Oxygen transfer mechanisms

The olefin oxygenations carried out with dioxygen seem to be metal-centered processes, which thus require the coordination of both substrates to the metal. Consequently, complexes containing the framework M (peroxo)(olefin) represent key intermediates able to promote the desired C-0 bond formation, which is supposed to give 3-metalla -l,2-dioxolane compounds (Scheme 6) from a 1,3-dipolar cycloinsertion. This situation is quite different from that observed in similar reactions involving middle transition metals for which the direct interaction of the olefin and the oxygen coordinated to the metal, which is the concerted oxygen transfer mechanism proposed by Sharpless, seems to be a more reasonable pathway [64] without the need for prior olefin coordination. In principle, there are two ways to produce the M (peroxo)(olefin) species, shown in Scheme 6, both based on the easy switch between the M and M oxidation states for... 

The hydroxylation of aliphatic C-H bonds can be performed exclusively by CPO [65-67]. A direct oxygen transfer mechanism has been suggested, as the oxygen atom arises from the peroxide. The reaction requires an activated C-H bond since hydroxylation occurs at the ally lie or benzylic positions. Both cyclic (cyclohexene)... 

The pore model is unable to describe this late reactivity maximum around X=0.7 as it foresees a possible maximum reactivity to occur only between 0 X < 0.393. In the literature, the late occurrence has been explained by intercalation of alkali metal species into the carbon stmeture, leading to a gradual release of active centres with conversion. We note, however, that intercalation effects have seldom been reported for charcoals (in contrast to graphite). In our opinion the cause for the "anomalous" reactivity behaviour stems from a combination of structural and catalytic phenomena emerging from the reaction mechanism involved. The most important mechanism proposed nowadays is the oxygen transfer mechanism in which the oxygen is extracted from the reactant gas (CO2) by the catalyst, which then supplies it in an active form to the carbon. 

Meou, A., Garcia, M. A., Brun, P. Oxygen transfer mechanism in the Mn-salen catalyzed epoxidation of olefins. J. Mol. Catal. A Chemical 1999, 138, 221-226. 

This mechanism was not able to explain the observed experimental features of the carbon gasification catalysed by iron. The oxygen-transfer mechanism assumes that the catalyst undergoes a cycle between two oxidation states such as metal and oxide or lower oxide and higher oxide. Walker et al extended the views of previous workers and proposed the following equation as representing the mechanism for the case of C-CO2 reaction ... 

With Ti-substituted Keggin polyoxometalates, for example, Na5 H PTiWn04o, the oxidation of cyclohexene with HP in an acetonitrile solvent yields traws-1,2-cydohexandiol as the main reaction product, via a heterolytic oxygen-transfer mechanism, when n> 2 in the compound. If the polyoxometalate contains only one proton, the main products are those of allylic oxidation, namely 2-cyclohexene-l-ol and 2-cydohexene-l-ol, produced via a homolytic oxidation mechanism [36tj. 

FIGURE 1.15 Accepted direct oxygen transfer mechanism of Mo-catalyzed epoxidation with H2O2. 

H. Shi, X. Wang, R. M. Hua, Epoxidation of alpha, beta-imsaturated acids catalyzed by tungstate (VI) or molybdate (VI) in aqueous solvents a specific direct oxygen transfer mechanism. Tetrahedron 61 (2005) 1297. 

Contamination of the deposit surface with Pb, Fe, Co and Ni (in the metallic state), Na, K, Li, Rb and Cs carbonates or oxides, and Ba and Sr oxides enhanced gasification, at 900°C, by up to a factor of 135. Catalysis derived from oxygen transfer mechanisms. A further nineteen elements had no significant influence. 

It seems to be widely accepted that oxygen transfer mechanisms are responsible primarily for the catalysed oxidation of carbon. These are based on the continuous cyclic variation of the oxidation state of the catalyst as it is oxidised by chemisorbed oxygen, from the dissociation of water vapour and then reduced by the underlying carbon, which in turn becomes gasified as carbon monoxide. The localised reaction of the catalyst particle with the carbon substrate results in particle motion and channelling of the surface. 

The likely mechanism of the CA[Mn]-catalyzed epoxidation of olefins is similar to that proposed by Burgess for free manganese involving peroxycarbonate as the key intermediate [41, 43 5], CA[Mn] forms a stable manganese-bicarbonate complex in the active-site [60], Hydrogen peroxide may add to carbonyl of this bicarbonate complex and displace water thereby forming a peroxycarbonate, which then may epoxidize a bound olefin. In contrast, heme peroxidases catalyze epoxidations either by a radical mechanism or by a ferryl-oxygen transfer mechanism [55]. 

Vallejos-Burgos F, Utsumi S, Hattori Y, Garcia X, Gordon AL, Kanoh H, Kaneko K, Radovic LR (2012) Pyrolyzed phthalocyanines as surrogate carlxHi catalysts initial insights into oxygen-transfer mechanisms. Fuel 99(1) 106-117... 

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