Methanesulfonic anhydride (Ms<sub>2</sub>O) is the acid anhydride of methanesulfonic acid. Like methanesulfonyl chloride (MsCl), it may be used to generate mesylates (methanesulfonyl esters).
Ms<sub>2</sub>O may be prepared by the dehydration of methanesulfonic acid with phosphorus pentoxide.
Ms<sub>2</sub>O can be purified by distillation under vacuum (distillation of a solid) or by recrystallization from Methyl tert-butyl ether/toluene.
Passage of hydrogen chloride through molten Ms<sub>2</sub>O yields MsCl.
Similar to MsCl, Ms<sub>2</sub>O can perform mesylation of alcohols to form sulfonates. Use of Ms<sub>2</sub>O avoids the alkyl chloride, which often appears as a side-product when MsCl is used. Unlike MsCl, Ms<sub>2</sub>O may not be suitable for mesylation of the unsaturated alcohols.
Examples of mesylation of alcohols with Ms<sub>2</sub>O:
Ms<sub>2</sub>O also converts amines to sulfonamides.
Assisted by Lewis acid catalyst, Friedel-Crafts methylsulfonation of aryl ring can be achieved by Ms<sub>2</sub>O. In contrast to MsCl, either activated or deactivated benzene derivatives can form the corresponding sulfonatesin satisfactory yields with Ms<sub>2</sub>O.
Examples of aromatic sulfonation with Ms<sub>2</sub>O:
Ms<sub>2</sub>O catalyzes the esterification of alcohols by carboxylic acids. 2-Naphthyl acetate was prepared from 2-naphthol and glacial (anhydrous) acetic acid in the presence of Ms<sub>2</sub>O. Both alcohols on ethylene glycol successfully benzoylated with benzoic acid and Ms<sub>2</sub>O. However, for free alcohols on monosaccharides, the acetylation was not completed.
Like PfitznerâÂÂMoffatt oxidation and Swern oxidation, with DMSO, Ms<sub>2</sub>O can oxidize primary and secondary alcohols to aldehydes and ketones, respectively, in HMPA. This method applies to benzylic alcohol. HMPA may be substituted by dichloromethane but may result in more side-products.