Mechanism of olefin oxidations

Oxidation of 2-Butene with Selenium Dioxide. The stoichiometry of the reaction of 2-butene with selenium dioxide shows that approximately 0.85 mole of l-acetoxy-2-butene plus 0.85 mole of bis(l-methyl-2-acetoxypropyl) selenide are produced per mole of selenium dioxide consumed. This suggests that, at least for this particular group of olefins, in the mechanism of olefin oxidation with selenium dioxide the formation of selenides should be considered as the final reduced state of the oxidant rather than elemental selenium. 

The investigation of the mechanism of olefin oxidation over oxide catalysts has paralleled catalyst development work, but with somewhat less success. Despite extensive efforts in this area which have been recently reviewed by several authors (9-13), there continues to be a good deal of uncertainty concerning the structure of the reactive intermediates, the nature of the active sites, and the relationship of catalyst structure with catalytic activity and selectivity. Some of this uncertainty is due to the fact that comparisons between various studies are frequently difficult to make because of the use of ill-defined catalysts or different catalytic systems, different reaction conditions, or different reactor designs. Thus, rather than reviewing the broader area of selective oxidation of hydrocarbons, this review will attempt to focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the following questions in mind ... 

Fig. 7. Mechanisms of olefin oxidation over tin-antimony oxides. [Reproduced from J. C. McAteer (/< ),]...

In summary, although much is known about the mechanism of olefin oxidation over cuprous oxide, the picture is far from complete. Electronic factors governing the rates of reaction at the surface are unquestionably important. Kinetics have not yet clearly defined the rate-limiting step. Initial attack on an olefin of appropriate structure occurs by abstraction of an allylic hydrogen, and is followed by further reaction at either end of an adsorbed allylic intermediate. Inclusion of a hetero atom (0, N) occurs after the second abstraction. 

The mechanism of olefin oxidation mediated by PdNO type complexes has been recently reviewed [44]. 

Bolland, J., G. Gee, Kinetic studies in the chemistry of rubber and related materials III. Thermo chemistry and mechanisms of olefin oxidation, Trans. Faraday Soc., 42, p. 244, 1946. 

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

A similar mechanism of chain oxidation of olefinic hydrocarbons was observed experimentally by Bolland and Gee [53] in 1946 after a detailed study of the kinetics of the oxidation of nonsaturated compounds. Miller and Mayo [54] studied the oxidation of styrene and found that this reaction is in essence the chain copolymerization of styrene and dioxygen with production of polymeric peroxide. Rust [55] observed dihydroperoxide formation in his study of the oxidation of branched aliphatic hydrocarbons and treated this fact as the result of intramolecular isomerization of peroxyl radicals. 

The reaction mechanisms of these transition metal mediated oxidations have been the subject of several computational studies, especially in the case of osmium tetraoxide [7-10], where the controversy about the mechanism of the oxidation reaction with olefins could not be solved experimentally [11-20]. Based on the early proposal of Sharpless [12], that metallaoxetanes should be involved in alkene oxidation reactions of metal-oxo compounds like Cr02Cl2, 0s04 and Mn04" the question arose whether the reaction proceeds via a concerted [3+2] route as originally proposed by Criegee [11] or via a stepwise [2+2] process with a metallaoxetane intermediate [12] (Figure 2). 

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. 

It has been reported occasionally that variable quantities of organo-selenium compounds are produced during the oxidation (3, 7). These compounds have been considered as side reaction products and have received little attention regarding their nature or possible role in the reaction. In this work we have studied the characteristics of these organoselenium compounds, their role in the mechanism of the oxidation of olefins with selenium dioxide, and their catalytic properties in the oxidation of olefins with other oxidants. 

It was also shown that the ratio of oxidized alcohol to oxidized Fe2+ could be greater then one. Baxendale and Wilson (1957) showed that hydroxyl radical initiating the chain polymerization of olefins by hydrogen peroxide was the same process as the rapid oxidation of glycolic acid. Merz and Waters (1947) confirmed that simple water-soluble alcohols are oxidized rapidly by Fenton s reagent. The primary alcohols are oxidized to aldehydes, which are further oxidized at comparable rates by exactly the same mechanism. Merz and Waters proposed a mechanism of chain oxidation of alcohols and aldehydes by sodium persulfate, hydrogen peroxide, and an excess of ferrous salt as follows ... 

This mechanism is consistent with a number of observations. Kinetic studies on prolyl 4-hydroxylase [223] and thymine hydroxylase (EC 1.14.11.6) [224] suggest that cofactor binds first, followed by 02. The bound 02 appears to have superoxide character, as superoxide scavengers are competitive inhibitors of 02 consumption [225,226], It is also clear that the oxidative decarboxylation of the keto acid is a distinct phase of the mechanism from the alkane functionalization step, as these two phases can be uncoupled, particularly when poor substrate analogs are employed [227-229], Evidence for an Fe(IV) = 0 intermediate derives from studies with substrate analogs. Besides the hydroxylation of the 5-methyl group of thymine, thymine hydroxylase can also catalyze ally lie hydrox-ylations, epoxidation of olefins, oxidation of sulfides to sulfoxides, and N-de-... 

Of special interest are olefm epoxidation with peroxy acids at the moment of their formation from acid anhydrides and hydrogen peroxide. Among the catalysts of olefin oxide formation from olefins interacting with H202 in the presence of metal (Ti, W, V, Ta, Ge, Mo, U, Ru) oxides or acids are tungstic acid and its salts [47], Epoxidation is described by the following mechanism ... 

Farmer, E.H., Bloomfield, G.F., Sundralingam, A., Sutton, D.A. 1942. The course and mechanism of auto-oxidation reactions in olefinic and polyolefinic substances, including rubber. 

The reaction mechanisms shown in Scheme 2 have been the subject of several computational studies. Of particular interest has been the dihydroxylation by osmium tetroxide,26-29 where the above mentioned controversy about the mechanism of the oxidation reaction with olefins could not be solved experimentally.10,12,13,16,19,22,24,25,35,36... 

Furthermore, a study of vinyl ester exchange, which proceeds by a mechanism analogous to the acetoxypalladation mechanism for olefin oxidation [see Section III, B, 1, Eqs. (174) and (175)], indicates that the dimer is the most catalytically active species, with the trimer next, and the monomer, (NaaPd(OAc)4), unreactive. In the study of Moiseev et ah, a maximum rate is attained at the point at which the concentration of Na2Pd2(OAc)g reaches a maximum. Thus the dimer is the reactive species rather than Na2Pd(OAc)4. However, the decrease in rate with increasing [NaOAc] is greater than can be explained on the basis of conversion of dimer to unreactive monomer [Eq. (5), Section II, A, 2]. 

Catalytic oxidative transformations of lower alkanes attract the attention as possible ways to transfer these substances into more suitable chemicals - olefins and oxygenates (alcohols, aldehydes, acids, etc.) - and to involve them into the industrial use as raw materials for chemical and petrochemical synthesis. However, the yields of desirable products reached up to date are not sufficiently high. The progress in the studies of intrinsic mechanism of catalytic partial oxidation of lower alkanes is not sustainable either. We believe that these two facts are correlated and that the analysis we performed in the present work can brighten up some important details of the mechanism of catalytic oxidation of lower alkanes. ... 

Fundamental studies directed toward the elucidation of the mechanism of olefin i.e.f isobutylene, polymerizations yielded a new method for the synthesis of novel linear and tri-arm star telechelic polymers and oligomers [1,2]. The synthesis involves the use of bi- or tri-functional initiator/transfer agents, so called inifers (binifers and trinifers), in conjunction with BCI3 coinitiator and isobutylene, and gives rise to polyisobutylenes carrying exactly two or three terminal -CH2-C(CH3)2Cl groups. These liquid telechelic polyisobutylene chlorides can be readily and quantitatively converted to telechelic polyisobutylene di- or tri-olefins [2,3] which in turn can quantitatively yield by hydroboration/oxidation telechelic polyisobutylene di- and triols [4,5]. 

Scheme XXII. Proposed oxidation mechanism of olefins.

The Wacker-type oxidation of the olefins is one of the oldest homogeneous transition metal-catalyzed reactions. The mechanism of the oxidation of ethylene to acetaldehyde by a PdCl2/CuCl2/02 system is shown in Figure 23. Interestingly, the selectivity of the oxidation of olefins with longer alkyl chains is dependent on their solubility in water. Furthermore, the production of chlorinated side-products and isomerized olefins has also occurred for olefins with low water solubility. In order to avoid the solubility issues, co-solvents such as DMSO, acetone, THF, dioxane, acetonitrile, DMF, and ethanol were used and DMF seemed to be the best. ... 

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]. 

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