Mesylate esters are a group of organic compounds that share a common functional group with the general structure CH3SO2O-R where R is an organic substituent. Mesylate groups are considered excellent leaving groups in nucleophilic substitution reactions.
Mesyl is the term for the methylsulfonyl or CH3SO2- functional group.
Mesylate esters may be prepared using an alcohol and methanesulfonyl chloride in the presence of a base, often amine bases such as triethylamine. The precise mechanism for this reaction has not been fully determined; either direct nucleophilic substitution at sulfur, addition-elimination in analogous fashion to attack at a carbonyl, or base-mediated elimination-addition via a sulfur ylide are all possible. It appears that the mechanism taken is dependent on the substrate alcohol and reaction conditions.
Alternatively, an alcohol, methanesulfonic anhydride and base may be used to prepare mesylates.