Olefin carbon pool

Olefins interconversion cycle olefin carbon pool ... 

Use of the chiral carbon pool for cyclopentenone preparation is also known. The fungal metabolite terrein [88] was selectively monoacetylated and then reduced with chromous chloride to enone [89]. Acetylation and olefin cleavage with ruthenium tetroxide aiwi sodium periodate led to aldehyde [90], which was readily decarbonylated to [65] (51). An alternative route (52) began with the less common S,S-tartaric acid [91], converted in four steps to diiodide [92]. Dialkylation of methyl methylthiomethyl sulfoxide with [92] gave the cyclopentane derivative [93]. Treatment of [93]... 

In the hydrocarbon pool mechanism (ie, aromatics carbon pool), two parallel routes, namely the side-chain route or exocyclic methylation route and the "paring route, have been suggested for the formation of olefins [97]. 

Elucidating rates of p-scission on various zeolites will be key to determining what firacfion of light olefins, particularly propylene and butenes, are products of the aromatic- versus olefin-based carbon pool however, the prevalence of secondary reactions, such as olefin oligomerization and dehydrocyclization, may hinder the measurement of kinetic parameters of p-scissioa... 

Two possible carbocation reaction schemes have been published. There is presently much debate over the nature of the intermediate from which the primary olefins—ethylene, propylene, and the butenes—are formed. According to Hutchings and Hunter [22] this would be an oxonium methylylide, produced from the methoxy carbenium ion, CHj generated out of methanol by dehydration (Fig. 24). According to Arstad et al. [102], Wei et al. [28], and Dahl and Kolboe [90] the intermediate(s) they call carbon pool with which methanol reacts would be the hexamethylbenzenium ion or a similar compound. They maiidy deduce this from the analysis of the catalyst after conversion of the methanol in a batch mode. 

The induction period predicted here and observed in the olefins yield in MTO on SAPO-34 depends as much on selectivation by deactivation as on the reaction scheme itself [119]. We should emphasize that the behavior described here, at the time, was based on the reaction scheme proposed by Hutchings and Hunter [22] and used by Alwahabi [101], not upon the carbon pool mechanism, often referred to for its explanation. 

The dissociative mechanism can explain both facts in that the hydrogen removed in the first step may recombine with an isomeric form of the ally lie intermediate to yield the isomeric olefin. Apparently syn and anti 7T-allylic complexes [67, 68) retain their configurations unless each may be converted into a common a-bonded complex in which the nonterminal carbon atoms of the allyl group are connected by a single bond and the isomerization of the intermediate can be represented as in Fig. 11. However, the recombination of the hydrogen atom with the allylic intermediate must be faster than the rate at which it enters the surface pool of... 

Scheme 5.29 Addition of a carbon radical generated from an N-acyliminium ion pool to an electron-deficient olefin...

A general synthesis for all diastereomeric L-hexoses, as an example for monosaccharides that often do not occur in the chiral pool, has been worked out. The epoxidation of allylic alcohols with tertiary butyl hydroperoxide in presence of titanyl tartaric ester catalysts converts the carbon-carbon double bond stereose-lectively to a diol and is thus ideally suited for the preparation of carbohydrates. The procedure is particularly useful as a repetitive two-carbon homologiza-tion in total syntheses of higher monosaccharides and other poly hydroxy compounds. It starts with a Wittig reaction of a benzylated a-hydroxy aldehyde with (triphenylphosphoran-ylidene)acetaldehyde to produce the olefinic double bond needed for epoxidation. Reduction with sodium-borohydride... 

Afterward, the notion of unspecified carbon deposition with an olefin-like composition (CH ) has been gradually transformed to Polymethylbenzenes (PMBs) by many research groups [87,97]. Those PMBs serve as scaffolds/cocatalysts, where methanol is added and olefins are eliminated in a closed catalytic cycle [87,98]. It is therefore indicated that the interplay between the inorganic framework and the organic reaction centers dictates the activity and selectivity. However, according to Ref. [97], the role of PMBs as the major hydrocarbon pool species appears to be independent of the zeotype catalyst chosen. Haw et al. [87] provided both experimental and theoretical evidence in favor of PMBs as the driving force for the hydrocarbon pool mechanism. In 1998, by means of pulse-quench reactions on an H-ZSM-5 catalyst and GC-MS and MAS NMR analysis, it was reported... 

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