And aryl radicals

An investigation of the competing halogen transfer from BrCCl3 and CCl45, 79 has shown that steric effects are also of importance in atom transfer reactions to alkyl and aryl radicals. Giese80 investigated very carefully the temperature depen-... 

Evidence for the formation of alkyl and aryl radicals in some cases following loss of SO2 (Scheme 1) has been obtained. Thus, a small amount of M-pentane was formed in the decomposition of M-pentanesulphonyl azide in mineral oil ). Thermolysis of diphenyl sulphone-2-sulphonyl azide (8) in dodecane at 150 °C gave diphenyl sulphone 9 (27%) and diphenyl sulphone-2-sulphonamide 10 (9%) which arise by hydrogen abstraction by the aryl radical and sulphonyl nitrene, respectively. When this thermolysis was carried out in Freon E-4 at 150 °C, the products were diphenylene sulphone 77 (1.3%) (Pshorr-type cyclization product of the aryl radical) and 10 (1.5%) together with tars 16h Ferro-... 

Unless indicated otherwise, the enthalpies of formation of alkyl and aryl radicals were taken from J. A. Martinho Simoes and J. L. Beauchamp, Chem. Rev., 90, 629 (1990). 

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

Alkyl, vinyl and aryl radicals can give homolytic substitution at a heteroatom either inter- or intramolecularly thus forming C-heteroatom bonds. 

Free radical substitution of pyridines usually occurs principally at position 2 (Table 25), which is in agreement with theoretical calculations (69CCC1110). 2-Substitution is more favored in methylation than in phenylation of pyridine. This suggests that the methyl has more nucleophilic character than the phenyl radical. Furthermore, methylation of pyridine in acidic solution gives 13-fold excess of 2- over 4-substitution, although the overall yield is low. Alkyl and aryl radicals have been generated from diverse sources (Table 25). 

In general, from among the protic solvents, only liquid ammonia (the first used)1 is particularly useful, and is still used more than any other solvent despite the low temperature at which reactions have to be carried out (b.p. -33 °C) and the fact that solubilities of some aromatic substrates and salts (M+Nu-) are poor. Ammonia has the added advantage of being easily purified by distillation, being an ideal system for production of solvated electrons, and has very low reactivity with basic nucleophiles and radical anions, and aryl radicals. Also, poor solubilities can sometimes be ameliorated by use of cosolvents such as THF. In addition it can be used as a solvent for the in situ reductive generation of nucleophiles such as ArSe- and ArTe- ions, e.g. the formation of PhTe- from diphenyl ditelluride (equation 16).54 55... 

The use of vinyl radicals (pioneered by Stork)90 and aryl radicals (introduced by Beckwith)91 in the tin hydride method is attractive because these radicals are extremely reactive,92 and often provide excellent yields of cyclic products that contain useful functionality for subsequent transformations. 

For an analysis of the relative reactivity of vinyl and aryl radicals towards cyclization and hydrogen abstraction from tin hydride, see ref. 34, p. 429. 

Techniques based on the replacement of hydrogen atoms in hydridechlorosilanes with alkyl, alkenyl and aryl radicals... 

An immediate consequence of the different ease with which Cy,-element bonds dissociate is that in radical substitution reactions, alkyl radicals are more easily formed. Vinyl and aryl radicals are less common, but can be generated productively. Alkynyl radicals do not appear at all in radical substitution reactions. In the following, we therefore limit ourselves to a discussion of substitution reactions that take place via radicals of the general structure R1 R2R3C. ... 

The reactivity of sulfur-centered nucleophiles such as thiourea anion [46] and thioacetate anion [17] in photoinduced SRN1 reaction has been reported as a one-pot, two-step method for the synthesis of several sulfur-aromatic compounds from moderate to good yields. Without isolation, the ArS ions obtained by the aromatic substitution are quenched with Mel to yield ArSMe in a one-pot procedure, together with Ar2S in variable yields, from an SRN1 between ArS- and aryl radicals (Scheme 10.3). 

Alkyl and aryl radical cyclizations were developed using catalytic amounts of MnCl2 and butylmagnesium bromide as a stoichiometric reductant (Fig. 44)... 

The photoreduction of 2- and 3-chloroanisole in alcoholic solvents has been studied and is considered to be best accounted for by invoking methoxyphenyl radicals which abstract hydrogen atoms from the solvent378.4-Chloroanisole probably reacts partly via a homolytic cleavage, but in view of the results of quenching and sensitization experiments, another pathway consists of electron transfer from the solvent (ROH) to excited aryl halide, followed by dissociation of the radical anion into chloride ion and aryl radical. 

Pyrrole anion is unreactive in liquid ammonia under irradiation with PhBr or 1-chloro-naphthalene. However, the reactions of aryl chlorides (p-chlorobenzonitrile, 3- and 4-chloropyridines and 4-chlorodiphenyl sulphone) with 2,5-dimethylpyrrole anion under electrochemical inducement in the presence of a redox mediator gave the C3-substituted product in moderate yields (35-40%) (equation 120)225. The rate constant of the coupling reaction between this nucleophile and aryl radicals is about 5-8 x 109 M"1 s 1 determined by electrochemical methods225. 

Sml2-mediated radical cyclisations involving alkyl, alkenyl and aryl radical intermediates can be used to construct efficiently five-membered and, in certain cases, six-membered ring systems. This approach provides a useful alternative to trialkyltin hydride-mediated methods as toxic reagents and problematic tin byproducts are avoided. In addition, the use of Sml2 to induce radical cyclisations has led to the development of a number of powerful, radical/anionic sequential processes for the construction of complex systems. Sequential reactions involving radical-alkene/alkyne cyclisations are discussed in Chapter 6. 

The absorption spectra of Pr(III) (3H4 -> 3Po transition), Nd(III) (4l9/2 -> 2P /2 transition) and Eu(III) (7Fo —> 5Do transition) with several /J-diketoncs having alkyl and aryl radicals were studied [241], The spectra of mono, bis, tris and tetrakis /J-diketonates were analysed by comparative absorption spectrophotometry. In the stepwise complexation... 

This approach to the thiophene ring seems most direct and involves (1) intramolecular nucleophilic addition of thiol, thiolate, and dithiocarboxylate sulfurs and, in a rare case, sulfide sulfur to sp and sp carbons (2) electrophilic attack of sulfenium and sulfonium ions and their equivalents on unsaturated carbon-carbon bonds (3) addition of thiyl radicals to unsaturated carbon-carbon bonds (4) addition of vinyl and aryl radicals to the sulfur atom of sulfides and (5) electrophilic attack of a carbocation on the sulfur atom of sulfides. 

Aliphatic iodides, and especially secondary and tertiary representatives, are subject to hydride ehmination and are not generally useful substrates in transition metal catalysed coupling reactions. The last reaction in Scheme 14 [31], for instance, cannot be executed using current transition metal-based technology. In contrast, vinyl and aryl iodides, which are superb partners in many classical metal-induced couphng reactions, are very poor substrates in the present radical process because of the high energy of vinyl and aryl radicals. The two methods thus nicely complement each other. 

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