Sulfides disubstituted, oxidation

In contrast, synthesis of 3,4-diphosphorylthiophenes requires more elaboration because of low reactivity of 3,4-positions of thiophene and unavailability of 3,4-dihalo or dimetallated thiophenes. Minami et al. synthesized 3,4-diphosphoryl thiophenes 16 as shown in Scheme 24 [46], Bis(phosphoryl)butadiene 17 was synthesized from 2-butyne-l,4-diol. Double addition of sodium sulfide to 17 gave tetrahydrothiophene 18. Oxidation of 18 to the corresponding sulfoxide 19 followed by dehydration gave dihydrothiophene 20. Final oxidation of 20 afforded 3,4-diphosphorylthiophene 16. 3,4-Diphosphorylthiophene derivative 21 was also synthesized by Pd catalyzed phosphorylation of 2,5-disubstituted-3,4-dihalothiophene and converted to diphosphine ligand for Rh catalysts for asymmetric hydrogenation (Scheme 25) [47],... 

A useful oxidizing agent. Will convert organic sulfides to sulfoxides without overoxidation and disubstituted enamines to a-aminoketones. Most useful is the oxidation of carbanions to hydroxyl groups. 

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). 

Thiiranes are very useful starting materials for synthesis of 1-mono- and 1,1-disubstituted taurines <200582122, 2006S315>. Reaction of thiiranes 220 with silver nitrate and ammonia in methanol at room temperature for 30 min gave 221. After reaction of 221 with hydrogen sulfide for 6h to form 222, oxidation of the latter with performic acid gave the corresponding mono- or disubstituted taurines 223 (Scheme 58). 

It has been shown in the previous sections that at least the polymerization of cyclic ethers, sulfides and amines proceeds via onium ions. The large majority of authors have agreed on this point (the mechanism of propagation of disubstituted cyclic ethers like isobutylene oxide is, however, still in dispute) . ... 

With 1 as catalyst, alkene bonds which have oxidation potentials less than 1.6 V (vs standard calomel electrode) are considered potentially susceptible to this transformation. With the stronger oxidant 2, the scope of the reaction can be extended to include, for example, tetraalkyl-substituted double bonds, but obviously not disubstituted alkenes such as cyclohexene. On the other hand, electron-rich alkenes such as enol ethers and vinyl sulfides cannot be cyclo-propanated by this method. In order to suppress cyclodimer formation from the alkene and its radical cation, the diazo ester is sometimes applied in a four- to fivefold amount with respect to the alkene. 

Hirobe and coworkers were the first to use an unusual class of oxidants, 2,6-disubstituted pyridine iV-oxides, in conjunction with ruthenium porphyrins for oxidation of olefins (Scheme 1.14), alcohols, and sulfides . 

A convenient and stereoselective, synthetic route was reported for the preparation of 3,7-disubstituted-l,2,5-trithiepanes 209 by the reaction of thiiranes with bis(trimethylsilyl)sulfide (HMDST) followed by intramolecular oxidative cyclization (14HAC678). In the reaction of substituted etha-nolamines with formaldehyde, the desired bis(oxazolidin-3-yl)methanes were prepared as a mixture of products, the ratio of the isomeric 3,8-dioxa-l,6-diazabicyclo[4.4.1]undecane 210 product present was dependent on the substituent on the ethanolamine (14CHE726). Whether the bis(oxazolidin-3-yl)methanes were isolated or the mixtures were taken on for further reaction, the subsequent product was formed exclusively. 

The sluggishness of direct oxidation under the standard conditions left room to set up a catalytic process. Because of the various equilibria involved, many titanium species are potential catalysts. After many variations in experimental conditions, it was found that cumene hydroperoxide allowed for catalytic conditions [24]. Table 6C.5 shows typical results for the enantiose-lective oxidation of methyl p-tolyl sulfide. The enantioselectivity remains high (8S% ee) until the amount of titanium complex is 0.2 mol equiv. A substantial decrease in enantioselectivity starts fi om 0.1 mol equiv. or below, although the chemical yield remains good. Obviously undesired catalytic species are quite active at low catalyst concentrations. It is curious that the addition of molecular sieves (pellets) helps to maintain good enantioselectivity, perhaps by efficient regulation of the amount of water. For preparative work at the 5-20 mmol scale, it is very convenient to use 0.5 mol equiv. of the titanium complex. A detailed procedure has been published [25]. It has been applied, for example, to the oxidation of 2,2-disubstituted 1,3-dithianes [29]. 

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