Alkane and Arene Oxidation

Both Pd and Pt are known to catalyze the oxidation of methane to methanol via C-H activation. This transformation poses several daunting challenges. First, the product of the initial oxidation, methanol, is significantly more reactive 

It should be noted that Pt-biscarbene complexes were also prepared. However, while 40 was an active methane oxidation catalyst, its Pt analogue decomposed in TFA. Aryl-substituted NHCs, however, were found to be substantially more stable to acid, and could be used for the conversion of methane to methanol, albeit in extremely low turnover (Equation (12.12)). 

N-Heterocyclic Carbenes in Synthesis, Wiley-VCH, New York, 2006. 

Glorius, (ed.), N-Heterocyclic Carbenes in Transition Metal Catalysis, Springer, Berlin, 2007. 

Pontes da Costa, M. Viciano, M. Sanaii, S. Merino, J. Tejeda, E. Peris and B. Royo, Organometallics, 2008, 27, 1305-1309. 

---

Platinum-Catalyzed Alkane and Arene Oxidations via Organometallic Intermediates... 

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... 

Many related complexes of iridium and rhodium undergo the oxidative addition reaction of alkanes and arenes [1]. Alkane C-H bond oxidative addition and the reverse reaction is supposed to proceed via the intermediacy of c-alkane metal complexes [4], which might involve several bonding modes, as shown in Figure 19.5 (for an arene the favoured bonding mode is r 2 via the K-electrons). 

The involvement of transition metal peroxo species in the oxidative functionalization of alkanes and arenes has been postulated for several metals with both hydrogen peroxide and alkyl hydroperoxides. 

The oxidation of primary and secondary alcohols by stable organic nitroxyl radicals has been reviewed.111 The kinetics of reactions of alkanes and arenes with peroxynitrous acid suggest the participation of the same active oxidizing species in both gas and aqueous phase HOONO or its decomposition product OONO. 112 The oxidation of the alkaloids reserpine and rescinnamine by nitric acid has been studied.113... 

Table III. Oxidation of Alkanes and Arenes by 4 Using Molecular Dioxygen...

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. 

We may note that the mechanisms of reactions included in the last two types are, in general, not the same for paraffins, on the one hand, and aromatic hydrocarbons, on the other hand, even if the products of these reactions are of the same type. For example, alcohols and phenols may be obtained from alkanes and arenes respectively by the reaction in air with hydroxyl radicals generated by the action of a metal complex. However, in the case of alkane, an alcohol can be formed by the reduction of alkyl peroxide, whereas hydroxyl is added to an arene with subsequent oxidation of a radical formed. Hence follows the possibility that arenes and alkanes may exhibit different reactivities in each specific reaction. 

Hydrogen peroxide [49] and alkyl peroxides [50] oxidize alkanes and arenes at relatively low (< 100 C] temperatures if heterogeneous catalysts, for exam-... 

The oxidation of cyclohexane in acetonitrile affords cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone in the ratio ca. 2 1 1. The reaction both with alkanes and arenes is accelerated upon irradiation with visible and, especially, UV tight [87f. ... 

Mechanisms of metal-catalyzed oxidations by hydrogen peroxide may be different for alkanes and arenes as well as for different metal complexes. Lor example, for the oxidation of alkanes by the complex [Ru(dmp)2(S)2 (PF6)2 (where S = MeCN or H2O) a mechanism analogous to the oxygen rebound radical mechanism, assumed for cytochrome P450 and its models (see Chapter XI), has been proposed (Scheme X.I) [I3h]. 

It is important to note that alkane oxidation reactions by various oxygen atom donors in cases when a metal-complex catalyst is a metalloporphyrin (or even any other complex) can he considered to he a model for biological hydrocarbon oxidation. The next chapter will be fully devoted to the oxidations of alkanes and arenes in living cells and modeling these processes using metal complexes. 

Therefore, the biomimetic approach, that is the aeation of chemical analogues of enzymes, seems to be especially promising in this respect. The aeation of chemical models of the enzymatic oxidation of alkanes and arenes makes it possible not only to understand its mechanisms better, but also to develop what are likely to be fundamentally new processes for the conversion of the hydrocarbon raw material. 

The current views about aerobic biological oxidation of alkanes and arenes involving various oxygenases. Its chemical simulations are discussed in this chapter, which gives only a brief survey of the most recent data for biological C-H activation and hydrocarbon oxidation. It is noteworthy that, somewhat unexpectedly, important and profound analogies exist between chemical activation by metal complexes and biological C-H oxidation. It is necessary to note that many books [14] and reviews [15] have been devoted to enzymatic oxidations and processes that more or less closely model these oxidations. 

These early discoveries form the basis for the growth of this field over the last 15 years. Advances have been made in understanding the fundamental basis for C-H activation, including both intermediates involved and the bond strengths of the metal-carbon bonds that are formed. A detailed understanding of both oxidative addition and electrophilic activation of C-H bonds is now in hand. New discoveries of alkane and arene functionalization allow the formation of useful derivatives in synthetically useful quantities. Many new examples of the use of transition metal C-H activation in organic transformations are being reported. 

---