Nucleophilic free radicals

In this review we have presented a complete and, as far as possible, up to date account of how C-glycosides may be synthesized using C(l) nucleophiles. Free radical based methods, which have provided numerous and useful procedures for C-glycoside synthesis, were beyond the scope of this review and only a few representative examples are mentioned. 

The C—H bond is normally not very polar. As a result, the <7Ch and ch orbitals are widely separated and more or less symmetrically disposed relative to a. A sluggish reaction is expected with carbon free radicals, but a rapid reaction may be anticipated with both electrophilic and nucleophilic free radicals. Examples of both kinds of reactions are ubiquitous in organic chemistry. An ab initio investigation of the former, involving oxygen-centered free radicals, has been carried out [237], The reactivity spectrum may be modified by substitution on the carbon bearing the hydrogen atom. As we have seen in Chapter 7, all three kinds of substituents stabilize the carbon-centered free-radical intermediate. 

Sibi and Chen [42] reported a related tandem intermolecular nucleophilic free-radical addition-trapping reaction of enoate 168 establishing chirality at both a and /(-centers with control over both absolute and relative stereochemistry (Scheme 9.30) using a Lewis acid catalyst and the bisoxazoline ligand 169. They observed... 

C60 chemically behaves as an electron-deficient polyalkene, with rather localized double bonds. Thus, the reactions that mainly take place involve cycloadditions, additions of nucleophiles, free radicals, and carbenes, as well as rp-complexation with various transition metal elements. 

Substitution reactions of heteroaromatic bases with nucleophilic free radicals 89H(28)489 90JHC79. 

Minisci, F., Galli, R., Cecere, M., Malatesta, V. and Caronna, T., Tetrahedron Lett, 1968, 5609 Substitutions by nucleophilic free radicals a new general reaction of heteroaromatic bases , Minisci, F, Fontana, F. and Vismara, E., /. Heterocycl. Chem., 1990, 27, 79 Minisci, F, Citterio, A., Vismara, E. and Giordano, C., Tetrahedron, 1985,41,4157 Advances in the synthesis of substituted pyridazines via introduction of carbon functional groups into the parent heterocycle , Heinisch, G., Heterocycles, 1987, 26, 481 Recent developments of free radical substitutions of heteroaromatic bases , Minisci, F, Vismara, E. and Fonatana, F, Heterocycles, 1989, 28, 489. 

A common aspect characterizing both the homolytic amination and the substitution of protonated hetero-aromatic bases by nucleophilic free radicals is the presence of a nitrogen atom with a positive charge. This presence determines strong polar effects whether it characterizes the radical (R2NH) or the aromatic... 

Minisci, F., Galli, R., Cecere, M., Malatesta, V., and Caronna, T.,Tetrahedron Lett., 1968, 5609 Substitutions by nucleophilic free radicals a new general reaction of heteroaromatic bases , Minisci, F., Fontana, F., and Vismara, E., J. Heterocycl. Chem.,... 

In the last decades, many protocols to promote the nucleophilic free-radical Minisci reaction have been developed. Stoichiometric amounts of cerium(IV) salts were used to afford the carbamoylation of heteroaromatic bases catalyzed by N-hydroxyphthalimide under anaerobic conditions [19]. At the same time, catalytic quantities of cobalt(ll) salts in the presence of oxygen were used together with N-hydroxyphthalimide to promote tiie acylation of bases [20]. Ethylene glycol in the presence of the oxidizing system made of Ag(l) and persulfate salts gave high yields of the corresponding hydroxymethylated bases [21]. Similarly, nucleophilic fluorinated radical intermediates allowed to insert perfluoroalkyl substituents onto heteroaromatic bases (Scheme 12, product 12) [22] and quinones (Scheme 12, product 13) [23]. 

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