Oxidation of Alcohols to Carbonyl Compounds via Alkoxysulfonium Ylides: The Moffatt, Swern, and Related Oxidations

Tidwell, Thomas T.


The use of dimethyl sulfoxide as an oxidizing agent began with the discoveries by Kornblum and co-workers that certain α-bromo ketones were converted into glyoxals under mild conditions by treatment with dimethyl sulfoxide, and that primary tosylates such as n-octyl tosylate were converted into the corresponding aldehydes using dimethyl sulfoxide and sodium bicarbonate at 150° for 3 minutes. The initial step of the reactions involves a displacement by dimethyl sulfoxide giving an alkoxysulfonium ion and this species undergoes a 1,2 elimination assisted by base to give the carbonyl product

A few years later it was discovered that alcohols were oxidized at room temperature to carbonyl compounds by dimethyl sulfoxide, dicyclohexylcarbodiimide (DCC), and phosphoric acid. This reaction was immediately recognized as an effective and mild procedure for sensitive substrates, and the extensive studies by this group and the development of alternative variations elsewhere have been the subject of several earlier reviews.

The course of the Moffatt procedure is summarized. The key intermediate in this process is the oxysulfonium ylide, which appears to be common to all of the variations of dimethyl sulfoxide oxidations, and which reacts intramolecularly to give the products.

Other related procedures that were soon developed utilize dimethyl sulfoxide activated by acetic anhydride, phosphorus pentoxide, sulfur trioxide/pyridine complex, and chlorine. Reaction of alcohols with phosgene to give chloroformates which react with dimethyl sulfoxide to give alkoxysulfonium ions, followed by reaction with triethylamine, also effects oxidation.

The reaction of the complexes of dimethyl sulfide and chlorine or N-chlorosuccinimide (NCS) with alcohols is proposed to give the same alkoxysulfonium complexes, which are efficiently converted into carbonyl products upon addition of triethylamine. Electrochemical activation of sulfides has also been successfully utilized.

The activation of dimethyl sulfoxide is effected by many reagents, and these reactions as well as those involving activated sulfides evidently involve the alkoxysulfonium ion and the decisive oxidation step which occurs via the alkoxysulfonium ylide in all the reactions. Activation of dimethyl sulfoxide by oxalyl chloride, has become the most used of these oxidation procedures, but several of the other methods are also convenient and efficient.