Mechanism of olefin epoxidation

Mechanism of Olefin Epoxidation by Transition Metal Peroxo Compounds... 

During the last three decades, peroxo compounds of early transition metals (TMs) in their highest oxidation state, like TiIV, Vv, MoVI, WV1, and Revn, attracted much interest due to their activity in oxygen transfer processes which are important for many chemical and biological applications. Olefin epoxidation is of particular significance since epoxides are key starting compounds for a large variety of chemicals and polymers [1]. Yet, details of the mechanism of olefin epoxidation by TM peroxides are still under discussion. 

The first quantum-chemical investigation of the mechanism of olefin epoxidation in flnoroalcohols was carried out by Shaik et al. [54], In the absence of kinetic data, a monomolecular mode of activation by the fluorinated alcohols for aU reaction pathways was assnmed [54],... 

In fact, the mechanism of olefin epoxidation by alkylperoxy radicals accepted in the literature is generally presented by the following scheme [143] ... 

Mechanism of olefin epoxidation catalyzed by cytochrome P450 enzymes 04CRV3947. 

Mechanism of Olefin Epoxidation byMn(salen) Complexes... 

J.T. Groves and Y. Watanabe, On the mechanism of olefin epoxidation by oxo-iron... 

Bartlett [206] in 1960 suggested a mechanism of olefin epoxidation with peracids, based on nonionized molecules and containing a three-atom cycle in the transition complex ... 

Mimoun, H., Mechanism of olefins epoxidation, Angcw. Chem., 94, 751, 1982. 

The mechanism of olefin epoxidation with iodosylbenzene in the presence of Cr(III) Schiff base complexes has been studied.The same reaction is catalysed by V0(acac)2 probably yij free radicals.Trans-stilbene is epoxidised by NalOi, in the presence of RuClg and substituted phenanthroline ligands. Vanadium(V) supported on a functionalised polystyrene resin is a good catalyst for the epoxidation of allylic alcohols by ButQOH a similar Mo(VI) catalyst is more suitable for cyclohexene epoxidation. ... 

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

From the energetics point of view, the epoxidation act should occur more easily (with a lower activation energy) in the coordination sphere of the metal when the cleavage of one bond is simultaneously compensated by the formation of another bond. For example, Gould proposed the following (schematic) mechanism for olefin epoxidation on molybdenum complexes [240] ... 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

An important improvement in the catalysis of olefin epoxidation arose with the discovery of methyltrioxorhenium (MTO) and its derivatives as efficient catalysts for olefin epoxidation by Herrmann and coworkers [16-18]. Since then a broad variety of substituted olefins has been successfully used as substrates [103] and the reaction mechanism was studied theoretically [67, 68, 80]. 

Fig. 8 Proposed mechanism of electrocatalytic epoxidation of olefins by manganese-porphyrin catalysts (reprinted with...

Mechanism. The following types of evidence ere pertinent in selecting on acceptable mechanism for olefin epoxidation by means of peroxy acids (1) the nature of the peroxy acid and the electronic effect of eubBtituents on its reactivity (2) the electronic effect of substituents on the reactivity of the olefin component (3) stereochemical factors affecting the reactivity of the olefin (4) the possibility of acid dialysis (5) solvent effects and (6) neighboring group effects. 

The majority of studies aimed at understanding the mechanisms of ethylene epoxidation rely on either experimental physical chemistry studies of single crystal-silver surfaces or computational studies looking at enthalpies of different reaction states. One unfortunate caveat of the experimental studies lies in the fact that the binding of ethylene to the silver surface is too weak for ultrahigh vacuum (UHV) studies to be of any use. Therefore, the majority of silver surface epoxidation reactions use olefins that stick to the surface better both before and after epoxidation (e.g., butadiene or styrene). 

Epoxidation of olefins with molecular O2, mechanism of 82UK1017. Epoxidation reagents 81H( 15)517,... 

As stated above, czs-stilbene is a frequently used substrate for the study of olefin epoxidation mechanisms [100] because of mechanistic information associated with the ratio of the cis- and frazzs-isomers in the stilbene oxide product. Catalytic f-BuOOH epoxidation of czs-stilbene was performed, with our Mn (Me2EBC)Cl2 catalyst, under and product analysis shows that czs-stilbene oxide contains 25 0.3% incorporation of from atmospheric 02 versus 1.7 0.3% in a control experiment using ordinary air, whereas frzzzzs-stilbene oxide contains 55.1 1% incorporation of O from O2 versus 2.6 1% incorporation in a control experiment. Similar incorporation ratios are expected for the cis- and frzzzzs-stilbene oxides if all epoxidation products result from only one reaction pathway and a single reactive intermediate. However, the incorporation of in czs-stilbene oxide (25 0.3%) is definitely different from that in frzzzzs-stilbene oxide (55.1 1%). This result leads to the conclusion that, at least two distinct reactive intermediates occur in these epoxidation reactions. This is also consistent with the results described above for norbomylene epoxidation. 

Mechanisms for olefin epoxidations catalyzed either by the enzyme or by model porphyrin complexes are not as well understood as those for hydroxyla-tion of aliphatic hydrocarbons. Some of the possibilities that have been proposed are represented schematically in Figure 5.13. 

Ti-MCM-48 is a catalyst for the epoxidation of olefins, the activity of which being strictly connected to the presence of tetrahedral Ti(IV) sites. The accepted mechanism, in the case of olefin epoxidation with H2O2 is reported in Scheme 3 [1],... 

It was noted that cross-linked polystyrene-telluric acid (a product of TeCl4 polycondensation with a copolymer of divinylbenzene and styrene and subsequent hydrolysis in an alkaline medium) is an active catalyst of olefin epoxidation by hydrogen peroxide [210]. At 333 K and in dioxane and terf-butanol solutions, the above catalyst quantitatively yielded epoxides from a wide range of unsaturated compounds such as aromatic and aliphatic hydrocarbons, alcohols and their derivatives. A plausible mechanism for the catalytic properties of a tellurium compound during polycondensation was discussed. 

The above mechanism suggested that the use of olefin activators other than paUadium(lI), which are not capable of promoting the p-hydride elimination, may lead to other types of olefin oxidization products, such as epoxides. Since thallium(in) is a known oxidant for olefin epoxidation, it was therefore postulated that replacement of the palladium(II) activator by T1(III) benzoate in the C0-NO2/NO redox system would lead to the accomplishment of olefin epoxidation [118]. 

The mechanisms of the epoxidations of olefins have been studied intensively.3 3 -376-378 In general, the reactions of metal-oxo complexes with olefins to form epoxides do not involve intermediates containing metal-carbon bonds. The 0x0 group tends to act as an electrophile and interact with the HOMO of the olefin during the transfer of the 0x0 to the olefin. After this initial interaction, the epoxide may form by a non-radical concerted process or by a stepwise process involving radical or cationic intermediates. ... 

Arsonated polystyrenes (9) in biphasic and triphasic systems may be used to catalyse the epoxidation of olefins by aqueous hydrogen peroxide. Yields with various olefins are uniformly high (58—100%) the mechanism of the epoxidation is similar to that of peroxycarboxylic acids (Scheme 9). 

Epoxidation reactions have been widely utilized for over 100 years with peradds, peroxides and, more recently, metal catalysts [7]. However, direct metal-catalyzed aerobic epoxidations are rare and generally require an aldehyde coreductant. In this case, the metal is proposed to catalyze radical formation (A-C, Scheme 5.2) followed by O2 insertion to form acyl peroxide D. Metal-catalyzed aerobic oxidation of aldehydes to peradds has previously been observed [8]. With the formation of species D, either an outer-sphere path similar to a peracid-type oxidation occurs (Path 1) or an inner-sphere metal-catalyzed oxidation in which the metal-based oxidant and substrate interact during oxygen transfer (Path 2 or 3). Mu-kaiyama and coworkers were the first to report an aerobic epoxidation of olefins catalyzed by transition metals using either a primary alcohol or an aldehyde as coreductants [9]. The role of the metal was probed through parallel studies of peracid and metal-catalyzed epoxidations of 2 which yielded different stereochemical outcomes. Therefore, a metal-centered mechanism for olefin epoxidation was proposed which implicates an oxygenase system. Path 2 or 3 (Table 5.1) [10]. 

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