Sulfur-centred Radicals

The reaction of thiyl radicals with silicon hydrides (Reaction 3.18) is the key step of the so called polarity-reversal catalysis in the radical-chain reduction of alkyl halides as well as in the hydrosilylation of olefins using silane-thiol couple (see Sections 4.5 and 5.1) [33]. The reaction is strongly endothermic and reversible (Reaction —3.18). 

---

The reactivity of TEMPO, and of other aminoxyl radicals, in the scavenging of carbon-or sulfur-centred radicals (equation... 

The sulfur-centred radical in anhydrous guanine dichloride with -tensor (2.028, 2.015, 2.002), indicated the presence of a 35C1 hyperfine interaction (1.98, 2.26, 6.79) mT and a large, highly anisotropic proton coupling tentatively assigned to a C8-H proton from a neighbouring molecule. This radical is likely an electron loss R-S-Cl a -type radical which, at low dose (3 kGy), accounts for ca. 50% of the total radical concentration. 

Radical formation in a mixed crystal system of cytosine monohydrate doped with small amounts of thiocytosine (ca. 0.5%) was investigated on order to gain insight into hole transfer in a well-defined crystalline system.31 Also of interest was whether the protonation state of the thiocytosine radical(s) was the same as that of the cystosine radical(s). Crystals were X-irradiated (ca. 30 kGy) and ESR and ENDOR spectra recorded at ca. 15 K. After irradiation, many types of free radicals were formed. Among these, the low field resonance from a sulfur centered radical (42), with g-tensor (2.132, 2.004, 2.002), was clearly visible. Radical 42 constituted approximately 10% of the total cohort of radicals formed in the crystal and is apparently the only sulfur-centred radical observed in this experiment. Six weakly coupled protons were observed, two of which are shown... 

Urbanski, S.P. and P.H. Wine Chemistry of gas phase organic sulfur-centred radicals, in Z.B. Alfassi (ed.), S-Centred Radicals, John WUey and Sons (1999), 97-140. 

Cyclization on to unsaturated C=N bonds has been reported by a number of workers to proceed in the unusual 6-endo fashion preferentially. Generation of the initial radical by addition of either a tin-centred or a sulfiir-centred radical to an alkyne or allene has been demonstrated. Thus cyclization of vinyl radicals (produced by the BusSnH-mediated addition to alkynes) on to imines proceeds in a 6-endo manner to give functionalized piperidines (Scheme 21). In a similar manner, allyl radicals (generated by Ts addition to allenes) add in a 6-endo manner to hydroxylamine derivatives. EUmination of the RO leads to the formation of dihydropyridines (Scheme 22). Functionalized bicyclic j8-lactams can be produced by an unusual 1-endo cyclization triggered by a similar addition of a tin-centred or sulfur-centred radical to substituted Af-propargyl )3-lactams. ... 

It appears that 4-hydroxypyrmtidine-2-lhiol does not react with CF3I under standard conditions. Similar to the reaction of 4-nitrolhiophenol noted above [57], this reaction requires more forcing conditions. Other 4-hydroxypyrimidine-2-thiols behave similarly. The irradiation of an anunonia solution of 2-mercapto-4-oxy-6-trifluoromethylyrimidine with CF3I must be conducted in a Pyrex ampoule at 30-45 °C to produce the S-trifiuoromethyl derivative (Scheme 25). Apparently, the reaction of these hydroxymercapto heterocyclic derivatives is complicated by stabilization of sulfur centred radicals. 

The Primary Donor Triplet State iP7a0 If in the charge separation process electron-transfer in PS I beyond the first acceptor A0 is blocked by treatment with sodium dithionite at high pH and illumination, which reduces the iron-sulfur centres (F) and the quinone (A, the triplet state of the donor, 3P7ao, is obtained via radical-pair recombination from the triplet RP according to ... 

Other pathways of 3PWr, formation have also been discussed.224 The RP mechanism leads to EPR spin polarization.22 3P7oo has first been observed by Frank et al,225. 3P7oo is short-lived (< 1 ms) and is thus studied by time-domain techniques like transient and pulse EPR.14 If only the iron-sulfur centres are pre-reduced, the radical pair P700, Aj is formed (see below). 

The monohydrate displayed an ESR spectrum associated with a thiyl (RS ) radical with g-tensor (1.997, 2.043, 2.004). The low field component, which probably contained some unresolved hyperfine structure, disappeared in hours at 180 K. The dihydrate spectrum, resulting in g = (1.998,2.007,2.087), displayed two low field singlets, one of much higher intensity than the other the line splitting present was not analyzed. Both lines disappear below 200 K with no successor radical. It is concluded that both hydrated crystals give rise to a ir-type sulfur-centred electron-loss radical, which were both expected to be de-protonated at 77 K. 

The thiocarbonyl group is excellent for radical addition, which takes place on the sulfur atom and leads to a carbon-centred radical stabilised by the a-sulfur atom. The Barton reaction has enjoyed a great many applications. It mainly involves xanthates and provides many useful processes, such as deoxygenation, decarboxylation, addition to multiple bonds, etc. A number of reviews by Crich et al. have appeared [188, 189], and the most recent is due to Zard [190]. 

A useful intermolecular radical reaction that avoids tributyltin hydride and excess alkene makes use of the ready p-ehmination of fin or sulfur radicals. Addition of the carbon-centred radical to the y-position of an unhindered allyl stannane gives an intermediate radical that eliminates a tin radical (4.38). The product is therefore the result of overall allyl addition and the released fin radical reacts... 

Some examples of intermolecular addition of carbon-centred radicals, followed by -elimination of tin or sulfur radicals were provided in Schemes 4.38-4.43 and this strategy is effective in intramolecular processes. Thus, in a synthesis of the antitumor agent CC-1065, the aryl radical generated from the bromide 66 underwent cyclization and subsequent p-ehmination to give the indoline 67 (4.59). An advantage of this type of elimination procedure is that it provides a new alkene in a defined position that is suitable for further elaboration. The p-elimination of a sulfur radical has found other applications, such as in syntheses of the alkaloid morphine and the neuroexcitatory amino-acid kainic acid. ... 

Studies on three different iron-sulfur enzyme systems, which all require S-adenosyl methionine—lysine 2,3-aminomutase, pyruvate formate lyase and anaerobic ribonucleotide reductase—have led to the identification of SAM as a major source of free radicals in living cells. As in the dehydratases, these systems have a [4Fe-4S] centre chelated by only three cysteines with one accessible coordination site. The cluster is active only in the reduced... 

The reaction of atoms, radicals or excited triplet states of some molecules with silicon hydrides is the most important way for generating silyl radicals [1,2]. Indeed, Reaction (1.1) in solution has been used for different applications. Usually radicals X are centred at carbon, nitrogen, oxygen, or sulfur atoms... 

The emphasis on the study of hemoproteins and the iron-sulfur proteins often distracts attention from other iron proteins where the iron is bound directly by the protein. A number of these proteins involve dimeric iron centres in which there is a bridging oxo group. These are found in hemerythrin (Section 62.1.12.3.7), the ribonucleotide reductases, uteroferrin and purple acid phosphatase. Another feature is the existence of a number of proteins in which the iron is bound by tyrosine ligands, such as the catechol dioxygenases (Section 62.1.12.10.1), uteroferrin and purple acid phosphatase, while a tyrosine radical is involved in ribonucleotide reductase. The catecholate siderophores also involve phenolic ligands (Section 62.1.11). Other relevant examples are transferrin and ferritin (Section 62.1.11). These iron proteins also often involve carboxylate and phosphate ligands. These proteins will be discussed in this section except for those relevant to other sections, as noted above. 

---