Dioxygen as Oxidant

8) The results of this study [71a] include the spectroscopic characterization of the ferryl complexes (ligands L, L, L ), a kinetic analysis of formation and decay of the ferryl complexes, labeling studies, and product analysis (TON and yield of O2 vs CO2) as a function of the three ligands and various oxidants. 

Much has been accomplished in the last decade in the field of high-valent nonheme iron chemistry and, in addition to the milestones noted in Section 6.1, the most relevant iron-bispidine-based achievements are that (i) with the bispidine ligands the highest potentials have been reported and that these poten- 

Two important questions related to this high-valent iron chapter remain to be addressed  

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The transformation of alcohols to the corresponding carbonyl compounds or carboxylic acids is one of the few examples in which a heterogeneous (solid) catalyst is used in a selective, liquid phase oxidation (7,2). The process, which is usually carried out in an aqueous slurry, with supported platinum or palladium catalysts and with dioxygen as oxidant, has limited industrial application due to deactivation problems. 

A. Bio-inspired catalysts using dioxygen as oxidant [Copper(II)-Schiff base]... 

A third topic, not specific to the use of dioxygen as oxidizing agent, concerns selectivity and, in particular, the closely related aspect of over-oxidation. This problem is most severe in the partial oxidation of saturated hydrocarbons, because activation of alkanes is usually more difficult than that of any of the oxygenated products (e.g., alcohols and aldehydes), which are easily oxidized further. The reason is simply that C-H bonds of functionalized alkanes are generally weaker than those of the parent hydrocarbons. Moreover, as far as metal catalysis is concerned, the polar oxidation products can... 

Scheme 13.7 Mechanism of the homolytic aromatic substitution under tin hydride/AIBN conditions. Dioxygen as oxidant of the cyclohexadienyl radical using (Me3Si)3SiH as radical reducing reagent.

An enantioselective a-alkylation of aldehydes (R-CH2CHO) gives a xanthenyl product (137, X = O) in up to 93% ee, using a simple organocatalyst (138) that activates the aldehyde via enamine catalysis, with subsequent addition of the stabilized benzyllc carbocation. This dehydrogenative alkylation uses dioxygen as oxidant and has been extended to the cases of thioxanthene and 10-methyl-9,10-dihydroacridine (i.e. 137, X = S and NMe). 29... 

Ketoreductases catalyze the reversible reduction of ketones and oxidation of alcohols using cofactor NADH/NADPH as the reductant or NAD + /NADP+ as oxidant. Alcohol oxidases catalyze the oxidation of alcohols with dioxygen as the oxidant. Both categories of enzymes belong to the oxidoreductase family. In this chapter, the recent advances in the synthetic application of these two categories of enzymes are described. 

Scheme 6.7 Oxidative Heck coupling of boronic acids and alkenes using dioxygen as a reoxidant.

Oligomeric polyperoxide is formed as a result of such copolymerization of the monomer and dioxygen. During oxidation of many unsaturated hydrocarbons, both reactions (abstraction and addition) occur in parallel to produce a mixture of hydroperoxides and oligomeric polyperoxides. The relative amounts of the products of R02 addition to the Tr-bond of olefins are given below [13] ... 

The very active unstable tin(III) ion is supposed to play an important role in this chain mechanism of tin(II) oxidation. Cyclohexane, introduced in the system Sn(II) + dioxygen, is oxidized to cyclohexanol as the result of the coupled oxidation of tin and RH. Hydroxyl radicals, which are very strong hydrogen atom acceptors, attack cyclohexane (RH) with the formation of cyclohexyl radicals that participate in the following transformations ... 

This methodology clearly enriches the tool box of the synthetic organic chemist. Other spin-offs from the studies described above, such as the incorporation of dioxygen as an oxidant and the use of alkyl carbon-het-eroatom coupling as a product release step for other metal-mediated organic transformations, may also emerge over the next several years. 

This scheme, set up with reactions in dichloromethane, gave spin adducts from several of the nucleophiles discussed above (F, Cl", AcO, CN, tetramethylsuccinimide anion and triethyl phosphite), provided UV light was employed. With filtered light of A > 435 nm, no spin adducts were detected. This is expected, since PBN cannot then be excited. With water as the nucleophile, only benzoyl nitroxide [9] was seen, indicating that any HO-PBN" disappears too rapidly to be detectable 10 s in acetonitrile) and/or that its rate of formation from PBN +ConW is too low (see above). The complication that nucleophilic addition-oxidation might compete was ruled out experimentally in dichloromethane, but detected for fluoride ion in chloroform, using dioxygen to oxidize the intermediate hydroxylamine anion. 

Again we see that an alkene isomerisation reaction has taken place. Another important, useful reagent applied in this field is also pictured in Figure 15.7, viz. the use of benzoquinone as the re-oxidant for palladium. Quinone takes the role of dioxygen as oxidising agent. It is very efficient and both quinone and hydroquinone are inert towards many substrates. Furthermore, no water is formed, as is the case when dioxygen is used. 

Finally, with some precursors, complete removal of contaminants can sometimes be troublesome. Consequently, some authors have chosen to use dioxygen as the reactive gas to avoid carbon contamination [62-64]. Thus, pure platinum films have been obtained on thermally oxidized silicon substrate by decomposition of [Pt(CH3)3(CpCH3)j or [Pt(K -acac)2] in ArjOj mixtures at 623 K. Pt films produced from [Pt(K -acac)2] contained less than lat% carbon, while oxygen contamination was not detectable [64]. Similarly, a significative reduction of carbon incorporation into Ru films was evidenced when oxygen was used as a reactive gas during CVD from [RuCp(CO)2]2 [62]. 

Subsequently, Backvall and coworkers developed triple-catalysis systems to enable the use of dioxygen as the stoichiometric oxidant (Scheme 3) [30-32]. Macrocyclic metal complexes (Chart 1) serve as cocatalysts to mediate the dioxygen-coupled oxidation of hydroquinone. Polyoxometallates have also been used as cocatalysts [33]. The researchers propose that the cocatalyst/BQ systems are effective because certain thermodynamically favored redox reactions between reagents in solution (including the reaction of Pd° with O2) possess high kinetic barriers, and the cocatalytic mixture exhibits highly selective kinetic control for the redox couples shown in Scheme 3 [27]. 

Palladium-catalyzed, Wacker-type oxidative cycHzation of alkenes represents an attractive strategy for the synthesis of heterocycles [139]. Early examples of these reactions typically employed stoichiometric Pd and, later, cocat-alytic palladium/copper [140-142]. In the late 1970s, Hegedus and coworkers demonstrated that Pd-catalyzed methods could be used to prepare nitrogen heterocyles from unprotected 2-allylanilines and tosyl-protected amino olefins with BQ as the terminal oxidant (Eqs. 23-24) [143,144]. Concurrently, Hosokawa and Murahashi reported that the cyclization of allylphenol substrates can be accomplished by using a palladium catalyst with dioxygen as the sole stoichiometric reoxidant (Eq. 25) [145]. 

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