Methyl thiyl radical formation

At present the exact details of the mechanisms leading to each of the products are far from well understood (7.81. However, it seems likely that oxidation of the common naturally-occurring organo-sulfur compounds (dimethyl sulfide, methyl mercaptan and dimethyl disulfide) proceeds, at least in part, through formation of the methyl thiyl radical, CHjS (4.9-111. That is the main reason for carrying out direct studies on CH3S radical reactions where the individual reactions are isolated. 

Consideration of the relative concentrations of various oxidants in the marine boundary layer leads to the conclusion that reaction with O3, along with CH3SOO adduct formation, is likely to be the dominant fate of the methyl thiyl radical under typical atmospheric conditions. The CH3S+O3 reaction has been the subject of several kinetic and mechanistic studies (see Table 3) the rate coefficient for this reaction has been found to exhibit a slight negative activation energy (295-367 K) and no dependence on pressure (75-325 Torr He) [59]. The mechanism of the CH3S-1-O3 reaction remains uncertain. The thermodynamically viable reaction channels include reactions (6a) to (6f) [61J ... 

The gas phase photolysis of methyl disulfide at 2537 or 2288 A in the presence of NO results in the formation of methylthionitrite without added NO, the only major product is CH3SH. The results are consistent with initial S-S cleavage into excited thiyl radicals... 

CTI reactions of methyl linoleate [35], y-linolenate [35], and arachidonate [36] catalyzed by thiyl radicals have been studied in some detail. Each isolated double bond in PUFA behaves independently as discussed above. Indeed, the time profiles of methyl linoleate disappearance and formation of mono-trans and di-trans isomers in these experiments indicated that the CTI occurs stepwise (Scheme 6.5). The number of possible geometrical isomers increases according to 2n, where n is the number of double bonds, and the complete analysis may be difficult and incomplete for high unsaturation as in the case of methyl arachidonate. 

Mechanisms have also been proposed where no methyl-Ni bond is formed, rather methane is generated initially concomitant with the formation of side-chain stabilized thiyl radicals that subsequently couple [152]. 

However, cyclization of unsaturated mercaptans was described in 1947 by Naylor who photolyzed hydrogen sulfide in diene 96, acetone being used as photosensitizer (Scheme 49). Since under ionic conditions other products are obtained, it can be assumed that the thiapyran 97 results from intramolecular addition of corresponding thiyl radical. Equally relevant are the results of Dyer and Osborne, " who, by distillation of 6-mercapto-l-hexene, obtained a mixture of tetrahydro-2-methyl-l-thiapyran (Cy6) and thiepan (Cy7). Also interesting, if resulting from thiyl radical intramolecular addition, is the result described by von Riihlmann, who, by submitting a mixture of 98 and 99 to BP obtained the dihydrothiazepin (Cy7) (100) as the only product (43% yield) (Scheme 50). Intermolecular addition of the thiyl radical followed by intramolecular imine formation, rather than the reverse, is also an attractive pathway in this case. 

In synthetic contexts, thiyl radicals are known to engage in a number of useful reactions [191-198], including C-H bond abstraction [197, 199-203]. Recently, MacMillan has demonstrated the capacity of catalytically generated thiyl radicals to cleave the C-H bond of benzylic ethers for radical coupling reactions (Fig. 27) to form diaryl methanols (a) [204] or p-amino ethers (b) [205]. The light source is either a Blue LED or a Compact fluorescent lamp (CFL), as designated above. In both reactions, a wide variety of arene components and ether substitutions are tolerated to furnish a diverse set of products and only a catalytic amount of methyl thioglycolate is necessary to affect the desired C-H HAT event. In the formation of diaryl ethers (Fig. 27a), phosphate serves as the base, whereas the P-amino ether... 

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