Oxidations of alkylaromatics

VF Tsepalov. The Study of Elementary Reactions of Liquid-Phase Oxidation of Alkylaromatic Hydrocarbons. Doctoral Thesis Dissertation, Institute of Chemical Physics, Chernogolovka, 1975, pp 3-40 [in Russian],... 

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... 

E0 = 40 kJ mol-1 at AH=0) is substituted by a few consecutive fast reactions with electron transfer. Russel [284-291] studied a few reactions of oxidation of alkylaromatic hydrocarbons in the presence of strong bases. He proved the chain mechanisms of these reactions. One of them includes a few stages with addition of dioxygen to carbanion. Another includes the electron transfer from carbanion to dioxygen. 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1], The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16], Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction [6], However, the addition of an aqueous solution of Na2C03 or NaOH in combination with vigorous agitation of this system accelerates the oxidation process [1-17]. The addition of an aqueous phase accelerates the oxidation and withdrawal of water retards it [6]. The addition of surfactants such as salts of fatty acids accelerates the oxidation of cumene in emulsion [3], The higher the surfactant concentration the faster the cumene autoxidation in emulsion [17]. The rates of cumene emulsion oxidation after an induction period are given below (T = 353 K, [RH] [H20] = 2 3 (v/v), p02 = 98 kPa [17]). 

Hence, the peculiarities of emulsion oxidation of alkylaromatic hydrocarbons can be formulated as follows. 

Table 1. Catalytic oxidations of alkylaromatics using Co(III)-CMSl in neat substrate at...

The electrochemical results described above indicate that unlike in the cases of other cobalt-catalyzed oxidation processes where the Co /Co redox couple is invariably involved [19b,38], in the present case where cubane clusters of the general formula Co4(p3-0)4( J,-02-CR)4(L)4 are to be employed as catalysts for the air/02 or TBHP oxidation of alkylaromatics, alcohols, etc., we have a catalytic system wherein the oxidation states of cobalt cycle between +3 and +4. The kinetic inertness of Co(lll) coupled with the inadequately explored reactivity of Co(lV) thus make the catalysts based on C04O4 cubanes quite interesting [36]. We shall now discuss the resulting materials prepared by supporting the cubane-like cobalt(lll)-oxo clusters discussed above in this section by following the chemical route in which the carboxylate anion derived from CMS-CH2CH2CO2H binds the in situ or preformed cobalt(III)-oxo tetramers at elevated temperatures. 

The calcined iron-grafted materials exhibit high selectivity as catalysts for oxidations of alkanes, alkenes and arenes with H2O2 as the oxidants [13a]. A similar method has been used by Tilley et al. to prepare a pseudotetrahedral (Co(II) [Co(4,4 -di Bu-bipy) OSi(0 Bu)3 2]) complex grafted onto the SBA-15 surface and subsequently use it in catalytic oxidation of alkylaromatic substrates with tert-butyl hydroperoxide [14]. Unfortunately, neither iron nor cobalt surface organometaUic compounds have been tested in the recycled catalytic system. 

Oxidations of alkylaromatic, phenol and hydroqninone snbstrates by macrocy-clic complexes containg the trans-Ru XO) unit have been reviewed, with emphasis on mechanistic aspects [242]. 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1]. The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16]. Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction... 

The cyclic sequence of Reactions 5 and 6 can be accelerated by adding a species which promotes the formation of Co(III) from Co(II). In the oxidation of alkylaromatic hydrocarbons ozone (7), acetaldehyde (8), and methyl ethyl ketone (4) act as promoters in this way. 

Studies in this field are just beginning, and the number of publications hardly exceeds a dozen. The most interesting results were obtained by the research groups of Yamada [160-162], Neumann [163,164] and Kozhevnikov [165, 166], Using various type catalysts (Ru porphyrene complexes, polyoxometalates, supported metals), the authors conducted selective oxidations of various types. These include epoxidation of alkenes, oxidation of alcohols, oxidation of alkylaromatics, oxidation and aromatiza-tion of dihydroanthracenes, and some other reactions. The experiments were typically conducted at 373—423 K under 1.0 MPa pressure of nitrous oxide. 

Roginskii, S. Z., Berlin, A. A., Kutseva, L. N., Aseeva, R. M., Cherkashina, L. G., Sherle, A. I., Matseeva, N. G. Catalytic Properties of Organic Polymers with a Conjugated Bond System. The Formation of Hydroperoxides by Oxidation of Alkylaromatic Hydrocarbons and Cychlohexane. Dokl. SSSR, Chemistry Section (English Transl.) 148,35 (1963) (1963),... 

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-... 

In general, liquid phase autoxidations on hydrocarbons after the initial stages take place, may be considered as co-oxidations with aldehydes, alcohols, ketones, carboxylic acids, etc. Often aldehydes or ketones are deliberately added to hydrocarbon autoxidations in order to promote the reaction. For example, in the cobalt-catalyzed oxidations of alkylaromatics (see Section II.B.3.b), aldehydes, or methyl ethyl ketone are usually added in commercial processes in order to attain high rates and eliminate induction periods. 

Co(III) acetate oxidation of alkylaromatics (entry no. 4) displays the same characteristics as those of entry no. 3, and, as concluded by several groups (Cooper and Waters, 1967 Hanotier and Hanotier-Bridoux, 1973), this reaction cannot be initiated by electron transfer. 

The Ce(IV) oxidation of alkylaromatic hydrocarbons has been extensively studied in recent years, and entry no. 13 indicates that this reagent should be a well-behaved electron-transfer reagent, in line with other kinetic evidence (Baciocchi etal., 1980, 1981). 

In principle, the ability of the polymers to catalyse mfld oxidations of alkylaromatic compounds by forming hydroperoxides at a secondary or tertiary carbon atom was also shown before ). 

In nonaqueous solvents, water is an impurity that, on a preparative scale, even rigorous drying fails to remove completely [20]. Hydroxylation products are therefore common by-products. An example is the oxidation of alkylaromatic compounds in a nominally anhydrous solution of NaC104 in MeCN [Eqs. (10) and (11)], which gives mainly the product of acetamidation [21-23]. 

In spite of its topicality, the history of the industrial transition metal-catalyzed oxidation of alkylaromatic compounds dates back to the early 1920s with the continuous oxidation of ethylbenzene to acetophenone using manganese acetate as catalyst. This process was developed by the IG Farben at Uerdingen [2]. 

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. 

Scheme 2. Cobalt-catalyzed oxidation of alkylaromatic compounds.

As a result, the rate of oxidation of alkylaromatic compounds is mainly dependent on the ratio of the rate of radical chain propagation and the rate of radical chain demolition with [ p/(2 t)° Jrei as a value for a relative oxidizability. For further discussion, cf. [11b, 19b, 23a, 27-30]. 

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