Q5(b). Baeyer–Villiger oxidation of an electron-rich aldehyde

Introduction. Peracids convert aldehydes and ketones into esters by Baeyer–Villiger oxidation. In an aldehyde the group that migrates is either H (giving a carboxylic acid) or the carbon group (giving a formate). An electron-rich aryl group (here 3,4-dimethoxyphenyl) migrates in preference to H, so the product is an aryl formate; this is the peracid version of the Dakin reaction.

Scheme: Ar in the peracid = 3-chlorophenyl; m-chlorobenzoic acid is released in the migration step.

1. Addition. The peracid adds to the C=O of 3,4-dimethoxybenzaldehyde to give the tetrahedral Criegee intermediate, ArCH(OH)-O-O-COAr'.
2. Migration (rate-determining). As the weak O–O bond breaks and m-chlorobenzoate leaves, a group on the carbinol carbon moves to the electron-poor oxygen. The aryl group migrates rather than H, because the 4-OMe group (para to the migrating carbon) stabilises the partial positive charge in the bridged (phenonium-like) transition state.
3. Product. Loss of the proton from OH gives X = 3,4-dimethoxyphenyl formate, HCOO-C₆H₃(OMe)₂.
4. Hydrolysis. Aqueous acid hydrolyses the formate ester: Y = 3,4-dimethoxyphenol + HCOOH.

Scheme: Acid hydrolysis of the formate ester.

Without the electron-donating groups (e.g. benzaldehyde itself) H migrates and the product is the carboxylic acid (here it would be 3,4-dimethoxybenzoic acid, the minor path).

Conclusion. X = 3,4-dimethoxyphenyl formate, formed through the Criegee intermediate by preferential migration of the electron-rich aryl group; Y = 3,4-dimethoxyphenol, by acid hydrolysis of the formate.

Application. This aldehyde-to-phenol sequence (Baeyer–Villiger/Dakin) is used to make catechol and hydroquinone ethers, e.g. 3,4-dimethoxyphenol from veratraldehyde for natural-product synthesis.