Disulfides from thiol oxidation

It should be noted that A-bromo compounds can act as oxidizing agents e.g., A-bromo-acetamide and NBS oxidize secondary alcohols to ketones, and use of this has been made in steroid syntheses.355 Aldehydes, semiacetals, and finally esters are formed from primary alcohols by NBS, and disulfides from thiols (for references see Homer and Winkelmann354). Iodine is liberated from acidified KI solution, a reaction that can be utilized for quantitative determination of NBS and for detection of unchanged NBS in a reaction. 

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

The principal mbbers, eg, natural, SBR, or polybutadiene, being unsaturated hydrocarbons, are subjected to sulfur vulcanization, and this process requires certain ingredients in the mbber compound, besides the sulfur, eg, accelerator, zinc oxide, and stearic acid. Accelerators are catalysts that accelerate the cross-linking reaction so that reaction time drops from many hours to perhaps 20—30 min at about 130°C. There are a large number of such accelerators, mainly organic compounds, but the most popular are of the thiol or disulfide type. Zinc oxide is required to activate the accelerator by forming zinc salts. Stearic acid, or another fatty acid, helps to solubilize the zinc compounds. 

Thiol (s), 652, 667-668 disulfides from, 668 electrostatic potential map of, 75 from alkyl halides, 667 hybridization of, 20 naming, 667 odor of, 667 oxidation of, 668 pKa of, 604... 

This reaction also protects proteins with cysteine residues from becoming oxidized to the disulfide since the GSH can be used to reduce the protein disulfide back to the thiol form ... 

The disulfide forms in the thiol oxidation from the recombination of the two RS radicals... 

Disulfide bond formation was introduced into DCC as a powerful reaction for the construction of dynamic systems in the late 1990s in separate reports from the groups of Still [19], Sanders [20], and Lehn [21]. Given the fundamental role played by thiol oxidation in biology, it is no surprise that the reaction is highly compatible with protein targets. Disulfide exchange... 

Similar behavior of other aromatic disulfides and thiols on gold electrodes has been described based on the SERS experiments [167]. Adsorption of benzenethiol, benzenemethanethiol, p-cyanobenzenemethanethiol, diphenyl sulfide, and dibenzyl sulfide was studied on the roughened gold electrode. All these species adsorb dissociatively as the corresponding thiolates. Monolayers formed from symmetric disulfides were exactly like those formed from the corresponding thiols. These monolayers were stable in a wide potential window from -1-800 to —1000 mV (versus SCE), which was limited by the oxidation of the Au surface from the positive side and hydrogen evolution at —1000 to —1200 mV at the negative side. 

In addition to the sulfur compounds listed above, hydrogen sulfide has been found in many crude petroleums. Elemental sulfur has been definitely found in several crude petroleums by API Research Project 48 (23). Although Birch and Norris (5) isolated several disulfides from the spent caustic used in treating gasoline from Iranian petroleum, these compounds may have resulted from the oxidation of the thiols and their presence in the original petroleum is regarded as doubtful. Other types of sulfur compounds, such as thiophenes and aromatic thiols, have been identified in cracked petroleum products, but the presence of such compounds in naturally occurring petroleums has not yet been established. 

Sulfur compound VIII, a carboxytrimethylbenzene sulfonic acid, could have come from an aryl disulfide, a thiol or could be derived from the further oxidation of compound IX, a carboxytrimethyldibenzothiophene-1,1-dioxide. This latter possibility is indicated by the lower concentration of compound IX relative to compound VIII in the oxidation products of coals containing mineral matter. Once again the catalytic effect of the mineral component of coal is indicated. 

In contrast to the free acids, sulfenate esters, amides and halides are more stable. Disulfides (52) can be obtained from thiols by mild oxidation (see p. 57), and sulfenyl chlorides () can in turn be prepared from disulfides (52) by treatment with chlorine (Scheme 30). Sulfenyl chlorides (51) react with alcohols to give esters, e.g. the methyl sulfenate (53) which on alkaline hydrolysis yields the sulfenic acid (45) (Scheme 30). 

Symmetrical disulfides (73) may be prepared by reaction of alkyl halides with disodium disulfide (Scheme 43). The product is contaminated with triand polysulfides owing to the presence of impurities in the disodium disulfide however, lower members of the series of dialkyl disulfides may be purified by fractional distillation. Disulfides can also be obtained from thiols by mild oxidation, e.g. by treatment with iodine or dimethyl sulfoxide (DMSO) (Scheme 44). In the reaction with iodine, the hydriodic acid formed must be removed, otherwise the disulfide is largely reduced back to the thiol by hydriodic acid which is a powerful reducing agent. Pure unsymmetrical disulfides are more difficult to prepare owing to their tendency to undergo disproportionation they can, however, be synthesised from thiols by treatment with imides (see p. 59) or sulfenyl halides (51) (Scheme 45). 

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