Methanesulfonic anhydride
# Methanesulfonic anhydride

Methanesulfonic anhydride $(Ms_2O)$ is the acid anhydride of methanesulfonic acid. Like methanesulfonyl chloride (MsCl), it may be used to generate mesylates (methanesulfonyl esters).

## Preparation & purification

$Ms_2O$ may be prepared by the dehydration of methanesulfonic acid with phosphorus pentoxide.

    $P_2O_5$ + 6 $CH_3SO_3H$ → 3 $(CH_3SO_2)_2O$ + 2 $H_3PO_4$

$Ms_2O$ can be purified by distillation under vacuum (distillation of a solid) or by recrystallization from Methyl tert-butyl ether/toluene.

## Reactions & Applications in synthesis

Passage of hydrogen chloride through molten $Ms_2O$ yields MsCl.

Similar to MsCl, $Ms_2O$ can perform mesylation of alcohols to form sulfonates. Use of $Ms_2O$ avoids the alkyl chloride, which often appears as a side-product when MsCl is used. Unlike MsCl, $Ms_2O$ may not be suitable for mesylation of the unsaturated alcohols.

Examples of mesylation of alcohols with $Ms_2O:$

* Octadecyl methanesulfonate was prepared from octadecanol in pyridine.
* Secondary alcohol at the anomeric carbon of 2,3,4,5-O-Benzyl-protected glucose reacted to form a glycosyl mesylate, which was found to be more stable than its triflate counterpart, in 2,4,6-collidine.

$Ms_2O$ also converts amines to sulfonamides.

### Aromatic sulfonation

Assisted by Lewis acid catalyst, Friedel-Crafts methylsulfonation of aryl ring can be achieved by $Ms_2O.$ In contrast to MsCl, either activated or deactivated benzene derivatives can form the corresponding sulfonatesin satisfactory yields with $Ms_2O.$

Examples of aromatic sulfonation with $Ms_2O:$

* Sulfonation of chlorobenzene resulted in addition of methylsulfonyl group at para and ortho positions (with respect to chloride), with a ratio of 2 to 1, respectively; while reaction with Meta-dichlorobenzene gave monosulfonylated product at C4 position.
* With sulfuric acid, di-aryl sulfones were synthesized.

### Esterification

$Ms_2O$ 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 $Ms_2O.$ Both alcohols on ethylene glycol successfully benzoylated with benzoic acid and $Ms_2O.$ However, for free alcohols on monosaccharides, the acetylation was not completed.

### Oxidation of alcohols

Like Pfitzner–Moffatt oxidation and Swern oxidation, with DMSO, $Ms_2O$ 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.

