Uses of sulfoxides

Most of the applications of sulfoxides in synthesis make use of the reactions of sulfur-stabilized carbanions with electrophiles [385,386]. Thus the methylsulfinyl methylene carbanion, conveniently generated through the interaction of sodium hydride with DMSO [387], is a powerlul nucleophile. 

It is also a strong base and an excellent reagent for the generation of ylides (Wittig and related reagents, for example). It reacts with esters to yield anions of p-ketosulfoxides. As the methylsulfinyl group of these compounds is easily removed by an aluminum amalgam, a useful synthesis of ketones is at hand. 

Many other uses of a-sulfmyl carbanions are found in the literature, and in the recent past the trend has been to take advantage of the chirality of the sulfoxide group in asymmetric synthesis. Various ways of preparation of enantiopure sulfoxides have been devised (see Section 2.6.2) the carbanions derived from these compounds were added to carbonyl compounds, nitriles, imines or Michael acceptors to yield, ultimately, with high e.e. values, optically active alcohols, amines, ethers, epoxides, lactones, after elimination at an appropriate stage of the sulfoxide group. Such an elimination could be achieved by pyrolysis, Raney nickel or nickel boride desulfurization, reduction, or displacement of the C-S bond, as in the lactone synthesis reported by Casey [388], 

Natural products were often the target of such asymmetric bond formation mediated by optically active sulfoxides [86-95,389]. 

The experimental procedures given below for the synthesis of both enantiomers of 4-substituted butenolides [390] emphasize some aspects of the reactivity of chiral p-ketosulfoxides their reduction with hydrides can 

---

The patent literature contains several references to the use of sulfoxide complexes, usually generated in situ, as catalyst precursors in oligomerization and polymerization reactions. Thus, a system based upon bis(acrylonitrile)nickel(0> with added Me2SO or EtgSO is an effective cyclotrimerization catalyst for the conversion of butadiene to cyclo-1,5,-9-dodecatriene (44). A similar system based on titanium has also been reported (407). Nickel(II) sulfoxide complexes, again generated in situ, have been patented as catalyst precursors for the dimerization of pro-pene (151) and the higher olefins (152) in the presence of added alkyl aluminum compounds. 

The use of sulfoxides in the separation of zirconium and hafnium has met with some success, and diheptyl sulfoxide has been patented as an extractant for this purpose (185). Mixtures of sulfoxides have also been used in the extraction of zirconium and hafnium from acid solutions (463). 

Studies on the use of sulfoxides as extractants for these metals have been reported (75, 457). 

Studies on the use of sulfoxides in the extraction of scandium (325, 440), yttrium (188), and the lanthanide series (456) have all been reported and seem to show considerable promise. 

The use of sulfoxides as chiral synthons has, over recent years, become a highly dependable protocol in synthetic organic chemistry. To some extent, however, the use of sulfoxides in asymmetric synthesis has been limited by the lack of a reliable and general method for their preparation in optically pure form. In this review we present the development of chiral sulfoxide synthesis via nucleophilic displacement at sulfur from the pioneering work of Andersen in 1962 to more recent methods. Sulfoxides have become associated with many diverse areas of synthetic chemistry indeed, their ability to act as a handle for the stereoselective generation of chirality at proximate centres has attracted much research worldwide. 

The use of sulfoxides as leaving groups in glycoside synthesis is less common, but compound 92 has been employed to couple with a secondary alcohol in the synthesis of hikizimycin (See Chapter 20). 

Chiral sulfoxides serve as important building blocks in many areas of organic synthesis [43, 44]. An apphcation is the use of sulfoxides as directing groups in the reduction of carbonyl functional groups [45]. Diastereomeric ketosulfoxides 101 and 105 were prepared by reacting cyclohexanone with (S)-menthyl-p-toluenesulfinate (Scheme 3.23). 

As might be expected from the results discussed in the previous paragraphs, coupling reactions between secondary a-sulfonyl carbanions and ketones are rarely encountered in the literature [12, 51, 57]. Even the use of sulfoxides proved to be of no avail [52, 53]. It is thus even more remarkable that the method reported by Falck and Mioskowsky affords tetrasubstituted yS-hydro-sulfones in excellent yields [56]. It is quite possible that the unique properties of the Sm " salt, which acts as a... 

---