Elimination, direction Carbon dioxide

In regard to transformation of the benzyne cycloadduct product between benzyne and furan into benzo[c]furan, the direct transformation by elimination of acetylene will not occur because computed activation barriers for this reaction were to high. The AMI computed activation barrier for acetylene elimination was 57.0 kcal/mol, while the activation barriers for the addition of a-pyrone (30.4 kcal/mol) and the elimination of carbon dioxides and benzene (29.9 kcal/mol) were much more energetically favorable (Scheme 1). These computational results are in full agreement with experimental evidence [38]. 

A dry combustion-direct injection apparatus was applied to water samples by Van Hall et al. [51 ]. The carbon dioxide was measured with a non-dispersive infrared gas analyser. Later developments included a total carbon analyser [97], a diffusion unit for the elimination of carbonates [98], and finally a dual tube which measured total carbon by combustion through one pathway and carbonate carbon through another. Total organic carbon was then calculated as the difference between the two measurements [99]. 

Daicel Chemical Industries in Japan patented a promising phosgene-free process involving the reaction of an aliphatic diamine with dimethyl carbonate (DMC) to produce carbamate esters, which are then thermally converted to the corresponding aliphatic diisocyanates [38] (Scheme 5.4). It is noteworthy that this process could be a total phosgene-free process since the reactant, DMC, can be made directly from methanol and carbon dioxide (or urea) and eliminates the use of phosgene [39]. 

The iodine-catalyzed reaction of aziridines with carbon dioxide leads to 2-oxazolidinones (251). Because carbon dioxide effectively polymerizes ethyleneimine, only low yields are obtained when unsubstituted ethyleneimine reacts with C02. However, direct insertion of carbon dioxide [124-58-9] into aziridines can be accomplished, with better yields, by ethoxycarbonylation of aziridines with subsequent elimination of ethylene under flash vacuum conditions (252). 1- Phenyl aziridine [696-18-4] can react with C02 under antimony [7440-56-0] catalysis to give AI-phenyl-2-oxazolidinone in good yields (253). At low temperatures and with the exclusion of atmospheric humidity, the reaction of ethyleneimine with carbon dioxide produces the unstable ethyleneiminium salt [51645-58-2] of IV-vinylcarbamic acid (254,255). 

The capillary plasma reactor consists of a Pyrex glass body and mounted electrodes which are not in direct contact with the gas flow in order to eliminate the influence of the cathode and anode region on CO2 decomposition. Analysis of downscaling effects on the plasma chemistry and discharge characteristics showed that the carbon dioxide conversion rate is mainly determined by electron impact dissociation and gas-phase reverse reactions in the capillary microreactor. The extremely high CO2 conversion rate was attributed to an increased current density rather than to surface reactions or an increased electric field. 

The preparation of anhydrous aluminum iodide by methods described previously1 2 involves direct union under conditions that invariably yield products contaminated with elemental iodine. Attempts to purify these products by sublimation under a variety of conditions (including sublimation in vacuo or in an atmosphere of carbon dioxide or helium) result in at least partial decomposition of the iodide and a final product that is colored, owing to the presence of iodine. The method described below also provides for direct union of the elements, but under conditions that eliminate contamination with elemental iodine and yield an initial product of exceptionally high purity. 

In water, ionization of the C-Br bond occurs first (Ei mechanism) to give the intermediate resonance-stabilized benzylic zwitterion C. After fast rotation about the C-C bond, carbon dioxide leaves conformer D perpendicularly to the plane of the car-benium ion, to give mainly the most stable ( )-isomer of / -bromostyrene. In butanone, after fast rotation about the C-C bond, elimination of CO2 and Br occurs in a concerted single-step (E2 mechanism) for stereoelectronic reasons (Br and C02 must be anti to one another) to give conformer B, which decomposes exclusively to the thermodynamically less stable (Z)-isomer. In more polar solvents, the partly zwitterionic activated complex, leading to zwitterion C in the rate-determining step, will clearly be more stabilized by solvation than the less dipolar activated complex leading directly to the (Z)-isomer of / -bromostyrene from conformer B [851]. 

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. 

The a-exomethylene-y-lactone framework has been successfully constructed via two electrosynthetic pathways, that is, both by the direct and by the concerted decarboxylation processes as mentioned earlier [Eq. (45)] [151]. The electrodecarboxylation of XCa is probably initiated by a one-electron oxidation of the sulfur atom, giving first the cation radical (XCb) and subsequently a concerted elimination of the thiyl radical and carbon dioxide to LXXXIX. On the other hand, the electrochemical decarboxylation of LXXXVIIIa involves an El-type elimination of a proton from the cation intermediate (LXXXVIIIb) generated from direct two-electron oxidation of the carboxyl group. The latter method generally requires a higher oxidation potential than that required for the concerted method. Therefore, the concerted electrodecarboxylation method becomes more advantageous, especially when the substrates or products are unstable under oxidative conditions. 

In this reaction one carbon atom is oxidized from the carboxyl stage to carbon dioxide. It is to be noted, therefore, that the carboxyl group cannot be directly reduced to the hydrocarbon without the loss of one carbon atom. The method is not practical. It is a traditional experiment, as it illustrates the difficulty in reducing the carboxyl group. The elimination of the carboxyl group is also effected by electrolysis of concentrated solutions of the alkali salts. The products in the case of sodium acetate are ethane and carbon dioxide ... 

Conclusive evidence for structure XVIII was obtained from a study of the action of alkali on it. While its UV-spectrum in ethanol had the absorption due to a saturated aliphatic ketone (A ,ax 220, 292 m/i, loge 1.98, 1.33), in alkaline ethanol the absorption was typical of an unsaturated ketone (An,ax 245,318m/Lt, loge 3.45,1.91). The latter function had clearly been generated by the elimination process illustrated. The a,)3-unsaturated ketone, isolated as a gum by acidification, on ozonolysis afforded acetaldehyde and an acidic gum. Mild pyrolysis of the latter gave carbon dioxide and a crystalline acid which was identified as (-I- )-)3-(propionic acid (XX) by direct comparison with the synthetic racemic modification. 

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