General Procedure for Oxidation of Alcohols by Pfitzner-Moffatt Method

General Procedure for Oxidation of Alcohols by Pfitzner-Moffatt Method 

Three equivalents4 of a carbodiimideb are added over a solution of 1 equivalent of the alcohol and 0.5 equivalents of pyridinium trifluoroacetate0 in 0.6-40 mL of neat dry DMSO (MW = 78.1, d = 1.10), or a mixture of DMSO and benzened, at room temperature.6 When most of the starting compound has been consumed/ the work-up can be made according to the following alternatives  

The solvent is removed at the rotary evaporator, and the resulting residue is purified by chromatography. It can be advisable to filter the precipitate of N, iV -dicyclohexylurea8—formed when DCC is used—before removing the solvent. In order to avoid interferences from unreacting carbodii-mide, it can be advisable to transform it in the corresponding urea by careful addition of oxalic acid either solid or in a solution in methanol—to the stirred reaction mixture. Addition of oxalic acid produces a copious evolution of gas that signals the duration of the hydrolysis of the carbodiimide. 

0 Very often more than 0.5 equivalents of pyridinium trifluoroacetate (MW — 191.1) are added. This practice is not advisable, as it can lead to a substantial decrease in the yield of the aldehyde or ketone. For instance, during the oxidation of testosterone (14), Moffatt el al. found that on changing from 0.5 to 2.0 equivalents of pyridinium trifluoroacetate, a decrease of ca. 20% occurs.14b On the other hand, the quantity of pyridinium trifluoroacetate can be diminished to 0.1 equivalents with no erosion of the yield, although leading to a slower reaction. 

Pyridinium trifluoroacetate can either be added as such, or formed in situ by the addition of pyridine (MW — 79.1, d — 0.98) and trifluoroacetic acid (MW — 114.0, d = 1.48). Very often pyridine is added in an excess of ca. 0.5-2 equivalents relative to trifluoroacetic acid for buffering purposes. 

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