Hydrogen transfer, mechanism

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

Several Rh, Ir, and Ru complexes follow the well-established hydrogen-transfer mechanism via a metal alkoxide intermediate and /3-elimination.116... 

Scheme 20.3 Schematic representation of the two different hydrogen transfer mechanisms (D = donor molecule ...

Among the classes of feedstock processed in the hydrocracker the most highly aromatics feed are light cycle oils produced in the FCC unit Once formed by cyclization and the hydrogen transfer mechanism discussed above, they accumulate in the product due to the absence of a metal function in the FCC catalyst and adequate hydrogen in the process environment. They are typically sold as low-value fuel oil, or hydrotreated to reduce sulfur content and improve their quality as diesel blend stocks. Another approach to upgrade their value even further... 

The radical addition and hydrogen transfer mechanisms of inhibition by chain-breaking antioxidants are now reasonably well understood in both qualitative and quantitative terms. The electron-transfer mechanism of inhibition deserves greater attention. 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

Whereas iV-tosylimines do not coordinate diethylzinc well in polar solvents (and thus tend to give ethylated product), solvents such as toluene favour coordination, leading to reduction of the imine to secondary amines under mild conditions, via a /3-hydrogen transfer mechanism 48... 

A free-radical hydrogen transfer mechanism is proposed that may lead to rapid H-exchange in polyaromatic systems. 

Acrylamino derivatives of nucleic add bases were found to be polymerized by hydrogen-transfer mechanism to (3-alanine derivatives46. ... 

Here hydrocarbon conversion reactions occur wholly or at least partly on the carbonaceous overlayer on the metal and oxide surfaces, as reported by others (13,15-20). Poly-condensed EDA complexes may behave as giant alkenes in which by reversible catalytic hydrogenation/dehydrogenation occurs. This mechanism is similar to the intermolecular hydrogen transfer mechanism proposed (IS) for hydrogenation of unsaturated hydrocarbons. 

It was believed that a sequential electron-proton-electron transfer mechanism would result in small isotope effects and that a hydrogen transfer mechanism would result in large isotope effects. Because small isotope effects were observed for amine dealkylation by cytochrome P-450, a sequential electron-proton-electron transfer mechanism was proposed for the reaction [67, 75-77]. The magnitude of a single isotope effect provides only limited information about a reaction mechanism, however, and isotope effects might be logically consistent with any type of C-H cleavage reaction. 

A total of 310.3 g of methyl ester were pyrolysed to yield 204.4 g of liquid product which corresponds to a 65.9% actual liquid yield. The theoretical yield based on a typical Cie, C composition of soybean oil, and the y-hydrogen transfer mechanism is approximately 76%. Therefore, the yield based on this limitation is approximately 87%. "niis shows that random cracking of the ester chain does not play a significant role in the reaction. The infrared spectrum shows no carbonyl peaks at 1720 cm. Peaks appearing at 907 cm" and 965 cm arc probably absorptions due to terminal alkenes and trans alkenes, otherwise the spectrum looks like that of a typical alkane. Consistent with these results, elemental analysis gave 83.7% carbon, 14.56%... 

In particular, it was reported that the rate decreases with increasing basicity and steric hindrance at the nitrogen atom. An exception to this rule was 7,8-BQ, which showed the highest relative rate and, in competitive reactions, was found to enhance the rate of hydrogenation of Q and other substrates as well. It was proposed that the rate enhancement effect is occasioned by a concomitant hydrogen transfer mechanism [7 a]. 

For identification and understanding of the hydrogen transfer mechanism the nature of transfer has been the subject of several studies either supporting ]14,15] or questioning [16] the above hypothesis. Several criteria were raised to test the dominant contribution for enzyme catalyzed reactions [17], such as ... 

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