Alkane under oxidative conditions

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... 

Cyclohexane, the six-carbon ring hydrocarbon with the molecular formula C6H12, is the most significant of the cyclic alkanes. Under ambient conditions it is a clear, volatile, highly flammable liquid. It is manufactured by the hydrogenation of benzene and is used primarily as a raw material for the synthesis of cyclohexanol and cyclohexanone through a liquid-phase oxidation with air in the presence of a dissolved cobalt catalyst. 

In the presence of strong acid activators, such as TFA, cobalt(III) acetate is capable of the selective oxidation of alkanes under mild conditions to alkyl acetates, ketones, or alkyl chlorides, depending on the reagents used.298 For example, the oxidation of n-heptane carried out at 25°C, is illustrated in the following examples ... 

Although the direct interaction of metal complexes with alkanes under mild conditions has been demonstrated by the foregoing results, selective and catalytic functionalization of alkanes remains to be found. Success is probably more likely to be achieved by way of mild dehydrogenation (e.g., transfer of hydrogen to a suitable donor)6113 b or carbonylation rather than by oxidation. By analogy with Co(lII) oxidations of alkanes, these reactions should be facilitated by increasing the electrophilicity of the metal ion, e.g., by using the metal tri-... 

Hydrogen peroxide is also the oxidant in the halogenations of alkanes under Gif conditions. In these systems, developed by Barton and his coworkers312, alkanes are selectively transformed into alkyl chlorides or bromides by polyhaloalkanes and H202 in the presence of FeCypicolinic acid catalyst in pyridine/acetic acid solvent313-315. It has clearly been established that the reaction mechanism does not involve a free-radical intermediate. 

Several heterogeneous catalysts have been developed for the hydroxylation of alkanes under mild conditions.68,69 One of them is the bi-catalytic system, which combines the ability of palladium to convert hydrogen and oxygen to hydrogen peroxide, with the capability of the iron ions to activate the hydrogen peroxide to hydroxylate hydrocarbons.70 Iron oxide and palladium supported on silica have been used as efficient catalysts for the oxidation of cyclohexane to the alcohol and ketone, via the in situ generation of hydrogen peroxide in an acetone solvent.71... 

It must also be pointed out, however, that initiation of acid-catalyzed alkane transformations under oxidative conditions (chemical or electrochemical) can also involve radical cations or radical paths leading to the initial carbenium ions. In the context of our present discussion, we shall not elaborate on this interesting chemistry further and limit our treatment to purely protolytic reactions. ... 

Barton and co-workers have developed a family of systems for oxidation and oxidative functionalization of alkanes under mild conditions exhibiting unusual selectivity (see reviews [50], polemics [50d,e], the first publication [51] and some recent publications[52], as well as the papers of other authors concerning these systems[53]). These oxidations occur in pyridine in the presence of an organic acid and are catalyzed by complexes of transition metals (mainly iron). If dioxygen is used as an oxidizing regent, a reductant must also take part in the reaction. The first such a system was invented in Gif-sur-Yvette [51]. Thus their name Gif systems. The systems with geographically based names are mentioned in Table IX.5. All Gif systems have the same chemical peculiarities ... 

Renneke and HilF have shown how heteropolyanions such as H3PWj2O40 can catalytically oxidize alkanes under photolytic conditions in wet CH3CN. The reduced... 

The catalytic system consisting of PhIO and various iron porphyrins hydroxylates unactivated alkanes under ambient conditions [83,88]. The substrates studied were cyclohexane, cycloheptane, adamantane, cis-decahydronaphthalene and norcarane, and the catalysts - Fe(TPP)Cl, Fe(TTP)Cl, Fe(TNP)Cl and Fe(TMP)Cl. Monohydroxylated products predominated in all cases, with maximum yields of 30-39 % based on PhIO. High selectivity for tertiary centers (adamantane), retention of configuration at carbon (cis-decaline), and a large kinetic isotope effect (12.9) have ben observed. The free radical trap bromotrichloromethane changes the product distribution, pointing to a radical mechanism. The mechanism (Scheme 4) is essentially the same as that proposed previously for alkane oxidation by cytochrome P-450 [89]. 

Functionalization of alkanes under mild conditions catalyzed by coordination complexes is known. For example, much of the cyclohexanol and cyclohexanone used in the synthesis of adipic acid is produced by the oxidation of cyclohexane with a cobalt catalyst. Chapter 18, however, focuses on the t) pes of alkane functionalization that occur by the reactions described... 

Abstract The Shilov system, a mixture of di- and tetravalent chloroplatinate salts in aqueous solution, provided the first indication of the potential of organotransition metal complexes for activating and functionalizing alkanes under mild conditions the participation of higher-valent species plays a crucial role. In this chapter, we discuss the experimental and computational studies that have led to detailed mechanistic understanding of C-H activation and functionalization by both the original Shilov system and the many subsequent modifications that have been developed, and assess the prospects for practical, selective catalytic oxidation of alkanes using this chemistry. 

Because, as shown above, in the oxidation catalysis of alkanes, one can replace a transition metal catalyst by a redox-inactive nontransition metal catalyst, the question arises whether it would be possible to go even further and eliminate completely the use of any metal catalyst, thus establishing a metal-free system capable of oxidizing alkanes under mild conditions. 

Despite the apparent attractiveness of the conversion of natural gas into high value chemicals as a cheap raw material, the nature of the alkanes (i.e., inertness in chemical reaction at mild temperatures) is a definite obstacle. The way to overcome this problem is by finding a cafalysf, which is able fo acfivafe fhe alkane under mild conditions and selectively transform it into a desired product. Many different reactions have been studied and within them, particularly, dehydrogenation, oxidative dehydrogenation (ODH), and partial oxidation. 

So far, a relatively limited number of works has been published on the oxidation of light alkanes under SC conditions, mostly on the oxidation of methane in supercritical water (SCW). The role of water, which under SC conditions even at moderate temperatures, below 600 °C, cannot be considered as an inert component, is of special interest. Unfortimately, only a few experimental studies have been performed on the oxidation of methane in other than water SC fluids, a comparison with which would be very interesting for understanding the role of the state of the medium in these reactions. 

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