Alkane elementary steps

The elementary steps (1) through (3) describe a free radical chain mech anism for the reaction of an alkane with a halogen... 

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

Note that, while this reaction (as well as alkane hydrogenolysis) involves known elementary steps of molecular organometallic chemistry, it has not been discovered in homogeneous catalysis. One reason might be that highly... 

The mechanism of alkane sulfoxidation includes the following elementary steps [13,21 24] initiation, three steps of chain propagation, and a few steps of chain termination. 

In summary, the advances of the past few years have well demonstrated both the challenges and the promise of the Pt(II)/Pt(IV) redox couple for alkane functionalization. It should also be mentioned that the emerging conceptual understanding of the elementary steps involved in this process has also contributed to the development of methods to activate and functionalize alkyl groups in complex organic molecules. 

In this chapter, we will study the elementary reaction steps of these mechanisms focusing primarily on the anthraphos systems. This chapter begins with a description of the impact of different methods (coupled cluster, configuration interaction and various DFT functionals), different basis sets, and phosphine substituents on the oxidative addition of methane to a related Ir system, [CpIr(III)(PH3)Me]+. Then, it compares the elementary reaction steps, including the effect of reaction conditions such as temperature, hydrogen pressure, alkane and alkene concentration, phosphine substituents and alternative metals (Rh). Finally, it considers how these elementary steps constitute the reaction mechanisms. Additional computational details are provided at the end of the chapter. 

The transfer reaction utilizes a sacrificial alkene to remove the dihydrogen from the pincer or anthraphos complex first, before the oxidative addition of the target alkane. The elementary reaction steps are slightly different from the thermal reaction, which is discussed in the next section, both in their order and their direction. For simplicity, we describe the symmetric reaction where the sacrificial alkene is ethylene and the reactant is ethane (21b). The elementary reaction steps for the mechanism of this transfer reaction involve IVR, IIIR, VIR, VI, III and IV, where the superscript R stands for the reverse of the elementary steps listed in Section III. These reverse steps (IVR, IIIR, and VIR) involve the sacrificial alkene extracting dihydride from the metal to create the Ir(I) species 8, while steps VI, III and IV involve oxidative addition of target alkane, p-H transfer and olefin loss. 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

Another effective way of staying clear of the thermodynamic barriers of C-H activation/substitution is the use of the c-bond metathesis reaction as the crucial elementary step. This mechanism avoids intermediacy of reactive metal species that undergo oxidative additions of alkanes, but instead the alkyl intermediate does a o-bond metathesis reaction with a new substrate molecule. Figure 19.13 illustrates the basic sequence [20],... 

Beta scission of a carbenium ion is an elementary step that is inihated by the weakening of the bond beta to the positive charge, leading to a smaller carbenium ion and an alkene. This elementary step is further discussed in Sections 13.8.1, 13.8.3.1 and 13.8.4 within the context of alkene skeletal isomerization, isobutane-2-butene alkylation and alkane cracking, respectively. 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

The early use of deuterium in place of hydrogen in the study of catalytic hydrogenation led to the recognition that the process was not simply the addition of H2 to the double bond. Horiuti and Polanyi proposed that both H2 and alkene (1) are bound to the catalyst surface and transformed to products by a sequence of elementary steps, which they represented as shown in Scheme 1, where an asterisk ( ) represents a vacant site on the catalyst.The last step, (d), is virtually irreversible under the usual hydrogenation conditions, but can be observed in the exchange reactions of D2 with alkanes. The mechanism accounts for the isomerization of an alkene if the reversal of step (c), which involves the formation of the alkyl intermediate (3), involves the abstraction of a hydrogen atom other than the one first added, and is coupled with the desorption of the alkene, (2) - (1). At present, the bond between the alkene and the metal often is represented as a ir-complex (4), as in equation (7). ... 

In order to account for the results obtained, a mechanism has been proposed which is indicated in Scheme 2. The key step of C-C bond cleavage is represented as one of simple bond metathesis between the Ta-alkyl bond and the C-C bond of an alkane. This elementary step has no precedent that we are aware of, and could easily be represented as, for example, oxidative addition and reductive elimination This (T-bond metathesis mechanism predicts product distributions based on steric constraints quite simply. Thus we find it useful and so will continue to use the hypothesis for discussion of our results. 

The thermodynamic analysis of a complex chemical process has both the advantage and the limitation of being concerned only with the reactants and products of the reaction. This blindness to the mechanistic details ensures that the analysis is correct even if the model designed for the path is not, but misses all the important chemical events that account for the transformation of the molecules. This point can be illustrated through Scheme 6, which represents the catalytic cycle of alkene hydrogenation. While the net thermodynamic efficiency of the process is only dictated by the enthalpies of formation of the olefin, hydrogen, and the alkane, its feasibility depends on the thermodynamics of each elementary step. 

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