Co-Br catalyzed oxidation

In very general terms, the Co-Br-catalyzed oxidation is a particular case of the free radical chain oxidation, common for all liquid phase oxidations of hydrocarbons [8-10]. The free radical chain oxidation occurs with four types of free radicals alkyl, alkoxy, alkylperoxy, and acylperoxy radicals [11, 12]. Other key active intermediates are hydroperoxides and peracids [11,12]. The nomenclature and structures are displayed in Figure 4.2. 

Figure 4.4 Hydrogen abstraction reactions in Co-only and Co-Br-catalyzed oxidation of p-toluic acid.

Figure 2. Schematic Diagram Illustrating the Difference Between Thermal, Cobalt Catalyzed, and Co/Mn/Br Catalyzed Oxidation of M-Chlorotoluene...

Observation of the colors during the metal/bromide oxygenation of toluene are consistent with very low steady state concentrations of the metals in their higher oxidation states. Previously we had estimated based on UV-VIS studies that only 0.6% of the cobalt in a Co/Mn/Br catalyzed oxidation of p-xylene consisted of Co(III) [10]. In the absence of... 

The reaction of alcohols with CO was catalyzed by Pd compounds, iodides and/or bromides, and amides (or thioamides). Thus, MeOH was carbonylated in the presence of Pd acetate, NiCl2, tV-methylpyrrolidone, Mel, and Lil to give HOAc. AcOH is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a Pd compound, an ionic Br or I compound other than HBr or HI, a sulfone or sulfoxide, and, in some cases, a Ni compound and a phosphine oxide or a phosphinic acid.60 Palladium(II) salts catalyze the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective.61 A novel Pd11 complex (13) is a highly efficient catalyst for the carbonylation of organic alcohols and alkenes to carboxylic acids/esters.62... 

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. 

This system may be contrasted with that of Hay and Blanchard (7), who found that the Co(OAc)Br-catalyzed air oxidation of alcohols was a radical chain process, showing an induction period and requiring a high oxygen flow. 

Table 4.7 Tolerance of functional groups in Co/Mn/Br-catalyzed aerobic oxidation of substituted toluenes.a)...

A third process was the liquid phase oxidation of o-xylene, catalyzed by Mn Co Br in acetic acid solution, via intermediate phthalic acid production. It was operated for a time in the 1970s and then discontinued. 

The synergy of Mn in the Co-Br system is one of the most puzzhng and complicated aspects of MC-catalyzed oxidation, and the reactions of Mn, particularly under commercial conditions and concentrations, are still not fully understood. The synergistic effect is seen from a significant increase in oxidation rate (up to three to five times) upon addition of small amounts of Mn (Mn/Co ranging from 0.01 to 0.1) to the Co-Br catalyst. Under industrial conditions, the synergy is also observed in significant improvements of oxidation selectivity to TA and a decrease in acetic acid solvent combustion. 

Fundamentally, Br s role in Co - Br and Co - Mn - Br catalysis is that it catalyzes the oxidation of alkylaromatics by Co(III) and Mn(III). Since the discovery of Co-Br catalysis, it has been postulated that Br is formed from Br" added initially as a salt (CoBr2, NaBr) or HBr [35, 36, 38]. Organic bromides could also serve as bromine sources, but in order to be active, the organic bromide must be converted into inorganic bromide [8, 38]. Thus, there are two questions related to the nature of the bromine species in Co-Mn-Br catalysis what is the major catalytically active form of bromide and what is the active bromine radical species ... 

Recently, great advancement has been made in the use of air and oxygen as the oxidant for the oxidation of alcohols in aqueous media. Both transition-metal catalysts and organocatalysts have been developed. Complexes of various transition-metals such as cobalt,31 copper [Cu(I) and Cu(II)],32 Fe(III),33 Co/Mn/Br-system,34 Ru(III and IV),35 and V0P04 2H20,36 have been used to catalyze aerobic oxidations of alcohols. Cu(I) complex-based catalytic aerobic oxidations provide a model of copper(I)-containing oxidase in nature.37 Palladium complexes such as water-soluble Pd-bathophenanthroline are selective catalysts for aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic... 

Vitamin Bi2-catalyzed intramolecular cathodic coupling leads to a regioselective 1,4-addition with formation of a spirocom-pound (Eq. 2) [95]. This chain reaction is initiated by the reduction of Co(III) to a Co(I) species, which reacts in an oxidative addition with the alkyl bromide. The resulting alkyl-Co(III)-Br species is then reduced to an alkyl anion that undergoes a Michael addition and yields Co(I) for the next cycle. 

Novel practical methods using various reagents, such as [Co(OAc)Br],1355 sulfur trioxide,1356 or ds-dioxoruthenium complexes,1357 were developed to transform alkynes to 1,2-diketones. Radical-catalyzed aerobic oxidation using A-hydro-xyphthalimide combined with a transition metal (Co, Cu, or Mn) affords a,P-acetylenic ketones in good yields.1358 Oxidation by the HOF. acetonitrile complex yields diketones, ketoepoxides, or cleavage products.1359 Ozonolysis of acetylenes combined with trapping techniques affords to isolate various derivatives.1360,1361 New information for the ozonolysis of acetylene was acquired by quantum-chemical investigatons.1362... 

Figure 8. Steady rate of oxidation of 4.07M ethylbenzene catalyzed by Co Ac and bromide ion in acetic acid as a function of the Br/Co molar ratio...

Numerous aryl bromides, iodides [203], borates [204] and triflates [205, 206] have been successfully carbonylated. Triflates could serve as a route for the synthesis of arenecarboxylic acid derivatives from phenols. This carbonylation using dppf in a catalytic mixture generally shows higher efficiency than PPhj or P(o-Tol)3 [207]. Poor performance is also noted for PPhj in a Pd-catalyzed vinyl substitution of aryl bromides [208]. Side-reactions involving the formation of [PPhjAr]Br and ArH are responsible. A system which is catalyzed effectively by PdCljfdppf) under 10 atm CO is the desulfonylation of 1-naphthalenesulfonyl chloride 58 in the presence of Ti(OiPr)4. Formation of isopropyl 1-naphthoate 59 can be explained in a sequence of oxidative addition, SOj extrusion, carbonylation and reductive elimination (Fig. 1-27) [209]. A notable side-product is di-l-naphthyl disulfide. 

A better synthesis (89% yield) of Fe(CO)3(PPh3)2 is reported from [PPN]2[Fe4(CO)i3], where PPN+ = bis(triphenylphosphine)iminium. The C0X2 (X = Cl, Br, I) catalyzed substitution of CO in Fe(CO)j is reported to yield Fe(CO)4L species in 15 to 99% yield and Fe(CO)3(PPh3)2 was prepared (net 62% yield) from Fe(CO)s in a two-step procedure that requires a chromatographic separation. Strohmeier and Muller report that irradiation of Fe(CO)s in the presence of several phosphines produces Fe(CO)3L2 and Fe(CO)4L complexes in yields that range from 13% for the synthesis of Fe(CO)3[P(/i-Bu)3]2 to 35% for Fe(CO)3[P(c-C6Hn)3]2. For some of the compounds synthesized, vacuum sublimation is necessary to separate the Fe(CO)3L2 species from Fe(CO)4L. The one-step photochemical procedure we report here employs cyclohexane as a solvent. That enables unreacted phosphine, Fe(CO)s, and Fe(CO)4L to remain in solution while pure Fe(CO)3L2 precipitates. It is essential that the phosphines used in these reactions be free of phosphine oxides, which labilize CO and yield products other than Fe(CO)3(PR3)2 complexes. 

Potassium peroxomonosulfate (Oxone ) is also an effective oxidizing agent for the epoxidation of various alkenes in the presence of Mn porphyrin and a PT catalyst [79]. The biphasic epoxidation of various olefins is readily catalyzed by Co and Ni" phthalocyanines with NaClO as the oxygen donor and Bu4N" Br" as the PT agent [80]. The PTC oxidation of alkenes with NaClO is also catalyzed by square-planar Ni complexes [81-83]. 

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