Thiyl radicals conditions

The -elimination of a thiyl radical (RS ) terminated a remarkably productive tandem radical bicyclization in Parker s formal total syntheses of ( )-codeine and ( )-morphine (see Scheme 14).29 Subjection of aryl bromide 72 to the conditions indicated generates transient aryl radical 73, an intermediate which engages the substi-... 

Traditionally thiols or mercaptans are perhaps the most commonly used transfer agents in radical polymerization. They undergo facile reaction with propagating (and other) radicals with transfer of a hydrogen atom and form a saturated chain end and a thiyl radical (Scheme 6.6). Some typical transfer constants are presented in Table 6.2. The values of the transfer constants depend markedly on the particular monomer and can depend on reaction conditions.4"1 44... 

The adduct formed in the first step (an equilibrium) is highly colored and decomposes either back to reactants or to a reduced species [Fe(CN)5NO]3 and the thiyl radical RS. In the presence of oxygen the reduced species is oxidised back to NP, giving a cyclised oxidation of RS to RS (and disulfide) but, under anaerobic conditions, NP reacts with eventual release of NO [46]. However, in spite of the desired... 

As an alternative to tin-mediated conditions, thiyl radicals are an attractive alternative they generate carbon-centered radicals upon reversible addition to a multiple bond. This was found to be effective for cyclizations of 49E and 49F (Scheme 13). Treatment with AIBN and PhSH, followed by fluoride-induced desilylation, afforded vinyl and 2-(thiophenyl)vinyl adducts 50E and 50F, respectively [122, 123, 125]. Interestingly, during fluoride-induced (TBAF) desilylation of the intermediate formed... 

The Jacobson thioanilide radical cyclization has been frequently used for the synthesis of benzothiazoles as shown by the preparation of benzothiazole derivative 32 <07JMC1087>. The harsh reaction conditions (K3Fe(CN)6, NaOH, H20, EtOH, 90°C) can be overcome by using 2,6-dichloro-3,5-dicyano-l,4-benzoquinone (DDQ) (CH2CI2, 25°C) <07TL669>. This reaction probably proceeds via thiyl radical 35, which undergoes 1,5-homolytic radical... 

The early woric with thiyl radicals primarily involved the intramolecular cyclization of alkenes. The reaction appears to be useful since high yields and stereoselective formation of products are observed (equation 7). Under a variety of conditions the diallyl diester (12) produces a 6 1 mixture of stereo-isomeric cyclopentanes (13) and (14) in high yield. ° When enynes are treated with thiyl radicals, low to moderate yields of addition products are observed (equation 8). ... 

The major fate of the thiyl radical under these conditions is disproportionation... 

The a-exomethylene-y-lactone framework has been successfully constructed via two electrosynthetic pathways, that is, both by the direct and by the concerted decarboxylation processes as mentioned earlier [Eq. (45)] [151]. The electrodecarboxylation of XCa is probably initiated by a one-electron oxidation of the sulfur atom, giving first the cation radical (XCb) and subsequently a concerted elimination of the thiyl radical and carbon dioxide to LXXXIX. On the other hand, the electrochemical decarboxylation of LXXXVIIIa involves an El-type elimination of a proton from the cation intermediate (LXXXVIIIb) generated from direct two-electron oxidation of the carboxyl group. The latter method generally requires a higher oxidation potential than that required for the concerted method. Therefore, the concerted electrodecarboxylation method becomes more advantageous, especially when the substrates or products are unstable under oxidative conditions. 

It has long been known that unsymmetrical ketones can be prepared by the reaction of aldehydes with alkenes under free-radical reaction conditions. Recently the revision of this chemistry has been reported by the Roberts group [42], They introduced thiols as a polarity reversal catalyst for the addition of aldehydes to alkenes. Thiyl radicals are electrophilic, and therefore a polar Sh2 type transition state for the hydrogen transfer step from an aldehyde would be ideal in this situation. Indeed, the addition of aldehydes to a variety of alkenes can be effected by... 

The involvement of a-and 7C-type dimeric radical-cations in one electron transfer to the aromatic sulfides (259) has been assessed. A laser-flash study of the behaviour of p-nitrobenzenethiol has shown that S-H fission is the dominant reaction with the formation of the corresponding thiyl radical. This occurs particularly when the irradiations are carried out in nonpolar solvents. The reactions encountered in polar solvents are different. Under these conditions the triplet state of p-nitrobenzenethiol is involved and this undergoes ready deprotonation. The amino acid derivatives (260) can be desulfurised by irradiation using triethylboron and triethylphosphite. ... 

Although in principle all free radical species are detectable by ESR spectroscopy, in practice detection may be difficult or impossible under a given set of experimental conditions. Problems with detection of a particular species will reflect m netic and/or kinetic factors. For example, oxygen-centered species such as OH, Oj, and RO [75] and sulfur-centered species such as thiyl radicals (RS ) [76] cannot be detected directly in fluid solution because of extreme anisotropy in their magnetic parameters which makes their ESR signal amplitudes vanishingly small. To detect radicals such as these, it is necessary to immobilize them in frozen solutions or to resort to indirect methods of detection (see below). The same applies to radicals that have a short lifetime. 

The conclusive picture emerging from the chemical studies under biomimetic conditions is that thiyl radicals are efficient catalysts for CTI of lipids in bilayers, and this process cannot be ignored when considering radical damage to biological components. 

The thiyl radicals (191) react via addition to both ends of the double bond and also by a rather unusual reaction, involving allylic abstraction, to give the thiazolidine (190). The proportions of the products were found to be very sensitive to the experimental conditions. 

Under aerobic conditions thiol could undergo slow and rate-limiting autooxidation to give thiyl radical followed by simple radical-radical addition of nitric oxide. 

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