Sulfonium

Sulfonium refers to a class of organosulfur cations of the form [R3S]+, in which a sulfur atom is bonded to three carbon substituents and carries a positive charge. These ions are typically isolated as salts, such as trimethylsulfonium tetrafluoroborate (Me3S+ BF4−). In many cases the sulfonium center is described as having a tetrahedral geometry.

Preparation and occurrence

Sulfonium salts are commonly prepared by alkylation of a sulfide (thioether) with an alkyl halide or related electrophile, for example R2S + R'X → [R3S]+X−. They can also arise in reactions where sulfides react with sulfonyl chlorides or other sulfonylating agents. In organic synthesis, sulfonium ions are used as valuable intermediates because the attached carbon substituents can be transferred in subsequent reactions.

Reactivity and applications

The sulfonium cation is a strong electrophile at sulfur and readily engages in S-alkylation and related transformations, making sulfonium salts important alkylating agents in chemical synthesis. Deprotonation at an α-carbon of a sulfonium salt can generate sulfonium ylides, which are key reagents in reactions such as the Corey–Chaykovsky epoxidation and cyclopropanation. Sulfonium salts also serve as photoinitiators for cationic polymerization and are employed in various polymer and materials chemistry applications. In addition, sulfonium-based ionic liquids and phase-transfer catalysts are studied for their distinctive solubility and reactivity profiles.

Types and examples

Common examples include trialkylsulfonium and triarylsulfonium salts, each with different counteranions (e.g., BF4−, PF6−). The choice of substituents and counterion influences stability, solubility, and reactivity, tailoring sulfonium reagents to specific synthetic or materials applications.

See also

Sulfoxonium and other onium ions; sulfonium ylides; photoinitiators for polymerization.