Baeyer-Villiger Reaction

by James

Description: Treatment of an aldehyde or ketone with a peroxyacid (RCO3H) results in the formation of an ester. [private_ReactionGuide]

Notes: A common peroxyacid for this purpose is m-chloroperoxybenzoic acid (m-CPBA), although other peroxyacids such as peroxyacetic acid can be used.

Examples:

Notes: The reagent in the second example is mCPBA, drawn out. Note that in the second example, the phenyl group “migrates” preferentially over the methyl group. The reason is often not explored deeply in introductory courses, but has to do with the superior ability of the phenyl group to stabilize positive charge in the transition state (relative to methyl). For more information on “migratory aptitudes” see this link.

Mechanism: This is shown with a generic R for the peroxyacid, since a variety of different reagents can be used. Addition (“1,2-addition”) of the peroxyacid oxygen to the ketone (Step 1, arrows A and B) to give a tetrahedral intermediate is followed by a proton transfer (Step 2, arrows C and D). Then, as the C-O π bond reforms, a 1,2-shift of carbon to oxygen occurs, breaking the C–C bond as well as the weak O–O bond, giving an ester (Step 3, arrows E, F and G). Then, deprotonation yields the neutral ketone (Step 4, arrows H and I)

Notes: The neutral carboxylic acid is a byproduct here. The group “B” for deprotonation could be any atom with a lone pair, such as another equivalent of the carboxylic acid. Finally, it is plausible to show protonation of the carbonyl oxygen first (this would result in a more reactive carbonyl carbon).

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