This reaction is particularly valuable to insert methylene groups in exocyclic positions and for the preparation of β,γ- unsaturated carboxylic acids and their derivatives (Scheme 14).

Scheme 14

It should be noticed that normally stabilized ylides give the E-alkene while non-stabilized alkenes give the Z-alkene.

6.2.2.10 Baeyer Villiger Oxidation

When ketones are treated with peracids, they rearrange to form esters, the reaction is known as Baeyer Villiger oxidation. Carboxylates are not such good leaving groups, but the oxygen–oxygen single bond is very weak and monovalent oxygen cannot bear to carry a positive charge so that, once the peracid has added, loss of carboxylate is concerted with a rearrangement driven, by formation of a carbonyl group (Scheme 15).

Scheme 15

The tendency of migration in this case of a group depends more on less the order in which the groups are able to stabilize a positive charge. Thus, tertiary alkyl is best in migrating followed by secondary alkyl, phenyl and methyl (Scheme 16).

Scheme 16