Alkylaromatics benzylic oxidation

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]. 

Hydrogen attached to a tertiary hydrogen is most readily substituted secondary hydrogens are only very slowly replaced, while primary hydrogens are quite unreactive. Alkylaromatics react preferentially at the benzylic position. The reaction is accompanied by oxidation to yield carbonyl compounds, acids, and carbon oxides. Other main byproducts are nitrites, alcohols, and large quantities of poly-nitro compounds. 

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). 

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. 

Cobalt chloride in diglyme or methyl ethyl ketone - diglyme mixtures catalyzes the oxidation of benzylic groups of alkylaromatics to the corresponding aldehydes or ketones under mild conditions (60 -... 

In addition, both I and the radical cation can trap O, leading to new radical species and complication of the overall mechanistic picture [29]. In case of alkylaromatics, radical cations unda-go side-chain fragmentation to benzyl radicals, giving rise to side-chain oxidation products [15, 28] (see Section 14.2.4). 

In an effort to develop a suitable perovskite catalyst for the oxidation of alky-laromatics to carbonyl compounds, the attention has been focused on the use of LaMOs perovskites. Thus, several ABOg-type perovskite oxides (A = La, Y, Nd, or Gd B = Fe, Mn, Cr, or Co) have been investigated as catalysts for the oxidation of 1,2-dichlorobenzene, a model compound for the highly toxic polychlorinated dibenzodioxins [39]. LaMOg (M = Cr, Co, Fe, Mn, Ni) perovskites were used as catalysts in the oxidation of alkylaromatics to benzylic ketones, under solvent-free conditions, using TBHP as oxidant [48]. LaCrOs was found to be an efficient catalyst for the oxidation of alkylarenes to phenyl ketones, under mild reaction conditions, in the presence of TBHP as oxidant. Perovskites... 

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