S-centered radicals

The C-centered radical is thought initially to rearrange to a S-centered radical via a 1,3-hydrogen shift, followed by a radical translocation from sulfur to the silicon surface on the neighboring row. The abstraction of a H-atom from... 

Alfassi. Z.B, The Chemixrn of S-Centered Radicals. John Wiley. Sons, Inc.. New... 

Armstrong DA (1999) Thermochemistry of sulfur radicals. In Alfassi ZB (ed) S-centered radicals. Wiley, New York, pp 27-61... 

Wardman P (1999) Thiyl radicals in biology their role as. molecular switch central to cellular oxidative stress. In Alfassi ZB (ed) S-Centered radicals. Wiley, New York, pp 289-309 Wardman P, von Sonntag C (1995) Kinetic factors that control the fate of thiyl radicals in cells. Methods Enzymol 251 31-45... 

S-centered radicals may exhibit g-values in the range of those observed in the enzyme and are therefore unreliable indicators of primarily metal vs. primarily ligand oxidation. 

The vast majority of radical cascades initiated by addition of higher main group (VI)-centered radicals to alkynes focus predominantly on S-centered radicals, mainly thiyl radicals, whereas considerably fewer intermolecular addition reactions involving radicals with the unpaired electron located on selenium or even tellurium are known. 

Cascade Reactions Initiated by Addition of S-Centered Radicals to Alkynes... 

One of the earliest radical cyclization cascades initiated by addition of S-centered radicals to alkynes was reported in 1987 by Broka and Reichert (Scheme 2.25). Thiophenyl radicals, PhS, which were generated under radical chain conditions, undergo addition to the terminal end of the C = C triple bond in enyne 138. The resulting vinyl radical 141 can undergo cychzation in both 6-endo (preferred) and 5-exo fashion, and reduction of the radical intermediates 142 and 143 leads to the final observed products 139 and 140, respectively. 

The apparent chemoselectivity for the addition of the electrophilic S-centered radicals to the less electron-rich alkyne moiety in enyne 138 can be rationalized by the fact that addition of S radicals to both aUcenes and alkynes proceeds smoothly (the rate constants for addition of S radicals to alkenes are about three orders of magnitude larger than those for the addition to alkynes), but is also reversible. However, the reversibility is less pronounced for the radical addition to alkynes, due to the high reactivity of the vinyl radicals formed (compared to alkyl radicals), which undergo subsequent reactions at faster rates than undergoing fragmentation back to the S... 

S-Centered Radicals, Alfassi, Z. B., Ed. John Wiley Sons Chichester, 1999. 

Asmus KD, Bonifacic M. (1999) Sulfur-centered reactive intermediates as studied by radiation chemical and complementary techniques. In S-Centered Radicals, Alfassi ZB. (ed.), pp. 141-191. John WUey Sons Ltd., Chichester. 

Alfassi ZB. (ed.). (1999) S-Centered radicals. John WUey 8t Sons, Chichester. 

Cherkasov, A.R. and Jonsson, M. (1999) Substituent effects on thermochemical properties of C-, N-, 0-, and S-centered radicals. Physical interpretation of substituent effects. /. Chem. Inf. Comput. Sci., 39, 1057-1063. 

Table 29.3.4 (continued) Heats of Formation of S-Centered Radicals ... 

A most important finding was the reversibility of the above mentioned repair reaction, i.e., the establishment of an equilibrium between C- and S-centered radicals 35-37... 

Z. B. Alfassi (ed.), S-Centered Radicals, John Wiley Sons, Chichester, New York, Weinheim, Brisbane, Singapore, Toronto, 1996. 

Ultraviolet radiation, X-rays, and y-rays can all produce free radicals whose characteristics are well studied by EPR spectroscopy. Solvent water can give rise to the H- and OH- radicals which, in turn, can produce radical species in proteins and nucleic acids. In some cases the sulfur-containing amino acids give rise to S-centered radicals. 

Finally, it must be said that while the main features concerning the cyclization of unsaturated 0-, N-, or S-centered radicals are beginning to be understood, very little is known about the behavior of other heteroatomic radicals, although some very interesting features emerge from the first reports published. 

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