrates of EH, in contrast to some narrow relationships that are valid for closely related series of substrates. A group of arene oxides, along with two alkene oxides to be discussed below (epoxyoctane and styrene oxide), are compared as substrates of human liver EH in Table 10.2 [119]. Clearly, the two alkene oxides are among the better substrates for the human enzyme, as they are for the rat enzyme (Table 10.1). 

The peculiar reactivity of epoxides of 5-vinylbarbiturates (10.65, Fig. 10.17) has been discussed in Sect. 10.5.3. Here, we note that hexobarbital epoxide (10.9) shows the same reactivity and decomposes via the same retro-aldol reaction to form 5-(l-methylbutyl)barbiturate [193]. 

Epoxides can react with alcohols via acidic or basic catalysed reaction mechanisms. However, since both strong acids and bases will degrade the cell wall polymers of wood, the reaction is usually catalysed via the use of amines, which are more strongly nucleophilic than the OH group. For example, whereas the production of epoxy-phenolic resins requires temperatures in the region of 180-205 °C, reaction between epoxides and primary or secondary amines takes place at 15 °C (Turner, 1967). Reaction of epoxides with wood often involves the use of tertiary amines as catalysts (Sherman etal., 1980). The sapwood is more reactive towards epoxides than heartwood (Ahmad and Harun, 1992). 

V. N. Shetti, P. Manikandan, D. Srinivas, and P. Ratnasamy, Reactive oxygen species in epoxidation reactions over titanosilicate molecular sieves, J. Catal. 216, 461 67 (2003). 

Week et al. [65] further reported the Co salen complex supported on norbomene polymers (23, 24) with stable phenylene-acetylene linker (Figure 8). The polymer-supported salen catalysts were investigated for HKR of the racemic terminal epoxides that showed outstanding catalytic activities and comparable selectivities to the original catalysts reported by Jacobsen. However, the polymeric catalyst was recycled only once after its precipitation with diethylether as the catalyst became less soluble and less reactive in subsequent catalytic... 

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