Acyloxysulfonium salt

The mechanism of the Pummerer reaction can essentially be broken down into four steps (equations 21-24). "" The initial step involves acylation of the sulfoxide oxygen to form an acyloxysulfonium salt (83). In the following step, removal of a proton from the ot-carbon of (83) produces an acylsulfonium ylide (84). This ylide subsequently undergoes concurrent electron reorganization and S—O bond cleavage to give the ion pair (85). In the final step, condensation of acetate ion with the sulfur-stabilized car-bocation gives the a-acyloxy sulflde product (86). 

The additive Pummerer reaction is a reaction in which displacement of the acyloxy group in the initially formed acyloxysulfonium salt by an internal nucleophile occurs to give a new positively charged sulfur species (Scheme 33). A number of pathways are open for the subsequent reaction of intermediate (138), including elimination of the proton a to the sulfur to give a thionium ion (pathway a), displacement of the sulfur from the a -carbon by an external nucleophile (pathway b), elimination of a 3-proton with formation of an alkene and (139) (pathway c) and loss of a proton fimm the carbon atom a to Nu with formation of (140 pathway d). 

The mechanism of the Pummerer rearrangement consists of four steps 1) acylation of the sulfoxide oxygen to form an acyloxysulfonium salt 2) loss of a proton from the a-carbon to afford an acylsulfonium ylide 3) cleavage of the sulfur-oxygen bond to give sulfur-substituted carbocation (RDS) and 4) capture of the nucleophile by the carbocation. 

Reduction of sulfoxides to sulfides. This reduction can be conducted at 0° by addition of TFAA to an acetone solution of a sulfoxide and sodium iodide. Yields are 90 987o- The reduction involves acylation of the sulfoxide to form an acyloxysulfonium salt, which is converted into a sulfide by iodide anion (equation I). ... 

Oxidation. The oxidation of primary and secondary alcohols to the corresponding carbonyl compounds can be achieved using dimethyl sulfoxide-acetic anhydride. The reaction proceeds through an acyloxysulfonium salt as the oxidizing agent (eq 12). 

Dimethyl Sulfoxide-Acetic Anhydride The reaction proceeds through an acyloxysulfonium salt as the oxidizing agent (eq 12). 

Electrophilic Aromatic Substitution. When activated with electrophilic reagents, such as trifluoroacetic anhydride (TFAA), diphenyl sulfoxide is converted into an acyloxysulfonium ion, which can effect electrophilic aromatic substitution. When this mode of sulfoxide activation is performed in the presence of simple aromatic substrates, such as o-xylene, the corresponding triarylsulfonium salt is formed in good yield (eq 4). The Ph2SO TFAA reagent combination reacts similarly with more complex substrates, such as indoles, affording the 3-indolylsulf-onium salts. 

---