Mechanisms aerobic dehydrogenation

This name is applied to the series of carriers along which pass the protons and electrons liberated in the course of a dehydrogenation in the tricarboxylic acid cycle or other aerobic dehydrogenation before they reach oxygen and unite with it to form water. As we have seen, it is along the respiratory chain that by a series of phosphorylations coupled to it by a still unknown mechanism (see p. 144) the main quota of energy-rich bonds is formed and placed at the disposition of the cells. 

This enzyme is commonly found in aerobic sulfur-oxidizing lithotrophs, but its importance as an alternative to a hydration-dehydrogenation electron-transport-linked energy-generating mechanism is still disputed (Kelly 1999) ... 

Moreover, we believe that the azo form helps in stabilizing several of the reactive copper complexes involved in this catalytic cycle such as the hydroxy copper complex 17. Thus, we surmise that this novel catalytic, aerobic oxidation procedure for alcohols into carbonyl derivatives proceeds via a dehydrogenation mechanism and relies on the effective role of hydrazine or azo compounds as hydrogen shuttles and stabilizing ligands for the various copper complexes (20). 

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. 

This substantiates the positive role of O2 in promoting benzaldehye and benzoic acid formation. From the above-mentioned two examples, it is apparent that there are many parallel reactions in the aerobic oxidation of benzyl alcohol, besides the direct dehydrogenation of benzyl alcohol to benzaldehyde, which result in the undesired by-products. On the basis of these studies and other literature evidence, a network of reactions occurring during the aerobic oxidation of benzyl alcohol over Pd/AljOj catalyst is presented in Scheme 12.1. After elucidating the mechanisms of reactions that lead to by-products, the next step in catalyst development is the complete elimination or at least suppression of these byproducts. It is logical to assume that all these reactions may not have the same active sites, and it is important to identify the different active sites for different reactions. 

Wigington BN, Drummond ML, Cundari TR, Thom DL, Hanson SK, Scott SL. A biomimetic pathway for vanadium-catalyzed aerobic oxidation of alcohols evidence for a base-assisted dehydrogenation mechanism. Chem Eur J. 2012 18 ... 

Intermolecular direct C(3)-alkenylation of indoles using palladium(II) as catalyst and oxygen as the oxidant has been achieved. The reaction shows complete regio- and stereo-selectivity all products are ii-isomers at the C(3)-position, and no Z-isomers or 2-substituted products are detected. Aerobic a, -dehydrogenation of aldehydes and ketones is catalysed by Pd(TFA)2/4,5-diazafluorenone. The cleavage of a-C-H bond of the ketone has been identified as the turnover-limiting step of the catalytic mechanism. ... 

---