Carbon of carbonyl groups

Most CO2 assimilation takes place at a- and 8-carbons of carbonyl groups of organic molecules to give a-hydroxy-, a-keto, and )8-keto acids 90, 91). There has been no report of a-hydroxyketo acid formation by artificial CO2 fixation with the intention to mimic CO2 assimilation in green plants, in which two moles of 3-phosphoglycerate (PGA) are produced hy CO2 fixation to ribulose-l,5-bisphosphate(RuDP) ... 

In the reductive carboxylic acid cycle of photosynthetic bacteria (92-94), which plays a role in the conversion of CO2 to precursors of fatty acids, amino acids, and porphyrins, four molecules of CO2 are fixed in one turn two of them are fixed to a-carbons of carbonyl groups of acetylcoenzyme A and succinyl-coenzyme A to generate pyruvate and a-keto glutarate, respectively ... 

Section 8-6 presents two u.seful reduction processes. Carbonyl compounds such as ketones and aldehydes arc useful precursors (starting materials) for the synthesis of alcohols. Either metal-catalyzed Ht addition or reaction with the hydride reagents NaBH and LiAIH converts aldehydes to primary alcohols. The same processes convert ketones to secondary alcohols. The.se hydride reductions are the lirst of many examples that you will. see of nucleophilic additions to the electrophilic carbons of carbonyl groups. This is one of the most important clas.ses of reactions in organic chemistry. 

The structures of Rh4(CO)i2 and Rh6(CO)j6 are shown in Figures 18.4 and 18.5, respectively [40,41]. These compounds form cluster structures in which rhodium atoms bond each other. These consist of Rh-Rh bond, Rh-CO bond and a bridged carbonyl bond in which the carbon of carbonyl group bonds to two or three rhodium atoms. As Rh6(CO)i6 has more Rh-Rh bonds compared with Rli4(CO)i2, it is more stable and has less solubility. 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

Table 7.55 Carbon-13 Chemical Shifts of Carbonyl Group 7.106...

MEK is a colorless, stable, flammable Hquid possessing the characteristic acetone-type odor of low molecular weight aUphatic ketones. MEK undergoes typical reactions of carbonyl groups with activated hydrogen atoms on adjacent carbon atoms, and condenses with a variety of reagents. Condensation of MEK with formaldehyde produces methylisopropenyl ketone (3-methyl-3-buten-2-one) ... 

The dynamic resolution of an aldehyde is shown in Figure 8.40. The racemization of starting aldehyde and enantioselective reduction of carbonyl group by baker s yeast resulted in the formation of chiral carbon centers. The enantiomeric excess value of the product was improved from 19 to 90% by changing the ester moiety from the isopropyl group to the neopentyl group [30a]. 

Reduction of carbon-carbon double bond Microalgae easily reduce carbon-carbon double bonds in enone. Usually, the reduction of carbonyl group and carbon-carbon double bond proceeds concomitantly to afford the mixture of corresponding saturated ketone, saturated alcohol, and unsaturated alcohol because a whole cell of microalgae has two types of reductases to reduce carbonyl and olefinic groups. The use of isolated reductase, which reduces carbon-carbon double bond chemoselectively, can produce saturated ketones selectively. 

Chapters 1 and 2 dealt with formation of new carbon-carbon bonds by reactions in which one carbon acts as the nucleophile and another as the electrophile. In this chapter we turn our attention to noncarbon nucleophiles. Nucleophilic substitution is used in a variety of interconversions of functional groups. We discuss substitution at both sp3 carbon and carbonyl groups. Substitution at saturated carbon usually involves the Sjv2 mechanism, whereas substitution at carbonyl groups usually occurs by addition-elimination. 

The palladium-catalyzed Heck carbonylation reaction is a powerful means of generating amides, esters, and carboxylic acids from aryl halides or pseudohalides [28]. The development of rapid, reliable, and convenient procedures for the introduction of carbonyl groups is important for the development of high throughput chemistry in general and high-speed microwave-mediated chemistry in particular. Unfortunately, the traditional method of introducing carbon monoxide into a reaction mixture via a balloon or gas tube is not practical because of the special requirements of microwave synthesis. 

Hydrolysis, or better, hydrogenolysis of the normal ozonide leads to the cleavage of the original carbon-carbon double bond with formation of carbonyl groups. The abnormal ozonide usually decomposes before it reaches room temperature and both the double bond and the adjacent carbon-carbon single bond are found to have cleaved. 

Enzyme reductions of carbonyl groups have important applications in the synthesis of chiral compounds (as described in Chapter 10). Dehydrogenases are enzymes that catalyse, for example, the reduction of carbonyl groups they require co-factors as their co-substrates. Dehydrogenase-catalysed transformations on a practical scale can be performed with purified enzymes or with whole cells, which avoid the use of added expensive co-factors. Bakers yeast is the whole cell system most often used for the reduction of aldehydes and ketones. Biocatalytic activity can also be used to reduce carbon carbon double bonds. Since the enzymes for this reduction are not commercially available, the majority of these experiments were performed with bakers yeast1 41. 

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