Metal-catalyzed Oxidations General Considerations

Mechanisms of Metal-catalyzed Oxidations General Considerations 

One way of circumventing this activation energy barrier involves a free radical pathway in which a singlet molecule reacts with 302 to form two doublets (free radicals) in a spin-allowed process (Fig. 4.1, Reaction (1)). This process is, however, highly endothermic (up to 50 kcal mol-1) and is observed at moderate temperatures only with very reactive molecules that afford resonance stabilized radicals, e.g. reduced flavins (Fig. 4.1, Reaction (2)). It is no coincidence, therefore, 

A second way to overcome this spin conservation obstacle is via reaction of 302 with a paramagnetic (transition) metal ion, affording a superoxometal complex (Fig. 4.1, Reaction (3)). Subsequent inter- or intramolecular electron-transfer processes can lead to the formation of a variety of metal-oxygen species (Fig. 4.2) which may play a role in the oxidation of organic substrates. 

Heterolytic oxidations generally involve the (metal-mediated) oxidation of a substrate by an active oxygen compound, e.g. H202 or R02H. Alternatively, stoichiometric oxidation of a substrate by a metal ion or complex is coupled with the reoxidation of the reduced metal species by the primary oxidant (e.g. 02 or H202). 

As noted above, dioxygen reacts with organic molecules, e.g. hydrocarbons, via a free radical pathway. The corresponding hydroperoxide is formed in a free radical chain process (Fig. 4.3). The reaction is autocatalytic, i.e. the alkyl hydroperoxide accelerates the reaction by undergoing homolysis to chain initiating radicals, and such processes are referred to as autoxidations [1]. 

---

Mechanisms of Metal-catalyzed Oxidations General Considerations 135... 

Oxidation Reactions. Metal oxides are effective catalysts for the total or deep oxidation reactions. Consideration of the types of total oxidation reactions that are catalyzed by metal oxides leads to three general classifications. These... 

In summary, we have witnessed a rapid development of transition metal-catalyzed, directing group-assisted unactivated C(sp )-H bond functionalization. It has become a competent tool for a sustainable constmction of chemical bonds. Deep understanding of the mechanistic pathway and design of chiral Ugands for enan-tioselective functionalization of general C(sp )-H bonds will gain considerable attention in near future. The exploitation of new catalytic system with cheap metals and sustainable conditions (external oxidant O2 solvent H2O) will be of particular interest both to academia study and industry development. 

The oxidative stability of biodiesel has been the subject of considerable research as it is, besides cold flow, one of the major technical issues with biodiesel. Some recent overview articles are (Knothe, 2007 Dunn, 2008 Jain and Sharma, 2010 Xin and Saka, 2010). This issue affects biodiesel primarily during extended storage. The influence of parameters such as the presence of air, heat, traces of metal, antioxidants, and peroxides as well as the nature of the storage container was investigated in the aforementioned studies. Generally, factors such as the presence of air, elevated temperatures, or the presence of metals facilitate oxidation. Studies performed with the automated Oil Stability Index (OSI) method confirmed the catalyzing effect of metals on oxidation, however, the influence of compound structure of the fatty esters, especially unsaturation, as discussed later, was even greater (Knothe and Dunn, 2(X)3). 

Hydroxycarbonylation and alkoxycarbonylation of alkenes catalyzed by metal catalyst have been studied for the synthesis of acids, esters, and related derivatives. Palladium systems in particular have been popular and their use in hydroxycarbonylation and alkoxycarbonylation reactions has been reviewed.625,626 The catalysts were mainly designed for the carbonylation of alkenes in the presence of alcohols in order to prepare carboxylic esters, but they also work well for synthesizing carboxylic acids or anhydrides.137 627 They have also been used as catalysts in many other carbonyl-based processes that are of interest to industry. The hydroxycarbonylation of butadiene, the dicarboxylation of alkenes, the carbonylation of alkenes, the carbonylation of benzyl- and aryl-halide compounds, and oxidative carbonylations have been reviewed.6 8 The Pd-catalyzed hydroxycarbonylation of alkenes has attracted considerable interest in recent years as a way of obtaining carboxylic acids. In general, in acidic media, palladium salts in the presence of mono- or bidentate phosphines afford a mixture of linear and branched acids (see Scheme 9). 

A catalyzed reaction follows a path of relatively low free energy of activation, in the sense of Eyring s theory, as compared with the same reaction proceeding without a catalyst. Where a comparison is possible, it would seem that inorganic catalysts lower the energy (heat content change) of activation (3), and the same holds for enzymes (131). The effect of catalysts on the entropy of activation, so far as the present author is aware, is, in general, less marked. In some way the molecules adsorbed on the metal, metal oxide, or other catalysts, or held to a specific protein, are converted into a more labile form, i.e., the potential hill for one specific mode of reaction (out of perhaps a number of such) is considerably lowered. 

The polymerization of epoxides, such as propylene oxide and cyclohexene oxide, in the presence of COj to produce polycarbonates has been an area of considerable interest and has been reviewed recently. A number of metal ion complexes have been found to catalyze the process. A general outline of the possible reactions is shown in Scheme 5.16, where the initial reactant is the result of COj insertion into an alkoxide, as shown in reaction (5.24). This Scheme does not show the many stereochemical possibilities if the epoxide is chiral. 

The metal-ion catalyzed, liquid-phase oxidation of SO2 has received considerable attention as a mechanism for SO2 conversion in plmes and contaminated droplets. In general, the mechanisms proposed are lengthy, and the derived rate expressions are largely empirical. Table VI STjmmarizes a variety of studies on this process. Observed rates vary substantially depending on the particular catalyst, relative humidity, and other conditions. 

One of the foremost methods for oxidation of olefins is stereospecific syn-di-hydroxylation by treatment with OSO4. Because of the toxicity and cost of osmium, however, the need for methods that would employ substoichiometric quantities of this reagent was identified early on [180], A number of stoichiometric oxidants for catalytic dihydroxylations were examined, including metal chlorates, HjOa, NMO [181], f-BuOOH [182], and KjFeiCN) [40, 183], The mechanism of 0s04-catalyzed dihydroxylation reactions has been the subject of much debate details of these mechanistic considerations are beyond the focus of this book and are amply discussed elsewhere [40-42,47,49,184-186]. A number of interesting stereoselectivity trends of general importance in substrate-controlled diastereoselective dihydroxylations [40-43] and catalytic asymmetric dihydroxylations are discussed below (Section 9.8) [42, 44-49]. 

---