Alkanes oxidation mechanisms

HO-initiated oxidation of the alkanes become complex with increase in carbon number. Namely, a large variety of alkyl radicals can be produced by the H-atom abstraction from the primary, secondary and tertiary C—H bonds in the parent alkane [88]. The resulting ROO ( C4) radicals have been shown by Atkinson et al. to yield R0N02 as well as RO + N02 upon reaction with NO [100-102]. A major complication in the alkane oxidation mechanism arises from the variety of competitive reaction channels that RO radicals can undergo, e.g., 02-reaction, unimolecular dissociation and internal isomerization. There have been a number of experimental and theoretical studies of these reactions [31,88]. 

Investigations by Knox and Wells [21, 22], devoted to the determination of alkane oxidation mechanism, led to the conclusion that alkane oxidation mainly proceeded via oxidation of corresponded olefin, transformed at the initial stage and oxidation via peroxide alkyl radicals yielded in only 20% ... 

In this connection, Stem el al. [23] studied the problem of which alkane oxidation mechanism should be preferred via olefin or peroxide alkyl radicals however, they did not come to any final conclusion. 

Oxidation of unfunctionalized alkanes is notoriously difficult to perform selectively, because breaking of a C-H bond is required. Although oxidation is thermodynamically favourable, there are limited kinetic pathways for reaction to occur. For most alkanes, the hydrogens are not labile, and, as the carbon atom cannot expand its valence electron shell beyond eight electrons, there is no mechanism for electrophilic or nucleophilic substitution short of using extreme (superacid or superbase) conditions. Alkane oxidations are therefore normally radical processes, and thus difficult to control in terms of selectivity. Nonetheless, some oxidations of alkanes have been performed under supercritical conditions, although it is probable that these actually proceed via radical mechanisms. 

It is proposed that hydrated or dehydrated titaniumperoxo compounds are formed in TS-1 by H2O2 chemisorption on the titanyl (Ti 0) group, and that these complexes constitute the actual oxidants [96]. In the particular case of alkane oxidation, a homolytic reaction mechanism is proposed, as is tentatively represented in scheme 6 [114]. 

It is clear from a recent review of the mechanisms of metal-catalyzed oxidations of hydrocarbons (89) that by far the most extensive studies have been on the oxidation of alkenes and aromatic compounds relatively little work on alkane oxidation is to be found. The studies on these reactions show that, if the reactivity is enhanced by a hard metal, it is often because the metal becomes involved in the free-radical reactions and generates further free radicals by the chain decomposition of hydroperoxides (39) ... 

The second major theory of the mechanism of alkane oxidation in the gas phase is the aldehyde theory. The products of gaseous oxidation almost invariably contain aldehydes, and it has been proposed (41) that these are formed by decomposition of alkylperoxy radicals. 

In the present work, therefore, a comparative study of the production of O-heterocycles during the cool-flame combustion of three consecutive n-alkanes—viz., n-butane, n-pentane, and n-hexane—was carried out under a wide range of reaction conditions in a static system. The importance of carbon chain length, mixture composition, pressure, temperature, and time of reaction was assessed. In addition, the optimum conditions for the formation of O-heterocycles and the maximum yields of these products were determined. The results are discussed in the light of currently accepted oxidation mechanisms. 

This potential-acidity diagram (Pourbaix s type) has been determined for a large series of alkanes.79 All of these results indicate two types of oxidation mechanism of the C—H bond (i) oxidation of alkanes into carbenium ion at high acidity levels and (ii) oxidation of alkanes into radicals at low acidity levels. 

This section deals with Gif and GoAgg systems that were discovered by Barton and coworkers in the 1980s. After the presentation of the various systems, we will focus on the mechanism of the reaction. The last section will focus on the latest applications of Gif and GoAgg type systems to alkane oxidation. 

Osmium-catalysed dihydroxylation has been reviewed with emphasis on the use of new reoxidants and recycling of the catalysts.44 Various aspects of asymmetric dihydroxylation of alkenes by osmium complexes, including the mechanism, acceleration by chiral ligands 45 and development of novel asymmetric dihydroxylation processes,46 has been reviewed. Two reviews on the recent developments in osmium-catalysed asymmetric aminohydroxylation of alkenes have appeared. Factors responsible for chemo-, enantio- and regio-selectivities have been discussed.47,48 Osmium tetraoxide oxidizes unactivated alkanes in aqueous base. Isobutane is oxidized to r-butyl alcohol, cyclohexane to a mixture of adipate and succinate, toluene to benzoate, and both ethane and propane to acetate in low yields. The data are consistent with a concerted 3 + 2 mechanism, analogous to that proposed for alkane oxidation by Ru04, and for alkene oxidations by 0s04.49... 

As with cytochrome P-450 oxidations of alkanes, the mechanism of P-450-catalyzed formation of oxiranes has been extensively studied, but certain details remain in dispute272. Examples of mechanistic schemes include formation of a transient [FeIV—0]+2-olefin complex followed by collapse to an [Fe—Oj-olefin adduct that collapses further to the epoxide and other products264. An alternative mechanism involves a 2a + 2s addition of olefin to the Fe=0 double bond to give an intermediate metallocene, followed by rearrangement to products273-275. 

Using the analogy of model reactions of alkane oxidation in mixtures ofFe(II) and dioxygen in solvents, a mechanism invoking the formation of intermediate with an iron-carbon bond followed by interaction with soxygen was proposed (Waller and Fimscomb, 1996 Shilov, 1997). 

Stereoselectivity differences were found between alkane and alkene oxidation in the presence of TS-1, which suggested that the oxidations proceeded via different mechanisms. Stereo-scrambling was present during alkane oxidation on TS-1, without any radical clock rearrangement, suggesting that the radicals formed may have had a very short lifetime or that their movements were restricted such that no rearrangement could occur. 

Figure 4.5 Possible mechanisms operating for alkane oxidation on TS-1.

Co(acac)3 in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols, to the corresponding carboxylic acids and ketones, respectively, e.g. Fig. 4.71 [205]. By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [206, 207], Fig. 4.71 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of al-kylperoxy radicals (see above for alkane oxidation). 

Shilov and coworkers discovered the oxidation of methane to methanol by mixtures of Pt and Pt, and aroused the Holy Graft-pursuing for electrophilic C H activation and subsequent alkane oxidation. The diimine complexes of Pt(II) methyl are indeed found to facilitate smooth benzene activation, resulting in formation of methane via Pt (Me)(Ph)(H) intermediates (Scheme 10). Such Pt(II)/Pt(IV) involved C-H activation reactions have been widely extended to a variety of nitrogen-donor ligands, whose electronic and steric effects shed light on the reaction mechanisms (see Section 7.1). 

For the purpose of establishing mechanisms and determining rate constants, classical direct investigations of alkane oxidation were severely handicapped in at least two ways. 

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