Oxidation of C-H Bonds in Alkanes

There are several reviews on alkane oxidation by Ru complexes (principally by RuO ) including [1-6]. Most of the oxidations catalysed are of C-H bonds C-C cleavage is considered in 4.2. We begin with what in principle is the simplest oxidation, that of the C-H bond in aldehydes and related substrates. 

As has been pointed out in Chapter 5, oxidation of some primary and secondary amines, and of amides, may equally well be regarded as alkane oxidations (cf. 4.1.2, 4.1.3 and 5.1.3.1). 

Griffith, Ruthenium Oxidation Complexes, Catalysis by Metal Complexes 34, DOI 10.1007/978-l-4020-9378-4 4, Springer Science+Business Media B.V. 2011 

Enantioselective hydroxylation of 2-benzyl (3-ketoesters was catalysed by [RuCl(OEt3)(PNNP)]/aq. H O /CH Cy thus ethyl 2-benzyl-3-oxo-butanoate gave ethyl 2-hydroxy-2-benzyl-yoxo-butanoate. Better results were obtained with cumyl hydroperoxide as co-oxidant [14]. The reagent Ru(CO)(TPP) or Ru(CO) (TMP)/(Cl3pyNO)/aq. HBr/C Hy40°C oxidised 5 3-steroids to the corresponding Sp-hydroxy derivatives with retention of configuration [15]. 

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The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

The importance of palladium acetate lies in its ability to catalyse a wide range of organic syntheses functionalizing C-H bonds in alkanes and in aromatics, and in oxidizing alkenes. It has been used industrially in the... 

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

Abstract This chapter covers oxidation of C-H and C-C bonds in alkanes. Section 4.1 concerns oxidation of C-H bonds aldehydes and other CH species (4.1.1), methylene (-CH groups) (4.1.2) and methyl (-CH ) groups (4.1.3). This is followed by the oxidation of cyclic alkanes (4.1.4) and large-scale alkane oxidations (4.1.5). Alkane oxidations not considered here but covered in Chapter 1 are hsted in Section 4.1.6. The final section (4.2) concerns oxidative cleavage of C-C bonds. 

The first step of the activation of butane and cyclohexane has been assumed to be the cleavage of a secondary C—H bond, with minor contributions from primary C — H bonds in the case of butane. This picture is supported only by indirect evidence. When the relative rates of reaction of various alkanes were compared on a V-Mg oxide and Mg2V207 catalyst (Table VIII), it was found that alkanes with only primary carbons (ethane) reacted most slowly. Those with secondary carbons (propane, butane, and cyclohexane) reacted faster, with the rate being faster for those with more secondary carbon atoms. Finally, the alkane with one tertiary carbon (2-methylpropane) reacted faster than the ones with either a single or no secondary carbon (26). From these data, it was estimated that the relative rates of reaction of a primary, secondary, and tertiary C—H bond in alkanes on the V-Mg oxide catalyst were 1, 6, and 32, respectively (26). 

Consider the C-H bond in alkanes. Carbon is a more electronegative element than hydrogen. Consequently, the electron pair that forms this bond is shifted towards the carbon atom. In the extreme, an ionic representation of this bond can be given as pictured in 122 (Scheme 2.45). Within these conventions the carbon atom in an alkane can be approximated as a carbanion (oxidation level 0 by definition). Using this definition it becomes possible to apply oxidation-reduction terminology to the processes as if they occurred to ion pair 122. Thus, oxidation of 122 with the loss of one electron leads to the radical 123. With the loss of two electrons, the oxidation leads to carbocation 124. Similarly, the conversion of an alkane to an alcohol and the alcohol into an aldehyde and the aldehyde eventually to a carboxylic acid can unambiguously be classified as an oxidation sequence with the loss of two, four, and six electrons. The oxidation levels 1, 2, and 3 are ascribed respectively to these functional derivatives. The conversion of an alkane to an alkene or alkyne can be interpreted in an analogous fashion. 

Carbon-hydrogen bonds are commonly formed by reductive elimination when an alkyl or aryl group and a hydride occupy mutually cis positions. Although intramolecular oxidative additions of C—H bonds and reactions of activated C—H bonds are well known for Ni, Pd, and Pt, additions of C—H bonds in simple alkanes and arenes are less common. 

Oxidative addition of a C-C bond (C-C bond activation) to a metal, like C-H addition, is potentially very important on both a laboratory and an industrial scale. If a metal complex were available that could react via OA with a specific alkane C-C bond, for example, the ultimate result would be exclusive functionalization of a normally unreactive carbon atom. Once that occurs, there are numerous transformations available to convert the M-C bond to other functionalities. Catalytic activation of C-C bonds in long-chain hydrocarbons found in petroleum could provide low-energy, efficient routes to production of compounds that are useful in gasoline refining. Unfortunately, unstrained C-C bonds—the type found in saturated hydrocarbons—typically do not readily undergo OA for the same reasons associated with lack of reactivity of C-H bonds. In fact, the situation ought to be worse with C-C bond activation, because OA now produces not one but two M-C bonds at the expense of breaking a robust C-C bond. There should, moreover, be more steric hindrance created when a C-C bond approaches... 

In Table 24-A-l we list types of molecules that have been added oxidatively to at least one complex. So far, the C—H bond in alkanes or alkenes cannot normally be broken under mild conditions in oxidative additions, although the saturated hydrocarbon cubane is isomerized by certain Rh1 complexes and initial breaking of a C—H bond by oxidative addition is involved.715 9 When no ligand loss is involved, there will be an equilibrium reaction L,M" + XY L,M" + 2XY... 

The direct oxidation of unfunctionahsed alkanes in an asymmetric fashion is a formidable challenge. However, oxidation of C—H bonds adjacent to suitable functional groups gives a handle on which to operate. In particular, the aUyKc oxidation of cyclic alkenes utilising asymmetric variants of the Kharasch—Sosnovsky reaction has received considerable attention. The reaction is catalysed by copper salts and requires a perester to give the allylic ester as product. 

Because of the favorable rates for reductive eliminations involving a hydride ligand noted in the introductory section, examples of complexes that undergo reductive elimination to form the C-H bonds in alkanes and arenes span the transition series. Thermally induced reductive eliminations to form dihydrogen (or dihydrogen complexes) from dihydride complexes can also be rapid, but this reaction occurs less frequently because the oxidative addition of dihydrogen is typically favored thermodynamically. Reductive elimination to form a C-H bond is the last step of many catalytic reactions, such as the hydrogenation and hydroformylation of olefins. 

The conversion of alkanes into more useful products is one of the most important practical problems in chemistry. The insertion of metals into C-H bonds was first discovered by Chatt and Davidson in 1965 [101] for low-valent ruthenium complexes. Following the discovery of the photochemically induced insertion of a transition metal into alkane C-H bonds [102, 103], a large number of organo-metallic complexes have been shown to activate C-H bonds in alkanes. A typical example is the photochemical oxidative addition of alkanes to complexes such as Rh(Cp)(CO)2- The reaction occurs via an initial loss of CO followed by a-... 

In 1982. Janowicz and Bergman al Berkeley -" and Hoyano and Graham at Albertat- reported (he first stable alcane intermolecular oxidative addition products. The Berkeley group photolyzed (q -MejCsMMejPlIrHj with the loss of H2. while the Alberta group photolyzed (ii -Me,C,)lr(CO)2 with the loss of CO to give highly reactive iridium imermediates which cleave C—H bonds in alkanes ... 

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