Carbon dioxide, elimination

The current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

Mercuric 2//-hexafluoroisobutyrate, [(CF3)2CHC02]2Hg, when heated in refluxing pyridine (30) or to 40-50° C in dmf (31) gave bis(l//-hexafluoroisopropyl)mercury, [(CF3)2CH]2Hg. Competition between mer-curation and decarboxylation was studied by repeating the reaction in dmf in the presence of mercuric acetate (31). The main path was found to be mercuration at the a carbon of the hexafluoroisobutyrate group followed by decarboxylation. The low yield of bis(l//-hexafluoroisopropyl)mercury showed that carbon dioxide elimination is slower than mercuration under these conditions. 

The most common and widely used supercritical fluid in SFC is carbon dioxide. It is inert, in that it is non-toxic and non-flammable, it also has mild critical parameters, a low critical temperature of 31.3°C and a critical pressure of 72.8 atm [1], Using pure, supercritical carbon dioxide eliminates organic solvent waste and with it waste disposal costs and concerns. This is extremely practical advantage in the industrial environment where the generation of waste requires special handling and significant cost. 

The oxadiazinone (50), obtained from the pyrimidine (49), undergoes a hetero Diels-Alder addition with carbon dioxide elimination on heating with enamines to yield the 6-alkylated lumazine derivatives regioselectively (Scheme 21) [95JHC807],... 

Flash vacuum pyrolysis (FVP) of 2-phenyl-1,3-oxazol-5(4//)-one at 600°C/1.5 x 10 3Torr proceeds via carbon dioxide elimination to give 3-phcnyl-2//-azirine as the only detectable product isolated in 34% yield. Under similar conditions,... 

The pyrolytic decomposition of the sodium salts of various fluorinated carboxylic acids to give isomeric unsaturated compounds has also been reported. The products were identified as alkenes with the C = C bond inside the carbon chain, mainly alk-2-enes. This isomerization may be catalyzed by the coal-like products formed during the pyrolytic decarboxylation of the salts, but the metal fluoride formed in the reaction may also be responsible for the isomerization. When potassium perfluoro(5-chloropentanoate) is heated in a rocking autoclave at 300 C for 2 hours, perfluorobut-2-ene (2b) is isolated in 82% yield.This is only possible by migration of the double bond away from the terminal position after carbon dioxide elimination and halogen exchange to form potassium chloride. ... 

The cycloadducts obtained in the oximinosulfonate Diels-Alder reaction are best converted directly to pyridines without purification. Exposure of the spiro-fused cycloadducts to a combination of NCS and sodium methoxide brings about cleavage of the dioxanedione ring with concomitant elimination of acetone and carbon dioxide. Elimination of tosylate from the resulting ester enolate then generates a dihydropyridine, and subsequent chlorination by NCS and elimination of HCl finally provides the desired aromatic pyridine product. 

An emulsion liquid membrane (ELM) system has been studied for the selective separation of metals. This system is a multiple phase emulsion, water-in-oil-in-water (W/O/W) emulsion. In this system, the metal ions in the external water are moved into the internal water phase, as shown in Fig. 3.4. The property of the ELM system is useful to prepare size-controlled aiKl morphology controlled fine particles such as metals, carbonates/ and oxalates.Rare earth oxalate particles have been prepared using this system, consisting of Span83 (sorbitan sesquioleate) as a surfactant and EHPNA (2-ethyl-hexylphospholic acid mono-2-ethylhexyl ester) as an extractant. In the case of cerium, well-defined and spherical oxalate particles, 20 - 60 nm in size, are obtained. The control of the particle size is feasible by the control of the feed rare earth metal concentration and the size of the internal droplets. Formation of ceria particles are attained by calcination of the oxalate particles at 1073 K, though it brings about some construction of the particles probably caused by carbon dioxide elimination. 

This di-ethyl ester of di-hydro collidine di-carboxylic acid is then treated with nitrous acid which removes the two added hydrogens. The ester is then hydrolyzed and carbon dioxide eliminated whereby collidine is obtained. This, by oxidation of the methyl groups to carboxyl groups - and elimination of carbon dioxide, yields pyridine. 

Examples of addition reactions of tributyltin methoxide are given in Table 14-3. Most of these reactions are reversible for example, the alkyl tin carbonate which is formed by the addition of a tin alkoxide to carbon dioxide eliminates C02 on heating to regenerate the tin alkoxide (equation 14-14). Again, the product of the reaction of an acyl halide with a dioxastannolane is involved in an exchange reaction in which the tin alkoxide adds reversibly to the carbonyl group (equation 14-31).41... 

Polymerization of fluoroolefins in liquid and supercritical carbon dioxide eliminates the use of water and C-8 surfactant that has been identified as a persistent organic pollutant (R. Carbonell) ... 

The primary purpose of the respiratory system is gas exchange. In the gas-exchange process, gas must diffuse through the alveolar space, across tissue, and through plasma into the red blood cell, where it finally chemically joins to hemoglobin. A similar process occurs for carbon dioxide elimination. 

There are also several mechanisms by which our body maintains the pH around 7.4. Some of these mechanisms use simple standard chemistry, some are more complex. These mechanisms are (i) the carbonic acid-bicarbonate buffer system, (ii) the protein buffer system, and (iii) the phosphate buffer system. Apart from these buffers, the pH of our body is also maintained by exhalation of carbon dioxide, elimination of hydrogen ions via the kidneys, etc. 

Photodecarbonylation is a well-established reaction and its recent applications involve synthesis of the tetra-t-butyltetrahedrane compound (equation 127) and [4.5]-coronane (equation 128) . Three examples of carbon dioxide elimination are exhibited in equations 129-131 to furnish corresponding alkanes . 

Reaction of diisocyanates with diamines by addition leads correspondingly to polyureas, and with dicarboxylic acids, to polyamides with carbon dioxide elimination. 

Decarboxylation is the main pyrolysis reaction of PC at temperatures above 500 °C. It is supposed that carbon dioxide elimination occurs according to the scheme ... 

Interestingly, despite the fact that RCOMn(CO)s compounds decarbonylate easily, an attempt to eliminate CO2 from C2HsOOCMn(CO)5 was unsuccessful (59). Carbon dioxide elimination from carboxylate complexes to give alkyl or aryl derivatives has not been reported. 

Another interesting synthetic approach which also constitutes a total synthesis was accomplished by Pace (4 ). Sodium acetoacetic ester was oxidized with iodine, the product saponified, and carbon dioxide eliminated to give 2,5-hexanedione. The diketone was reduced to the diol, which was transformed to the dibromide. The hexadiene was formed and converted to the tetrabromide and subsequently to the hexabromide, which upon treatment with alcoholic potassium hydroxide gave DL-mannitol. 

Treatment of the a,a-disubstituted bromo-esters (421) with triphenyl-phosphine at 170 °C resulted in carbon dioxide elimination and ethyltri-phenylphosphonium bromide and olefin formation, a reaction formulated as proceeding through the cyclic transition state (422). Thus the bromomalonate (423) was converted into the ajff-unsaturated ester (424) in 60% yield. ... 

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