Free radicals electrophilic character

In agreement with the theory of polarized radicals, the presence of substituents on heteroaromatic free radicals can slightly affect their polarity. Both 4- and 5-substituted thiazol-2-yl radicals have been generated in aromatic solvents by thermal decomposition of the diazoamino derivative resulting from the reaction of isoamyl nitrite on the corresponding 2-aminothiazole (250,416-418). Introduction in 5-position of electron-withdrawing substituents slightly enhances the electrophilic character of thiazol-2-yl radicals (Table 1-57). 

The carbon atoms of azole rings can be attacked by nucleophilic (Section 4.02.1.6 electrophilic (Section 4.02.1.4) and free radical reagents (Section 4.02.1.8.2). Some system for example the thiazole, imidazole and pyrazole nuclei, show a high degree of aromati character and usually revert to type if the aromatic sextet is involved in a reaction. Othei such as the isoxazole and oxazole nuclei are less aromatic, and hence more prone to additio reactions. 

It has been mentioned that some free radicals (e.g., chloro) are electrophilic and some (e.g., tert-hvAy ) are nucleophilic. It must be borne in mind that these tendencies are relatively slight compared with the electrophilicity of a positive ion or the nucleophilicity of a negative ion. The predominant character of a free radical is neutral, whether it has slight electrophilic or nucleophilic tendencies. 

As with the silanes, some of the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C—Sn bond and it is a weaker bond.103 104 There are also useful synthetic procedures in which organotin compounds act as carbanion donors in palladium-catalyzed reactions, as discussed in Section 8.2.3 Organotin compounds are also very important in free-radical reactions, which will be discussed in Chapter 10. 

Generally, as the potential energy level of SOMO increases (becomes a more reactive radical), free radicals have nucleophilic character, while as the potential energy level of SOMO decreases (becomes a stable radical), free radicals have electrophilic character. Thus, when effective radical reactions are required, small energy difference in SOMO-HOMO or SOMO-LUMO interactions is necessary. For example, the relative reactivities of radical addition reactions of a nucleophilic cyclohexyl radical to alkenes,... 

There is no concrete evidence that tin or lead alkyls can initiate coordinated anionic polymerization, and this is in accord with their covalent bond character and low electrophilicity. Polymerizations which have been initiated with these catalysts are most probably free radical in nature. 

However, when the free radical is p-nitrophenyl, the proportion of meta isomer increases. This is explained by the fact that the radical p-NO C Hi is somewhat electrophilic in character. 

Electrophilic fluorination by substitution of hydrogen at unactivated tertiary carbon has been achieved by use of either CF3OF or elementary fluorine (diluted with N2). Electron-attracting substituents direct the reaction to remote C—H bonds, suggesting that the reaction has electrophilic rather than free-radical character. Examples include the 9a-fluorination of 5a-androstane-3/8,17/8-diol esters (309), 14a-fluorination of various 5a,6j8-dichloro-3,17-disubstituted steroids of type (310), and 17a-fluorination of 5a-cholestan-3j8-yl esters or their 5a,6/8-dichloro-derivatives. Hypobromite and other hypohalite reactions for the functionalization of unactivated carbon atoms are reviewed. ... 

This review focuses on free radical-mediated stereoselective bond construction in which the carbonyl group plays a key role. Reaction at the carbonyl group as well as on carbons alpha and beta are described. The general reaction characteristics of these reactive intermediates are as follows. The acyl radicals are nucleophilic in character and thus they react easily with electrophilic acceptors. On the other hand, radicals on carbon alpha to the carbonyl are electrophilic in nature and their reactivity matches with nucleophilic partners. The majority of reactions at carbon beta to the carbonyl are in a, -unsaturated systems and in these the beta carbon is electrophilic. 

A free radical, or univalent atom, is a chemical system like any other x and ri can be found for it, and Table 3.11 shows a listing of such data for a number of important radicals. The acid-base character of free radicals has been recognized for some time. It is common to speak of electrophilic radicals, such as Cl, and nucleophilic radicals, such as (CH3)C. Table 3.11 is a quantitative ordering of these descriptions. The alkali metal atoms could also be added to the list. These would be the most nucleophilic, or best electron donors. 

Despite the rr-electron-rich character of tetrathiafulvalenes, very few reactions with electrophilic reagents have been described, since they usually lead to oxidation rather than to substitution and/or addition. The only known products of attack on sulfur are the 5-oxides [78JOC4394 79JCS(P2)862] and a formally 5-alkylated derivative produced in the reaction of TTF+ with a free radical, rather than by electrophilic attack (Section II.B.4). 

Halogenated hydrocarbons differ in their chemical reactivity as a result of the electron-withdrawing properties of the halogens on adjacent carbon atoms, resulting in the a-carbon developing an electrophilic character. The halogen atoms also have the ability to stabilize a-carbon cations, free radicals, carbanions, and carbenes. 

In sum total, the types and the amounts of side reactions that can take place are a function of the structures of the peroxides, the stability of the formed radicals, the solvent, and the monomer that is being polymerized. The stability of the radicals that form can also affect the amount of radicals being captured by the monomers. Also, it was reported that while generally the character of free radicals is neutral, some of them are electrophilic (such as chloro) and others are nucleophilic (such as f-butyl). This tendency, however, is relatively slight when compared with positive and negative ions [15]. 

Oxygen possesses two characteristic properties, its paramagnetic nature and its electrophilic character (5). As a paramagnetic diradical, it tends to react readily with other paramagnetic species. It combines with organic compounds in their triplet state, with certain transition metal complexes, and even with itself (72). It also reacts with strained compounds which might have free radical character (75, J4) and readily with free radicals in the familiar process of chain autoxidation (75, 76). 

Alkylation and Acylation by Free-radical Reactions. Heteroaromatic bases, being electron-deficient compounds, readily react with nucleophilic reagents, particularly when protonation enhances their electron-deficient nature. Because of the nucleophilic character of the wide range of available alkyl radicals, homolytic alkylation of heteroaromatics is of interest comparable to that of electrophilic alkylation in the homocyclic series. Homolytic acylation is of no less importance because of the wide applicability of this general reaction in the heterocyclic field. 

The examples reported in this section provide clear evidence that free radical intramolecular addition to polar bonds occurs. These reactions may have genuine synthetic utility even when cyclization is followed by -scission of the (Cy ) radical. As most of the examples have been reported only recently, little is known concerning the scope of these reactions or of the origin of the selectivity of addition to one or the other terminus of the polar bond. It may be expected that features such as the nucleophilic or electrophilic character of the cyclizing radical must be taken into consideration along with the factors which govern intramolecular addition to C=C double bonds. 

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