Autoxidation alkanes

Polymers with chelated Co have also been used as catalysts for alkane autoxidation. Kulkarni et al. (166) employed a tyrosine-based polymer for autoxidation in pure cyclohexane, but very different conditions were used by Shen and Weng (167,168) in the autoxidation of cyclohexane or cyclohexanone. The latter authors, used glacial acetic acid as a solvent and a Co-exchanged weak acid resin as the catalyst. At high conversions, adipic acid is formed ... 

Cobalt compounds are among the most efficient catalysts of alkane and aryl alkane autoxidation [16], Toluene oxidation by dioxygen at 60 °C in acetic acid in the presence of Co(III) at the initial stages gives exclusively benzaldehyde [16d]. The reaction proceeds without an induction period, reaching the maximum rate at the begiiming of the reaction. The kinetic equation for the oxidation is... 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

Autoxidation of alkanes generally promotes the formation of alkyl hydroperoxides, but d4-tert-huty peroxide has been obtained in >30% yield by the bromine-catalyzed oxidation of isobutane (66). In the presence of iodine, styrene also has been oxidized to the corresponding peroxide (44). 

Selectivity of alcohol formation can be substantially increased by carrying out the autoxidation in the presence of a stoichiometric quantity of boric acid that reacts with the intermediate hydroperoxide to form alkyl borate. This observation gained practical importance in the commercial oxidation of alkanes (see Section 9.5.1). 

Autoxidation of alkanes may be carried out by metal catalysis.2,14 17 Although metal ions participate in all oxidation steps, their main role in autoxidation is not in their ability to generate free radicals directly by one-electron oxidation [Eq. (9.14)] but rather their activity to catalyze the homolytic decomposition of the intermediate hydroperoxide according to Eqs. (9.15) and (9.16). As a result of this decomposition, metal ions generate chain-initiating radicals. The overall reaction is given in Eq. (9.17) ... 

We can suggest only one phase effect that might offset these effects. In the autoxidation of several hydrocarbons, Howard and Ingold (21) found solvent effects on kp and kt consistent with previously reported solvent effects on over-all rates (10, 19). We might then expect (25) that kp would be slightly larger and/or kt slightly smaller in the gas phase than in alkanes. 

Heteroatoms or functional groups can either increase or diminish the rate of autoxidation of alkyl groups. Haloalkanes and alkanes substituted with electron-withdrawing groups are usually more resistant toward homolytic C-H bond cleav-... 

Surprisingly, alkanes containing tertiary C—H bonds showed poor reactivity in these reactions.2943 b 29Sa d Thus, isobutane was less reactive than n-butane, and methylcyclohexane less reactive than cyclohexane (cf., lower reactivity of cumene to toluene). In the series of normal alkanes, n-butane reacted faster than n-pentane. n-Undecane was unreactive. These results are inconsistent with a normal free radical autoxidation. The authors used the analogy with arene oxidations to postulate that formation of radical cations by electron transfer from the alkane to Co(III) was a critical factor ... 

The use of mixed-metal catalysts can also dramatically affect the products of autoxidations. An example mentioned earlier is the selective oxidation of acetaldehyde to acetic anhydride in the presence of a mixture of cobalt and copper acetates. Another example is the co-oxidation of alkanes and olefins in the presence of both an autoxidation and an epoxidation catalyst (see Section III.B) ... 

As a further example the four hydroperoxides obtained in the autoxidation of oleate would be expected to give either the aldehydes and radical esters shown in the following equation or, alternatively, the oo-oxoesters and alkane and alkene radicals if the fi scission takes place on the other C-C bond. The free radicals can then react with neutral molecules or inactivate one another (Figure 2.12). 

Systematic examination of the catalytic properties of dimeric complexes was initiated shortly after the identification of dinuclear iron sites in metalloenzymes. The first report of a reactive dimeric system came from Tabushi et al. in 1980, who examined the catalytic chemistry of [Fe3+(salen)]20, 1 (salen is N,N -(salicylaldehydo)-l,2-ethylenediamine) (12). They reported interesting stereoselectivity in the oxidation of unsaturated hydrocarbons with molecular oxygen in the presence of mercaptoethanol or ascorbic acid and pyridine as a solvent ([l]<<[alkane]<<[2-mercaptoethanol]). With adamantane as substrate, they observed the formation of a mixture of (1- and 2-) adamantols and adamantanone (Table I) (12). Both the relative reactivity between tertiary and secondary carbons (maximum value is 1.05) and final yield ( 12 turnovers per 12 hr) were dependent on the quantity of added 2-mercaptoethanol. Because autoxidation of adamantane gave a ratio of 3°/2° carbon oxidation of 0.18-0.42, the authors proposed two coexisting processes autooxidation and alkane activation. 

Classical autoxidation of tertiary C-H bonds in alkanes can afford the corresponding hydroperoxides in high selectivities. This is applied industrially in the conversion of pinane to the corresponding hydroperoxide, an intermediate in the manufacture of pinanol (Fig. 4.43). 

Air oxidation of /i-butane to maleic anhydride is possible over vanadium phos(4tate and, remaiicably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in conffast to the situation found in the liquid, n-allcanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

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