Phenolates, radical arylation

Materials that promote the decomposition of organic hydroperoxide to form stable products rather than chain-initiating free radicals are known as peroxide decomposers. Amongst the materials that function in this way may be included a number of mercaptans, sulphonic acids, zinc dialkylthiophosphate and zinc dimethyldithiocarbamate. There is also evidence that some of the phenol and aryl amine chain-breaking antioxidants may function in addition by this mechanism. In saturated hydrocarbon polymers diauryl thiodipropionate has achieved a preeminent position as a peroxide decomposer. 

Radical arylations of phenols differ in some respects from those of phenolates (Scheme 37). First, the decreased nucleophilicity of the phenol, such as 100, allows the use of unmasked aryl diazonium chlorides 101 as radical sources. Given that an efficient reductant is present in the reaction mixture and that the diazonium salt is added slowly, biphenyl alcohols 102 can be prepared in moderate to good yields [153,154]. In this way, the concentration of the salt 101 is kept low at any time and homocoupling reactions (addition of the aryl radical to diazonium ions) as well as azo coupling to the phenol 100 can be successfully overcome. 

Contrary to the easy synthesis of alkylated phenols, e.g. via the electrophilic alkylations of phenols using alkyl halides, alkenes or other alkyl derivatives, there is no universal method for the preparation of arylated phenols. Radical or other arylation procedures usually lead to a mixture of difficult to separate isomers. Aryl-substituted phenols have, therefore, to be prepared by special routes. 

The widely used hindered phenol and aryl amine antioxidants contain reactive 0-H and N-H functional groups capable of reacting with oxy radicals by transfer of hydrogen (4). Electron transfer is also a possibility, and some antioxidants, or their reaction products, may function as traps for alkyl radicals. The hydrogen donation mechanism is capable of terminating two kinetic chains ... 

The oxidative coupling reactions of certain electron-rich arenes under suitable reaction conditions proceed, at least partially, via free-radical mechanism. Scheme 3. The phenolate anion is oxidized by suitable one-electron oxidant to the phenoxyl-radical whose tautomeric form is aryl-radical on the adjacent carbon atom. The symmetrical biaryl is formed by coupling of the latter, whereas the unsymmetrical one is produced by free-radical arylation of the second arene molecule, usually in an intramolecular... 

A sequential one-pot synthesis of unsymmetrical aryl urea derivatives was developed by arylation of N-substituted cyanamides under metal-free conditions, followed by a second N-arylation under copper-catalyzed conditions [96]. Chen and Chen devised the arylation of pyridine N-oxides with At21X via initial O-arylation followed by a 1,3-radical rearrangement to o-pyridinium phenolates, and arylation of pyridine N-amidates delivered o-pyridinium anilines (Scheme 5d) [97]. 

Fragmentation Loss of alkyl by fragmentation of the C-O bond with concomitant double H rearrangement to form the protonated sulfonic acid ion (m/z 97 for methanesulfonates), which then loses water. Loss of the alkoxyl residue (fragmentation of the S-O bond). Formation of an alkene ion from the sulfonate alkyl by a McLafferty-type rearrangement. In aryl esters, the phenoxy ion and the phenol radical cations dominate the spectrum. 

Aliphatic enol esters and aryl esters Formation of alk-CO+ (m/z 43, 57, 71,...). Elimination of a ketene to give the enol/phenol radical cation. The rearrangement occurs prodominantly, but not exclusively, through a 4-membered transition state. 

Antioxidants function by preferentially reacting with the radical intermediates, thereby protecting the polymer and extending its usage life. There are two types of antioxidants that are typically used in commercial polymer stabilization primary and secondary. The majority of primary antioxidants are either hindered phenolics or secondary aryl-amines. Both hindered phenolics and aryl-amines have one or more reactive OH and NH groups. The hydrogen atoms which are liberated from the alcohol or amine groups readily react with free radicals to form stable species. Secondary antioxidants are usually phosphites or thioesters. 

The second mechanism involves the removal of the unstable hydroperoxide which is the main source of new free radicals in the system, a reaction which does not involve radical formation (2b). Many sulphur and phosphorus compounds act in this way and are used in synergistic combination with phenols and aryl amines... 

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Until fairly recently, there were no antioxidants which could trap alkyl radicals. Recently Sumitomo has developed an acrylated phenol (Sumilizer GM and GS) [26,27] and Ciba-Geigy has developed an aryl benzofuranone (HP 136) [28]. Both... 

Aromatic compounds can also be arylated by aryllead tricarboxylates. Best yields ( 70-85%) are obtained when the substrate contains alkyl groups an electrophilic mechanism is likely. Phenols are phenylated ortho to the OH group (and enols are a phenylated) by triphenylbismuth dichloride or by certain other Bi(V) reagents. O-Phenylation is a possible side reaction. As with the aryllead tricarboxylate reactions, a free-radical mechanism is unlikely. ... 

Dimroth, K. Arylated Phenols, Aroxyl Radicals and Aryloxenium Ions Syntheses and Properties. /29, 99-172 (1985). 

By-products from capture of nucleophilic anions may be observed.53 Phenols can be formed under milder conditions by an alternative redox mechanism.98 The reaction is initiated by cuprous oxide, which effects reduction and decomposition to an aryl radical, and is run in the presence of Cu(II) salts. The radical is captured by Cu(II) and converted to the phenol by reductive elimination. This procedure is very rapid and gives good yields of phenols over a range of structural types. 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

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