mesylates

Mesylates are salts and esters derived from methanesulfonic acid (MsOH). In organic chemistry the term most often refers to alkyl mesylates, R–O–SO2–CH3, which are prepared by treating alcohols with methanesulfonyl chloride (MsCl) in the presence of a base to give alkyl methanesulfonate esters (R–OMs). The corresponding mesylate salts contain the mesylate anion CH3SO3− paired with a cation such as Na+, K+, or a quaternary ammonium.

Alkyl mesylates are among the best leaving groups available for nucleophilic substitution. The mesylate group is relatively small and highly electron-withdrawing, which promotes SN1 and SN2 reactions. This combination makes RO–Ms a common activation step for converting poor leaving groups such as alcohols into reactive substrates for carbon– or heteroatom–nucleophile attack. Mesylates are typically prepared under anhydrous conditions and can be hydrolyzed back to the alcohol under aqueous workups.

In practice, mesylates are used to promote substitutions with a wide range of nucleophiles, including thiols, amines, organometallics, and halides. They are often preferred over other sulfonate esters when higher reactivity is desired, though their high reactivity also requires careful handling and dry conditions.

Safety and handling: Methanesulfonyl chloride is a corrosive reagent; methanesulfonic acid is a strong, hygroscopic acid. Appropriate protective equipment and ventilation are recommended when working with mesylates and their reagents.