The Swern oxidation, discovered by Daniel Swern, is a highly selective and mild method for the oxidation of primary or secondary alcohols to aldehydes or ketones, respectively. The organic reaction is carried out with oxalyl chloride, dimethylsulfoxide and triethylamine.
Major advantages of this mild method are the high selectivity: Unlike other oxidations (such as Jones oxidation), an aldehyde is not further oxidized to a carboxylic acid. The reaction temperature is between -78 and 20 ° C. An additional advantage over the chromium (IV) compounds normally used for oxidations is the low toxicity of the reagents. Response mechanism
The first step of the Swern oxidation is to activate dimethylsulfoxide by reacting one of both resonant structures (1b) with oxalyl chloride (2) to a sulfoxonium intermediate (3). This intermediate rapidly decomposes into carbon dioxide and carbon monoxide, thereby forming the dimethyl chlorosulfonium chloride (4), the activated dimethyl sulfoxide.
Next, the alcohol (5) is added to this dimethylchlorosulfonium chloride to give an intermediate alkoxysulfonium compound (6). Addition of at least 2 equivalents of an organic base (usually triethylamine) produces a sulfurylidene (7). This ylid can be decomposed through a cyclic transition state to dimethylsulfide (a gas) and the final carbonyl compound (8). Also see
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