Reaction of Sulfoxides with Acylating Reagents. IV. The Catalytic Effect of Both the Brönsted and Lewis Acids in the Oxygen Exchange Reaction of Diaryl Sulfoxides with Acetic Anhydride

Diaryl sulfoxides undergo oxygen exchange and racemization reaction with acetic anhydide. The rate of racemization of p-tolyl phenyl sulfoxide was found to be twice that of oxygen exchange. The remarkably small substituents effect is also in keeping with the SN2 type mechanism. Addition of acetic acid increases the rate of both the racemization and oxygen exchange. Racemization of sulfoxides is also known to be catalyzed markedly by a small amount of Lewis acid. When the reaction was carried out in the presence of a small amount of both acetic acid and Lewis acid, the ratio of the rate constants, kex⁄krac becomes unity. This may mean that the presence of both Brönsted and Lewis acids facilitates the cleavage of S–O bond by easy protonation (or hydrogen-bonding) or coordination on the sulfoxide oxygen, and the assistance of the nucleophilic attack by acetate becomes less important in the reaction. The larger substituent effect is also in keeping with the SN1 type mechanism.