Activation in alkanes

These differences underline a larger steric hindrance in the coordinahon sphere of Hf metal. Thermolysis experiments prove that (=SiO)Hf(CH2Bu )3 is more stable and more active in alkane hydrogenolysis than (=SiO)Zr(CH2Bu )3. 

Tungsten surface organometallic chemistry has also been studied because a silica grafted tungsten hydride had been previously shown to exhibit some activity in alkane metathesis reaction (see below) [75, 76]. 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

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. 

Two studies in 1985 and 1992 focused on the use of superacids in isomerization.20,21 An important advantage of these catalysts is that they may be used at lower temperature because of their greatly increased ability to bring about carbo-cationic process. For example, aluminum chloride, still one of the important isomerization catalysts in industry, can be used at about 80-100°C. In comparison, superacids are active in alkane isomerization at room temperature or below. In addition to avoid side reactions under such conditions, lower reaction temperatures favor thermodynamic equilibria with higher proportion of branched isomers, which is fundamental in increasing the octane number of gasoline. 

Summing up, it is conceivable that both the state of surface topography as well as the presence of other elements on the Pd surface are decisive for its activity in alkane reactions. However, two things seem obvious and unquestionable. First, all these catalytic variations result from the fact that hydrogenolysis, rather than isomerization, is sensitive to changes in... 

Kushch, S.D., Fursikov, P.V., Kuyunko, N.S., Kulikov, A.V. and Savchenko, V.I. (2001) Fullerene black relationship between catalytic activity in -alkanes dehydrocyclization and reactivity in oxidation, bromination and hydrogenolysis. Eurasian Chemico-Technol. Journal, 3, No. 2, 131-139. 

Later it was found that to get a successful activity in alkane metathesis, catalysts need to have a multifunctionality, i.e., (i) activation of the C-H bond resulting in a metal alkyl, (ii) a-H elimination leading to a metallocarbene, (iii) p-H elimination leading to an olefin, (iv) olefin metathesis, and (v) finally successive hydrogenations of the olefins or the carbenes leading to alkanes. The selectivity of products is a consequence of the relative stabilities of metaUacyclobutanes intermediates formed during the olefin metathesis [52]. 

Using a procedure related to that for the preparation of tantalum catalysts, supported W-polyhydride species were prepared to evaluate their catalytic activity in alkane metathesis. Hydrogenolysis of the grafted, alkylidyne d°-complex W(=fBu)(CH2tBu)3 [52] led to W-polyhydride complexes (Scheme 2.8) [53]. 

These early examples provided the impetus for the development of C-H bond activation in alkanes and arenes. An important advance in this respect was the example reported by Chatt for the [Ru(0)(dmpe)2] 12 catalyzed C-H bond activation of naphthalene 13 (also some amount of C H activated product of the alkyl phosphines could be observed). It is the first reported example of the C-H bond activation of (hetero)arenes by a transition metal complex and Chatt has therefore been credited to have laid the foundation for the rapid development of synthetically viable and environmentally attractive C C bond forming technologies via C-H bond activation (Fig. 5). Later Chatt suggested that the above transformation could be occurring through an oxidative addition mechanism which has been since then commonly accepted. 

Steps have been taken toward eco-friendly catalytic systems active in alkane functionalization under mild conditions, preferably in aqueous media, by using hydrosoluble catalysts obtained from ligands that are water soluble. Systems can operate usually with a green oxidant (aqueous hydrogen peroxide) and in partially aqueous media. 

In either VPO or MoVTeNbO catalysts, V-sites are directly involved in the selective oxidative activation of paraffins, while the presence of a second element is generally required in order to facilitate the formation of a defined crystalline phase (i.e. vanadyl pyrophosphate or orthorhombic metal oxide bronze, respectively) (Figs. 24.2a and 24.2c), but also for facilitating the multifuctionality of the catalysts. However, although both catalytic systems are active for the oxidative activation in alkanes, different selectivities are achieved depending on the alkane feed. Thus, a... 

The same authors have found that (Me5C5)2Yb is capable to form a complex with methyl-(pentamethylcyclopentadienyl)beryllium [48]. In this case the methyl group is bridged that allows to consider these compounds as a model of the transition state of the C-H bond activation in alkanes by lanthanoid complexes. 

A long controversy also grew up over the work of Shilov et al [28]. In 1969 these authors published the first example of homogeneous CH bond activation in alkanes the hydrogen—deuterium exchange reaction catalyzed by platinum(II) complexes (reaction 13). 

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