Dimethylchlorosulfonium chlorid

Another method of aetivation is known as the Swern oxidation. Under these conditions a reactive dimethylchlorosulfonium chloride is formed from the reaction of dimethyl sulfoxide and oxalyl chloride (Scheme 2.28b). This then reacts with an alcohol to give an alkoxysulfonium salt. In the presence of a base (triethylamine) this salt fragments with the formation of a carbonyl compound (Scheme 2.28c). 

The mechanism of the Swem oxidation has been studied in depth and the formation of an initial adduct 7 from the reaction between dimethyl sulfoxide and oxalyl chloride which then collapses to give a dimethylchlorosulfonium species 8 is clearly indicated by mechanistic studies.3,8 Reaction of 8 with an alcohol then produces the alkyoxysulfonium ion 9 which upon treatment with an amine base gives the ylide 10. Subsequent proton extraction gives the carbonyl product 2 with the release of dimethyl sulfide. 

To understand how dimethyl sulfoxide and oxalyl chloride react to form the dimethylchlorosulfonium ion, see Problem 64. 

Oxalyl chloride reacts with dimethyl sulfoxide at low temperatures to initially form adduct 1790a, which collapses to a dimethylchlorosulfonium species 1790b. Reaction of 1790b with an alcohol at 78 °C produces the alkoxysulfonium ion 1791, which is converted into the product by reaction with an amine base to give yhde 1792, which further reacts intramolecularly to give the carbonyl product. 

The Swem oxidation oxidizes primary alcohols to aldehydes and secondary alcohols to ketones using dimethyl sulfoxide and oxalyl chloride, followed by reaction with triethylamine. The actual oxidizing agent is the dimethylchlorosulfonium ion, (CH3)2SC1. (To see how this compound is formed from the reactants, see Problem 77 in Chapter 17.) Propose a mechanism for the oxidation. (Hint the first step is an Sn2 reaction, the last step is an E2 reaction.)... 

Propose a mechanism to explain how dimethyl sulfoxide and oxalyl chloride react to form the dimethylchlorosulfonium ion used as the oxidizing agent in the Swern oxidation, (see Chapto" 11, problem 71). 

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