Cobalt-catalyzed oxidation

The flow-cell design was introduced by Stieg and Nieman [166] in 1978 for analytical uses of CL. Burguera and Townshend [167] used the CL emission produced by the oxidation of alkylamines by benzoyl peroxide to determine aliphatic secondary and tertiary amines in chloroform or acetone. They tested various coiled flow cells for monitoring the CL emission produced by the cobalt-catalyzed oxidation of luminol by hydrogen peroxide and the fluorescein-sensitized oxidation of sulfide by sodium hypochlorite [168], Rule and Seitz [169] reported one of the first applications of flow injection analysis (FTA) in the CL detection of peroxide with luminol in the presence of a copper ion catalyst. They... 

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 Enhancement in Activity and Selectivity in Cobalt Catalyzed Oxidation of MCPBA. 

The Co/Mn/Br now eliminates the bottleneck caused by the presence of Co(in)s. The steady state concentration of Co(III) is also much lower caused by its rapid reduction by Mn(II). This reduces carboxylic acid decomposition. We have measured the rate of Mn(III) oxidaion of bromide in the presence and absence of p-xylene and do not find any difference in rate. Hence the system also eliminates the slow Co(III) + chlorotoluene reaction. This sequence of reactions is overall faster and more selective than either the thermal or cobalt catalyzed oxidation of m-chlorotoluene. 

The cobalt-catalyzed oxidation of cyclohexane takes place through cyclohexyl hydroperoxide with the cobalt catalyst acting primarily in the decomposition of the hydroperoxide to yield the products 870 877... 

Y. Kamiya illustrates the influence on catalytic activity of the form of the catalyst. Thus, in the cobalt-catalyzed oxidation of hydrocarbons in acetic acid solution, introduction of bromide ions increases the activity of the catalyst, especially when the metal ion concentration is fairly high. The presence of bromides also results in a marked increase in the proportion of carbonyl compounds among the products and it is believed that these are formed as a result of a propagation step in which bromine-containing cobaltous ions react with alkylperoxy radicals. 

M is shown in Table I. The effect of bromide ion on the cobalt-catalyzed oxidation of methylbenzenes is quite large. 

Industrially important cobalt-catalyzed oxidation of hydrocarbons 385... 

Homolytic liquid-phase processes are generally well suited to the synthesis of carboxylic acids, viz. acetic, benzoic or terephthalic acids which are resistant to further oxidation. These processes operate at high temperature (150-250°C) and generally use soluble cobalt or manganese salts as the main catalyst components. High conversions and selectivities are usually obtained with methyl-substituted aromatic hydrocarbons such as toluene and xylenes.95,96 The cobalt-catalyzed oxidation of cyclohexane by air to a cyclohexanol-cyclohexanone mixture is a very important industrial process since these products are intermediates in the manufacture of adipic acid (for nylon 6,6) and caprolactam (nylon 6). However, the conversion is limited to ca. 10% in order to prevent consecutive oxidations, with roughly 70% selectivity.97... 

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. 

A stereocontrolled synthesis of the /ra j-tetrahydrofuran units in Annonaceae acetogenins that relies on the Sharpless asymmetric dihydroxylation protocol is outlined in Scheme 60 <1999TA2551>. In the first step, the disubstituted double bond of the starting material is dihydroxylated followed by monoprotection as a methoxymethyl ether. Einally, a cobalt-catalyzed oxidation using molecular oxygen as oxidant furnishes the /ra j-tetrahydrofuran. 

Quantitative kinetic analysis was performed on all the catalytic oxidation sequences and are presented in the form of an Arrhenius plot. Figure 2. Included in this plot are the data obtained for the cobalt catalyzed oxidation of graphite reported previously (lef. 17). Inspection of this data clearly demonstrates that although sulfide treatment did not completely inhibit catalytic oxidation, it had a significant retarding effect on the rate of reaction. 

Scheme 2. Cobalt-catalyzed oxidation of alkylaromatic compounds.

In the high-concentration cobalt systems, cobalt ions are reported to be present in several configurations, with the Co" dimer being the most active [71]. Some non-varivalent metal salts, such as ZrO(OAc)2, are reported to promote cobalt catalyzed oxidations. This may be the result of the influence of the promoter on coordination number and monomer-dimer equilibria [72, 73]. 

Reddy, M. M. Punniyamurthy, T. Iqbal, J., Cobalt Catalyzed Oxidation of Cyclic Alkenes with Molecular Oxygen Allylic Oxidation Versus Double BondAttack. Tetrahedron Lett. 1995, 36,159. 

Since the catalytic decomposition of ROOH with Co(II) is much faster than the thermal decomposition of ROOH, the latter reaction becomes almost irrelevant in the overall kinetics of oxidation. The steady-state concentrations of RO and ROOH are reached much faster, significantly reducing the observed induction period compared with uncatalyzed oxidation. The theoretical maximum rate in this case is described by Eq. (4-II) [11,13]. A comparison between Eqs. (4-1) and (4-II) shows that the theoretical maximum rate of uncatalyzed oxidation is four times higher than that of cobalt-catalyzed oxidation. Practically, the theoretical maximum rate of oxidation is rarely reached due to very long induction periods. 

The reduction of Co(III) by ROOH is a relatively slow reaction and, therefore, Co(III) is a predominant state of cobalt in a typical cobalt-catalyzed oxidation. 

The addition of bromide to cobalt-catalyzed oxidation (reactions (4.3-4.11)) results in all Co(III) formed during oxidation being now reduced by inorganic bromide in a fast reaction (4.12), forming bromine radical species and Co(II). We shall... 

Biannic B, Bozell JJ (2013) Efficient cobalt-catalyzed oxidative conversion of lignin models... 

Zakzeski J, Bruijnincx PCA, Weckhuysen BM (2011) In situ spectroscopic investigation of the cobalt-catalyzed oxidation of lignin model compounds in ionic liquids. Green Chem 13 671... 

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. 

According to Baier (2001), the ABLR yields the same mass transfer performance (defined as oxygen transfer rate obtained by cobalt-catalyzed oxidation of sodium sulfite) at half the power input as compared to the conventional BLR. 

Oxygen scavenging reaction (Cobalt catalyzed oxidation) ... 

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. 

Scheme 2.43 Iron- or cobalt-catalyzed oxidation with molecular oxygen in the presence of BIPA or 2-BPEPA...

With a slight modification of the catalytic system, olefins can also be used as reaction partners for the cobalt-catalyzed oxidative annulation with the benzamide substrates (Scheme 10.17) [43]. The reaction features mild room-temperature conditions and is applicable to terminal olefins including styrene. 

Scheme 10.16 Cobalt-catalyzed oxidative annulatlon of 8-amlnoquinoline-bearing benza-mlde and alkyne (Q = 8-qulnollnyl).

Scheme 10.18 Cobalt-catalyzed oxidative carbonylation of 8-aminoquinoline-bearing benza-mide (Q = 8-quinolinyl).

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