Electrophilic character radicals generated

The (thermal) decomposition of thiazol-2-yldiazonium salts in a variety of solvents at 0 C in presence of alkali generates thiazol-2-yl radicals (413). The same radicals result from the photolysis in the same solvents of 2-iodothiazole (414). Their electrophilic character is shown by their ability to attack preferentially positions of high rr-electron density of aromatic substrates in which they are generated (Fig. 1-21). The major... 

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 electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

The enolic double bond of a difluorovinyl group in the anomeric position is able to add a radical. It can thus afford a difluoro-C-glycoside, either directly or indirectly. When the alkylating radical is generated from a 6-halogenopyranoside, the reaction can lead to difluoro-C-disaccharide. These reactions are even more efficient if the radical has an electrophilic character (e.g., with an a-halogenoester instead of an alkyl radical). 

In any case, greater synthetic utility has been observed by increasing the electrophilic character of A -centered radicals. The precise control of reaction conditions (particularly the presence of Bronsted or Lewis acids) and the nature of the substituent on the nitrogen atom are thus very important in determining which type of intermediate is generated and consequently which efficiency and which selectivity should be expected. 

The carbenes or carbenoids can be generated in a variety of ways. It is not always clear whether the reaction is concerted or stepwise and whether the carbene behaves as an electrophilic, nucleophilic or radical species. For instance, a carbenoid generated from bismuthonium ylide (19) in the presence of copper(I) chloride would behave as a triplet and add as a radical to a terminal alkyne" (equation 17). The dicarbonyl structure and the absence of reaction with methyl propionate to a furane might well indicate electrophilic character of this carbene. 

In simplified terms, the difference between WHPCO and WACO mechanisms can be explained as follows. In WHPCO, the rate of reaction depends on the redox reaction of H2O2 with iron (or other redox metals) to form the active radical species. In WACO, the rate depends on the electrophilic character of the catalyst, e.g. its rate of generation of surface radical species. Although this property also depends on the presence of redox sites, the Fenton mechanism is much more effective to close the cycle. 

Nitrobenzenesulfonylperoxy intermediate 51 had shown a larger oxidizing ability towards olefins compared with the 4-nitrobenzenesulfonylperoxy intermediate. When 2-nitrobenzenesulfonyl chloride reacts with 02" at low temperature (—20 to —35°C) in CH3CN, the corresponding sulfonylperoxy radical 51 or anion 52 is generated. The radical character of the sulfonylperoxy intermediate 51 was further confirmed by the electrophilic oxidizing nature of a 2-nitrobenzenesulfonyl chloride/KOi mixture. 

Another method consists in generating an electrophilic carbon-centered radical (e.g. the CH3COCH2- radical from acetone, peroxydisulfate and Ag(I)) which, instead of reacting with the protonated heteroarene, readily adds to simple alkenes forming a radical adduct that, owing to its nucleophilic character, selectively reacts with the heterocyclic ring (Scheme 4) [2]. 

---