Carboxylation mechanism

The presence of various types of Au sites and carbonate/carboxylate species as well as variation in the OH intensity of Ti02 (not shown here) during the reaction suggests that CO oxidation over AuCb catalyst could follow the carboxylate mechanism which involves the reaction of adsorbed CO with OH to produce a carbonate/carboxylate species on Au cations and the decomposition of carboxylate to COz-iS, 10) Transient infrared study needs to be employed to further verily the role of carbonate/carboxylate species in the reaction pathway. (13)... 

Concerning the mechanism of metal-catalyzed carboxylations, two basic pathways—the metal hydride and the metal carboxylate mechanisms—gained general acceptance.14 16 118 119122151 The actual mechanism depends on the catalyst used and the reaction conditions particularly in the presence of additives (acids, bases). It seems reasonable to assume that both mechanisms may be operative under appropriate conditions. 

Despite less direct evidence, the metal carboxylate mechanism has also been widely proposed 134 156-159... 

Both mechanisms are predicted to show syn addition of hydride and caiboxylate to the alkene. In the metal hydride mechanism (equation 36) alkene insertion is syn and CO insertion proceeds with retention of configuration at carbon. In the metal carboxylate mechanisms (equation 37) alkene insertion is syn and cleavage of the metal-carbon bond can take place with retention at carbon. The palladium-catalyzed hydroesterification reaction produces the erythro ester from (Z)-3-methyl-2-pentene (equation 38) and the threo ester from ( )-3-methyl-2-pentene (equation 39).w... 

The transition metal carboxylate mechanism (Scheme 10), illustrated with palladium, requires the generation of this type of complex in the initial steps. This can occur by the alcoholysis of dicobalt octacarbonyl (Scheme 6) or by, for example, the reaction of methanol with a palladium(II) carbonyl (equa-... 

Although the RPP cycle is the fundamental carboxylating mechanism, a number of plants have evolved adaptations in which CO2 is first fixed by a supplementary pathway and then released in the cells in which the RPP cycle operates. One of these supplementary pathways, the C4 pathway, involves special leaf anatomy and a division of biochemical labor between cell types. Plants endowed with this path-... 

The reductive pentose phosphate cycle is the only fundamental carboxylating mechanism in plants. In C3 plants the entire process of photosynthesis (the light reactions and the RPP cycle) occurs within chloroplasts. The enzymes catalysing steps in the RPP cycle are water-soluble and are located in the soluble portion (chloroplast stroma or extract). 

Although in each type of C4 pathway there is an initial carboxylation catalyzed by PEP carboxylase, the plant s ability to produce a net increase in fixed carbon depends on subsequent release of CO2 and refixation by the RPP cycle. In this sense, the RPP cycle is still the fundamental carboxylating mechanism of these plants. It should be noted that C3 plants also contain a cytosolic PEP carboxylase which is capable of fixing CO2. However, C3 plants lack the biochemical and structural specialization as well as the division of labor between cell types that make possible the classical C4 type of photosynthesis. 

The isovaleryl CoA derived from leucine is dehydrogenated to yield -methylcrotonyl CoA. This oxidation is catalyzed by isovaleryl CoA dehydrogenase. The hydrogen acceptor is FAD, as in the analogous reaction in fatty acid oxidation that is catalyzed by acyl CoA dehydrogenase. -Methylglutaconyl CoA is then formed by the carboxylation of P-methylcrotonyl CoA at the expense of the hydrolysis of a molecule of ATP. As might be expected, the carboxylation mechanism of P-methylcrotonyl CoA carboxylase is similar to that of pyruvate carboxylase and acetyl CoA carboxylase. 

For the first time, in 2008, Gokhale et al. [34] proposed the concept of carboxylate mechanism. They did several density functional theory (DFT) calculations to investigate the mechanism of WGSR over Cu(l 11). The typical mechanism involves the oxidation of CO by OH to form carboxyl (COOH) species. The carboxyl so formed may then yield CO2. This mechanism is very similar to the redox mechanism. The typical reaction mechanism is as follows ... 

FIGURE 9.6 (a) Carbonate/carboxylate mechanism for the reverse WGS reaction, (b) Carbon-... 

Thus it may have happened that conditions favored bacterial mutants which had developed carboxylation mechanisms, in which the products of carboxylation were reduced by electrons obtained... 

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