Carbanions sulfur-containing

This chapter describes the chemical behavior of a-sulfinyl and a-sulfonyl carbanions. The stereoelectronic effects of these sulfur-containing groups have been the subject of much controversy for more than a decade which has now gradually settled down. Meanwhile, the special features of the chemical behavior of these groups have been utilized for syntheses of thousands of useful organic substances. This chapter deals with the... 

Sulfur-containing groups at an a-position stabilize carbanions. All these species, i.e., the a-sulfonyl 6281-102, a-sulfmyl 6346-80, a-sulfenyl 645,103,104 and a-sulfonio carbanions 65105-1 ancj those derived from sulfonates 66 and sulfonamides 6795 may retain their... 

For reviews of sulfur-containing carbanions, see Oae, S. Uchida, Y. in Patai Rappoport Stirling The Chemistry ofSulphones and Sulphoxides, Wiley NY, 1988, p. 583 Wolfe, S. in Bemardi Csizmadia Mangini Organic Sulfur Chemistry Elsevier, NY, 1985, p. 133 Block, E. Reactions ofOrganosulfur Compounds Academic Press NY, 1978, p. 42 Durst, T. Viau, R. Intra-Sci. Chem. Rep., 1973, 7 (3), 63. For a review of selenium-stabilized carbanions, see Reich, H.J. in Liotta Organoselenium Chemistry Wiley NY, 1987, p. 243. For support for this theory, see Wolfe, S. LaJohn, L.A. Bemardi, F Mangini, A. Tonachini, G. Tetrahedron Lett., 1983, 24, 3789 Wolfe, S. Stolow, A. LaJohn, L.A. Tetrahedron Lett., 1983, 24, 4071. 

So far we have considered sulfur-containing carbanions obtained by ... 

So far we have considered sulfur-containing carbanions obtained by metallation with appropriate bases of a to sulfur atom(s) acidic C-H bonds. For synthetic purposes this method has been by far the most used. Among the variety of other routes to such carbanions [203] we shall consider two methods which seem to be general and promising the reductive lithiation of phenyl thioethers and the thiophilie addition of organometallics to thiocarbonyl compounds (vide infra Section 4.1.2). 

Chiral thiols 59 were derived from 10-camphorsulfonic add by reduction of the sulfony] chloride with lithium aluminum hydride, yielding a 1 4 mixture of enrfo/exo-diastereomers, which can be separated by chromatography60. Containing a hydroxy and a thiol functionality, 59 can be used for the formation of sulfur-containing heterocycles such as oxathianes which can react with carbanions (Section D.l.3.4.4.) or form enolates (Section D. 1.5.3.4.2.1.) and can direct osmium tetroxide catalyzed dihydroxylations (Section D.4.4.). 

Lyases. The lyase class of enzymes consists of a diverse group of enzymes cleaving C-C, C-0, and C-N bonds by means other than hydrolysis or oxidation. Some of the enzymes catalyzing C-C bond cleavage are called aldolases, decarboxylases (when carbon dioxide is released from a substrate), and thiolases (when the sulfur-containing nucleophile of cysteine or CoASH is used to break a carbon-carbon bond) (Fig. 8.21). The structures amenable to carbon-carbon bond cleavage usually require a carbonyl carbon that can act as an electron sink to stabilize the carbanion transiently formed when the carbon-carbon bond breaks. 

A number of phase transfer reactions of sulfur containing substrates have been reported. These include substitution reactions in which sulfur is nucleophile, electrophile or an ancillary function. The synthesis of a variety of thioethers has been reported, as have the alkylation of numerous sulfur stabilized carbanions. In addition, reduction of both disulfides and N-tosylsulfilimines have been conducted under phase transfer catalytic conditions. The unifying theme of this chapter is, therefore, the synthesis and reactions of sulfur containing molecules. Inherent in this approach is some redundancy, but it is hoped that the ability to locate reactions of interest compensates. 

Generation of a-Silyl Carbanions Bearing a Sulfur-Containing Croup... 

Sulfur ylides contain a carbanion, which is stabilizea oy an adjacent positively-charged sulfur. Ylides derived from alkylsulfonium salts are usually generated and utilized at low temperatures. Oxosulfonium ylides are, however, stable near room temperature. The most common method of ylide formation is deprotonation of a sulfonium salt. What has been said... 

Olefin synthesis starts usually from carbonyl compounds and carbanions with relatively electropositive, redox-active substituents mostly containing phosphorus, sulfur, or silicon. The carbanions add to the carbonyl group and the oxy anion attacks the oxidizable atom Y in-tramolecularly. The oxide Y—O" is then eliminated and a new C—C bond is formed. Such reactions take place because the formation of a Y—0 bond is thermodynamically favored and because Y is able to expand its coordination sphere and to raise its oxidation number. 

Stabilization by Sulfur or phosphorus. Attachment to the carbanionic carbon of a sulfur or phosphorus atom causes an increase in carbanion stability, though the reasons for this are in dispute. One theory is that there is overlap of the unshared pair with an empty d orbital" (pn-dn bonding, see p. 45). For example, a carbanion containing the SO2R group would be written... 

Related to these diphosphine dichalcogenides are the triphosphine trisulfides [R2P(S)]3CH (12) which can be prepared from lithiated methylene diphosphine disulfides upon treatment with thiophosphinic chlorides (Equation 9). Deprotonation of 12 with tBuLi gives a resonance-stabilised anion 13 containing a planar central carbanion whose charge is delocalised onto the three neighbouring phosphorus and sulfur atoms.32... 

The results of ab initio calculations are compatible with the conclusion that -SH stabilizes an adjacent carbanion more than —OH363 Furthermore, the above discussion represents a viable alternative to the d-orbital model for explaining the enhanced kinetic and equilibrium acidities of molecules containing sulfur groups. 

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