Mechanism allylic oxidation

Figure 7.10 (top) shows the FT-IR spectrum of freshly prepared C60D36. The exposure to air after 1 day causes alterations in the spectrum (Fig. 7.10, middle). In particular it can be noticed the reduction of the intensity of the C-D stretching band at 2,092 cm-1 and the complete disappearance of the C-D bending at 966 cm-1. Evidences of oxidation can be inferred by the C=0 stretching band at 1,710 cm-1 and by the C-OH and C-OOH bending at about 1,040 cm-1 supporting the allylic oxidation mechanism. After 3 days exposure to air an increase in the relative intensity of the ketone, hydroxyl and hydroperoxide bands can be observed (Fig. 7.10, bottom). 

Figure 18. Allylic oxidation mechanism. (Reproduced with permission from Ref. 6. Copyright 1981, Adv. Catal. )...

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

Selenium dioxide is a useful reagent for allylic oxidation of alkenes. The products can include enones, allylic alcohols, or allylic esters, depending on the reaction conditions. The mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a [2,3]-sigmatropic rearrangement that restores the original location of the double bond, and (c) solvolysis of the resulting selenium ester.183... 

Scheme 8. General mechanism of the copper-catalyzed allylic oxidation of alkenes (Kharasch-Sosnovsky reaction).

Catalytic amounts of this addend (4 equiv relative to Cu) increase the selectivity of the allylic oxidation when TBHP is used as the oxidant. No change was observed with terf-butyl perbenzoate. This observation suggests a dichotomy in the mechanism of this reaction when using the two oxidants. Furthermore, in the absence of anthraquinone, a small negative nonlinear effect (78) is observed while in its presence, a small positive nonlinear effect appears. The reasons for this reversal are not clear, although the authors observed that low enantiopurity catalysts lead to turbid... 

Scheme 12. Proposed mechanism leading to the allylic imide observed as a side product in the allylic oxidation of alkenes in nitrile solvents. [Adapted from (120).]...

Song F, Gamer AL, Koide K (2007) A highly sensitive fluorescent sensor for palladium based on the allylic oxidative insertion mechanism. J Am Chem Soc 129 12354—12355... 

Several catalytic systems based on copper can also achieve allylic oxidation. These reactions involve induced decomposition of peroxy esters. (See Part A, Section 12.8, for a discussion of the mechanism of this reaction.) When chiral copper ligands are used, enantioselectivity can be achieved. Table 12.1 shows some results for the oxidation of cyclohexene under these conditions. 

Allylic hydroperoxides are primary products in the autoxidation of - olefins, and lack of definite information on their reactivity and chemical behavior has hampered efforts to understand olefin oxidation mechanisms (2). This deficiency is most strongly felt in determining the relative rates of addition and abstraction mechanisms for acyclic olefins since assignment of secondary reaction products to the correct primary source is required. Whereas generalizations about the effect of structure on the course of hydroperoxide decompositions are helpful, most questions can be answered better by directly isolating the hydroperoxides involved and observing the products formed by decomposition of the pure compounds. 

The second synthetic approach to oidiolactone C (61) is summarized in Scheme 20. This route also commences with the ozonolysis of trans-communic acid 180. Now, when this compound was exposed to ozone in excess, keto aldehyde 187 was obtained in 76% yield. The key step in this approach was the y-lactone closure via chemoselective reduction of the lactone moiety on compound 189 through a SN2 mechanism. Compound 189 could be prepared by saponification of the corresponding methyl ester with sodium propanethiolate. Once the primary alcohol is oxidized, the completion of the synthesis of key lactone 103 only requires the allylic oxidation of the C-17 methyl with concomitant closure of the 8-lactone. This conversion was achieved with Se02 in refluxing acetic acid to give 103 in 51% yield. 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

The redox mechanism applies not only to allylic oxidation of olefins and to the oxidation of aromatic hydrocarbons, but also to the oxidation of methanol and sulphur dioxide, as well as the oxidation of ammonia to nitrogen. Only in the case of ethylene oxidation and oxyhydration of olefins do catalysts act according to another mechanism. The latter processes seem to be always low temperature reactions, occurring below 300° C, whereas redox mechanisms are possible above this temperature (e.g. 400—500°C). 

The allylic oxidation of propene is catalyzed by (compound) metal oxides, which essentially contain metal ions of variable valency. It is commonly accepted that a redox mechanism is operative in such a way that the catalyst acts as the oxidizer and that lattice oxygen is incorporated in the oxidation products. The assumptions have been proved for several catalysts by the analysis of cation valency changes and by experiments with labelled oxygen. 

For specific cases such as olefin oxidation over Bi-Mo oxide combinations some information concerning the oxidation mechanism is available. The work of Adams and Jennings (2), of Sachtler (16), and of Adams (1) has led to the general acceptance of an allylic intermediate. The discoverers of the Bi-Mo catalyst system (21) showed that propene is converted to acrolein, while Hearne and Furman (9) proved that butene forms butadiene. The allylic intermediate therefore can in principle react in two different ways (1) formation of a conjugated diene... 

Further to its ability to perform allylic and benzylic oxidations,149 /-butylpcroxy-iodane (6) effects radical oxidation of 4-alkylphenols to give 2,5-cyclohexadien-l-ones under mild conditions in good yields.150 o,o-Coupling dimers as side products and inhibition of the reaction by added galvinoxyl radical scavenger support a radical oxidation mechanism. 

A chemo- and highly regio-selective Pd-catalysed allylic oxidation reaction that proceeds via a novel mechanism where two different ligands interact serially with palladium to promote different steps of the catalytic cycle has been reported. Initial formation of a dimeric 7r-allylpalladium acetate complex has been proposed.41... 

As observed from reaction (6.19) and experimental data [41,120,121], ROOH satisfactorily replaces molecular oxygen and the reducer. When oxidized with hydroperoxides in the presence of iron porphyrin catalysts (cytochrome P-450 analogs), olefins mostly convert to allyl oxidation products, namely unsaturated alcohols and ketones, whereas the quantity of epoxides does not exceed 1% [122], According to current suggestions [121] such behavior of iron porphyrin catalysts is explained by olefin epoxidation with the cata-lyst-ROOH complex by the heterolytical mechanism according to the following equation ... 

The selenium-dioxide mediated allylic oxidation of alkenes was explored by means of 2H and 13C KIEs to clarify the mechanism of ene step.85 Changes of isotopic composition were determined for unreacted 2-methyl-2-butene 33 in reaction with Se02 at 25°C in ferf-butyl alcohol (Equation (49)). 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

Thus, the isotope effect for the allylic oxidation of cyclohexene by cytochrome P 450 is about 5 and is the same for the reconstituted, NADPH-dependent and the peroxide-dependent paths. This similarity suggests that although product ratios may change from one oxygen donor to another, the mechanism of oxygen transfer may be invariant. Efforts to develop a clearer understanding of the relationships between the 02-dependent and peroxide-dependent pathways for oxygen transfer catalyzed by cytochrome P 450 are currently underway. 

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