Indirect reaction branching

Mechanism of Combustion The burning mechanism of polyolefins is similar to that of gaseous hydrocarbons, i.e., a chain radical mechanism with both direct and indirect branching (via aldehyde, hydroperoxide, etc., acting as a molecular intermediate) [284-286], as a function of temperature, which nearly makes impossible any generalization. Moreover, in the gaseous phase, some ionic reactions have been postulated [252], but the molar fraction of these species is only 10 at atmospheric pressure whereas that of the radical species is 10 —10 ". ... 

While the data provide clear evidence for the formation of incomplete oxidation products, and help to identify the nature of the stable adsorbate(s) formed upon interaction with the respective Ci molecules, the molecular-scale information on the actual reaction mechanism and the main reaction intermediates is very indirect. Also, the reaction step(s) at which branching into the different reaction pathways occurs (e.g., direct versus indirect pathway, or complete oxidation versus incomplete oxidation) cannot be identified directly from these data. Nevertheless, by combining these and the many previous experimental data, as well as theoretical results, conclusions on the molecular-scale mechanism are possible, and are substantiated by a solid data base. 

Steckhan and coworkers found that the indirect anodic oxidation of N-pro-tected dipeptide esters 56, in which the C-terminal amino acid is a-branched, can afford methyl imidazolidin-4-one-2-carboxylate 57 in 45-84% yields [86], This reaction can be performed at a Pt-anode by using Et4NCl as an electrolyte in the presence of 5% methanol in an undivided cell (Scheme 30). 

Cathodic corrosion inhibitors reduce the corrosion rate indirectly by retarding the cathodic process which is related to anodic dissolution. In this process, access to the reducible species such as protons, to electroactive site on the steel, is restricted. Reaction products of cathodic inhibitors may not be bonded to the metal surface as strongly as those used as anodic inhibitors. The effectiveness of the cathodic inhibitor is related to its molecular structure. Increased overall electron density and spatial distribution of the branch groups determine the extent of chemisorption on the metal and hence its effectiveness. Commonly used cathodic inhibitor materials are bases, such as NaOH, Na2C03, or NH4OH, which increase the pH of the medium and thereby also decrease the... 

The inorganic phosphorus (Pi) produced in the reaction serves as an indirect measure of the branching enzyme activity [7]. 

The Stacker reaction has been employed on an industrial scale for the synthesis of racemic a-amino acids, and asymmetric variants are known. However, most of the reported catalytic asymmetric Stacker-type reactions are indirect and utilize preformed imines, usually prepared from aromatic aldehydes [24]. A review highlights the most important developments in this area [25]. Kobayashi and coworkers [26] discovered an efficient and highly enantioselective direct catalytic asymmetric Stacker reaction of aldehydes, amines, and hydrogen cyanide using a chiral zirconium catalyst prepared from 2 equivalents of Zr(Ot-Bu)4, 2 equivalents of (R)-6,6 -dibromo-1, l -bi-2-naphthol, (R)-6-Br-BINOL], 1 equivalent of (R)-3,3 -dibromo-l,l -bi-2-naphthol, [(R)-3-Br-BINOL, and 3 equivalents of N-methylimida-zole (Scheme 9.17). This protocol is effective for aromatic aldehydes as well as branched and unbranched aliphatic aldehydes. 

From this type of schematic representation it would be expected that a more highly branched xylan chain would give more potential for reaction of the arabinose residues, either through crosslink formation or by Maillard-type reactions. Indirect evidence supporting this concept can be derived from pentosan analysis of the hemicellulose fraction of grasses and legumes. 

The greatest inhibition effect in the gaseous phase is due to partial indirect suppression of the branching step (Equation 4.2). Hence, N20 flames would be less susceptible to poisoning because reaction (Equation 4.11) is a nonbranching reaction. 

Griffiths and Skirrow [42] have discussed various estimates of the rate for (2) and concluded that it was around 10 1. mole . sec . The most recent value, based on the kinetics of the final stage of the oxidation at 60—80 °C (with large excess of aldehyde) [43], is lower, (1.2 0.2) x 10 1. mole" . sec. However, it is clear that 2 is high enough to ensure that RCO radicals produced directly or indirectly in the branching step will react by (2) rather than by (4b) or (4c) except at very low oxygen pressures. Thus reaction (3) for which the rate coefficient is [42, 62, 73]... 

Many additives, e.g. N2, CO2, H2O [45], have little or no effect on the low temperature oxidation rate. Others may promote reaction or give rise to retardation or, possibly, inhibition. Promotion or acceleration is usually associated with additives which are themselves directly or indirectly radical sources at the temperature of the system (e.g. ditertiary butyl peroxide [58], peracetic acid [19], HBr [59]), and the effect is understandable in terms of an increased (induced) rate of initiation. The most important additive in this category is peracetic acid. This is a product in the oxidation of acetaldehyde, and the effect of its addition on the oxidation kinetics has been used by Combe et al. [19] to obtain supporting evidence for the now accepted branching step. 

L-Amino acid transaminases are ubiquitous in nature and are involved, be it directly or indirectly, in the biosynthesis of most natural amino acids. All three common types of the enzyme, aspartate, aromatic, and branched chain transaminases require pyridoxal 5 -phosphate as cofactor, covalently bound to the enzyme through the formation of a Schiff base with the e-amino group of a lysine side chain. The reaction mechanism is well understood, with the enzyme shuttling between pyridoxal and pyridoxamine forms [39]. With broad substrate specificity and no requirement for external cofactor regeneration, transaminases have appropriate characteristics to function as commercial biocatalysts. The overall transformation is comprised of the transfer of an amino group from a donor, usually aspartic or glutamic acids, to an a-keto acid (Scheme 15). In most cases, the equilibrium constant is approximately 1. 

Indeed, branches formation was postulated 103), although on the basis of indirect evidence, for the polymerization at 80 °C. At higher temperatures, the reaction shown in Scheme (7-29) (involving cleavage) predominates. 

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