Hydrogen-Bonding Catalysis with Sulfonium Salts

Abstract

aAlthough quaternary ammonium and phosphonium salts are known as important catalysts in phase-transfer catalysis, the catalytic ability of tertiary sulfonium salts has not yet been well demonstrated. Herein, we demonstrate the catalytic ability of trialkylsulfonium salts as hydrogen-bonding catalysts on the basis of the characteristic properties of the acidic α hydrogen atoms on alkylsulfonium salts.The importance of onium salt compounds has been established in the field of organic chemistry. 1 Alkyl-ammonium, phosphonium, and sulfonium salts are some of the most important and reliable onium salt reagents in organic synthesis (Fig. 1). These compounds are often utilized as very useful reagents in the construction of organic building blocks, and the reactions using these reagents appear in textbooks of organic chemistry as important, named reactions. 2,3 Furthermore, quaternary ammonium and phosphonium salts are also known as reliable catalysts, which are used to promote a wide variety of organic transformations as phase-transfer and/or base catalysts. 4 Despite the wide synthetic utility of onium salt compounds as reagents and catalysts, the catalytic ability of tertiary sulfonium salts has not yet been demonstrated well in organic synthesis. 5 The limitation of sulfonium salt catalysts has been attributed mainly to the high reactivity and instability of the compounds with acidic α hydrogen atoms. To create new possibilities for sulfonium salts as catalysts, we focused on the hydrogen-bonding abilities of α hydrogen atoms on alkylsulfonium salts when we reported the use of type 1 tetraalkylammonium salts as hydrogen-bonding catalysts on the basis of the characteristic properties of the α hydrogen atoms (Fig. 2). 6,7 Herein, we demonstrate the catalytic ability of trialkylsulfonium salts in hydrogen-bonding catalysis. 8 Fig. 1 Onium salts in organic synthesis.Based on the design of type 1 tetraalkylammonium salts as effective hydrogen-bonding catalysts, 6 we focused on simple type 2 trialkylsulfonium salts (Fig. 2). The structures of the α hydrogen atoms that binded to the iodide anion compared favorably to the X-ray crystal structures of ammonium iodide 1a and sulfonium iodide 2a. 9,10 Furthermore, we expected the acidity of the α hydrogen atoms of 2a to approximate the acidity of 1a, based on the reported pKa values. 11 Fig. 2 Comparison between ammonium iodide 1a and sulfonium iodide 2a.