Hydrogen abstraction sulfoxide

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. 

The total yield of OH radicals is 6.0 and hence the yield of sulfmic acid of G = 2.7 indicates that 45% of the OH radical produced in the radiolysis attack the sulfmyl group of MTMSO. The authors said that the residual 55% attack the sulfide S— bond, but this claim ignores completely other routes, such as hydrogen abstraction, which was found for other sulfoxides. ... 

The first reported40 photorearrangement of a sulfoxide occurs in 2,2-dimethylthiachroman 1-oxide on irradiation in benzene, this is converted into 2-isopropylbenzo[6]thiophene. The detailed mechanism of this transformation is uncertain, but it has been rationalized by assuming that intramolecular hydrogen abstraction by the excited... 

The facts that singlet oxygen is needed for this dealkylation process and that the reaction is about 10 times faster then the sulfoxide production do not support the postulated hydroxylation in the a-posi-tion (26). Hydrogen abstraction at that position would produce a highly stabilized radical intermediate (Scheme 34). 

At atmospheric pressure, autoxidation of thioethers occurs primarily by abstraction of the a-hydrogen and sulfoxides are produced in low yields by reaction of the unreacted thioethers with intermediate a-hydroperoxides (36,37). 

Type B reactivity was observed for systems in which substitution was found P to the sulfoxide [37]. Compound 57 is shown as a representative case. It was shown that 64 and 65 are probably derived from secondary photolysis of 63. Two mechanisms were proposed for the Type B transformations, each involving a-cleavage. First, alkyl-S cleavage can lead to the sultene 59. Further photolysis leads to S-0 homolysis. The subsequent loss of atomic sulfur is the difficulty with this mechanism but may result from attack by other radicals in solution. Intramolecular hydrogen abstraction gives the major isolated product 65. The other proposed mechanism has aryl-S cleavage to give the sulfme 62, presumably followed by photochemical desulfurization [25,35]. 

Moving the methyl groups a to the sulfoxide in the thiochromanones gave entirely different chemistry, Type C. The transformation of 68 to 71 is an example. This too could be rationalized by a-cleavage, but hydrogen abstraction mechanisms were also suggested [37], This reaction is discussed further in Section V. 

Probably in analogy to the well-known hydrogen abstraction reactions of carbonyl and nitro compounds, hydrogen abstraction has been proposed as a primary process of sulfoxide excited states. It is safe to say that while some of these suggestions appear quite reasonable, the acmal evidence for hydrogen abstraction is much thinner than for a-cleavage. 

To date, all results of these studies have been negative. We are skeptical that hydrogen abstraction is truly a significant primary photochemical process of aromatic sulfoxides, but this remains an open question. 

The Jenks laboratory has investigated two other mechanisms for sulfoxide deoxygenation, but all of the work has been on dibenzothiophene sulfoxide 9 [99,100]. It is conceivable, given other properties of this molecule [101], that 9 is an exceptional case. First, one additional considered possibility is that the sulfoxide undergoes a hydrogen abstraction, followed by hydroxyl transfer by 199 to the resultant solvent radical. 

The hydrogen abstraction mechanism would be expected to produce quantum yields that correlated qualitatively with the hydrogen donating ability of the solvent. In fact, the quantum yields for dexoygenation of 9 are extremely similar for solvents as varied in this quality as acetonitrile, benzene, toluene, 2-pro-panol, and tetrahydrofuran. A significant increase in quantum yield was noted for tetrahydrothiophene and cyclohexene, two solvents that would be expected to be more reactive with an electrophilic O atom. Quenching experiments show that a long-lived sulfoxide triplet is not involved [100]. 

The sulfoxides 215 and 216 also show what is essentially alkene photochemistry [109], Photostationary states of E/Z isomeri2ation were obtained for the analogous sulfides and sulfones as well. Interestingly, if the sulfoxide is replaced by an ether, isomerization is followed by internal ketone hydrogen abstraction from R and five-membered ring formation. 

Hydrogen abstraction from sulflnic acids by alkoxy radicals represents another widely used approach to sulfonyl radicals [61]. This process is involved in sulfonyl radical production in course of hydroxyl radical scavenging by aliphatic sulfoxides [65] ... 

The surmoimting role of configuration and conformation on the regioselectivity of this elimination emerged convincingly from the clearresults observed with the epimeric steroidal sulfoxides 504 and 505. Only in the transition states 504 and 505 is the bulky adamantyl residue kept away from any interaction with the steroid framework and this leads to hydrogen abstraction from Cj in the first case and from C4 in the latter [178]. 

Arylhydroxylamines are readily converted into nitroso compounds with DAD at 0°C (eq 1). One example of the conversion of an A,A-dimethylhydrazone to a nitrile has been reported. Sulfur-containing amino acids like methionine and S-ethylcysteine can be oxidized to their sulfoxides in virtually quantitative yields, although another reaction pathway occurs with most other thioethers. Thioethers and ethers usually react with DAD to yield a-hydrazo derivatives by hydrogen abstraction. The initial ether/DAD adducts can be formed thermally at 100 °C or photochemically at much lower temperatures. ... 

G(CH3S02 ) is the yield of CH3S02 radicals, AOH- and AH+ are the decrease and increase of the conductivity in basic and acidic solutions respectively, l is the specific conductivity and l(Haq+) and i(OH ) are known to be 315 and 178 fl-1 cm2, respectively. For dimethyl sulfoxide G(RS02 ) was found to be 5.46 comparing this to G(OH) = 6.0 for N20 -saturated aqueous solution leads to the conclusion that 91% of the OH radicals were added to the sulfoxide bond. There is no proof for the fate of the other 9% it is probable that they abstract hydrogen atoms from the methyl groups. 

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