Tosylate

Tosylate refers to the p-toluenesulfonate group, typically encountered as either the tosylate ester of an alcohol (R-OTs) or as the p-toluenesulfonate anion (OTs−). The term is most often used in the context of organic synthesis, where tosylates serve as highly effective leaving groups. The tosylate ester is formed when an alcohol reacts with p-toluenesulfonyl chloride (TsCl) in the presence of a base, producing R-OTs and a chloride salt byproduct. The corresponding counterion, p-toluenesulfonate, is a stable sulfonate.

Tosylates are valued because the sulfonate group stabilizes negative charge and greatly enhances the ability of an attached group to depart in substitution reactions. As a result, tosylates are common intermediates for converting poor leaving groups (usually alcohol OH) into good leaving groups, enabling SN1 or SN2 transformations. They are also used to enable selective substitutions, including inversion of stereochemistry at secondary centers by SN2 displacement with suitable nucleophiles.

In practice, tosylation is a standard step in synthetic sequences. Tosylates may be displaced by a wide range of nucleophiles (e.g., halides, thiolates, amines) to furnish diverse products. The tosylate group itself can be removed later if needed, or retained as part of a larger molecule where appropriate.

See also: sulfonate esters, p-toluenesulfonyl chloride, tosylate salts, leaving groups in organic synthesis.