Arylalkanes, oxidation

The hydroxylation of cyclohexane, of potential interest for the production of cyclohexanone, is exceedingly slow at near room temperature and has low selectivity at 100 °C [27, 28]. Tertiary C—H bonds yield tertiary alcohols, with little or no oxidation observed at the secondary carbons that may be present in the alkyl chain t-C—H sec-C—H (Table 18.3). The steric constraints introduced by alkyl substitution strongly favor the competition of side reactions, at the expense of hydroxylation. On arylalkanes, oxidation occurs on both the aromatic ring and the alkyl chain, with a general preference for the latter. Consistently, the competitive hydroxylation of benzene and n-hexane or cyclohexane mainly occurs on the alkane. However, benzylic methyls, despite the relative weakness of their C—H... 

A possible mechanism of oxidation of methylene groups to carbonyl groups involves autoxidation (oxidation by molecular oxygen) at the benzylic position. Autoxidation of arylalkanes is a facile reaction with low activation energies for example, 6.0 kcal/mole for 1,1-diphenylethane and 13.3 kcal/mole for toluene. ... 

The oxidative effects of silver(II) complexes of pyridine carboxylates have been studied for a variety of substrates. With ar-amino acids, a rapid reaction occurred at 70 °C in aqueous solution with bis(pyridyl-2-carboxylato)silver(II). 4 The product was the next lower homologous aldehyde and yields were generally greater than 80%. Other substrates included primary and secondary amines, alcohols, monosaccharide derivatives, alkenes, arylalkanes and arylalkanols.90 Only minor differences were detected in efficiencies when 2-, 3- or 4-mono-, or 2,3-di-carboxylates were used as the oxidant. 

Because r is larger than r then a linear plot ofln k vs. 1 /eR with a positive slope will be obtained irrespective of the charge on the ion. The observation3 that a positive slope of log/c vs. l/sR was obtained for the oxidation of triphenylmethane and diphenylmethane by [Fe(CN)6]3 in various mixtures of aqueous acetic acid was taken as proof that the rate determining step was of the ion dipole type, i.e. involved [Fe(CN)6]3- and the arylalkane molecule. 

Aryl ketones or aldehydes.1 Reaction of arylalkanes with DDQ in aqueous acetic acid can result in carbonyl products. Thus reaction of 1 with DDQ in HOAc-H20 results in the ketone 2, believed to be formed by acetylation at the benzylic position followed by acid-catalyzed hydrolysis to the corresponding alcohol, which is oxidized to a ketone by DDQ. 

Arylalkanes can react with electrophihc oxidants [58] to produce alkyl radicals via simultaneous electron transfer and deprotonation ... 

Oxidations of arylalkanes by "Bu4NMn04 in toluene solvent led to the conclusion that rate-limiting hydrogen atom transfer from the substrate to a permanganate oxo group is consistent with all of the experimental evidence [69a] ... 

Arylalkanes have been oxidized by a molybdenum(VI) complex dioxodithiocyanato-4,4"-di-terf-butyl-2,2"-bipyridylmolybdenum [73], The reaction can also be carried out under catalytic conditions using DMSO as the oxo-donor reactant. 

The hypothesis that the amine-catalysed oxidation of thiols is a particular case of the more general reaction of oxidation by molecular oxygen of thiolate ions is confirmed by the finding that arene-thiols, which are more acidic and hence more dissociated, are oxidized faster than arylalkane-and alkane-thiols in the presence of amines . 

Pb(C2H5)4 is employed as a chain-starter for the alkylation of alkanes or cycloalkanes by alkenes or cycloalkenes [697], and in the liquid-phase oxidation of alkylaromatic hydrocarbons to give hydroperoxides [892]. Pb(C2Hs)4 catalyzes the photochemical addition of HBr [661, 681] or of H2S or mercaptans to alkenes, such as propene [661, 679, 681], and it catalyzes the intermolecular condensation of arylalkanes in sunlight or on heating [684]. 

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