Mesylat

Mesylat, or mesylate, is the term used for compounds containing the methanesulfonate group, derived from methanesulfonic acid (CH3SO3H). In practice, it refers to the methanesulfonate anion (CH3SO3−) in salts and to esters formed with alcohols (alkyl mesylates, R–O–SO2CH3). The mesylate moiety is widely employed in organic synthesis as a versatile leaving group.

The methanesulfonate moiety is highly electron-withdrawing, and methanesulfonic acid is a strong, non-volatile acid with a pKa around −1.9. The resulting mesylate anion is a stable leaving group, which makes mesylates valuable for nucleophilic substitution and elimination reactions. As esters, mesylates are formed when alcohols are converted to alkyl mesylates, enabling subsequent transformations.

Preparation of alkyl mesylates is typically accomplished by reacting an alcohol with methanesulfonyl chloride (MsCl) in the presence of a base such as triethylamine or pyridine, yielding the corresponding alkyl mesylate and a salt of HCl. Metal mesylates (e.g., sodium mesylate) can be prepared by neutralizing methanesulfonic acid with a suitable metal base or by metathesis.

Applications of mesylates center on their role as leaving groups in organic synthesis. Alkyl mesylates readily undergo SN2 substitutions and, under suitable conditions, elimination to form alkenes. They are often preferred over other leaving groups for certain substrates due to their relative stability and reactivity. Mesylate esters are also used as intermediates in the preparation of pharmaceuticals and other fine chemicals.

Safety considerations include the handling of methanesulfonyl chloride and methanesulfonic acid, both of which are corrosive and should be used with appropriate personal protective equipment and in well-ventilated areas.