Química Orgânica 2 · Adição e condensação aldólica
4-(Dimethylamino)butan-2-one by the Mannich reaction
Condições: HCHO, Me2NH·HCl, EtOH, Δ
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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Dimethylamine adds to formaldehyde
Dimethylamine, a good nucleophile, adds to the very reactive formaldehyde carbonyl. Dimethylammonium ions from the HCl catalyst are the acid in every protonation.
As setas curvas
- The nitrogen lone pair forms the C–N bond.
- The C=O π pair moves onto oxygen.
Proton transfer to oxygen
Dimethylammonium protonates the alkoxide oxygen of the zwitterion.
As setas curvas
- The alkoxide takes the ammonium proton.
- The N–H pair stays on nitrogen.
Deprotonation of nitrogen
Dimethylamine removes the N–H proton, giving the neutral hemiaminal (carbinolamine).
As setas curvas
- Dimethylamine takes the N–H proton.
- The N–H pair stays on nitrogen.
Protonation of the hydroxyl
Dimethylammonium protonates the OH so it can leave as water.
As setas curvas
- The hydroxyl lone pair takes the ammonium proton.
- The N–H pair stays on nitrogen.
Water leaves: the iminium ion
The nitrogen lone pair pushes out water, forming the electrophilic iminium ion (the Eschenmoser-type CH2=NMe2+).
As setas curvas
- The nitrogen lone pair forms the C=N+ bond.
- The C–O pair leaves as water.
Protonation of acetone
The HCl catalyst protonates the acetone carbonyl, the first step of acid-catalysed enolization; dimethylammonium is far too weak an acid for this.
As setas curvas
- The carbonyl lone pair takes the HCl proton.
- The H–Cl pair stays on chlorine as chloride.
Enol formation
Dimethylamine removes an α-hydrogen from the protonated ketone, giving the enol of acetone.
As setas curvas
- Dimethylamine takes an α-hydrogen.
- The C–H pair forms the enol C=C bond.
- The C=O+ π pair returns to oxygen.
Which electrophile?
Three carbonyl-type electrophiles could be present: acetone, formaldehyde and the iminium ion.
The enol attacks the iminium ion
The enol oxygen lone pair pushes the C=C electrons onto the iminium carbon, forming the new C–C bond and a protonated ketone.
As setas curvas
- The enol oxygen lone pair forms the C=O+ bond.
- The C=C π pair forms the new C–C bond.
- The C=N+ π pair moves onto nitrogen.
Deprotonation gives the Mannich base
Dimethylamine removes the proton from the protonated ketone, giving the β-amino ketone and regenerating the ammonium catalyst.
As setas curvas
- Dimethylamine takes the O–H proton.
- The O–H pair stays on oxygen.
4-(Dimethylamino)butan-2-one
4-(Dimethylamino)butan-2-one, the Mannich base, forms with water; under the acidic conditions it is isolated as its hydrochloride.
O que você vai praticar
Draw the Mannich reaction of acetone, formaldehyde and dimethylamine: iminium ion formation, acid-catalysed enolization of acetone, C–C bond formation and deprotonation to 4-(dimethylamino)butan-2-one.
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