Cobalt -catalyzed autoxidation

Consequently, as a result of increasing environmental pressure many chlorine and nitric acid based processes for the manufacture of substituted aromatic acids are currently being replaced by cleaner, catalytic autoxidation processes. Benzoic acid is traditionally manufactured (ref. 14) via cobalt-catalyzed autoxidation of toluene in the absence of solvent (Fig. 2). The selectivity is ca. 90% at 30% toluene conversion. As noted earlier, oxidation of p-xylene under these conditions gives p-toluic acid in high yield. For further oxidation to terephthalic acid the stronger bromide/cobalt/manganese cocktail is needed. 

Hydrocarbon Concentration. The steady rate of hydrocarbon oxidation is exactly first order with respect to hydrocarbon concentration, but it tends to be independent of this concentration below l.OAf (Figure 4). The cobalt-catalyzed autoxidation of Tetralin (6) and ethylbenzene at 0.05M cobalt in the absence of bromide is exactly second order with respect to hydrocarbon concentration. 

Other workers348 d 124 have reported that commencement of the cobalt-catalyzed autoxidation of pure hydrocarbons, i.e., nonpolar solvent, is accompanied by oxidation of Co(II) to Co(III). The transformation is easily observed by the change in color from pale violet or pink [Co(II)] to intense green [Co(III)]. Similarly, manganese-catalyzed autoxidations were observed to start when Mn(II) was converted to Mn(III). The concentration of Co(III) reached a maximum during the course of autoxidation and then decreased. This maximum coincided with the appearance of aldehydes in the reaction mixtures. The authors also showed by calculation124 that reduction of Co(III) by a secondary product accounted for the observed kinetics much better than reduction by hydroperoxide. Hence, the decrease in concentration of Co(III) after it had reached a maximum was attributed to reduction by aldehydes (see Section II.B.3.e), which was much more facile than reduction by alkyl hydroperoxide [reaction (96)]. 

In support of the electron transfer mechanism [Eqs. (139)—(141)], the ESR spectra of various radical cations have been observed during reaction of alkenes with Co(III) in trifluoroacetic acid mixtures.218 However, a very different situation may obtain in the cobalt-catalyzed autoxidation of olefins in neutral non-... 

In the presence of oxygen, reaction (186) is replaced by reaction (187). The rate of reaction is first order in Co(III) under these conditions.253,258 Other workers241 254"256 have observed, however, a second-order dependence on Co(III) concentration, which is more difficult to explain. A rate law containing both second-order and half-order terms in Co(III) has also been reported257 for the cobalt-catalyzed autoxidation of toluene in acetic acid. The mixed kinetic expression was explained by the participation of reactions of both a Co(III) monomer and a Co(III) dimer with the substrate. 

Such a scheme by itself is insufficient to explain the accelerating effect of hydrogen bromide on cobalt-catalyzed autoxidations since optimum rates are achieved only in the presence of both hydrogen bromide and cobalt. One of the functions of the Co(II) is to maintain the concentration of hydrogen bro-... 

In the presence of bromide ion there is apparently no direct reaction of Co(III) with the hydrocarbon substrate, in contrast to cobalt-catalyzed autoxidations carried out in the absence of bromide. That different mechanisms are operating is illustrated by the relative rates of oxidation of alkylbenzenes catalyzed by cobalt acetate alone compared to those obtained in the presence of added bromide ion (Table VIII). In the presence of bromide ion, the relative reactivities are consistent with a mechanism involving attack by bromine atoms but not one involving electron transfer. Individual discrepancies in selectivities between bromine atom and the species active in the Co(0 Ac)2-NaBr system (Table VIII) were attributed to a bromine complex,... 

Write equations to explain (a) Use of Br /Br as co-catalyst in cobalt-catalyzed autoxidation. (b) Hydroxylation of hydrocarbon by Cyt P450. 

The cobalt-catalyzed autoxidation of toluene in acetic acid at 363 K is accelerated by butan-2-one and benzaldehyde because peroxy radicals play a minor role in ratecontrolling propagation reactions. High rates of autoxidation are also obtained in the presence of Br because bromine atoms are important chain-propagating species. ... 

The effects of manganese on the cobalt/bromide-catalyzed autoxidation of alkylaromatics are summarized in Figure 17. The use of the Mn/Co/Br system allows for higher reaction temperatures and lower catalyst concentrations than the bromide-free processes. The only disavantage is the corrosive nature of the bromide-containing system which necessitates the use of titanium-lined reactors. 

However, the rate of oxidation in the presence of bromide ion (Figure 2) is exactly first order with respect to cobalt. The autoxidation of hydrocarbons catalyzed by cobalt and bromide ion is characterized by the fact that the rate increases with increasing cobalt concentration, while the rate at high cobalt concentrations reaches a limiting value in the absence of bromide ion. 

Many authors128, 142-146 have proposed reaction (112), or a variant of it, in an attempt to explain kinetic data. For example, Uri145 proposed the following mechanism for the initiation of cobaltous stearate-catalyzed autoxidation of methyl linoleate in benzene ... 

Similarly, the cobalt acetate-catalyzed autoxidation of p-cymene afforded mainly p-isopropylbenzoic acid,243,244 246 derived from selective oxidation of the methyl group. Manganese acetate, on the other hand, was involved in a catalyzed autoxidation,... 

Bawn and co-workers carried out detailed investigations of metal-catalyzed autoxidations of acetaldehyde313,314 and benzaldehyde.315 316a b The rate of chain initiation in the autoxidation of benzaldehyde catalyzed by cobalt acetate in acetic acid was equal to the rate of reaction of Co(III) with benzaldehyde in acetic acid in the absence of oxygen. Moreover, the onset of oxygen absorption coincided with the conversion of Co(II) to Co(III). The catalyst was maintained... 

In the cobalt- and manganese-catalyzed autoxidation of acetaldehyde, direct reaction of the latter with the catalyst in its higher oxidation state constitutes the rate-determining step. 

Depending on the conditions, metal-catalyzed autoxidation of acetaldehyde can be utilized for the manufacture of either acetic acid or peracetic acid.321 In addition, autoxidation of acetaldehyde in the presence of both copper and cobalt acetates as catalysts produpes acetic anhydride in high yield.322 b The key step in anhydride formation is the electron transfer oxidation of acetyl radicals by Cu(II), which competes with reaction of these radicals with oxygen ... 

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. 

Bromide, as hydrogen bromide, alkali bromide, NH4Br, or CoBr2, or organically bound bromide as in bromoform, tetrabromoethane, or monobromoacetic acid, has an expressed effect on the cobalt- and manganese-catalyzed autoxidations of al-kylaromatic hydrocarbons. The catalytic activity of the metal ions is drastically increased by an addition of bromide ions in the right molar ratio, mostly n(metal)/n(Br) = 1 1. 

There have been detailed studies on the relative reactivities of hydrocarbons in cobalt catalyzed oxidations. Relevant data have been collected by Sheldon and Kochi [3]. Remarkably, toluene is about 3 times more reactive than cumene in cobalt mediated oxidation. The situation is reversed in uncatalyzed autoxidation. 

Labuza, T.P., J.F. Maloney, M. Karel, Autoxidation of methyl linoleate in freeze-dried model system II. Effect of water on cobalt catalyzed oxidation, J. Food Sci., 31, p. 885, 1966. 

Alkoxycyclopropane ring rearrangements have been -used in the synthesis of several terpenes. Steroids can be obtained by a cobalt-catalyzed stereospecific triple ring closure of enediynes . Practical routes for large-scale preparation of Cinchona alkaloids and their analogs, including the preparation of quinine and quinidine by a highly stereospecific autoxidation, have been founds . 

Copper complexes were used as efficient catalysts for selective autoxidations of flavonols (HFLA) to the corresponding o-benzoyl salicylic acid (o-BSH) and CO in non-aqueous solvents and at elevated temperatures (124-128). The oxidative cleavage of the pyrazone ring is also catalyzed by some cobalt complexes (129-131). 

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

Autoxidation of Hydrocarbons Catalyzed by Cobalt and Bromide Ions... 

Oxidation Products. Although the ratio of hydroxyl to carbonyl products is 1/1 or nearly so in the ordinary metal salt-catalyzed autoxi-dation of hydrocarbons, higher proportions of carbonyl compounds are obtained in autoxidations catalyzed by cobalt and bromide ion—e.g.,... 

The effect of bromide ion was more pronounced in polystyrene oxidation. Although polystyrene in a 1/1 mixture by volume of chlorobenzene and acetic acid is barely autoxidized at 100°C. in the presence of cobalt salt or initiators, the oxidation catalyzed by cobalt is so strongly accelerated by bromide ion that it proceeds rapidly even at temperatures as low as 45°C. (Figure 10). 

Allan S. Hay During the autoxidation of p-xylene catalyzed by cobalt acetate bromide, a potentiometric titration for bromide ion of an aliquot of the reaction mixture at 0°C. shows that only a fraction of the bromide is present in ionic form. If the titration is performed at room temperature, there is a gradual drift of the end point until it finally corresponds to the calculated total amount of bromide. The implication thus is that benzylic bromides are present during the reaction, and at room temperature during the titration they are slowly solvolyzed. 

Similarly, cobalt(ll)-pyridine (CoPy) complexes bound to copolymers of styrene and acrylic or methacrylic acid, cross-linked with divinylbenzene, catalyze the autoxidation of tetralin dispersed in water at 50°C and 1 bar.45 The rate of oxidation with the colloidal CoPy catalyst was twice as fast as with homogeneous CoPy and nine times as fast as with cobalt(II) acetate in acetic acid. 

Rhodium and iridium, which are in the same group as cobalt in the Periodic Table, are also expected to effect reactions analogous to Eqs. (95) and (96) this view is supported by recent studies of autoxidations catalyzed by Rh and Ir complexes136-140 (see Section II.B.2). Complexes of these metals rapidly decompose hydroperoxides in a catalytic reaction.141... 

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