Thiols as hydrogen donors

A more efficient and general procedure once again involves the <7-acyl thiohydroxamates. Decomposition in the presence of triplet oxygen, and r-butyl thiol as hydrogen donor, provides norelkyl hydroperox-... 

Higher enantioselectivities were generally found when 3-mannose thiol 13 was used as hydrogen donor. The extra bulkiness provided by the gem- -diphenyl groups in alkene 14 compared to alkene 11 is indicated to be responsible for the high enantiomeric purity observed (Reaction 5.12) [31]. 

The ease of fission of the S H bond under homolytic conditions means that thiols are often used as hydrogen donors in free radical reactions. 

Thioredoxin (1,2) are small (M. 11-12,000) ubiquitous redox proteins with two half-cystine residues in the conserved active site structure Trp-Cys-Gly-Pro-Cys. The oxidized form Trx-S2 is reduced by NADPH and thioredoxin-reductase the reduced rorm Trx(SH)2 is a powerful protein disulfide oxido-reductase which regulatesthe activity of enzymes by thiol redox control it serves as hydrogen donor for various reductive enzymes such as ribonucleotide reductase or enzymes reducing sulfate or methionine sulfoxide. Also, Trx(SH)2 is essential for phage T7 DNA replication as a subunit of T7 DNA polymerase and assembly of filamentous phages (fl and M13), at least in E, ooti. 

Pulse radiolysis of penicillamine in aqueous solution gives thiyl radicals RS% Amax 330 nm, and the radical anion RSSR-, Amax 450 nm, formed from RS and RS / Photolysis of propanethiol tritiated at SH, in the presence of a hydrogen donor, results in tritium scrambling in the propyl group and in the hydro n donor, suggesting that such a system is a useful source of H atoms in solution. In eased yields of the products of sensitized photodecarboxylation of R X CHj COaH (X = O, S, or NH) are obtained in the presence of thiophenol as hydrogen donor. Butane-thiol quenches triplet acetophenone only slowly (At = 1.4 x 10 1 mol s ), and it can be included in the reaction systems of ketone photo-reactions as a radical trap without interfering with the photo-excited substrate. ... 

Aldehydes, both aliphatic and aromatic, can be decarbonylated by heating with chlorotris(triphenylphosphine)rhodium or other catalysts such as palladium. The compound RhCl(Ph3P)3 is often called Wilkinson s catalyst.In an older reaction, aliphatic (but not aromatic) aldehydes are decarbonylated by heating with di-tert-peroxide or other peroxides, usually in a solution containing a hydrogen donor, such as a thiol. The reaction has also been initiated with light, and thermally (without an initiator) by heating at 500°C. 

Scheme 3.14 is discussed as follows. Thiols react with the aliphatic phospine cation-radicals as hydrogen atom donors (Romakhin et al. 1995), but they come out as donors of thiolate anions,... 

Norrish Type II biradicals have also been trapped. For example, irradiation of a benzene solution of 58, to which a high concentration of butanethiol-S-Norrish Type II reaction and extensive incorporation of deuterium at the y carbon of recovered 58. The thiol, a good hydrogen donor, supplies a deuterium to the radical 59 before 59 can give products or return to starting material as shown in Scheme 6.93... 

Dihydroanthracene has been used as one of the hydrogen transfer reagents in a series of homolytic hydrogen transfer reactions by Braude, Jackman, and Linstead and as a hydrogen donor for the hydrogenation of thiyl radicals to form thiols. ... 

Consider a reduction of a slab of coal in a hydrogen-donor solvent and assume that the temperature is increased linearly. The more easily reducible groups release H2S at low temperatures. This H2S may diffuse out of the particle and then be determined, or it may react with the organic matrix and not be detected as part of the sulfur in that group. For example, thiols release H2S at low temperatures ... 

Allyl radicals substituted at only one of the terminal carbon centers usually react predominantly at the unsubstituted terminus in reactions with nonradicals. This has been shown in reactions of simple dienes such as butadiene, which react with hydrogen bromide, tetrachloromethane or bromotrichloromethane to yield overall 1,4-addition products . The reaction of allyl radicals with hydrogen donors such as thiols or tin hydrides has been investigated and reviewed repeatedly. In most cases, the thermodynamically more favorable product is formed predominantly. This accords with formation of either the higher substituted alkene or the formation of conjugated tt-systems. Not in all cases, however, is the formation of the thermodynamically more favorable product identical to overall 1,4-addition to the diene. In those cases in which allyl radicals are formed through reaction of dienes with tin hydrides or thiols, the... 

The extension and wide application of this work has come from the work of Chatgilialoglu and co-workers on the use of (Me3Si)3SiH as a hydrogen donor suitable for water-soluble and water-insoluble precursors, in systems comprising substrate, silane and initiator (ACCN) mixed at 100 °C. This system was proposed as it worked well for both hydrophilic and hydrophobic substrates, with the only variation that an amphiphilic thiol was also needed in the case of water-soluble compounds (Scheme 4.10).25... 

One frequent observation is that radicals can more frequently be reduced instead of undergoing the usually more fancy but desired transformation. The unexpected hydrogen donor can be a reagent (such as a stannane, silane, thiol, hypophosphorous acid, etc., or solvent). If the hydrogen shift is suspected to be a culprit, it could be easily unmasked through a deuterium experiment. It may then be necessary to block the unwanted... 

A new decarboxylative route to free radicals, which has proved particularly successful in preparative work, embodies the thermal (or photochemical) decomposition reaction of 1-hy-droxypyridine-2(l/f)-thione esters 23 with tributyltin hydride, /er/-butanethiol, or a similar hydrogen donor.These esters can be easily prepared from acyl halides and the sodium salt of l-hydroxypyridine-2(l//)-thione, or from the carboxylic acid, dicyclohexylcarbodiimide and l-hydroxypyridine-2(l/f)-thione. The intermediate radicals were readily reduced to the corresponding hydrocarbons 24 in efficient chain reactions with organotin hydrides or thiols as reaction partners, and the proportion of rearranged to unrearranged products could be controlled by the choice of hydrogen donor, its concentration and the temperature. This system was sufficiently quantitative and well behaved for use in kinetic studies, and the rate constants of the (S-scission reactions of the listed cyclopropylmethyl species were determined. 

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