Oxidation, basic conditions sulfide

Generally, isolated olefinic bonds will not escape attack by these reagents. However, in certain cases where the rate of hydroxyl oxidation is relatively fast, as with allylic alcohols, an isolated double bond will survive. Thepresence of other nucleophilic centers in the molecule, such as primary and secondary amines, sulfides, enol ethers and activated aromatic systems, will generate undesirable side reactions, but aldehydes, esters, ethers, ketals and acetals are generally stable under neutral or basic conditions. Halogenation of the product ketone can become but is not always a problem when base is not included in the reaction mixture. The generated acid can promote formation of an enol which in turn may compete favorably with the alcohol for the oxidant. 

These workers also prepared the thio analog of 17 (R = H) by treating 16 (R = H) with aqueous ammonia to provide the P-oxoamide, which was converted into the corresponding enolized P-thioxoamide 18 by treatment with hydrogen sulfide and hydrogen chloride in ethanol. Compound 19 was synthesized by oxidation of 18 with iodine in ethanol under basic conditions. 

The overall transformation is the conversion of the carbon-sulfur bonds bond to a carbon-carbon double bond. The original procedure involved halogenation of a sulfide, followed by oxidation to the sulfone. Recently, the preferred method has reversed the order of the steps. After the oxidation, which is normally done with a peroxy acid, halogenation is done under basic conditions by use CBr2F2 or related polyhalomethanes for the halogen transfer step.92 This method was used, for example, to synthesize 1,8-diphenyl-1,3,5,7-octatetraene. 

In solution, sulfides and polysulfides exist in an equilibrium between their basic and protonated forms. Since the equilibrium position is dependent upon the pH of the solution (Figure 5.7), this nature of the reaction with hydrogen peroxide is also influenced by pH.33 Under acidic to neutral conditions, sulfidic species are oxidized predominantly to sulfates. The key reactions that occur are outlined in Figure 5.8. 

The interpretation of these two schematic views confronts some difficulties. Option (1) requires the extrusion of a hydrogen from a methylene hardly active in acidic medium (acetic acid). Under basic conditions, it would be necessary first to convert the sulfide to a sulfoxide such as V to achieve sufficient activation of this methylene. This oxidation may be accomplished by LTA as indicated previously. Nevertheless, the ensuing cyclobutane fragmentation in VI— the actual oxidative step—by concurrent attack of acetate and departure of Pb(II) diacetate (X = PbOAc2 in Scheme 27.1) has no precedent in LTA-amine chemistry, although it is electronically balanced. Besides, the final product of this sequence would be sulfoxide VII. Having no reductive work-up procedure, this sulfoxide should survive until the isolation step. Since this is not the experimental fact, option (1) must then be discarded. 

The cleavage is achieved by oxidation of the sulfide to the sulfone, followed by p-elimination under basic conditions to give free amines. 

C-S bond formation through reaction between aryl halide (Cl, Br, I) and aromatic or aliphatic thiols has been catalysed by a Ni-NHC complex under basic conditions. The reaction is of wide scope and high efficiency. Vinyl sulfides have been synthesized by hydrothiolation of alkynes catalysed by Rh-NHC complexes. Mechanistic investigations have established that the catalysis proceeds via an oxidative addition of the S-H bond to Rh(I) followed by alkyne insertion and reductive elimination. Interestingly, the regioselectivity could be controlled by the nature of the complex (mono or dinuclear precursors), and the use of a strong donor such as the NHC prevents the deactivation of the catalyst. 

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). 

---