Hydrogen abstraction by thiyl radicals

Schmittel M, Ghorai MK (2001) Reactivity patterns of radical ions - a unifying picture of radical-anion and radical-cation transformations. In Balzani V (ed) Electron transfer in chemistry, vol 2. Organic molecules. Wiley-VCH, Weinheim, pp 5-54 Schoneich C, Bonifacic M, Dillinger U, Asmus K-D (1990) Hydrogen abstraction by thiyl radicals from activated C-H-bond of alcohols, ethers and polyunsaturated fatty acids. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 367-376... 

Schoneich C, Bonifacic M, Dillinger U, Asmus K-D (1990) Hydrogen abstraction by thiyl radicals from activated C-H-bond of alcohols, ethers and polyunsaturated fatty acids. In Chatgilialoglu C,... 

An instructive example for hydrogen abstraction by thiyl radicals, in this case from an ether, has been reported by Akhlaq et al. Experimental evidence is a racemisation of 2,5-dimethyl-tetrahydrofurane according to a reaction sequence which involves an intermediate planar radical site at the carbon a to the ether s oxygen. 

Hydrogen atom abstraction by thiyl radicals has also been shown to occur with various aliphatic and aromatic compounds [57]. Extensive studies have been conducted with respect to the thiyl reaction with alcohols and ethers in which the C-H bond of interest is activated by a-positioned hydroxyl or alkoxy groups. In these systems, an equilibrium is established, as formulated in equation (36) [9, 55] ... 

However, this is likely not to be the only process. A photolytic decomposition of di(r-butyl)disulfide in low temperature glasses is quite efficient and produces substantial amounts of 2-methyl-2-propanethiol as product which cannot be explained assuming the reaction shown above 187]. The only way to explain the production of this product is to admit the possibility of hydrogen abstraction by the r-butylthiyl radical from the matrix which is considered next. The other option is to admit the existence of hot thiyl radicals generated by photolysis. 

It has been found that 365 nm irradiation of 3-methylpentane glasses containing trapped thiyl radicals results in production of matrix radicals [27]. Because neither parent thiols nor hydrocarbons absorb in this spectral region, the production of alkyl radicals was assigned to hydrogen abstraction by photoexcited thiyl radicals from the matrix ... 

In an analogous process, the reactions of unsubstituted and 2-substituted allyl phenyl sulfides with (TMSlsSiH give a facile entry to allyl fns(trimethylsilyl) silanes in high yields (Reaction 26). In this case, the addition of (TMSlsSi radical to the double bond is followed by the S-scission with ejection of a thiyl radical, thus affording the transposed double bond. Hydrogen abstraction from (TMSlsSiH by PhS radical completes the cycle of these chain reactions. ... 

Thiyl radicals, RS-, may be obtained by hydrogen abstraction from RSH, and will then add readily to alkenes by a chain reaction analogous... 

The plan for a tandem cyclization as the key step in the synthesis of morphine alkaloids has been successfully performed [95]. Three examples are given in Scheme 7.10. Specifically, the initially formed aryl radical, generated by bromine abstraction from compound 74, underwent a tandem cyclization to construct the desired carbocyclic skeleton. Depending on the nature of the Z substituent to the double bond, the product radicals either abstracted hydrogen from silane (Z = C02Me or CN) or eliminated thiyl radical (Z = SPh). 

Under photo-stimulation, isoindolyloxyl radical (5) abstracts primary, secondary, or tertiary hydrogens from unactivated hydrocarbons including cyclohexane, isobutane, or n-butane (Scheme l).23 The nitroxide (5) traps the resultant carbon-centred radical (R ) and so afford the A -aI koxyisoindo les (6). Blank photolysis experiments with no added hydrocarbon have shown some unprecedented / -fragmentation of (5) to afford the nitrone (7). A number of C60 nitroxide derivatives have been synthesized and characterized by ESR spectroscopy which show features common to nitroxide radicals.24 Reaction of nitroxide and thionitroxide radicals with thiyl radicals have been observed, from which sulfinyl, sulfonyl, and sulfonyloxy radicals were generated.25 The diisopropyl nitroxide radical was generated in the reaction of lithium diisopropylamide with a-fluoroacetate esters.26... 

The addition of thiols to olefins (thiolene reaction), to form thioethers, is a well-known reaction. The process can occur by either free-radical or ionic mechanisms. The free-radical reaction can be initiated thermally via a peroxide or by UV irradiation with benzophenone. The initiation step involves the formation of a thiyl radical by hydrogen atom abstraction. Both of these species are capable of starting polymer chains (Table 2.30). 

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