Alkyl radical with thiols

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

It is important to note that the rate of reaction of alkyl radicals with thiols does not simply correlate with the exothermicity of the reaction, i.e., with the BDE of the C-H bond to be formed. For example, the tertiary 2-hydroxypropyl radical reacts more readily with thiols than the primary hydroxymethyl radical, and this reacts even faster than the methyl radical (Table 6.4). The reason for this surprising behavior has been discussed in terms of the charge and... 

Electrochemical reduction of the salts (4) provides radicals (18) which dimerize or undergo further reduction to anions (32) or dianions (80MI43100). The reduction potentials are not much affected by substituents. Reduction with zinc in aprotic conditions gives bi(l,2-dithiol-3-yls) (59), and 3-chloro-l,2-dithiolylium salts (35a X = Cl) are converted into bi(l,2-dithiol-3-ylidenes) (20) (75TL3473). Divalent chromium converts the 3,5-dimethyl-l,2-dithiolylium cation into a dithioacetylacetonate ligand (72AJC2547). The reaction of 3,5-diamino-l,2-dithiolylium salts (8) or alkyl derivatives with thiols provides dithiomalonamides (60) by electron transfer (63ACS163). 

Another surprise was that interstrand cross-Unks were also formed independently of O2. However, this was initially rationalized by demonstrating that O2 reacted reversibly with 102 (Scheme 43). Nonlinear regression analysis of the ratio of thymidine to oxygenated products (eg 107) as a function of GSH concentration provided an estimated rate constant for GSH trapping of 102 (fecsH = 6.9 X 10 M s ) consistent with expectations for reaction of an alkyl radical with the thiol.The accuracy of the thiol trapping rate constant validated the extracted rate constant for O2 loss from 106 to reform 102 (feo2 = d.4 s ), which is consistent with rate constants reported for O2 loss from other peroxyl radicals. 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

Phenols are important antioxidants, with vitamin E being the most important endogenous phenolic membrane-bound antioxidant. Membrane levels of vitamin E are maintained through recycling of the vitamin E radical with ascorbate and thiol reductants. Vitamin E is a mixture of four lipid-soluble tocopherols, a-tocopherol being the most efiective radical quencher. The reaction of a-tocopherol with alkyl and alkylperoxyl radicals of methyl linoleate was recently reported. These are facile reactions that result in mixed dimer adducts (Yamauchi etal., 1993). 

The reaction of thiyl radicals with silicon hydrides (Reaction 3.18) is the key step of the so called polarity-reversal catalysis in the radical-chain reduction of alkyl halides as well as in the hydrosilylation of olefins using silane-thiol couple (see Sections 4.5 and 5.1) [33]. The reaction is strongly endothermic and reversible (Reaction —3.18). 

Irradiation of aliphatic thiols, sulfides, and disulfides with a mercury lamp produces gaseous products identified by the mass spectrograph. Thiols are the least stable to light, with the formation of hydrogen95-129 as the main product. Sulfides and disulfides yield, as the predominant products, saturated hydrocarbons of structures corresponding to the smallest alkyl radical attached to sulfur. Haines et al.95-97 have offered a mechanism to explain the predominant production of hydrogen during photolysis of thiols. 

The (oxidizing) peroxyl radicals behave in a similar fashion. They do not react readily with thiols, not even with thiophenols whose H atoms are very weakly bound, but they are readily reduced by the corresponding thiolate ion in contrast to alkyl radicals which are poor electron acceptors and hence do not react with thiolate ions (Simic and Hunter 1986). 

Reduction of RX to RH.1 In the presence of di-r-butylhyponitrite (initiator) and a thiol,2 triethylsilane reduces alkyl chlorides, bromides, or iodides to alkanes in >91% yield by a chain reaction in which the thiol effects transfer of H from the silane to an alkyl radical. This reduction is generally effected with a R3SnH, which is toxic and more difficult to remove from the products. 

Likewise, perfluorinated radicals react more rapidly with electron-rich aUcenes (X=H) than with electrophilic alkenes (X=F) in some intramolecular processes [124] (Figure 4.52). Similarly, rates of hydrogen abstraction by perfluoroalkyl radicals from a series of aromatic thiols were greatest from the most nucleophilic thiol [125] clearly, taken together, these data show that perfluoroalkyl radicals are highly electrophilic in character, in comparison with alkyl radicals, which are of course more nucleophilic. 

Plourde [17] has described the radical addition of alkyl, substituted alkyl and benzyl thiols to polymer-supported cychtol allyl ethers. Treatment of immobilized allyl ether 117 with benzyl thiol in the presence of AIBN provided, after base-induced cleavage from the support and purification, sulfide 119 in high yield and purity (Scheme 25). Similar reactions with hydroxyl- and carboxyl-substituted alkyl thiols also resulted in good yields of products as single regioisomers [17]. 

The 1,3-cyclohexadiene could not be prepared with higher purity than 98% and hence the analysis based on the final products is less meaningful. The yield of 3- and 4-hydroxycyclohexenes show that only 31% (0.18 pmolJ /0.58 pmolJ ) of the OH radicals add to the double bonds. There is no information about the missing 44% (100% — 25%—31%). Von Sonntag and coworkers suggested that the yield of hydroxy cyclohexenes is not indicative of the OH addition to the double bonds due to non-quantitative reaction of the ally lie radical 1 (equation 10) with RSH. Since, in the case of 1,4-cyclohexadiene, they found complete material balance, they concluded that the alky lie radical formed in reaction reacts quantitatively with the thiolic compound. Thus, radical 2 formed in reaction (11) will react quantitatively with RSH. The inefficiency of the reduction of the allylic radical by the thiol is probably due to the weak ally lie C—H bond which leads to a six orders of magnitude lower rate constant for the RSH-I- allylic radical reaction compared with the RSH-I- alkyl radical reaction. If all the material imbalance is due to incomplete reduction of the allylic radical, its formation is the main path of reaction of OH with 1,3-cyclohexadiene. 

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