Oxidative dehydrogenation mechanism

The aerobic oxidation of alcohols catalysed by low-valent late-transition-metal ions, particularly those of group VIII elements, involves an oxidative dehydrogenation mechanism. In the catalytic cycle (Fig. 5) ruthenium can form a hydridometal species by /1-hydride elimination from an alkoxymetal intermediate, which is reoxidized by dioxygen, presumably via insertion of 02 into the M-H bond with formation of H202. Alternatively, an alkoxymetal species can decompose to a proton and the reduced form of the catalyst (Fig. 5), either directly or via the intermediacy of a hydridometal intermediate. These reactions are promoted by bases as cocatalysts, which presumably facilitate the formation of an alkoxymetal intermediate and/or /1-hydride elimination. 

Different formalisms were proposed for the oxidative dehydrogenation mechanism. For reducing sugars in basic solutions it was suggested [25] that aldose anions are adsorbed by the surface and then transfer a hydride (Section 9.3). Because oxidation reactions can also be conducted in neutral or acidic media, a dehydrogenation pathway occurring entirely on the metal surface was proposed... 

In conclusion, the oxidative dehydrogenation mechanism is widely accepted, but the reaction pathway will be under debate for some time because the nature and concentration of adsorbed species are not known. 

Thus, the process of dehydrogenation most often proceeds through the oxidizing dehydrogenation mechanism as represented by the following equation ... 

The above examples show the ability of microsome reductases to oxidize substrates in the processes where the first step is a one-electron reduction, which may or may not be accompanied by superoxide formation. However, cytochrome P-450 can directly oxidize some substrates including amino derivatives. For example, mitochondrial oxidation (dehydrogenation) of 1,4-dihydropyridines apparently proceeds by two mechanisms via hydrogen atom abstraction or one-electron oxidation [48 50]. Guengerich and Bocker [49] have shown that... 

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

Further studies are necessary to clarify these aspects and the details of the mechanism of propane oxidative dehydrogenation on V-containing silicalite. However, it should be noted that the amount of sites which appears to be stabilized... 

Oxidative dehydrogenation reactions of alcohols and amines are widespread in enzymatic biochemistry, and are of potential importance with regard to the operation of fuel cells based on simple alcohols such as methanol. The nature of products, and their rates of formation, may vary depending on the reaction conditions, and a role of metal ions has been recognized. The oxidation of amines may lead to a variety of products (nitriles, nitro species, etc.) although dehydrogenated diimine products are obtained quantitatively when the oxidation of the amine occurs via coordination to metal centers. A review is available on the mechanisms of oxidative dehydrogenations of coordinated amines and alcohols (93). 

M-butane proceeds via an intermolecular mechanism with 2-butene involved intermediately.300-303 The role of the transition metal promoters such as Fe and Mn was shown to increase the surface concentration of the intermediate butene 304 The formation of butene is speculated to occur through an oxidative dehydrogenation on the metal site305 or by one-electron oxidation.306... 

Scheme 16. Mechanism of oxidative dehydrogenation of [OsIV(en)2(en-H)]3+ produced from the disproportionation of[Os(en)3J3+. Reprinted with permission from the Journal of the American Chemical Society, Ref. 204. Copyright 1982, American Chemical Society.

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