Photolysis of thiols

Photolysis of Thiols. Thiols undergo photolytic reactions (46—48). These reactions proceed as shown in equations 31 and 32. A secondary mechanistic pathway is also observed. This is shown in equations 33 and 34. 

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. 

Flavins (8), 8-azaflavin (9) and their analogs are characteristic oxidants toward thiols under anaerobic and other specific conditions (equation 12). S-Arylidene-l,3-dimethylbarbituric acid derivatives (10) also work as the oxidant in dioxane at 12(X-1S0 C, and the method was applied to synthesis of unsymmetrical disulfides. Electrochemical oxidation and photolysis of thiols to disulfidqs are also known. 

Photolysis of thiol 152 (R = H) gave disulfide 192, 1,3-benzodithiole 193, and thianthrene 194, whereas the photolysis of the acetyl, alkenyl,... 

Quite similar processes occur also in aqueous solutions, as well as in other solvents sometimes complicated, however, by interaction of the radical initially formed (equation 110) with other species present. Indeed the photolysis of thiols has been used as a source of hydrogen atoms to study their reactions with several compounds . 

Thio-Claisen rearrangement 746-748 Thioketene 5-oxides photolysis of 878 reactions of 277 Thiolane dioxides formula of 382 NMR spectra of 460 reactions of 604 synthesis of 461 Thiolane oxides 747 formula of 382 IR spectra of 461 synthesis of 461, 462, 752 Thiolanes, photolysis of 881 Thiolene dioxides formula of 382 NMR spectra of 460 reactions of 464, 465 synthesis of 461 Thiolene oxides formula of 382 synthesis of 461, 746 Thiols... 

The kinetics of the reaction of 2 -deoxyuridin-L-yl radicals (11) with thiols, with superoxide release from the peroxyl radical (13) generated, have been reported. Radical (11) is produced by photolysis of precursor (10). When the radical is produced in the presence of thiols, (12) is formed. Second-order kinetics were found for the reactions with thiols. Peroxyl radical (13) is formed in the presence of oxygen. This undergoes heterolytic fragmentation to the superoxide anion O2 and cation (14), which ultimately leads to 2-deoxyribonolactone (15). 

Thiepane (35) has been synthesized by an intramolecular radical addition of the thiyl radical (equation 59) which was generated by photolysis of a thiol (71TL2025). Similarly, C—S bond formation has been achieved (equation 60) by an intramolecular condensation of 6-mercaptohexanoic acid to give the thiolactone, thiepan-2-one (135) (64MI51700). A Dieckmann-type base-catalyzed cyclization of a diester precursor followed by acid-catalyzed hydrolysis and decarboxylation has been used in the synthesis of thiepan-3-one (41) as indicated in equation (61) (52JA917). 

The best evidence for the photolytic decomposition of mercaptans and disulfides into free radicals involves photoinitiation of polymerization of olefins. Thus, photolysis of disulfides initiates the copolymerization of butadiene and styrene,154 as well as the polymerization of styrene207 and of acrylonitrile.19 Thiophenol and other thiols promote polymerization upon ultraviolet irradiation.19 Furthermore, the exchange of RS-groups between disulfides and thiols is greatly accelerated by light. Representative examples are benzothiazolyl disulfide and 2-mercapto-thiazole,90 tolyl disulfide and p-thiocresol, and benzyl disulfide and benzylmercaptan.91 The reaction probably has a free radical mechanism. Similar exchange reactions have been observed of RS-groups of pairs of disulfides have been observed.19... 

Certain divinyl disulfides (a) are readily available by oxidation of dithioic esters in basic solution (Scheme 9). Heating (a R = Me or Ph) in toluene converted it to a mixture of the 3,4-disubstituted 2,5-di(methylthio)thiophene (b) and the corresponding 5-methylthio-2-thiophenethiol (c). Addition of potassium f-butoxide to the toluene resulted in a nearly quantitative yield of the thiol (c), but further addition of methyl iodide in a second step converts (c) to (b) so that the yield of either product can be maximized (74RTC258). The mechanism is the same as that shown in Scheme 7 (Scheme 9). Photolysis of enethiol esters gives divinyl sulfides such as those shown in Scheme 7, and these form thiophenes under the conditions of photolysis (77JOC1142). 

Photolysis of thiatriazoles was first described by Kirmse,25 who found that aryl cyanides, aryl isothiocyanates, and sulfur are formed on irradition of 5-arylthiatriazoles with UV light. Recently Okazaki et al.n reported the formation of sulfur atoms on irradiation of thiatriazoles. Photolysis of 5-phenylthiatriazole (A lax= 280 nm, e — 10800)25 in the presence of cyclohexene yielded cyclohexene episulfide. Cyclohexene thiols were not formed this was taken as an indication that the reactive species is a triplet sulfur atom. Tetramethylethylene behaves similarly.22... 

Unlike naphtho[6c]oxete, described previously, its thio analog 20 is a stable compound which can be obtained in high yield (97-100%) on photolysis of naphtho[c[Pg.8]

On the other hand, it is now well established that relative kinetic methods which rely on the photolysis of systems containing NO, for OH production lead to erroneously high rate constants for the reaction of OH with thiols (2) and organic sulfides (16.171 due to uncontrolled processes. In the present work the O2 partial pressure was varied between SO and 760 Torr and the NOx concentration was varied by a factor of 4. The trend in the measurements was towards higher values of the rate constants with increase in the O2 partial pressure. However, the observed increase never exceeded the total error limits of the experiments. Changing the NOx concentration had vety little effect upon the measured rate constant... 

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