Formation of aliphatic carbon–carbon bonds: acid-catalyzed reactions
The principle which is applied in acid-catalyzed reactions is the generation of an electrophilic species, with the aid of an acid, in the presence of a nucleophile with which the electrophile then reacts. Aldehydes and ketones which are capable of enolization undergo self-condensation when treated with acids. The acid has two functions: first, it enhances the reactivity of the carbonyl group towards the addition of a nucleophile. Second, it catalyzes the enolization of the carbonyl compound. As in aromatic Friedel–Crafts reactions, the electrophilic reagent can be either a complex of the acid chloride and Lewis acid or an acylium cation derived from it. The Mannich reaction normally gives the hydrochloride of the Mannich base. These salts are usually stable at room temperature but those derived from aliphatic compounds undergo elimination on being heated. The use of a pre-formed immonium salt avoids the need for an acid catalyst and thereby increases the scope of the reaction.