Transition metal-promoted oxidations

Few other areas of modern synthetic organic chemistry offer the diversity shown by homogeneous catalytic oxidation reactions. Practically all the transition metals have complexes showing oxidation activity widely disparate mechanisms of action are standard. 

It is the aim of this chapter to present in detail a few selected examples of useful organic transformations promoted by Group 4-11 (Ti-Cu) metals rather than to give a comprehensive listing of all possible transformations, as this information is available in several other excellent books. - The protocols are selected to demonstrate the most common oxygenation (addition of O atoms) or oxidation (removal of H atoms) pathways encountered in transition metal-promoted reactions of organic substrates. 

Caution As all oxidation reactions represent controlled highly exothermic reactions, and most involve the handling of toxic materials, all of the protocols in this chapter should be carried out in an efficient hood with explosion resistant sashes. Eye protection and disposable gloves must be worn. Clean reaction flasks are essential to avoid the accidental inclusion of materials known to bring about the rapid decomposition of high energy oxidants. 

2-substitution - slow (10 h), catalytic reaction avoids epoxide ring opening, ee suffers (-85%) if R branched 

Although the AE reaction tolerates many functional groups, it is incompatible with RCO2H, RSH, ArOH, PR3, and most amines. If the substrate is free of these functions and the procedure fails, moisture contamination of the dialkyl tartrate or Bu OOH solution is usually to blame. The former should be distilled quickly below 100 C (higher temperatures lead to tartrate polymerisation, resulting in lower product optical yields). The latter should be dried over a fresh supply of molecular sieves just before use. Cumene hydroperoxide may be substituted for Bu OOH in most AE reactions. Although its removal can complicate workup of the reaction mixture, its use normally results in slightly improved enantioselectivities. 

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W. Liu, and M. Flytzani-Stephanopoulos, Transition metal-promoted oxidation catalysis by fluorite oxides A study of CO oxidation over Cu—Ce02, Chem. Eng. J. 64, 283—294 (1996). 

While there are differences in mechanistic detail, studies by others (13, 25) substantiate the existence of allylic hydroperoxides as intermediates in the homogeneous, transition-metal promoted oxidation of cyclohexene. 

Woodward, S. Transition metal-promoted oxidations. Transition Metals in Organic Synthesis 1997, 1-34. 

Eithei oxidation state of a transition metal (Fe, Mn, V, Cu, Co, etc) can activate decomposition of the hydiopeioxide. Thus a small amount of tiansition-metal ion can decompose a laige amount of hydiopeioxide. Trace transition-metal contamination of hydroperoxides is known to cause violent decompositions. Because of this fact, transition-metal promoters should never be premixed with the hydroperoxide. Trace contamination of hydroperoxides (and ketone peroxides) with transition metals or their salts must be avoided. 

Transition metal carbide catalysts have also been explored as methane partial oxidation catalysts [110] promising results were obtained over M02C systems and enhancements were reported with the addition of transition metal promoters. 

M-butane proceeds via an intermolecular mechanism with 2-butene involved intermediately.300-303 The role of the transition metal promoters such as Fe and Mn was shown to increase the surface concentration of the intermediate butene 304 The formation of butene is speculated to occur through an oxidative dehydrogenation on the metal site305 or by one-electron oxidation.306... 

We studied the oxidation of cyclohexene at 70°C in the presence of cyclopentadienylcarbonyl complexes of several transition metals. As with the acetylacetonates, the metal center was the determining factor in the product distribution. The decomposition of cyclohexenyl hydroperoxide by the metal complexes in cyclohexene gave insight into the nature of the reaction. With iron and molybdenum complexes the product profile from hydroperoxide decomposition paralleled that observed in olefin oxidation. When vanadium complexes were used, this was not the case. Variance in product distribution between the cyclopentadienylcarbonyl metal-promoted oxidations as a function of the metal center were more pronounced than with the acetylacetonates. Results are summarized in Table V. 

Especially important are the radical-redox reactions by which transition metals promote free radical generation by catalyzing the decomposition of the peroxides formed during bleaching. This faster free radical generation results in higher oxidation rates and a more extensive depolymerization of the cellulose. 

Therefore, at least on titania, transition metals promote the spillover of hydrogen to the support this is a necessary step in the reduction of the support (and hence modification of the global solid s catalytic properties). In other words, hydrogen spillover is a prerequisite in each of these recently recognized metal-support interactions (SMSI and IFMSI). Evidently these very specific metal-support interactions are, from the point of view of the spillover phenomena, merely the reduction of more or less easily reducible metal oxides, as mentioned in the preceding subsection. 

Punniyamurthy T, Velusamy S, Iqbal J. Recent advances in transition metal catalyzed oxidation of organic substrates with molecular oxygen. Chem. Rev. 2005 105 2329-2363. McGarrigle EM, GUheany DG. Chromium- and manganese-salen promoted epoxidation of alkenes. Chem. Rev. 2005 105 1563-1602. 

The exchange and reduction chemistry is particularly prevalent in the early transition metals (groups III-VII). Some examples are shown in Eqs. (49-51) [150-152]. Along with / -elimination, these deleterious reactions are the major reason for poor to moderate yields (20-60%) in early transition metal chemistry. It has been suggested that MgX2 acts as a Lewis acid site that will promote the reduction of the transition metal s oxidation state... 

Transition metal-promoted hydroperoxide deconposition is inqiortant to the oxidative stability and quality of foods for several reasons. First, the abstraction of hydrogen from an unsaturated fatty acid results in the formation of a single alkyl radical. Followii hydrogen abstraction, oxygen adds to the alkyl radical to form a peroxyl radical and subsequent abstraction of a hydrogen from another fatty acid or antioxidant to form a lipid hydroperoxide (Figure 1). These reactions by themselves do not result in an increase in free radical numbers. If these reactions were tiie only steps in tiie lipid oxidation reactions, the rapid exponential increase in oxidation that is commonly observed in lipids would not occur. Transition metal-promoted decomposition of lipid hydroperoxides results in the formation of additional radicals (e.g. alkoxyl and peroxyl) which exponentially increase oxidation rates as they start to attack otiier unsaturated fatty acids. 

Figure 2. A schematic of the differrat physical environments of an oil-in-water onulsion and the potential location of the reactants in transition metal-promoted decon sition of lipid hydrcq)eroxide. Mn and Mn are transition metals in their reduced and oxidized states, respectively ROOH is a lipid hydroperoxide and RO , and ROO are alkoxyl and peroxyl radicals, respectively. Conq)onents are not drawn to scale.

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