12.24:
Reactions of Aldehydes and Ketones: Baeyer–Villiger Oxidation
Baeyer–Villiger oxidation uses peracid and an acid catalyst to convert aldehydes to carboxylic acids and ketones to esters via the insertion of an oxygen atom next to the carbonyl carbon.
The substrate's carbonyl oxygen is first protonated, which makes the carbonyl carbon more electrophilic.
A nucleophilic attack by the terminal oxygen of the peracid, followed by the loss of a proton, produces a tetrahedral intermediate.
Protonating the carboxylate facilitates a concerted intramolecular process wherein reformation of the carbonyl bond, migration of a group from carbon to oxygen, and cleavage of the weak O–O bond take place, releasing the final product.
With unsymmetrical ketones, a question that arises is which group undergoes migration? With two possible outcomes, one of the products predominates.
The reaction result depends on the migratory aptitude of different groups; thus, Baeyer–Villiger oxidations are regioselective. Also, note that the reaction retains the stereochemistry at the migrating center.
For aromatic substrates, an electron-donating group accelerates the migration process, while an electron-withdrawing group retards it.
12.24:
Reactions of Aldehydes and Ketones: Baeyer–Villiger Oxidation
Baeyer–Villiger oxidation converts aldehydes to carboxylic acids and ketones to esters. The reaction uses peroxy acids or peracids and is often catalyzed by acid. The reaction is named after its pioneers, Adolf von Baeyer and Victor Villiger. The reaction is achieved by a wide range of peracids such as m-chloroperoxybenzoic acid (mCPBA), perbenzoic acid (C6H5COOOH), peracetic acid (CH3COOOH), hydrogen peroxide (H2O2), and tert-butyl hydroperoxide (t-BuOOH).
The carbonyl center is activated by protonating the carbonyl oxygen. The peracid then adds across the C=O bond to give a tetrahedral intermediate called the Criegee intermediate. During the concerted intramolecular rearrangement that follows, the carbonyl group is restored, a group migrates from carbon to oxygen, and the weak peroxide bond cleaves to give an acid (if the substrate is an aldehyde) or an ester (if the substrate is a ketone).
The migratory aptitude of different groups follows the order of –H > –CR3 > –CHR2 ≈ –Ph > –CRH2 > –⁠CH3. This makes Baeyer–Villiger oxidation regioselective. Also, the reaction retains the stereochemistry at the migrating center for asymmetrical substrates. Reactions involving cyclic ketones produce lactones.