Radical attack on sulfur

7 Reactions with Radicals S.06.3.7.1 Radical attack on sulfur 

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The converse situation in which ring closure is initiated by the attack of a carbon-based radical on the heteroatom has been employed only infrequently (Scheme 18c) (66JA4096). The example in Scheme 18d begins with an intramolecular carbene attack on sulfur followed by rearrangement (75BCJ1490). The formation of pyrrolidines by intramolecular attack of an amino radical on a carbon-carbon double bond is exemplified in Scheme 19. In the third example, where cyclization is catalyzed by a metal ion (Ti, Cu, Fe, Co " ), the stereospecificity of the reaction depends upon the choice of metal ion. 

Both cationic and anionic precursors were used to generate the 02SOH radical (44) which is the presumed intermediate of OH radical attack on S02 to form S03 in the last step of the sulfur oxidation cascade [198]. The hydrogen sulfite cation was produced by dissociative ionization of methanesulfonic acid and used to generate radical 44, albeit in a low yield [209]. A improved preparation and NR mass spectrometric characterization of 44 relied on an anion precursor, 02SOH , which was made by termolecular associative reaction of S02 with OH-[210]. Neutralization with Xe of 02SOH- produced stable radical 44 that gave rise to a dominant survivor cation in the -NR+ mass spectrum [211]. 

The reaction of thietane with A7-chlorosuccinimide to give 102 may proceed via attack of a nitrogen radical on sulfur, but an ionic mechanism via an 5-halo-sulfonium salt intermediate is also possible. Alkoxy radicals attack the sulfur atom of thietanes to give ring-opened alkyl radicals 103 whose electron-spin resonance spectra have been obtained. Trifluoromethylthio and other sulfur... 

Despite the rr-electron-rich character of tetrathiafulvalenes, very few reactions with electrophilic reagents have been described, since they usually lead to oxidation rather than to substitution and/or addition. The only known products of attack on sulfur are the 5-oxides [78JOC4394 79JCS(P2)862] and a formally 5-alkylated derivative produced in the reaction of TTF+ with a free radical, rather than by electrophilic attack (Section II.B.4). 

Sulfur—sulfur bonds occur in a wide variety of compounds, and disulfide bridges are especially important in certain proteins and enzymes. Sulfur bonds can be cleaved homolytically by dissociation or by radical attack on S and heterolytically by nucleophilic or electrophilic attack. 

The presence of excess tetrafluoroethylene promotes reaction of the fluorocarbon radical end of 1 with another molecule of tetrafluoroethylene to form a new diradical, 2. Stabilization of 2 occurs by intramolecular attack of the carbon radical end on sulfur with formation of a favored thiolane ring, 3, and displacement of the sulfur chain. Yields of 20% of 3 have been attained. 

The propagation step in this free radical polymerization has been demonstrated to involve radical attack on episulfide sulfur (as depicted above) by telomerization studies with cyclohexane. From the structures of the two major products, 20 and 21, cyclohexyl radical attacks at the sulfur atom by displacing fluoroalkyl radical. This new radical, a prototype of the growing polymer chain, then abstracts hydrogen from cyclohexane to form 20 or attacks another molecule of 6 (n = 1) at sulfur to give 21. 

The photo-induced electron transfer of l,4-bis(methylene)cyclohexane in acetonitrile-methanol solution with 1,4-dicyanobenzene (DCB) affords two products, both consistent with nucleophilic attack on the radical cation followed by reduction and protonation or by combination with DCB ).63 In the absence of a nucleophile, the product mixture is highly complex, as is the case under electro-oxidative conditions. Under UV irradiation, /nmv-stilbene undergoes dimerization and oxygenation (to benzaldehyde) by a single-electron mechanism in the presence of a sensitizer such as 2,4,6-triphenylpyrilium tetrafluoroborate (TPT).64 This reaction was found to yield a similar product mixture with the sulfur analogue of TPT and their relative merits as well as electrochemical and photophysical properties are discussed. 

One of the most useful and widely used applications in the synthesis of natural product derivatives relies on the efficient photoaddition of RS-H onto a double bond (a reaction known as thiol-ene coupling) [55], The reaction exploits the weakness of the S—H bond that can be cleaved homolytically under irradiation (atca. 254nm). The electrophilic sulfur-centered radical attacks a nucleophilic double bond, thus starting a radical chain reaction. 

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