Free radicals elimination

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

Free-radical mechanisms are mostly found in pyrolyses of polyhalides and of primary monohalides,though they also have been postulated in pyrolysis of certain carboxylic esters/ Much less is known about these mechanisms and we shall not consider them further. Free-radical eliminations in solution are also known but are rare. ... 

Free radical eliminations are frequent during pyrolytic reactions, and they are common for linear chain polymers. At higher temperatures (600° C-900° C) this type of reaction is also common for small molecules and explains the formation of unsaturated or aromatic hydrocarbons from aliphatic ones. As an example, butane decomposition may take place as follows ... 

The other type of pyrolytic elimination reaction involves free radicals. The steps are similar to those that we studied in the free radical substitution reactions, i.e. there is an initiation step, followed by several propagation steps, and then there are some termination steps. Free radical elimination is found in polyhalides and primary monohalides. For the general primary monohalide, R2CHCH2X,... 

Free radical elimination reactions in solution are rather rare instead they tend to be confined to the pyrolysis of polyhalides and primary monohalides as illustrated above. 

Also, elimination reactions (mostly, free radical elimination Er) are widespread in thermal decomposition of organic residue on the modified oxides surface. Degradation of organosilicon cover of modified silica surface at high temperatures presents an example of such transformation [65]. 

They react intermolecularly with a substrate (i. e., with an H-donor) in the ground state. Cycloaddition of the excited quinoic system may be assumed in unsaturated polymers. In saturated polymers (RH), a free-radical elimination of H occur at bonds of low strength. Quinone ist reduced in this reaction to hydroquinone (QH2) in the same way as in a thermally induced reaction. 

China. Traditionally, the extract has been nsed not only as a sweetener but also as a folk medicine for remediation of fever and congh. In recent years, the free radical-eliminating activity and antitumor activity of MgV and related glycosides have been elncidated [19], In Japan, an extract comprised mainly of MgV is used as an ingredient for a commercialized sweetener with low caloric value. This section deals with the farther glycosylation of MgV to improve the quality of sweetness. 

The effect of carvacrol on hepatocellular carcinoma has also been the subject of Jayakumar et al. [88]. The authors examined the preventive effect of the substance against carcinoma induced by diethyinitrosamine in rats. The main topic of research was the strong antioxidant effect and free radical elimination as an anti-cancer mechanism. Carvacrol modulated LPO levels and enhanced the endogenous antioxidant defense in cancerogenesis and decreased the high levels of serum markers. 

The free radicals are formed by the thermal decomposition of the initiating organic peroxide. For example, if R = CgH5-C=0, then this free radical eliminates a CO molecule to provide the CgH radical that reacts with ethylene as shown below to provide the polymerization initiation free radical. 

Primary alkyl phenyl tellurides undergo elimination to form terminal olefins in high yields on treatment with an excess of iV-chloro-iV-sodio-4-methyl-benzenesulphonamide (chloramine-T) in refluxing THF/ ufc-Dinitro compounds and /3-nitrosulphones are converted into olefins via free-radical elimination processes on treatment with tributyltin hydride in the presence of catalytic quantities of azobis(isobutyronitrile) (AIBN). Elimination from the dinitro compounds shows no stereocontrol by contrast, elimination from /3-nitrosulphones is highly stereoselective, e.g. (26)- (27), presumably because elimination from the intermediate radical is faster than rotation about the central carbon-carbon bond. 

Oral E, Godleski Beckos C, Muratoglu OK. Free radical elimination in irradiated UHMWPE through crystal mobility in [ ase transition to the hexagonal phase. Polymer 2008 49 4733-9. 

R = H) undergoes a variety of enzyme-catalyzed free-radical intramolecular cyclization reactions, followed by late-stage oxidations, eliminations, rearrangements, and O- and N-alkylations. Working from this generalization as an organizing principle, the majority of known AmaryUidaceae alkaloids can be divided into eight stmctural classes (47). 

Oligomeric Vinylphosphonate. A water-soluble oligomer, Fyrol 76 [41222-33-7] is produced by reaction of bis(2-chloroethyl) vinylphosphonate and dimethyl methylphosphonate with elimination of all the chlorine as methyl chloride (127,128). This Hquid, containing 22.5% P, is curable by free-radical initiation, on cotton or other fabrics. Nitrogen components, such as A/-methylolacrylamide or methylolmelamines, are usually included in the finish, which can be durable to multiple launderings (129,130). 

Flame Retardants. Flame retardants are added to nylon to eliminate burning drips and to obtain short self-extinguishing times. Halogenated organics, together with catalysts such as antimony trioxide, are commonly used to give free-radical suppression in the vapor phase, thus inhibiting the combustion process. Some common additives are decabromodiphenyl oxide, brominated polystyrene, and chlorinated... 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Braun and Schurek [9] assumed that during polymerization a reaction can occur between the polymer and free radicals that leads to the elimination of hydrogen chloride and formation of a double bond. The formation of HCl during the polymerization of vinyl chloride has been observed [10],... 

An interesting free radical carbon-carbon bond formation with concomitant elimination of a /5-thio substituent was achieved during the course of Boger s impressive synthesis of CC-1065.26-27 In the event, treatment of aryl bromide 70 (see Scheme 13) with tri-n-... 

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