Halosulfonium salts

Reaction at the C atom of nitronate salts is known with a variety of electrophiles, such as aldehydes (Henry reaction) and epoxides (191-193). Thus the incorporation of the nitro moiety and the cyclization event can be combined into a tandem sequence. Addition of the potassium salt of dinitromethane to an a-haloaldehyde affords a nitro aldol product that can then undergo intramolecular O-alkylation to provide the cyclic nitronate (208, Eq. 2.17) (59). This process also has been expanded to a-nitroacetates and unfunctionalized nitroalkanes. Other electrophiles include functionalized a-haloaldehydes (194,195), a-epoxyaldehydes (196), a-haloenones (60), and a-halosulfonium salts (197), (Chart 2.2). In the case of unsubstituted enones, it is reported that the intermediate nitronate salt can undergo formation of a hemiacetal, which can be acetylated in moderate yield (198). 

The reaction of sulfides bearing a-methylene hydrogens with halogenating agents, to form a-halo sulfides, can be classified as a Pummerer reaction of halosulfonium salts. With chlorine and bromine it is sometimes difficult to limit the reaction to monosubstitution. However, monosubstituted products are produced in high yields using N-halosuccinimides, sulfuryl and thionyl chloride and cyclic phosphoms... 

Halosulfonium salts are unstable in the absence of Lewis acids. The instability arises from their liability to nucleophilic attack by the counterion, which results in bond cleavage (Scheme 8). 

Barton et al. (134) achieved the alcohol oxidation by converting the substrates into the chloroformate esters which are capable of acylating DMSO. Subsequently, Corey and Kim (135, 136) reported a much less laborious method utilizing halosulfonium salts derived from dimethyl sulfide. The reaction of these highly electrophilic species with alcohols produces the same dimethylsulfoxonium intermediates. 

Annibale and coworkers have reported a kinetic study of the reduction of Au(III) to Au(I) by various dialkyl sulfides. In their evaluation of the reactions, the authors concluded (0 The first step of the interaction establishes the substitution equilibria (h) The second step involves transfer of a chlorine atom from Au(III) to free sulfide. This leads to a digonal gold(l) complex and a halosulfonium salt, [R2SC1]C1, as the primary reaction products. Scheme 9.5 ... 

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