Alkanes, dehydrogenation kinetics

Five-coordinate alkyl Pt(rv) complexes have been proposed as short-lived intermediates in platinum-catalyzed alkane functionalization cycles. Hence, interest in preparing suitable complexes to enable their chemistry to be studied has arisen.Novel five-coordinate platinum(iv) alkyl complexes with a variety of /3-diiminate ligands have been prepared and have been shown to be useful precursors to unsaturated Pt(ll) species for alkane. Stoichiometric alkane dehydrogenation was observed using either a five-coordinate Pt(iv) species or an olefin hydride complex. Mechanistic proposals were based on NMR spectroscopic measurements and, in one case, on X-ray crystallographic characterization of a product. Kinetic details were not reported in this communication, but the systems hold potential promise, and conversion to a catalytic system may be feasible upon further investigation. 

Alkanes are relatively stable species thermodynamically and so many reactions of alkanes (dehydrogenation, dehydrodimerization, carbonylation) are unfavorable under ambient conditions. This means we often need to couple some favorable process with the unfavorable alkane conversion in order to drive it. We look at the details in Section III.B, but only note here that the common appearance of photochemistry in alkane chemistry can be seen as a way to drive reactions thermodynamically and to access highly reactive transition metal fragments that are kinetically competent to react with alkanes. 

A fast isomerization of the kinetic product 1-octene to internal octenes has been observed. The stoichiometric alkane dehydrogenation mediated by 17 is not inhibited by the presence of N2 and a-olefins, and the rate of the reaction seems to be accelerated by the presence of water. Complex 17 can be regenerated from 19 by reacting with O2 and HO Ac [92]. This suggests that a catalytic cycle involving O2 as hydrogen acceptor is plausible. However, preliminary attempts to make this reaction catalytic have been thwarted by catalytic instability towards O2 at the temperature required for alkane dehydrogenation. 

To illustrate how a bifunctional catalyst operates, we discuss the kinetic scheme of the isomerization of pentane [R.A. van Santen and J.W. Niemantsverdriet, Chemical Kinetics and Catalysis (1995), Plenum, New York]. The first step is the dehydrogenation of the alkane on the metal ... 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

In contrast to the results obtained for dehydrogenation reactions, kinetic energy release distributions for alkane elimination processes can usually be fit with phase space theory. Results for the loss of methane from reaction 9 of Co + with isobutane are shown in Figure 10b. In fitting the... 

Kinetic analyses and deuterium-labeling experiments have demonstrated that, remarkably, the reductive elimination of TEA and the formation of intermediate C is the rate-determining step in the (de)hydrogenation cycle. Accordingly, hydrogenation of the acceptor appears to be slower than dehydrogenation of the alkane substrate. This contrasts with the fact that catalytic olefin hydrogenation is well-established in transition-metal-mediated chemistry [10]. 

Among these examples, probably the most notable case is the dehydrogenation of linear alkanes to their corresponding terminal alkenes (a-olefins), this being the kinetically favored process over the production of the internal alkenes. However, the same complex slowly catalyzes an isomerization of the terminal alkene to internal alkenes, as the latter are the thermodynamic products (Scheme 13.15) [33]. 

Specific dehydrogenation at the terminal positions of alkanes is a reaction that would be of high utility. The 1-alkenes obtained by such a reaction are the basis of a variety of additional products. Felkin and co-workers discovered that metal complexes are able to mediate the transfer of hydrogen from alkanes 13 to olefins 14 (Scheme 4) [17]. The specific advantages of a transition metal catalyst can be applied to the benefit of the chemoselectivity of this reaction. In a kinetically controlled process, it is predominantly primary C-H bonds that add to the metal complex. A subsequent /Miydride elimination affords the terminal alkenes... 

Higher alkenes can be obtained from thermal cracking of wax, and although a thermodynamic mixture of internal alkenes might have been expected, the wax-cracker product contains a high proportion of 1-aIkenes, the kinetically controlled product. For the cobalt-catalyzed hydroformylation the nature of the alkene mixture is not relevant, but for other derivatizations the isomer composition is pivotal to the quality of the product. Another process involves the catalytic dehydrogenation of alkanes over a platinum catalyst. 

Oxidative dehydrogenation is kinetically fast and may relieve this limitation [6]. Previous research has shown that substantial amounts of olefins are formed by catalytic oxidative dehydrogenation in a high temperature (1000°C), short contact time (1-10 milliseconds) reactor for small alkanes [7, 8]. 

H-abstraction reactions of cyc/o-alkanes follow the same rules and apply the same reference kinetic parameters as the analogous reactions of normal and branched alkanes. For example, Fig. 6 shows the main cyclo-hexyl radical pyrolysis pathways. For simplicity s sake, most of the dehydrogenation reactions are not reported. 

Quantitative studies of the kinetics of dehydrogenation of lower alkanes are few and far between, but those few have considerable significance. 

---