Radicals elimination, primary

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. ... 

In coumarins which bear large alkyl side chains, such as osthol (122), the primary fragmentation does not involve carbon monoxide loss. The dominant processes are side chain cleavage, methyl radical elimination from the side chain, and loss of a methoxyl radical. The mass spectra of this and related naturally occurring coumarins have been intensively studied and reviewed (75RCR603). 

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. 

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. 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

The nucleophilic displacement reactions with azide, primary amines, thiols and carboxylatc salts arc reported to be highly efficient giving high (>95%) yields of the displacement product (Table 9.25). The latter two reactions are carried out in the presence of a base (DBU, DABCO). Radical-induced reduction with tin hydrides is quantitative. The displacement reaction with phenolates,61j phosphines,6M and potassium phthalimide608 gives elimination of HBr as a side reaction. 

It seems that no general mechanistic description fits all these experiments. Some of the reactions proceed via an addition-elimination mechanism, while in others the primary step is electron transfer from the arene with formation of a radical cation. This second mechanism is then very similar to the electrochemical anodic substitution/addition sequence. 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

The competitive kinetics of Scheme 3.1 can also be applied to calibrate the unimolecular radical reactions provided that kn is a known rate constant. In particular the reaction of primary alkyl radicals with (Mc3Si)3SiH has been used to obtain kinetic data for some important unimolecular reactions such as the p-elimination of octanethiyl radical from 12 (Reaction 3.5) [12], the ring expansion of radical 13 (Reaction 3.6) [8] and the S-endo-trig cyclization of radical 14 (Reaction 3.7) [13]. The relative Arrhenius expressions shown below for the... 

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