Química Orgânica 2 · Condensação de Claisen e adição conjugada
2-[(Dimethylamino)methyl]cyclohexan-1-one by Mannich reaction
Condições: HCl (cat.)
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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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Amine adds to formaldehyde
Dimethylamine's nitrogen lone pair attacks the carbonyl carbon of formaldehyde, giving a zwitterionic adduct.
As setas curvas
- The nitrogen lone pair forms the C–N bond.
- The C=O π pair moves onto oxygen.
Protonate the alkoxide
Hydronium (from the catalytic HCl) protonates the alkoxide oxygen.
As setas curvas
- The alkoxide takes a proton from hydronium.
- The O–H pair returns to oxygen as water.
Deprotonate nitrogen
Water removes the N–H proton, giving the neutral carbinolamine and regenerating hydronium.
As setas curvas
- Water's lone pair takes the N–H proton.
- The N–H pair returns to nitrogen.
Protonate the OH
Hydronium protonates the hydroxyl group so it can leave as water.
As setas curvas
- The OH oxygen takes a proton.
- The O–H pair returns to oxygen.
Form the iminium ion
The nitrogen lone pair pushes out water, giving the electrophilic N,N-dimethylmethaniminium ion.
As setas curvas
- The nitrogen lone pair forms the C=N π bond.
- The C–O bond breaks and water leaves.
Which species is the electrophile?
Mannich electrophile.
Protonate cyclohexanone
Hydronium protonates the carbonyl oxygen of cyclohexanone, the first step of acid-catalysed enolization.
As setas curvas
- The carbonyl oxygen takes a proton.
- The O–H pair returns to oxygen as water.
Form the enol
Water removes an α-hydrogen; the C–H pair becomes the C=C bond of the enol as the C=O π pair returns to oxygen.
As setas curvas
- Water takes the α-hydrogen.
- The C–H pair forms the C=C π bond.
- The C=O π pair moves onto oxygen.
Enol attacks the iminium ion
The enol C=C π electrons attack the iminium carbon, pushed by the enol oxygen lone pair; the C=N π pair returns to nitrogen.
As setas curvas
- The enol oxygen lone pair forms the C=O π bond.
- The C=C π pair forms the new C–C bond to the iminium carbon.
- The C=N π pair moves onto nitrogen, giving the neutral amine.
Deprotonate the carbonyl
Water removes the proton from the protonated ketone, giving the neutral Mannich base.
As setas curvas
- Water takes the O–H proton.
- The O–H pair stays on oxygen.
2-[(Dimethylamino)methyl]cyclohexan-1-one
The product is the Mannich base 2-[(dimethylamino)methyl]cyclohexan-1-one (racemic): a CH2NMe2 group has been added to the α-carbon. In the acidic medium it is largely present as its ammonium salt.
O que você vai praticar
Draw the acid-catalysed Mannich reaction: iminium formation from dimethylamine and formaldehyde, enolization of cyclohexanone and C–C bond formation.
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