Alkane activation routes

Scheme 8.4 Postxilated routes to alkane activation over solid acids.

The overall mechanism shown in Scheme 2, outlined by Shilov at an early stage in the research, has been essentially vahdated by all subsequent work. The reaction begins with activation of a C-H bond at a Pt(II) center. (There are examples of arene, but not alkane, activation by Pt(lV) these probably involve classical electrophihc routes via 7i-complexes and Wheland intermediates [10].) This fact seems incontrovertible since Pt(ll) by itself catalyzes H/D exchange the detailed mechanism of the activation is much less obvious, as discussed in Sect. 3. The resulting RPt(II) complex is extremely sensitive to electrophilic cleavage - no [RPt Cl c(H20)3 J species can be observed in the presence of any protmi source - so using Pt(II) alone, no alkane conversion beyond isotopic exchange would be feasible. However, [Pt Cle] effects oxidation to RPt(IV), which is virtually completely inert to protonolysis but quite susceptible to nucleophilic... 

Apart from alkene production, n-alkanes are applied as feedstock in the technical production of alkyl sulfonate surfactants (see Section 5.3.5 for details about surfactants and their application). Two routes are established for the production of the alkyl sulfonate sodium salts [R-S03]Na from higher alkanes (for this application typical C-numbers are 12-18). In both routes, alkane activation proceeds via alkyl radicals, which are generated by UV-irradiation at room temperature. In the sulfo-chlorination route, the higher alkane is contacted with SO2 and chlorine to form the alkylsulfonyl chloride, which is later neutralized with NaOH to give the... 

An alkane CH bond can oxidatively add to a variety of low-valent tiansi-tion metals preferentially to give the linear product, n-Pr-M-H, however, and in any subsequent functionalization, the linear product, n-PrX, is often obtained. In addition, methane activation holds promise as methane seems likely to become a more important feedstock for the chemical industry. Methane conversion to methanol or a derivative (e.g., MeOCH20Me) would make a conveniently transportable fuel. Partial oxidation such as this is particularly hard. Methanol is much more easily oxidized than methane, so classical oxidation procedures give CH2O, CO and CO2. In the CH activation route, the CH bond of methanol is not much more reactive than in methane, so the overoxidation problem is less severe. 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

A number of examples have been reported documenting the use of palladium phosphine complexes as catalysts. The dialkyl species [PtL2R2] (L2 = dmpe, dppe, (PMe3)2 R = Me, CH2SiMe3) catalyze the reaction of [PhNH3]+ with activated alkenes (acrylonitrile, methyl acrylate, acrolein).176 Unfunctionalized alkenes prove unreactive. The reaction mechanism is believed to proceed via protonation of Pt-R by the ammonium salt (generating PhNH2 in turn) and the subsequent release of alkane to afford a vacant coordination site on the metal. Coordination of alkene then allows access into route A of the mechanism shown in Scheme 34. Protonation is also... 

Although the activation and functionalization of C-H bonds of alkanes are the important, promising routes for synthesis of functionalized materials, it is difficult to achieve the functionalization of alkanes because they are unreactive due to the low reactivity of alkane C-H bonds. Carboxylation of alkanes to carboxylic acids is one of the interesting and important functionalization processes. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

The hydrogenation activity is an added advantage of this route, since most of the hydroformylation-products are converted to the alcohols anyway. Concurrent with the hydrogenation of aldehyde to alcohol, however, hydrogenation of alkene feedstock to alkane occurs, which may be as high as 15% under certain conditions (versus 2-3% for the non-ligand-modified... 

In spite of significant fundamental studies and its significant economic potential as an alternate route to alkenes, the oxidative dehydrogenation of alkanes to alkenes is not currently practiced.383 The main reason is that the secondary oxidation of the primary alkene products limits severely alkene yields, which becomes more significant with increasing conversion. This is due mainly to the higher energies of the C—H bonds in the reactant alkanes compared to those of the product alkenes. This leads to the rapid combustion of alkenes, that is, the formation of carbon oxides, at the temperatures required for C—H bond activation in alkanes. 

Under the same conditions, several types of hydrocarbon are also converted to fully deuterated compounds. The results are summarized in Table 1. Cydooctene was also transformed into fully deuterated cydooctene without a skeletal rearrangement. As shown in entries 2 and 3, saturated hydrocarbons have also been transformed into fully deuterated compounds. As described above, an interaction between allylic C-H bonds and palladium hydride induces the H-D exchange reaction for alkenes. H-D exchange in alkanes, however, cannot be explained in this way. Direct C-H activation without assistance from any functional group may be a route to the formation of fully deuterated alkanes. 

Acyclic azo compounds are obtained by usual routes only as frans-isomers. Photochemical isomerisation provides an easy and effective way to the cis-isomers. The cfs-azo compounds are however thermally quite labile and decompose with elimination of nitrogen 41>, some of them even at —120 °C 5>. Activation energies for the thermal decomposition of cfs-azo-alkanes are of the order of 20... 

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