Heteroaromatic radicals

The substitutions with nucleophilic radicals become particularly interesting only with electron-deficient aromatic substrates, as the ionic nucleophilic substitutions. Carbon free radicals are the most common nucleophilic radicals and they are obviously among the most important organic free radicals. Heteroaromatic bases on the other hand are electron-deficient aromatic substrates which readily react with nucleophilic species. The protonation of heteroaromatic bases strongly increases their electron-deficient nature and therefore the reactivity towards nucleophilic reagents, while the reactivity towards electrophilic species is strongly... 

Further development of the reaction led to zinc bis(sufinate) salts as stable, scalable reagents for this reaction, many of which are now commercially available. Treatment of the zinc salt with t-butyl hydroperoxide generates a trifluoromethyl radical. Heteroaromatic systems can undergo HAS with this electrophilic radical at innately nucleophilic sites in the aromatic ring, though choice of solvent can impact the regioselectivity of this reaction. 

Scheme 11.16 Proposed mechanism of the Cu(ii)-catal zed DDQ-mediated atyla-tion of isochroman, and comparison with a Minisci radical-heteroaromatic ring coupling.

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 thiazolyl radicals are, in comparison to the phenyl radical, electrophilic as shown by isomer ratios obtained in reaction with different aromatic and heteroaromatic compounds. Sources of thiazolyl radicals are few the corresponding peroxide and 2-thiazolylhydrazine (202, 209, 210) (see Table III-34) are convenient reagents, and it is the reaction of an alky] nitrite (jsoamyl) on the corresponding (2-, 4-, or 5-) amine that is most commonly used to produce thiazolyl radicals (203-206). The yields of substituted thiazole are around 40%. These results are summarized in Tables III-35 and IIT36. 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Free-Radical Substitutions of Heteroaromatic Compounds R. O. C. Norman and G. K. Radda... 

In Volume 2, in the chapter on Free-Radical Substitutions of Heteroaromatic Compounds by R. O. C. Norman and G. K. Radda, p. 166, Table VI, 6-R-Acridine should read 9-R-Acridine p. 167, lines 17 and 18, 6-phenylacridine should read 9-phenyl-acridine... 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

Direct perfluoroalkylation of heteroaromatics occurs with RFI when sodium hydroxymethane sulfinate (Rongalite) is present. 3-Perfluoroal-kylcoumarins can be obtained (90CC1781). The distribution of isomers from substituted pyridines is compatible with a radical reaction (90TL2711). 

Alkyl radicals generated efficiently from allylsulfones in 80% aqueous formic acid induced a cyclization reaction on aromatic and heteroaromatic compounds to provide polycyclic aromatic and heteroaromatic derivatives (Eq. 7.17).37... 

---