Radicals methyl, relatively nucleophilic

On the basis of the reaction of alkyl radicals with a number of polycyclic aromatics, Szwarc and Binks calculated the relative selectivities of several radicals methyl, 1 (by definition) ethyl, 1.0 n-propyl, 1.0 trichloromethyl, 1.8. The relative reactivities of the three alkyl radicals toward aromatics therefore appears to be the same. On the other hand, quinoline (the only heterocyclic compound so far examined in reactions with alkyl radicals other than methyl) shows a steady increase in its reactivity toward methyl, ethyl, and n-propyl radicals. This would suggest that the nucleophilic character of the alkyl radicals increases in the order Me < Et < n-Pr, and that the selectivity of the radical as defined by Szwarc is not necessarily a measure of its polar character. 

We can see from these data that benzyl radical significantly easier adds to electrophilic methyl acrylate and acrylonitrile than to relatively nucleophilic hex-1-ene. This is one of the arguments for a nucleophilic character of benzyl radical. The polar factors affect essentially kinetic parameters of the processes, as judged from the ratios / KadC4H9 = g3 and K dCOOCHa / k C4H9 = 41. The... 

Nucleophilic Trapping of Radical Cations. To investigate some of the properties of Mh radical cations these intermediates have been generated in two one-electron oxidant systems. The first contains iodine as oxidant and pyridine as nucleophile and solvent (8-10), while the second contains Mn(0Ac) in acetic acid (10,11). Studies with a number of PAH indicate that the formation of pyridinium-PAH or acetoxy-PAH by one-electron oxidation with Mn(0Ac)3 or iodine, respectively, is related to the ionization potential (IP) of the PAH. For PAH with relatively high IP, such as phenanthrene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene, no reaction occurs with these two oxidant systems. Another important factor influencing the specific reactivity of PAH radical cations with nucleophiles is localization of the positive charge at one or a few carbon atoms in the radical cation. 

In hydrogen atom abstractions, alkyl radicals change, as the degree of substitution increases, from being mildly electrophilic (the methyl radical) to being mildly nucleophilic (the ferf-butyl radical). In addition reactions to pyridinium cations, the Minisci reaction, they are all relatively nucleophilic, as shown by their... 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

The relative basicity and nucleophilicity of the azobenzene radical-anion has been assessed by comparing the relative effectiveness of methyl iodide and water in quenching the reversibility of the first reduction wave For HMPA solution it was found that alkylation competed favourably with protonation although inter-... 

The addition of functionalized alkyl radicals to protonated heteroaromatics was more difficult (because the radicals could not be generated by H-atom abstraction), but a recent development holds promise to resolve this problem. Generation of a methyl radical in the presence of an alkyl iodide sets up a relatively rapid equilibrium as indicated in Scheme 85. This equilibrium will favor any more highly substituted alkyl radical over methyl, and further, this latter radical will be significantly more nucleophilic. Thus when methyl radicals are generated in the presence of cyclohexyl iodide and a protonated quinaldine, die... 

Competition studies from Szwarc s group provided excellent quantitative insights into the relative affinities of methyl and trifluoromethyl radicals for a host of alkenes [86-88], and from this work came the first general recognition that substituted alkyl radicals could exhibit polar characteristics ranging from nucleophilic to electrophilic. On the basis of such early work, methyl and trifluoromethyl were taken to be the prototypical nucleophilic and electrophilic radicals, respectively, characterizations which it turns out are somewhat exaggerated in both cases. 

Goldschmidt and Beer.372 The 2-alkylated product is the main one formed. This orientation is not unexpected since, compared with the phenyl radical, an alkyl radical should have some nucleophilic character. The j8-/y- ratio is also lower than in the phenylation, as expected for a more nucleophilic radical.371 Pyridine has a methyl affinity of 3 compared with benzene.373 This, however, does not represent the relative amount of picolines and toluene formed with acetyl peroxide.371... 

The more substituted radicals continue to be measurably the more nucleophilic. The relative rates with which the various alkyl radicals react with the 4-cyan-opyridinium cation (7.33, Y = CN) and the 4-methoxypyridinium cation (7.33, Y = OMe) are given in Table 7.2. The LUMO of the former will obviously be lower than that of the latter. The most selective radical is the ferf-butyl, which reacts 350000 times more rapidly with the cyano compound than with the methoxy. This is because the ferf-butyl radical has the highest-energy SOMO, which interacts (B in Fig. 7.4) very well with the LUMO of the 4-cyanopyridi-nium ion, and not nearly so well (A) with the LUMO of the 4-methoxypyridinium ion. At the other end of the scale, the methyl radical has the lowest-energy SOMO, and hence the difference between the interactions C and D in Fig. 7.4 is not so great as for the corresponding interactions (A and B) of the ferf-butyl radical. Therefore, it is the least selective radical, reacting only 50 times more rapidly with the cyano compound than with the methoxy. 

A third possibility of chemical modification is conversion into an acylsilane which reduces the oxidation potential of the corresponding ketone by approximately 1 V. A peak potential of 1.45 V (relative to Ag/AgCl) for the oxidation of undecanoyltrimethylsilane has been reported. Preparative electrochemical oxidations of acylsilanes proceed in methanol to give the corresponding methyl esters. A two-step oxidation process must be assumed because of the reaction stoichiometry —oxidation of the acylsilane results in the carbonyl radical cation which is meso-lytically cleaved to give the silyl cation and the acyl radical, which is subsequently oxidized to give the acyl cation as ultimate electrophile which reacts with the solvent. A variety of other nucleophiles have been used and a series of carboxylic acid derivatives are available via this pathway (Scheme 49) [198]. 

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