Alkanes, activation dehydrogenation

Keywords Alkane metathesis Borylation C-H bond activation Dehydrogenation Hydroarylation Iridium catalyst Silylation... 

Until now, for most of the systems described here it has been accepted that alkane activation occurred through oxidative addition to the 14-electron intermediate complexes. Yet, Belli and Jensen [26] showed, for the first time, evidence for an alternative reaction path for the catalytic dehydrogenation of COA with complex [lrClH2(P Pr3)2] (22) which invoked an Ir(V) species. Catalytic and labeling experiments led these authors to propose an active mechanism (Scheme 13.12), on the basis of which they concluded that the dehydrogenation of COA by compound 22 did not involve an intermediate 14-electron complex [17-21], but rather the association of COA to an intermediate alkyl-hydride complex (Scheme 13.12). 

The catalysts based on vanadium oxide are one of the better studied systems. A V-Mg oxide in which Mg orthovanadate (Mg2(V04)2) and MgO were the only identifiable phases was a rather selective catalyst (27). Since MgO was relatively inactive in alkane activation, Mg orthovanadate was assumed to be the active component. Indeed, Mg orthovanadate prepared as a stoichiometric compound showed high selectivities for the oxidative dehydrogenation of propane (29). In this latter study, it was shown interestingly that Mg orthovanadate was the only alkali or alkali earth orthovanadate that... 

Supercritical C02 has also recently attracted much attention as a reaction medium for C-H bond activation, because C02 is miscible with organic compounds, including organometallic compounds, and potentially stable toward alkane activation conditions. We have successfully applied supercritical C02 to the carbonyla-tion and dehydrogenation of hydrocarbons [34], The technique is effective for conversion of gaseous substrates such as methane and ethane [35]. 

Hydride complexes have been important precursors in the study of Alkane Activation. For example, alkanes can be catalytically dehydrogenated by ReH7(PR3)2 or [hH2(OH2)2(PR3)2] or (PCP)hH2 thermally or photo-chemically. Cyclooctane is the best snbstrate, presumably because it has the least unfavorable heat of dehydrogenation of all common alkanes (equation 27). 

In spite of their high formal oxidation states, the polyhydrides behave as low-oxidation species because of the low charge on the metal, which is close to the charge found in the free metal. From the organometallic point of view, the most remarkable consequence of this reactivity is found in the reactions of polyhydrides with alkanes leading to alkane dehydrogenation. Alkane Activation or C-H Bond... 

Since the introduction of the Cyclar process by BP(1), much work has been carried out on the Influence of the procedure for incorporation of dehydrogenating components (Ga or Zn) into the ZSM-5 zeolite, as well as on the mechanism of the conversion of light alkanes to aromatics (2-7). The differences observed among the different catalyst preparations are related to the dispersion and the stoichiometry of the metal oxide. Indeed changes in selectivity to aromatics with time on stream (8) or with a pretreatment of the catalyst with have been associated with the formation of lower valences of Ga as the active dehydrogenating species (6). 

A key issue for synthetic chemists is the direct and selective functionalization of alkanes under mild conditions. A major problem in C-H bond activation by molecular catalysis is the lack of a suitable reaction medium, because most organic solvents are not inert under alkane activation conditions and therefore prevent the desired reactions. In this context, dense carbon dioxide seems to be a promising reaction medium as it is miscible with organics, including organometallics, and potentially stable under alkane activation conditions. Indeed, methane carbonylation and alkane dehydrogenation by molecular catalysis have been reported using dense carbon dioxide as the reaction medium (Scheme 67). " ... 

Keywords Alkane functionalization C-H activation Dehydrogenation Iridium... 

Active Dehydrogenation Site and Structure-Activity Reiationship for Cr/Ai20z CataiyStS. The catalytic dehydrogenation of alkanes is of great... 

Photocatalyzed reactions have been very important in alkane activation chemistry[l-3] because, other than oxidation, most reactions of alkanes are thermodynamically unfavorable. Such is the case for alkane dehydrogenation (eq. 1), carbonylation (eq. 2) and dehydrodimerization (eq. 3), and these are three reactions that have been the best studied to date and are covered in this chapter. 

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