Relative oxidation rate manganese oxide

Fueled by the success of the Mn (salen) catalysts, new forays have been launched into the realm of hybrid catalyst systems. For example, the Mn-picolinamide-salicylidene complexes (i.e., 13) represent novel oxidation-resistant catalysts which exhibit higher turnover rates than the corresponding Jacobsen-type catalysts. These hybrids are particularly well-suited to the low-cost-but relatively aggressive-oxidant systems, such as bleach. In fact, the epoxidation of trans-P-methylstyrene (14) in the presence of 5 mol% of catalyst 13 and an excess of sodium hypochlorite proceeds with an ee of 53%. Understanding of the mechanistic aspects of these catalysts is complicated by their lack of C2 symmetry. For example, it is not yet clear whether the 5-membered or 6-membered metallocycle plays the decisive role in enantioselectivity however, in any event, the active form is believed to be a manganese 0x0 complex <96TL2725>. 

Reaction 22a is important only with cobalt acetate catalyst and accounts for the fast rate of methane formation during the reaction of peracetic with acetaldehyde. It can also explain how methane is produced only from the methyl group of peracetic acid. This reaction path is more important with cobalt probably because of the higher oxidation potential of the cobalt (III)-cobalt (II) couple relative to that of the manganese (III) -manganese (II) couple. 

Cumene oxidized relatively slowly, at about 1/13 the rate of p-xylene. This was not caused by the formation of phenol, as might be expected by an acid-catalyzed rearrangement of cumene hydroperoxide. No phenol or product clearly derived from phenol, as by radical attack or by oxidation to a quinone, was detected at any time in the reaction mixture. The two major products were a-methylstyrene and 2-phenylpropylene oxide their concentrations increased with time. The group at Shell also observed the formation of a-methylstyrene and 2-phenylpropylene oxide among the products of cumene oxidation in butyric acid at 140°C. with cobalt and manganese catalysts (30). 

The electron transfer reactivity of Ceo has been compared with that of p-benzoquinone which has a slightly more negative one-electron reduction potential ( °red relative to the SCE = -0.50 V) [44] than Ceo (E°red —0.43 V). The rate constants of electron transfer from Cgo and Ceo to electron acceptors such as allyl halides and manganese(III) dodecaphenylporphyrin [45] correlate well with those from semiquinone radical anions and their derivatives. Linear correlations are obtained between logarithms of rate constants and the oxidation potentials of... 

The use of manganese-ion catalysis frequently results in increased production of formic acid. This is probably the result of increased rate of production and a decrease in the relative rate of attack on formic acid [57, 58]. Part of the increased production would be the result of an enol mechanism for the oxidation of methyl ketones. For example, the manganese-ion catalyzed oxidation of methyl ethyl ketone (MEK) gives increased amounts of formic and propionic acids at the expense of acetic acid. 

There is an interesting exception to this observation. As noted above, aldehyde oxidations tend to be very fast and to have relatively long kinetic chain lengths. Most chain terminations occur via bimolecular reactions of acylperoxy radicals (eq. (7a)) these reactions result in carbon dioxide generation and are inefficient. If one adds manganese catalyst, some of the acylperoxy radicals will be reduced to peroxy acid and Mn " will be produced. Mn can carry the chain via an analog of reaction (18), but does not participate in chain termination reactions. As a result, kinetic chain lengths and rates tend to increase. 

Arsenic is generally present as As(V) in oxidizing environments and as As(in) in moderately reducing environments. However, the rate of As(III) oxidation can be relatively slow in the presence of oxygen and As (III) can coexist with As(V). The rate of As(III) oxidation is greatly increased by the presence of other oxidants such as manganese oxides. 

Iron and manganese can both be oxidized by oxygen alone, but the rate is relatively slow and is sensitive to pH. For iron, the rate law is second order with respect to hydroxide concentration, and varies greatly in the neutral pH range. At a pH of about 6.56, very little conversion was observed in a 50 min period. At pH 7.24 approx 95% conversion was obtained in 10 min. 

---