Oxygen decarbonylation

The formation of such multiply coordinated surface intermediates would be expected to be enhanced by adsorption of multi-functional reagents, e.g., oxygenates with hydrocarbon chains more reactive than saturated alkyl ligands. To test this hypothesis, we have also examined the adsorption and reaction of allyl alcohol (CH2=CH-CH20H) and acrolein (CH2=CH-CHO) on the Rh(lll) surface. While these molecules do exhibit evidence for interaction with the surface via both their oxygen and vinyl functions, and while they appear to preserve the divergence of decarbonylation pathways observed for their aliphatic counterparts, their reactivity patterns add yet another layer of complexity to the puzzle of oxygenate decarbonylation. 

WILKINSON Cartxinylalion decarbonylation catalyst Rh catalyst lor cartxinylation, decarbonylation, oxygenation, benzyl cleavage... 

Experiments were carefully designed to study the effect of the oxygen concentration in the system on the competition between hydrogenation and decarbonylation, as being shown in... 

Fig.4. Decarbonylation of 4-CBA under different oxygen concentrations (220 "C, 2 g 20-60 mesh catalyst)...

The 4-CBA decarbonylation does not weaken much the function for purification. However, it undoubtedly competes with its hydrogenation. The competition is greatly influenced by the amount of the dissolved oxygen in the system. The more is the dissolved oxygen, the easier the decarbonylation takes place. However, the hydrogen has an opposite effect. 

A possible explanation for this is decarbonylation of the isobutyryl radical prior to its further reaction with oxygen (reaction (10)) ... 

Hydrocarbon formation involves the removal of one carbon from an acyl-CoA to produce a one carbon shorter hydrocarbon. The mechanism behind this transformation is controversial. It has been suggested that it is either a decarbonylation or a decarboxylation reaction. The decarbonylation reaction involves reduction to an aldehyde intermediate and then decarbonylation to the hydrocarbon and releasing carbon monoxide without the requirement of oxygen or other cofactors [88,89]. In contrast, other work has shown that acyl-CoA is reduced to an aldehyde intermediate and then decarboxylated to the hydrocarbon, releasing carbon dioxide [90]. This reaction requires oxygen and NADPH and is apparently catalyzed by a cytochrome P450 [91]. Whether or not a decarbonylation reaction or a decarboxylation reaction produces hydrocarbons in insects awaits further research on the specific enzymes involved. 

In order to establish the identity of the trimethylcarbonium ion, the t-butyl fluoride-antimony pentafluoride system was investigated. It was found that when the vapour of t-butyl fluoride was passed over the surface of purifled liquid antimony pentafluoride (with exclusion of moisture and oxygen) a stable complex layer is formed on the top of the antimony pentafluoride. When this layer was separated and its proton magnetic resonance investigated (see subsequent discussion) the spectrum was found to be identical with that of the least-shielded species formed by decarbonylation of the t-butyloxocarbonium salt... 

Figure 27. Mechanism of radiation induced chain scission in PMMA. Homolysis of the mainchain-carbonyl carbon bond is indicated as the initial step. Acylcarbon-oxygen, sigma bond homolysis also occurs but rapid decarbonylation ultimately leads to the same indicated products.

The influence of the decomposing atmosphere on the final species obtained has been studied for an oxygen-free carbon-supported Ru3(CO),2 [106]. A DRIFT study showed that under H2 the fast formation of H4Ru4(CO),2 species results under He, Ru3(CO),2 decomposes via a progressive decarbonylation and fragmentation. Complete decarbonylation under both H2 or He produced well-dispersed Ru particles resistant to sintering under CO hydrogenation conditions [106]. 

CO hydrogenation carried out over decarbonylated silica-supported RhCo3(CO)i2 and Rh2Co2(CO)i2 showed that the produchon of oxygenates, mainly ethanol, was favored over the catalyst with a Rh/Co = 1 raho [142, 186]. 

Carbohydrate resources. Carbohydrate resourees, sueh as hydrolyzed stareh and suerose as well as xylose and glueose, ean be proeessed into hydroearbons in a proeess similar to the one performed with bio-oils as described above (section 3.2.2), i.e. by using a HZSM-5 catalyst operated at around 510 °C and ambient pressure." This process is perhaps a little surprising since carbohydrates do not resemble the desired hydrocarbon product as much as the bio-oils do. However, formation of hydrocarbon compounds was found to occur as a result of oxygen removal from the carbohydrate by decarbonylation and decarboxylation reactions." This process is probably one of the first attempts to conduct catalytic cracking of biomass. 

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