1.3- Dithianes sulfides from

Dithioacetals of aldehydes are sources of carbanions and hence may be used to form new C-C bonds in reactions in which the formerly electron-deficient character of the aldehydic carbon has been reversed. The 1,3-dithianes derived from formaldehyde or a higher aldehyde may be metallated and then alkylated (Scheme 2.27). Hydrolysis of the dithioac-etal is usually carried out in the presence of a thiophilic (sulfur seeking) metal salt such as a mercury salt. The insoluble sulfides cause the equilibrium to move in favour of the parent carbonyl compound. 

Crown thioether chemistry dates from 1886, when Mansfeld reported the synthesis of 9S3 [40]. To determine whether ring sizes greater than six could be prepared ( ) he allowed ethylene bromide to react with sodium sulfide. From this reaction he isolated a product that differed in properties from p-dithiane he suggested it might be 9S3 (Table 4). A similar procedure with 1,3-dibromopropane led to a compound tentatively proposed to be 12S3. 

Oxathiane and -dithiane are formed from ethylene oxide and hydrogen sulfide at 200°C in the presence of an aluminum oxide catalyst (65). 

In 1863 Husemann prepared an intermediate, to which he assigned the formula C2H4S, by the action of sodium sulfide on ethylene bromide. From it he obtained the cyclic dimer, dithiane, by distillation. Mans-feld (1886) reinvestigated the intermediate and concluded that it is a polymer. As a reminder of the significance of the term polymer at that time it is to be noted, however, that Mansfeld suggested the cyclic trimeric formula for the intermediate, which is now known to be a linear polymer. Other polymers prepared similarly by Husemann (1863) include methylene sulfide (—CH2—S—)a and methylene tri-thiocarbonate (—CH2—S—CS—S—) . Neither was recognized as a polymer, and neither has since been investigated from this standpoint. 

The cationic polymerization of styrene sulfide has been reinvestigated by Van Craeynest (15). With triethyloxonium tetrafluoroborate as initiator, a rapid and quantitative polymerization was observed, followed by a slow degradation of die polymer to a mixture of cis and tram 2,5-diphenyl-l, 4-dithiane and as and tram 2,6-diphenyl-1,4-dithiane. Since the BF4 counter ion is not capable of forming a covalent bond, a back-biting reaction via sulfonium ions seems the plausible mechanism for the dimer formation. The polymerization initiated with dimethyl sulfate showed the same characteristics a fast polymerization is followed by degradation to the same mixture of isomeric diphenyl- 1,4-dithianes. However, the mwts-2,5-diphenyl derivative was the only isomer that crystallized from the solution. It is therefore reasonable to accept that with dimethyl sulfate also, the cyclic dimers of styrene sulfide are formed by a back-biting type of degradation of the polymer and not by the mechanism shown above. 

Another advantage over other Lewis acids is the thiophilic nature of copper this has led to its utility in the hydrolysis of thioacetals. Copper(II) chloride in conjunction with copper(II) oxide was introduced by Mukaiyama [16] for the deprotection of 1,3-dithianes and this method has found utility in a variety of synthetic protocols (Sch. 6) [17]. This combination, in which copper oxide plays the role of a buffer to prevent the medium from becoming too acidic, has also found application in the hydrolysis of a-heteroatom substituted and vinyl sulfides [18]. Acetals, which are prone to epimerization under acid-catalyzed hydrolysis conditions (21), can be con-... 

Among organic sulfides, those derived from 1,3-dithiane occupy an important place. The interest in these reagents lies not only in their reactivity with electrophilic substrates but also in the synthetic principles which have been developed from work on these compounds. By masking the aldehyde group by the formation of a dithiane, the carbon atom may participate in nucleophilic additions or substitution reactions and after hydrolysis of the thioacetal, the carbonyl group can then be regenerated (Scheme 1). 

The availability of f j-di-r-butyl disulfide monoxide from selective oxidation catalyzed by vanadyl-88 enables the preparation of chiral r-butyl sulfinamides, sulfoxides, and sulfinimines. Very similar reaction conditions bring about the transformation of various 1,3-dithianes to chiral monoxides. (/ ,Rl-2,2,5,5-Tetramethyl-3,4-hexanediol is a chiral ligand for the Ti(IV)-catalyzed oxidation of sulfides with cumene hydroperoxide. ... 

In 1995, Bolm and Bienewald introduced a new, very practical method for the asymmetric catalytic oxidation of sulfides [44]. In the presence of vanadium complex prepared in situ from VO(acac)2 and 23 reactions of various sulfides or dithianes fike 24 with aqueous hydrogen peroxide afforded the corresponding sulfoxides with enantiomeric excesses of up to 85% (Eq. 2). Only traces of the corresponding sulfones were observed. The transformation can easily be carried out in open vessels at room temperature using inexpensive H2O2 as oxidant. 

---