Reaction of Sulfoxides with Acylating Reagents. III. Mechanism of the Reactions of Phenyl Methyl Sulf oxide with Acetic Anhydride

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Abstract
The Pummerer reaction, viz., the reaction of sulfoxides having at least one methyl or methylene group with carboxylic acid anhydride gives α-acyloxy derivatives of the corresponding sulfide. Earlier, we suggested on the basis of 18O tracer experiments with 18O-labeled acetic anhydride that the Pummerer reaction of dimethyl sulfoxide with acetic anhydride proceeds through an intermolecular nucleophilic attack of acetoxy group on the methylene carbon of the ylide-ylene intermediate. Noticeable acetoxy interchange has been found to take place during a similar Pummerer reaction of aryl methyl sulfoxide. However, the Pummerer reaction is the main path when the reaction with acetic anhydride is performed at around 120°C. In the Pummerer reaction of aryl methyl sulfoxides, the substituent effect is large (ρ=−1.6) and the kinetic isotope effect (kH/kD) is also substantial, i.e., 2.9. The reaction is assumed to involve the initial acetylation and the subsequent slow proton removal steps. Addition of sodium perchlorate or acetic acid accelerates the acetoxy exchange reaction rather than the Pummerer reaction.

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