Sulfoxide systems

Thus, the observed stereoselectivity means the exclusive formation of the anti-isomer (a). This conclusion was confirmed by NMR analysis63 (see Section III.B.4.b) and, clearly, can be extended and generalized with respect to larger cyclic sulfoxide systems. 

Stereochemical constraints in cyclic sulfones and sulfoxides impart increased weight to strain and conformational factors in the generation of carbanions and their stability, causing distinct differences between the behavior of cyclic and open-chain systems233, due primarily to the prevention of extensive rotation about the C —S bond, which is the major way that achiral carbanions racemize. Study of the a-H/D exchange rate fce and the racemization rate ka may provide information concerning the acidity-stereochemical relationships in optically active cyclic sulfone and sulfoxide systems. 

The synthesis takes advantage of the well-documented sulfoxide - sulfenate rearrangement , as well as of its retro-process, leading to cyclization and formation of the desired four-membered ring sulfoxide system (i.e. 211, 212). A closely related ring enlargement is based on the reversibility of this rearrangement and has found wide use in penicillin chemistry . 

The relatively minor alteration in reaction conditions can be seen to drastically alter the nature of the product (468). In addition many sulfoxide complexes are thermally degraded, and in consequence the extent of drying can alter the nature of the product. Thus, the complex [Co(0-Me2SO)8][I]2 is isolated from a cobaltous iodide-dimethyl sulfoxide system, but extensive drying in vacuo causes degradation to yield [Co(0-Me2SO)6][CoI4] (128). 

The use of palladium(II) sulfoxide complexes as catalyst precursors for polymerization has met with mixed results thus a report of a palla-dium(II) chloride-dimethyl sulfoxide system as a catalyst precursor for phenylacetylene polymerization suggests similar results to those obtained using tin chloride as catalyst precursor (421). However, addition of dimethyl sulfoxide to solutions of [NH fPdCh] decreases the activity as a catalyst precursor for the polymerization of butadiene (100). Dimethyl sulfoxide complexes of iron have also been mentioned as catalyst precursors for styrene polymerization (141). 

Oae (251,252) as well as Darwish and Datta (253) investigated the process of thermal racemization of chiral alkylarylsulfimides and diarylsulfimides. It was found to proceed at temperatures as low as 65 to 100°C with a rate constant of the order 1 to 10 X 10" sec" , which corresponds to an activation energy of about 23 to 30 kcal/mol. These data indicate that the thermal racemization of sulfimides is much faster than that of analogous sulfoxide systems. The racemization of sulfimides is a unimolecular reaction practically independent of the polarity of the solvent this property, coupled with the absence of decomposition products, supports the view that racemization of sulfimides occurs by pyramidal inversion. 

The catalytic sulfoxidation system developed by Uemura and coworkers in 1993381,386 consisted of Ti(OPr-i)4, (f )-BINOL, H2O and TBHP in different compositions. The best results (highest ee) were obtained with low amounts of catalyst 0.025 eq. Ti(OPr-i)4, 0.05 eq. (f )-BINOL, 0.5 eq. H2O and 2 eq. TBHP. With this method sulfoxides could be obtained with excellent enantioselectivities (ee 96%), although yields were low (28-44%) due to kinetic resolution of the formed sulfoxides to give the corresponding sulfones. More detailed investigations by Uemura and coworkers showed that an enantiomeric excess of 50% of the sulfoxides is obtained at the initial stage of the reaction and that an increase in ee results the longer the reaction takes place . So the Ti-(i )-BINOL complex catalyzes... 

The use of chiral tertiary phosphine ligands has been studied most widely, but other chiral ligands such as carboxylic acids (15), imines (8,16), amides (17), amines (18), alkoxides (19), and hydroxammates (13) have been investigated, and we reported recently on some sulfoxide systems (29, 21). 

While many enantioselective processes utilizing optically pure acyclic sulfoxide derivatives have been studied extensively among the synthetic community, their cyclic counterparts have, by comparison, received little attention. Cyclic sulfoxide systems which have received the most attention are five- and six-membered ring derivatives, and it is the chemistry of such compounds which forms the subject of this discussion. 

Disulfide Bond Formation Using Silyl Chloride-Sulfoxide System. Akaji, K. Reviews on Heteroatom Chemistry 1997, 76, 85-100. 

For the water-dimethyl sulfoxide system, values of the eight corrected equilibrium constants are all encompassed by a factor of 1.6. The one-parameter equation... 

Allyl sulfoxide systems appropriate for a transformation to allylic alcohols can also be generated by other preparative methods, e.g. by isomerization of aryl vinyl sulfoxides. Such an isomerization occurs in connection with a Knoevenagel reaction of (10) with aldehydes leading finally to y-hydroxy-a,3-unsatu-rated systems (11), which can easily be transformed into a,3-unsaturated y-lactones (12 Scheme 17). ... 

Clean conjugate addition of appropriate nucleophiles to allenic sulfoxides has been used to produce allylic sulfoxide systems. Homer has described the formation of functionalized allylic alcohols when adding nucleophiles like amines, alcohols or thiols to allenic sulfoxides in excess. If the addition is performed with equimolar amounts at lower temperature, so that the 2,3-sigmatropic rearrangement is avoided, the intermediate addition products (enamines or enol ethers) can be hydrolyzed to the synthetically valuable 3-keto sulfoxides (Scheme 20). ... 

E. Guittet and S. JuliS, Synth. Commun., 1981, 11, 723. Recently the high transmission of stereochemistry in the 2,3-sigmatropic rearrangement of allyl sulfoxide systems has been used in an allylic hydroxylation process via an asymmetric ene reaction of N-sulfinylcarbamates J. K. Whitesell, J. F. Carpenter, H. K. Yaser and T. Machajewski, J. Am. Chem. Soc., 1990, 112, 7653. 

Benedict, D. R., Bianchi, T. A., Cate, L. A. Synthesis of simple unsymmetrical ethers from alcohols and alkyl halides or sulfates the potassium hydroxide/dimethyl sulfoxide system. Synthesis 1979,428-429. 

Highest yields are obtained when the molar ratio of dimethyl sulfoxide to organic halide is 4 1. As the ratio is decreased, conversion and yield are both reduced. Kinetic studies under optimal conditions indicate that maximal conversion and yield are obtained in 8-9 hrs. When the reaction is continued beyond this time, the aryl halide reacts wilh the solvent to give hy-pruducls. The optimum temperature for the dimethyl sulfoxide system Is 17.5-18(1". At the reflux temperature (189") the solvent... 

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