Science of Synthesis

Yan, L.; Lin, S.; Liu, P., Science of Synthesis, (2007) 20, 751.

Pertrifluoroacetic acid[‌102‌] is a remarkably efficient reagent for the oxidation of acyclic ketones to the corresponding esters in essentially quantitative yields,[‌103‌] but the addition of a base, sodium monohydrogen phosphate, is necessary to neutralize the trifluoroacetic acid that is formed. Without the base this acid catalyzes the transesterification of the newly formed esters, and this reaction then becomes a major source (5–10%) of trifluoroacetate byproducts. Other bases, such as sodium hydrogen carbonate, can also be used to repress transesterification.[‌104‌] The mild reaction conditions are compatible with many acid-sensitive functional groups. Some illustrations of the use of pertrifluoroacetic acid in Baeyer–Villiger reactions are collected into ‌Scheme 27‌. For example, the sterically congested ketone ‌85‌, which cannot be oxidized using either 3-chloroperoxybenzoic acid or 3,5-dinitroperoxybenzoic acid, is transformed into the corresponding acetate ‌86‌ using pertrifluoroacetic acid.[‌105‌] Pertrifluoroacetic acid also converts the cyclopropyl methyl ketone ‌87‌ stereoselectively into the cyclopropyl acetate ‌88‌ in 77% yield;[‌106‌] note, this ketone does not react successfully with peroxybenzoic acid.[‌107‌] In a similar manner the highly functionalized cyclobutyl methyl ketone ‌89‌ is transformed into the acetate ‌90‌ without triggering a retro-aldol reaction.[‌108‌] Although these reactions are very successful, there is a problem: the pertrifluoroacetic acid specified is generated using concentrated hydrogen peroxide solution (ca. 90%). This reagent is extremely hazardous and is now no longer commercially available. Fortunately, there are alternative procedures in which trifluoroacetic anhydride is added either to a solution containing urea and hydrogen peroxide[‌109‌] or to sodium percarbonate (sodium carbonate and hydrogen peroxide).[‌110‌,‌111‌] Two examples of these modified procedures are also depicted in ‌Scheme 27‌.

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References

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• 8.Meeeee-Meee, (8888) 8/8e8, 888.
• 8.Meeeee-Meee, (8888) M 88-8, 8888.