Aromatic oxidation alkylaromatics

In addition, phenols are formed by the reaction of hydroxyl radical addition to the aromatic ring of oxidized alkylaromatic hydrocarbon [56]. 

The c-organyl complexes formed in oxidative addition [12] of alkanes, arenes as well as alkenes and monosubstituted acetylenes can be fairly stable and in many cases have been isolated. Thus, upon heating or photolysis, the complexes CP2WH2, Cp2 VCO,and CpjWHCHj give rise to a coordinatively unsaturated tungstocene species CpjW, which readily combines with aromatic or alkylaromatic hydrocarbons [13]. 

Aerobic oxidation is not going to be limited to alcohol oxidation. Apparently we are closer to finding suitable catalysts for alcohol oxidation, but oxidation of alkenes, aromatic hydrocarbons, alkylaromatics, imines, amines, sulfur compounds etc. will require much work in the forthcoming years. In some cases, the presence of radical initiators, even though in minor quantities, may serve to promote the oxidation catalyzed by noble metal nanopartides, and gold in particular [56, 108]. Thus, there is no doubt that the next years will witness exciting developments in the field of metal nanopartides for aerobic oxidation. 

The aromatic core or framework of many aromatic compounds is relatively resistant to alkylperoxy radicals and inert under the usual autoxidation conditions (2). Consequentiy, even somewhat exotic aromatic acids are resistant to further oxidation this makes it possible to consider alkylaromatic LPO as a selective means of producing fine chemicals (206). Such products may include multifimctional aromatic acids, acids with fused rings, acids with rings linked by carbon—carbon bonds, or through ether, carbonyl, or other linkages (279—287). The products may even be phenoUc if the phenoUc hydroxyl is first esterified (288,289). 

An additional curious feature of alkylaromatic oxidation is that, under conditions where the initial attack involves electron transfer, the relative rate of attack on different alkyl groups attached to the same aromatic ring is quite different from that observed in alkane oxidation. For example, the oxidation of -cymene can lead to high yields of -isopropylbenzoic acid (2,205,297,298). 

In the presence of anunonium bromide cobalt (ref. 22) and manganese (ref. 23) have been shown to catalyze the ammoxidation of methylaromatics to the corresponding aromatic nitriles (Fig. 20). It is interesting to compare this homogeneous, liquid phase system with the more well-known vapour phase ammoxidation of alkylaromatics over oxidic catalysts (ref. 4). 

Finally, it makes possible the oxidation of hydrocarbon to a significant depth, and when the RH molecule contains several methyl groups, the catalyst allows all these groups to be transformed into carboxyls. This last specific feature is insufficiently studied so far. Perhaps, it is associated with the following specific features of oxidation of alkylaromatic hydrocarbons. The thermal decomposition of formed hydroperoxide affords hydroxyl radicals, which give phenols after their addition at the aromatic ring... 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

The question arises as to whether inner-sphere complexes of the aromatic hydrocarbon with cobalt(III) are involved in electron transfer. An investigation260 was carried out of the oxidation of alkylaromatic hydrocarbons by the heteropoly compound Ks [Co(III)04W12036] H20. Electron exchange between the Co(III) complex, which contains tetrahedral cobalt, and the corre-... 

Therefore the catalysis of the oxidation of the alkylbenzenes to the corresponding aldehydes is kept alive by the formation of an excess of Co ", formed by the oxidation of the aldehydes with oxygen. In general, oxidation intermediates like aromatic aldehydes and peroxides, which are normally more reactive than the corresponding toluenes, can regenerate highly oxidized metal species. Besides the free-radical mechanism stoichiometric and ionic reaction pathways also play an important role in the oxidation of alkylaromatic compounds. This is shown with Co " as oxidant on the left-hand side of Scheme 2. 

This basic mechanism was originally proposed for oxidation of alkylaromatics but was thought to require the /r-system of the aromatic ring e.g. according to eq. (26) ... 

Partial catalytic oxidation of alkylaromatic hydrocarbons is interesting both from the industrial and the scientific point of view. The industrial interest is due to the availability of these substances from the petrochemical industry and to a number of applications for the possible oxidation products. Conventional gas phase oxidation concerns the side chain and leads mainly to benzoic acid or even to destruction of the aromatic ring. Oxidation under mild conditions could cease the reaction at earlier stages and reduce the number of the products formed. However, the appropriate catalyst for such partial oxidation has not been found yet. 

There are two routes to the partial oxidation of alkylaromatic hydrocarbons - oxidation of the side chain or of the aromatic ring. In both cases the oxidation could proceed at different positions (except for the methyl group of toluene) and to different extents. Titanium silicalites were found to activate the oxidation of the secondary or tertiary saturated carbon atoms, the terminal methyl groups remaining unaffected [2,6-8]. This behaviour limits the number of the... 

Similarly, the CrAPO-5- and chromium silicalite-1 (CrS-l)-catalyzed oxidation of aromatic side-chains with TBHP or O2 as the primary oxidant [27-31] almost certainly arises as a result of soluble chromium(VI) leached from the catalyst. The same probably applies to benzylic oxidations with TBHP catalyzed by chromium-pillared montmorillonite [32]. More recently, a chromium Schiff s base complex tethered to the mesoporous silica, MCM-41, was claimed [33] to be an active and stable catalyst for the autoxidation of alkylaromatic side-chains. It would seem unlikely, however, that Schiff s base ligands can survive autoxidation conditions. Indeed, on the basis of our experience with chromium-substituted molecular sieves we consider it unlikely that a heterogeneous chromium catalyst can be developed that is both active and stable to leaching under normal oxidizing conditions with O2 or RO2H in the liquid phase. Similarly, vanadium-substituted molecular sieves are also unstable towards leaching under oxidizing conditions in the liquid phase [6,34]. 

Few studies refer to the oxidation of aromatics. The hydroxylation of benzene to phenol [81] and the oxidation of alkylaromatics to arylcarboxylic acids [82] have been claimed. The oxyfunctionalization of saturated C-H bonds has not been reported. 

Knowledge of how aluminum chloride oxidizes aromatics to cation radicals is practically non-existent. At one time it seemed that a nitro compound was a necessary co-acceptor (Buck et al., 1960) and that, whereas with mononuclear alkylaromatics, the Lewis acid-nitro compound pair formed only charge transfer complexes (Brown and Grayson, 1953), complete electron transfer occurred with more easily oxidized aromatics. But, cation-radical formation from perylene, anthracene, and chrysene was found to occur in carbon disulfide, chloroform, and benzene solutions, too (Rooney and Pink, 1961) and even occurs on warming anthracene and naphthacene with solid aluminum chloride (Sato and Aoyama, 1973). There is no doubt that a nitro compound enhances electron transfer, however (Sullivan and Norman, 1972). Cation radical formation in AlCl3-nitromethane has been estimated as approximately 100% as compared with 1% in sulfuric acid oxidation of dialkoxybenzenes (Forbes and Sullivan, 1966). Unfortunately, aluminum halide salts have not been isolated and, therefore, even the beginnings of analytical data have yet to be collected. There is no definite knowledge of either the nature of the counter ion or the fate of the electrons in these cation-radical formations. 

Oxidation of aromatics by thallium trifluoroacetate in TFA at 20° also gives the cation radicals of alkylaromatics, and it is thought, because of this, that thallation of aromatics may involve cation radical formation first (Elson and Kochi, 1973). 

Metal-ion catalysed air oxidations of aromatic compounds are industrially important. Oxidation of a methyl to a carboxylic acid group, such as in the first stage of the manufacture of terephthalic acid from p-xylene, is believed to involve peroxidation of benzylic carbon [formed in eqn (29)] (Andrulis et al., 1966). The reactions leading to benzyl acetates [eqns (29)-(31) for example] must therefore be carried out in the absence of air. Oxidations of alkylaromatics in the presence of oxygen and involving cation radical intermediates have been reported (Onopchenko et al., 1972 Holtz, 1972 Scott and Chester, 1972). 

The mechanism via bromine atoms is supported by molecular bromine formation in the interaction of with Br in the absence of a hydrocarbon (Bf2 is apparently formed by bromine atom recombination). This mechanism is also consistent with the fact that bromide ions, while catalyzing the oxidation in the case of alkylaromatic compounds, are not particularly effective in the case of simple alkanes. This corresponds to the difference of bromine atom reactivity with respect to alkylaromatic and aliphatic hydrocarbons. The bond energy in the H-Br molecule (85 kcal mole ) is practically equal to the energy of the C-H bond in the n.-position to the aromatic ring, so that the reaction... 

Our primary interest is to oxidize hydrocarbons with air, at low temperatures, in the absence of sacrificial reductants. While neither aliphatic nor aromatic hydrocarbons are significantly oxidized under our conditions, alkylaromatics containing benzylic hydrogen are oxidized. For example, in the presence of 1, cumene is oxidized to its hydroperoxide at 65 C, conversions of about 1.3 weight %/hour can be easily obtained with moderate air flows. 

Water-soluble macromolecular metal complexes based on terminally functionalized ethylene oxides and ethylene oxide-propylene oxide block copolymers have been used as catalysts for hydroformylation, hydrogenation, Wacker oxidation of imsaturated compounds, hydroxylation of aromatic compounds, oxidation of saturated and alkylaromatic hydrocarbons, metathesis, Heck reaction, and some asymmetric reactions. 

Wathever the actual mechanism, oxide clusters, including aklali oxides appeared to be able to catalyze the formation of a carbide species which could react with methanol, thus resulting in an overall alkylation of the side chain of substituted aromatics. Further dehydrogenation of the resulting alkylaromatic was also achieved with such oxide species. 

Other applications for aromatic hydroxylation are tiny in comparison with phenol, but the same technology can in principle be used. Side-chain oxidation is a possible side reaction for alkylaromatics. 

Jary, W. Pemdorfer, E. Roessler, M. Poechlauer, P. Hartmann, M. Alsters, P Method for producing aromatic aldehydes and ketones by the catalytic oxidation of alkylaromatic compounds. PCT Int. Appl. WO 2002096849, 2002 Chem. Abstr. 2002,138, 13957. 

Extensive discussions of these substitution reactions may be found elsewhere (see Refs. 19, 22, 23, and 24, for example) providing the information to support the mechanistic conclusions and indicating the areas of controversy. A comprehensive literature survey through 1971 was presented in tabular form by (N.L.) Weinberg to include experimental conditions such as electrolyte, electrode material, temperature, and current density as well as the reaction products. The oxidations of alkylaromatics " and polynuclear aromatics are also reviewed in separate chapters in the recently published encyclopedia. 

There are two directions in the development of supramolecular catalytic compositions, that is, (1) creation of systans based on macrocyclic compounds as host molecules that bind substrates with their hydrophobic cavity and (2) development of the systems that bind substrates using aggregates formed by am-phiphihc compounds. Compounds that form host-guest complexes like modified cahxarenes are able to aid transport of substrates into the aqueous phase. This approach has been implemented in the Wacker oxidation [40,41], oxidation of alkylaromatic compounds [42], hydroxylation of aromatic compounds [43], hydrogenation [44,45], hydroformylation [45-48], and carbonylation [49]. In this case, the substrate is transported into the aqueous phase in the form of the corresponding inclusion complex. This not only affects the activity of the catalyst, but also provides selectivity of the process. Thus, in the Wacker oxidation of 1-alkenes the maximum yield of methyl ketone was achieved when 1-hexene is used, and for systems based on calix[6]arene with 1-octene among catalytic systems with modified calix[4]arenes [50]. 

---