The Mannich reaction is an example of nucleophilic addition of an amine to a carbonyl group followed by elimination of a hydroxyl anion to the Schiff base. The Schiff base is an electrophile which reacts in step two in a second nucleophilic addition with a carbanion generated from a compound containing an acidic proton. Therefore the Mannich reaction contains both an electrophilic and a nucleophilic nature. The Mannich reaction is also considered a condensation reaction.
In the Mannich reaction ammonia or primary or secondary amines are employed for the activation of formaldehyde. Tertiary amines and aryl amines stop at the Schiff base because it lacks a proton to form the intermediate imine. α-CH-acidic compounds (Nucleophiles) are Carbonyl compounds, Nitrile compounds, Acetylene compounds, aliphatic Nitro compounds, α- alkyl-pyridine compounds or Imine compounds. It is also possible to use heterocycles such as furan, pyrrole, and thiophene, as their structure simulates the enol form of a carbonyl quite nicely.
The Mannich reaction requires high reaction temperatures, long reaction times and a protic solvent. Formation of undesired reaction by-product is a common phenomenon.
Because the reaction takes place under acidic conditions, the compound with the carbonyl functional group (in this case a ketone) can tautomerize to the enol form, after which it can attack the iminium ion.
The reaction taking place is between a simple aldehyde such as propionaldehyde and an imine derived from ethyl glyoxylate and para-methoxy-aniline (PMP = paramethoxphenyl) catalyzed by (S)-proline in dioxane at room temperature. The reaction product is diastereoselective with a preference for the syn-Mannich reaction 3:1 when the alkyl substituent on the aldehyde is a methyl group or 19:1 when the alkyl group the much larger pentyl group. Of the two possible syn adducts (S,S) or (R,R) the reaction is also enantioselective with a preference for the (S,S) adduct with enantiomeric excess larger than 99%. Scheme 5 explains this stereoselectivity.
Proline enters a catalytic cycle by reacting with the aldehyde to form an enamine. The two reactants (imine and enamine) line up for the Mannich reaction with Si facial attack of the imine by the Si-face of the enamine-aldehyde. Relieve of steric strain dictates that the alkyl residue R of the enamine and the imine group are antiperiplanar on approach which locks in the syn mode of addition. The enantioselectivity is further controlled by hydrogen bonding between the proline carboxylic acid group and the imine. The transition state for the addition is a nine-membered ring with chair conformation with partial single bonds and double bonds. The proline group is converted back to the aldehyde and a single S,S isomer is formed.
By modification of the proline catalyst to it is also possible to obtain anti-Mannich adducts
An additional methyl group attached to proline forces a specific enamine approach and the transition state now is a 10-membered ring with addition in anti-mode. The diastereoselectivity is at least anti:syn 95:5 regardless of alkyl group size and the S,R enantiomer is preferred with at least 97% ee.
The Mannich Reaction is also used in the synthesis of medicinal compounds e.g. Rolitetracycline (Mannich base of Tetracycline), Fluoxetine (Antidepressant) and Tolmetin (Antiinflammatory drug).