Thiols oxidative coupling

Keywords thiol, oxidative coupling, active manganese dioxide, barium manga-nate, disulfide... 

The thiol form (12) is susceptible to oxidation (see Fig. 2). Iodine treatment regenerates thiamine in good yield. Heating an aqueous solution at pH 8 in air gives rise to thiamine disulfide [67-16-3] (21), thiochrome (14), and other products (22). The disulfide is readily reduced to thiamine in vivo and is as biologically active. Other mixed disulfides, of interest as fat-soluble forms, are formed from thiamine, possibly via oxidative coupling to the thiol form (12). 

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

Oxidation of thiol 287 with air in glyme gave disulfide 288, which on intramolecular oxidative coupling with A,)V-dibromoisocyanurate formed macrocycle 289 in 42% yield (Scheme 188) (99MI3). 

The synthesis of polysulfide elastomers involves the use of a small amount of trichloroalkane in addition to dichloroalkane and sodium sulfide in order to form a branched polymer. The prepolymer is treated with a mixture of sodium hydrosulfide and sodium sulfite followed by acidification to convert all end-groups to thiol groups. Further polymerization and crosslinking is achieved by oxidative coupling of the thiol end-groups by treatment with lead dioxide, p-quinone dioxime, or other oxidizing agent... 

These iniferter sites containing an N-H group can be easily transformed into the corresponding thiol which leads to disulfide by oxidative coupling and can form chelation with metal ions (Eq. 47) [171,172]. Poly(St) prepared for polymerization with 44 and 45 was applied to the chain-extension reaction by the S-S bond or chelation bond formations. 

Catalysts of this type are also extraordinarily reactive for the oxidative coupling of thiols to disulfides, and the reaction is quantitative in a wide variety of organic solvents. In this case, the analogous first step would be... 

The most significant improvement came in the early 1940s when a method for preparing thiol-terminated liquid polysulfides was developed. Cure of the liquid polysulfides could be accomplished by oxidative coupling. Thus, in effect, a mbber could be compounded without the need of heavy mixing equipment. One of the first large-scale applications of the liquid polysulfides was as a binder for solid rocket fuel. From about 1946 until 1958, these binders were used in various rocket systems and the aliphatic polysulfides achieved commercial success. The switch to predominately liquid-fueled rockets in 1958 ended this phase of the polysulfide business. 

Another very important factor in protein architecture is the disulfide —S—S— link. Remote parts of the polypeptide chain can be held close together through the oxidative coupling of two cysteine thiol groups to form a disulfide bridge ... 

The oxidative coupling of thiols catalyzed by Fem-exchanged montmorillonite in phosphate buffer (pH 7.2) has also been demonstrated (Scheme 3.48) [151]. This system gives the corresponding disulfide as the sole product, which is in contrast to a previous report [152], where oxidation of thiols catalyzed by ion-exchanged clay gave the sulfide as the major product. 

Scheme 3.48 Oxidative coupling of thiols catalyzed by Felll-montmorillonite.

Only derivatives of the 1,2-oxathiole series have been obtained by direct heteroatom coupling. The successful bond formation involves some type of activation of the sulfur atom in 3-hydroxypropanethiol or its oxidized derivatives. TTie parent 1,2-oxathiolane system (106) was recently reported (81CC741) to be obtained from the vacuum pyrolysis of (105), as shown in Scheme 23. A cyclic oligomer is believed to be generated first and (106) is formed on standing. Oxidation of (106) yields the mono- and di-oxide which have been known for some time. The monoxide may be obtained by the oxidative coupling of 3-hydroxypropane thiol (equation 22). Chlorine is the most frequently used oxidant and... 

Figure 3.91 Oxidative coupling of thiols with amines.

Saxena, A., Kumar, A., and Mozumd, S. 2007. Ni-nanoparticles An efficient green catalyst for chemo-selective oxidative coupling of thiols. Journal of Molecular Catalysis A Chemical, 269 35 0. 

Disulfides. Thiols are oxidized to disulfides by dimethyl sulfoxide. This oxidative coupling is particularly useful for coupling of aryl thiols (equation I). 

An attractive reaction has been reported for BY, which is able in ethanol to catalyze the oxidative coupling of various thiols to disulfides (eq 16). ... 

Tetrabutylammonium chlorochromate, (C4H9)4XCr03CI, is formed by adding a solution of chromium trioxide in concentrated hydrochloric acid to a solution of tetrabutylammonium hydrogen sulfate in 5 N hydrochloric acid at 0 °C. The dry compound is used in chloroform for the oxidation of alcohols to carbonyl compounds and the oxidative coupling of thiols to disulfides [619],... 

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