Oxidation reactions sulfides

Like ammonia, hydrogen sulfide (oxid. no. S = —2) can act only as a reducing agent when it takes part in redox reactions. Most often the H2S is oxidized to elementary sulfur, as in the reaction... 

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

This process of creating ATP, known as electron transport phosphorylation, then, involves two half-cell reactions, one at the electron donation site and the other where the electrons are accepted from the transport chain. Taking aerobic sulfide oxidation as an example, the donating species H2S(aq) gives up electrons, two at a time, to a series of redox complexes. With the loss of each pair of electrons, the sulfide oxidizes first to S°, then thiosulfate, sulfite, and finally sulfate. 

Fig. 22.7. Thermodynamic driving forces for various anaerobic (top) and aerobic (bottom) microbial metabolisms during mixing of a subsea hydrothermal fluid with seawater, as a function of temperature. Since the driving force is the negative free energy change of reaction, metabolisms with positive drives are favored thermodynamically those with negative drives cannot proceed. The drive for sulfide oxidation is the mirror image of that for hydrogentrophic sulfate reduction, since in the calculation 02(aq) and H2(aq) are in equilibrium.

Taking sulfide oxidation (Reaction 22.19) as an example, when the fluid mixture reaches 25 °C, there are about 5 mmol of H2S(aq) and 0.6 mmol of 02(aq) in the unreacted fluid, per kg of vent water. The 02(aq) will be consumed first, after about 0.3 mmol of reaction turnover, since its reaction coefficient is two it is the limiting reactant. The thermodynamic drive for this reaction at this temperature is about 770 kJ mol-1. The energy yield, then, is (0.3 x 10-3 mol kg-1) x (770 x 103 J mol-1), or about 230 J kg-1 vent water (Fig. 22.8). In reality, of course, this entire yield would not necessarily be available at this point in the mixing. If some of the 02(aq) had been consumed earlier, or is taken up by reaction with other reduced species, less of it, and hence less energy would be available for sulfide oxidation. 

Dibenzothiophene is among the sulfides oxidized, and its monoxide was obtained in 89% yield albeit in a longer (6h) time. Thiophene itself was also oxidized, but its monoxide is known to be too labile for isolation (44-46). Instead, it was trapped by a Diels-Alder reaction, as shown in Scheme 8. 

Scheme 10 shows the course taken by these reactions. No trace was found of the 5,5-dioxide, the 5,5,10-trioxide, or the 5,5,10,10-tetraoxide. This reaffirms that sulfide oxidation precedes sulfone formation (42). 

This reaction is mediated by bacteria called sulfide oxidizers. 

Scheme 6. Reaction mechanism for the photochemical sulfide oxidation reaction.

A further catalytic method for asymmetric sulfoxidation of aryl alkyl sulfides was reported by Adam s group, who utilized secondary hydroperoxides 16a, 161 and 191b as oxidants and asymmetric inductors (Scheme 114) . This titanium-catalyzed oxidation reaction by (S)-l-phenylethyl hydroperoxide 16a at —20°C in CCI4 afforded good to high enantiomeric excesses for methyl phenyl and p-tolyl alkyl sulfides ee up to 80%). Detailed mechanistic studies showed that the enantioselectivity of the sulfide oxidation results from a combination of a rather low asymmetric induction in the sulfoxidation ee <20%) followed by a kinetic resolution of the sulfoxide by further oxidation to the sulfone... 

Recently, Feng and co-workers reported an asymmetric sulfide oxidation" catalyzed by titanium complexes bearing HydrOx ligands, for example, 576 (Scheme 8.199). ° Enantioselectivities approached a level of synthetic utility for oxidation of aryl alkyl sulfides 632 although the yields of the sulfoxide 633 were poor due to overoxidation to the sulfone 634. The overoxidation is especially significant for reactions with high enantioselectivity. 

The carboxylic group of 6-aryl-2-methylthiopyrido[2,3- s1pyrimidine-7-carboxylic acid 563 was amidated with (i )-2-(aminomethyl)-l-( /t-butoxycarbonyl)pyrrolidine 564, followed by sulfide oxidation of the resulting amide 565 and reaction with 4-morpholinoaniline to give the substituted pyridopyrimidine 566 as a kinase inhibitor (Scheme 26) <2005W02005090344>. 

Pericas and Jeong demonstrated independently that sulfur-tethered substrates, when subjected to the PKR conditions, afforded the desired bicyclic products. The sulfur tether is removed cleanly by Pummerer reaction after oxidation of sulfur to sulfoxide or 1,4-addition of bisalkyl cuprate followed by hydrogenolysis of sulfide with Raney nickel. It is worth mentioning that the regioselectivity regarding the acetylene part is opposite to that of the intermolecular version (Equation (30)). 

Electrochemistry (Continued) purely organic compounds, 342 sulfide oxidation, 361 Electrode materials, 342 Electrophilic allylation, 192 attractive interaction, 196 mechanism, 192, 197 turnover-limiting step, 197 Electroreaction, asymmetric, 342 Electrostatic interaction, 328 Elimination and insertion, 3 Enamide reactions ... 

The Schmidt reaction affords both possible isomers when applied at the sulfide oxidation level, and also with the sulfoxide, no trace of sulfoximine being found (75CJC276). Reduction of the oxime to the amine with lithium aluminum hydride is significantly improved by the presence of titanium tetrachloride (78KGS1694). 

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