Thiols chain reactions involving

Radiolysis of mixtures of thiols and olefins in the absence of oxygen leads to anti-Markovnikov addition across the double bond in a long chain reaction involving free radicals. The propagation steps for a terminal... 

Another very important way that a growing chain may be terminated is by chain transfer. This stops the chain but starts a new one. Thiols, such as phenyl-methanethiol and dodecanethiol, are efficient chain-transferring agents. The reactions involved are as follows (where M represents monomer and RSH represents the chain-transfer reagent) ... 

Viscosity of the medium can also play a role in the kinetics due to the importance of diffusion in the observed rate constants. In the bulk radical polymerization of 2-phenoxyethyl methacrylate, thiol chain-transfer reagents operate at rates close to those observed for MMA while the rate of CCT catalyzed by 9a is an order of magnitude slower (2 x 103 at 60 °C) than that of MMA.5 The thiol reactions involve a chemically controlled hydrogen transfer event, whereas the reaction of methacrylate radicals with cobalt are diffusion controlled. The higher bulk viscosity of the 2-phenoxyethyl methacrylate has a significant influence on the transfer rate. 

Methanethiol results among others from the pyrolysis of methionine (Merritt et al., 1970). Like short-chain aliphatic thiols, it is also produced in model reactions involving cysteine and sugars, and also by heat treatment of methional (Q.22). 

The reaction in Entry 12 is related and uses thioglycolate esters as a chain transfer agent. In this particular reaction involving an electron-rich alkene, the yield is only 8% in the absence of the thioglycolate. Entry 13 is another example of the addition of an acyl radical to relatively electrophilic alkene. Entry 14, involving the addition of formamide was done with acetone photosensitization. The 2-dioxolanyl radical involved in Entry 15 would be expected to be nucleophilic in character and higher yields were obtained with diethyl maleate than with typical terminal alkenes. The addition of 1,3-dioxolane to various enones has been done using benzophenone sensitization.The radicals in Entries 17 and 18 are electrophilic in character. Entries 19 and 20 are examples of thiol additions. 

Under free-radical conditions (often induced by photochemical means), thiol-ene reactions proceed via a typical chain process with initiation, propagation and termination steps (Scheme 6.3). The mechanism involves a thiyl radical cis-trans isomerisation, with concurrent formation of a thiol-ene product. Initiation requires treatment of a thiol with a photo-initiator under irradiation, and results in formation of a thiyl radical (R-S ). Propagation involves direct addition of the thiyl radical across... 

Addition of dimethylsulphoxide (DMSO) is often useful for promoting oxidation of free thiols to disulphides (87, 88). The oxidant is miscible with water, so concentrations of up to 20% (v/v) can be used to promote faster reactions. Oxidation occurs over an extended pH range (pH 3-8), meaning that conditions can be found under which the substrates that undergo oxidation have improved solubility characteristics. Side reactions involving oxidation of sensitive side-chains (Met, Trp, or Tyr) have not been observed. Sometimes, complete removal of DMSO presents problems (30). 

Transfer Agents. In free radical polymerization, thiols are often employed as chain transfer agents. Chain transfer reactions involving thiols proceed via hydrogen atom abstraction as illnstrated in Figure 4. 

This is illustrated in Fig. 6 for the reaction of succinyl CoA acetoacetate coenzyme A transferase with its specific substrate succinyl CoA, compared with the corresponding nonenzymic reaction through the transition state [34]. Both reactions involve thiol interchange to form a new thiol ester of coenzyme A. Catalysis by the enzyme increases the reaction rate by a factor of 5 x 10, which corresponds to a stabilization of the transition state by 18.7 kcal moP Most of this stabilization arises from the utilization of noncovalent binding interactions between the coenzyme A moiety of the substrate and the enzyme. A short chain thiol ester of succinate with methyl mercaptopropionate has the same chemical properties as succinyl CoA but lacks the specific binding groups of the normal substrate this compound reacts with the enzyme some 10 more slowly than succinyl CoA, only an order of... 

The photoinduced addition of a thiol (RSH) to an olefinic double bond has been used to produce polymer networks by taking multi-functional monomers [37-44]. The thiol-ene polymerisation proceeds by a step growth addition mechanism which is propagated by a free radical, chain transfer reaction involving the thiyl radical (RS ). The initial thiyl radicals can be readily generated by UV-irradiation of a thiol in the presence of a radical-type photoinitiator. The overall reaction process can be schematically represented as follows ... 

Under radical-mediated conditions, a thiol also adds to a yne. The radical thiol-yne reaction can be considered as a sister reaction to the radical thiol-ene reaction because it possesses characteristics virtually identical to tiiose of the radical thiol-ene reactions. As with the thiol-ene reaction, the thiol-yne reaction, in general, proceeds rapidly under a variety of experimental conditions in an air atmosphere at ambient temperature and humidity, selectively yielding the mono or bisaddition products with little to no clean up required. In the case of the double addition products formed imder radical conditions, the reaction of two equivalents of thiol with a terminal alkyne is itself a two-step process (Fig. 12.32). The rst, slower, step 1 involves the addition of thiol to the CsC bond to yield an intermediate vinylic radical that subsequently imdergoes chain transfer to... 

The most frequently used reactions involve the following side chains of the amino acids lysine (e-amino group), cysteine (thiol group), and aspartic and glutamic acids (carboxylic group). 

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