Hydrogen atom from thiols

Roberts had reported that the low reactivity of alkyl and/or phenyl substituted organosilanes in the reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols (vide supra)49. The general reaction mechanism (Scheme 11) shows that alkyl radicals abstract hydrogen atom from thiols and the resulting thiyl radicals abstract hydrogen from the silane. This procedure has been applied in dehalogenation, deoxygenation and desulphurization reactions. This approach has also been extended by the same... 

Von Sonntag and coworkers14 repeated Michael and Hart s study of the reaction of OH radical with 1,3- and 1,4-cyclohexadienes and extended it. They found that in the case of 1,4-cyclohexadiene, 50% of the OH radicals abstract an hydrogen atom, while only about 25% of the OH radicals abstract an hydrogen atom from 1,3-cyclohexadiene. The remaining OH radicals probably add to the double bond. The addition to the double bond was confirmed by final products analysis in the case of the 1,4-isomer. When N20-saturated aqueous solution of 1,4-cyclohexadiene (10-2 M) together with lower (10-4 M) concentration of the thiol (1,4-dithiothreitol) was y-radiolysed, it was found that 4-hydroxycyclohexene was produced with a yield of 0.29 prnol J 1, i.e. a yield of 50% of the OH radicals (equation 9). 

Although no definitive study of scavengers in photoaffinity labeling experiments has yet been made, the present indications are that thiols are the most suitable reagents. It is expected that as strong nucleophiles they should react with most photogenerated species which are electrophiles as well as with free radicals which can abstract hydrogen atoms from the... 

The reactions of a-tert-butylthioacrylonitrile with various radicals were thoroughly studied by Viehe and his coworkers [51-56]. Abstraction of a hydrogen atom from various substrates (aldehydes, thiols,. ..) was induced by di-t-butyl peroxide (DTBP) or di-r-butyl peroxalate (DTBPO). The results are described in Table 2. With the exception of the IBN-radical addition described above, all radicals gave adduct-dimers 3 in substantial yields. 

RNRs catalyze the reduction of ribonucleotides to deoxyribonucleotides, which represents the first committed step in DNA biosynthesis and repair.These enzymes are therefore required for all known life forms. Three classes of RNRs have been identified, all of which turn out to be metalloenzymes. The so-called class I RNRs contain a diiron site (see Cobalt Bn Enzymes Coenzymes and Iron-Sulfur Proteins for the other two types of RNRs). As diagrammed in Figure 5, these enzymes generate first a tyrosyl radical proximal to the diiron site in the protein subunit labeled R2, and then a thiyl radical in an adjacent subunit (Rl) that ultimately abstracts a hydrogen atom from the ribonucleotide substrate. This controlled tyrosine/thiol radical transfer must occur over an estimated distance of 35 A, and a highly choreographed proton-coupled electron transfer (PCET) mechanism across intervening aromatic residues has been proposed. Perhaps, even more remarkably,... 

Substituent groups (or substituents), considered as replacing hydrogen atoms in parent hydrides, are named using appropriate suffixes ( ol , thiol , peroxol , carboxylic acid , etc.) and prefixes ( hydroxy , phosphanyl , bromo , nitro , etc.). Substituent suffixes are ranked in Section P-43 of Ref. 1. Prefixes are extensively listed in Appendix 2 of Ref. 1. The case of substituents formed by removal of one or more hydrogen atoms from a parent hydride is explained briefly, with examples, in Section IR-6.4.7, and prefixes for many common inorganic substituents are included in Table IX. 

Binding of NTP allows the thiyl radical access to substrate C3 (H), which it abstracts to initiate reduction. Elimination of water by the 3 -radical, followed by or concurrent with reduction by an enzymatic dithiol, leads to the 3 -radical of the product dNTP. Abstraction of the thiol hydrogen atom from Cys408 by the product radical regenerates the thiyl radical and produces dNTP. The radical cationic species following dehydration shown in Figure 31 is a hypothetical intermediate. 

This concept turned out to be useful for a variety of transformations, including reductions, epimerizations [14], hydrosilylations of alkenes, hydrostannations of al-kynes [15] and carbon-carbon bond formation [13]. As an example, thiols catalyze the cyclization of acyl radicals [16a] and masked acyl radicals [16b] these reactions have no preparative value in the absence of thiols because the nucleophilic radicals resulting from the cyclization step are not able to abstract efficiently a hydrogen atom from the starting material, whereas thiyl radicals can (Scheme 2). 

In this mechanism the excitation of the sensitizer by the absorption of light produces the exclted singlet state from which the photochemlcally reactive n-ir triplet state Is formed by Intersystem crossing. This triplet state abstracts a hydrogen atom from the thiol molecule and forms a ketyl radical and a thlyl radical. The ketyl radicals dimerize to plnacols and the thlyl radicals add to the polymer by a chain mechanism. The ketyl radicals may also Initiate and terminate chains either by direct reaction and/or by hydrogen transfer. The preferred ene compounds are those having the allyllc structure because of their Inherent chemical stability (38). 

In the first cycle (Scheme 6.17), the thiyl radicals are generated from thiol groups by photo-initiation or thermo-initiation. The thiyl radical then adds across the ethynyl group to form a vinyl sulfide radical that abstracts a hydrogen atom from a thiol group to produce the vinyl sulfide and another thiyl radical simultaneously. In the second cycle (Scheme 6.17), which can occur or not depending on system conditions, a... 

Silyl derivatives, which are widely used for gas chromatographic applications, are formed by the replacement of active hydrogens atoms from acids, alcohols, thiols, amines, amides, enolizable ketones, and aldehydes with trimethylsilyl groups. A wide variety of reagents are available. These... 

---