Hydrogen Transfer from Thiols

In kinetic NMR experiments, rate constants for the intermolecular hydrogen transfer from several carbohydrates to cysteinyl radicals were found to be of the order of k29 = (1-3) x lO s at 37 °G [77]. These values agree with previous, pulse radiolytically determined rate constants for thiyl radical-mediated hydrogen abstraction from various model alcohols and ethers [74, 78, 79]. In contrast, the reverse reaction, hydrogen transfer from thiols to carbohydrate radicals, proceeds with k 29 > 10 s [80, 81], indicating that equilibrium (3.29) is normally lo-... 

Carbon-centred radicals formed on the carbohydrate moieties of DNA can be repaired through hydrogen transfer from thiols (von Sonntag 1987) ... 

In contrast, because of similar electronegativities, the transition state for hydrogen transfer from benzene thiol to a perfluoro-n-alkyl radical should have little polar character. In confirming the important role of polar effects in these hydrogen transfer processes, a good Hammett correlation was observed for the reduction of n- c7f15i by a series of arene thiols [141,143]. 

Here, the chemistry depends upon the rate of hydrogen transfer from the thiol being competitive with the rate of alkene polymerization. By employing polyfunctional thiol compounds, very tough abrasion-resistant coatings are formed [8]. 

In the investigation of species resulting from the indirect effect, calculations of the electron affinities and ionization potentials of the hydrogen and hydroxyl adduct DNA base radicals allow for further understanding of possible electron transfer processes. In conjunction with experimental observations, our ab initio results provide us with a complete list of redox properties for the DNA radical intermediates and predict that only a fraction of the base adduct radicals can be reduced via electron transfer from thiols, while DNA sugar radicals will be reduced. 

Roberts and co-workers have employed a number of chiral carbohydrate-derived thiols as polarity reversal catalysts in the radical hydrosilylation of electron-rich prochiral alkenes [68-70]. In these thiols, the SH group is attached to the anomeric carbon atom. Scheme 21 demonstrates the non-catalyzed reaction and in step b, the hydrogen atom transfer from the silane... 

This substitutes very well for most tin hydrides in many reactions. Roberts, on the other hand, has demonstrated that thiols act as polarity reversal catalysts for hydrogen-atom transfer from organosilanes to alkyl radicals.185 The use of triethyl borane and oxygen as radical initiators166 has allowed the reaction to be carried out at much lower temperatures and hence mechanistic elucidations have been possible.187... 

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,... 

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... 

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