Sulfur-containing groups carbanion stabilization

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... 

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... 

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. 

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. 

In the last decade more chemical attention has been focused on those Nuphar alkaloids which contain sulfur. The chemical behavior of this group is very much dependent on the presence of sulfur. Sulfur introduces additional steric hindrance to the molecule, creates a new nucleophilic center, and increases the stability of carbanions in sulfoxides the C—S bond in sulfonium salts is strongly polarized. 

Quite a number of other electron withdrawing groups containing multiple heteroatomic bonds, such as the ester carbonyl, nitrile, carboxamide, etc., can likewise stabilize carbanions. The lithium salt of tert-butyl acetate 32 is an example of an enolate anion sufficiently stable as a salt to be used as a shelf reagent. Substituents containing easily polarizable atoms, such as sulfur or selenium, are also capable of stabilizing an adjacent anionic center. 

Similar to the sulfoxides, sulfoximides are able to stabilize carbanions and transfer chirality from sulfur to carbon. They have, therefore, found many applications as chiral reagents. To obtain chiral sulfoximides, one may start either with a chiral sulfoxide, which is treated with azide [e.g., tosyl azide under copper catalysis, e.g., formation of (/ )-1820], or with a racemic sulfoximide, which often can be resolved with a chiral acid, in particular (lS,4/ )-10-camphorsulfonic acid, e.g., formation of (S )- )20. The reagents used almost exclusively contain a methyl and a phenyl group at the stereogenic sulfur which allows easy modification employing the acidity of the methyl group. 

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