Oxidation of alkylarenes

Aerobic oxidation of alkylbenzenes is a promising subject in industrial chemistry. Many bulk chemicals such as terephthalic acid, phenol, benzoic add, and so on are 

Representative results for the NHPI-catalyzed aerobic oxidation of various alkylbenzenes in the presence of Co(OAc)2 in acetic acid under ambient conditions are listed in Table 6.1. Both p- and o-xylenes are selectively oxidized to p- and o-toluic adds without the formation of dicarboxylic acids. o-Ethyltoluene undergoes selective oxidation to form a mixture of the corresponding alcohol and ketone in which the ethyl moiety was selectively functionalized. It is of interest to examine the effect of substituents on the aromatic ring in the oxidation of substituted toluenes. p-Methoxytoluene is more rapidly oxidized than the toluene itself, while 

Terephthalic acid (TPA) as well as dimethyl terephthalate (DMT) have recently become very important as raw material for polyethylene terephthalate [50]. In 1999, about 17 x 10 tonnes of TPA were manufactured worldwide, and its 

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Ambulgekar GV, Samant SD, Pandit AB (2004) Oxidation of alkylarenes to the corresponding acids using aqueous potassium permanganate by hydrodynamic cavitation. Ultrason Sonochem 11 191-196... 

Sodium cyclopentadienide, 41, 96 Sodium dichromate for oxidation of alkylarenes to aromatic carboxylic acids, 43, 80 Sodium ethoxide for condensation of diethyl oxalate with diethyl succinate, 44, 67... 

Despite the ionic reactions, other reactions (presented in Scheme 44) show that free-radical mechanisms can also take place. Catalyzed by ebselen, TBHP oxidation of alkylarenes to alkyl aryl ketones (141) [240], anthracene to anthraquinone (142), 1,4-dimethoxyarenes to 1,4-quinones (e.g., menadione 143) [244], and oxidative coupling of 2-aminophenol to phenoxazinone (144) [245] gave results similar to these when one-electron oxidants such as Ce(IV), Ag(II), or Mn(III) were the reagents. Moreover, oxidation of azine derived from 2-acetylpyridine gave a mixture of ketone (145) and condensed triazole (146) [240], The same result was found when cerium ammonium nitrate was used as the reagent. This suggests that the... 

The selective oxidation of hydrocarbons into hydroperoxides, primary products of oxidation is the most difficult problem because of the high catalytic activity of the majority of applied catalysts in ROOH decomposition. At the same time, the problem of selective oxidation of alkylarens (ethylbenzene and cumene) with molecular 02 in ROOH, is of current importance from the practical point of view in connection with ROOH use in large-tonnage productions such as production of propylene oxide and styrene (a-phenylethylhydroperoxide, PEH), or phenol and acetone (cumyl hydroperoxide) [1],... 

Much higher yields are found for the oxidation of alkylarenes to aldehydes, ketones and carbon acids by oxygen in the presence of transition metal compounds (MnBr2, CoBr2, CuBr) as catalysts [57]. For example, a yield of 30 % benzaldehyde and only 10 % benzoic acid was found for the oxidation of toluene. Also the dehydrogenation of cyclohexene and cyclohexane to benzene with Pt02 as catalyst was investigated at 375 °C [34]. 

Scheme 4. Selective radical decay in the oxidation of alkylarene with 2...

IBX is an efficient and selective reagent for the oxidation of alkylarenes 855 at the benzylic positions to give ketones 856 (Scheme 3.341) [1174]. This reaction is quite general and can tolerate various substituents within the aromatic ring. Overoxidation to the corresponding carboxylic acids is not observed even in the presence of electron-rich substituents. 

HOMOGENEOUS CATALYTIC OXIDATIONS OF ALKYLARENS WITH MOLECULAR OXYGEN... 

The various catalytic systems on the base of transition metal compounds have been used for the alkylarens oxidation with molecular oxygen. And all of them catalyzed alkylarens oxidations mainly to the products of deep oxidation [6, 34]. One of the most striking examples is the oxidation of alkylarens into carbonyl compounds and carbonic acids by dioxygen in the presence of so-called MC-catalysts (Co(ll) and Mn (II) acetates, HBr, HOAc) [6]. 

For the first time the phenomenon of significant rise of not only initial rate (W j), but also the selectivity (S = [PEH] / A[RH] 100%) and conversion degree (C = A[RH]/[RH]p 100%) of oxidation of alkylarens (ethylbenzene, cumene,) into ROOH by molecular O2 under catalysis by transition metals complexes M(L )2 (M = Ni(ll), Co(ll), L =acac ) in the presence of additives of electron-donor monodentate ligands (L = HMPA (hexameth-ylphosphorus triamide), dimethyl fomiamide (DMF), N-methyl pyrrol-idone-2 (MP)), MSt (M = Li, Na, K) was found by authors of the articles [55-57]. 

Hanotier, J. Hanotier-Bridoux, M. de Radzitzky, P. Effect of Strong Acids of the Oxidation of Alkylarens by Manganic and Cobaltic Acetates inAcetic Acid, J. Chem. Soc., Perk. Trans. II, 1973, 381. 

Potassium permanganateItriethylaminejsulfuric acid Benzylic oxidation of alkylarenes... 

Transition metal-catalyzed oxidations of alkylarenes may result in a wide variety of products. For example, MN gives not only target MNQ but also isomeric 6-MNQ as well as products of C—C coupling, side-chain oxidation, and ovCToxidation (Scheme 14.12). 

Toluene reacts at a similar rate to that of benzene and prodnces ring-hydroxylated compounds (56% 4-methyI- and 41% 2-methylphenol) as the sole produets [99], In contrast, mesoporons titaninm-silicates favor oxidation of the tolnene side-chain [100], The rate of hydroxylation of xylene isomers over TS-1 was sterically controlled, orthopores zeolite Ti-MOR, it inereased as expected for an electrophilic attack, parasterically hindered aromaties beeause of pore size limitation. The development of an efficient heterogeneons catalyst for the seleetive oxidation of alkylarenes remains a great challenge [100],... 

Also, the vanadium-substituted polyoxomolybdate, Q5[PV2Moio04o] (see Figure 9.1b, Q = (C4Hg)4N ) was catalytically active and highly selective in the oxidation of primary and secondary alcohols with nitrous oxide to yield aldehydes and ketones, respectively. Scheme 9.3 [25]. In addition, the same catalyst under similar conditions catalyzed the oxidation of alkylarenes. Scheme 9.3. In the oxidation of alkylarenes with hydrogen atoms at P positions, the substrate is dehydrogenated, for example, cumene to a-methylstyrene, whereas in the absence of p hydrogen atoms ketones are formed, for example, diphenylmethane to benzophenone. 

Scheme 9.3 Oxidation of alkylarenes and alcohols with N2O catalyzed by Q5[PV2Mo3o04ol-...

The oxidation of benzylic alcohols was quantitative within hours and selective to the corresponding benzaldehydes, but the oxidation of allylic alcohols was less selective. The oxidation of aliphatic alcohols was slower but selective. In mechanistic studies considering oxidation of benzylic alcohols, similar to the oxidation of alkylarenes, a polyoxometalate-sulfoxide complex appears to be the active oxidant. Further isotope-labeling experiments, kinetic isotope effects, and especially Hammett plots showed that oxidation occurs by oxygen transfer from the activated sulfoxide and elimination of water from the alcohol. However, the exact nature of the reaction pathway is dependent on the identity of substituents on the phenyl ring. 

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... 

Sulfoxides have been exploited as oxygen donors. Dimethylsulfoxide has been used in the presence of [PMoi204o] , under anaerobic conditions, for the oxidation of alkylarenes such as xanthene and diphenylmethane to xanthen-9-one and benzophenone, respectively, with up to 96% yield, in 15h at 170°C in 1,2-dichlorobenzene. 

Khenkin, A. and Neumann, R. (2004). Oxidation of Alkylarenes by Nitrate Catalyzed by Poly-oxophosphomolybdates Synthetic Apphcations and Mechanistic Insights, J. Am. Chem. Soc., 126, pp. 6356-6362. 

The problem of selective oxidation of alkylarens to hydroperoxides is economically sound. Hydroperoxides are used as intermediates in the large-scale production of important monomers. For instance, propylene oxide and styrene are synthesized from a-phenyl ethyl hydroperoxide (PEH), and cumyl hydroperoxide is the precursor in the synthesis of phenol and acetone. The method of modifying the Ni" and Fe° complexes used in the selective oxidation... 

Scheme 8.4 Oxidation of alkylarenes with phenylmethylsulfoxide catalyzed by Q3[PMoi204o]...

FIGURE 19.36 Oxidation of alkylarenes to aromatic carboxylic acids. 

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