Química Orgânica 2 · Adições à carbonila: revisão para a prova
Pyrrolidine enamine of cyclopentanone
Condições: Cyclopentanone, pyrrolidine (1.2 equiv), TsOH·H2O (0.01 equiv), toluene, reflux with Dean–Stark removal of water
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Pyrrolidine adds to the carbonyl carbon
The nitrogen lone pair of the secondary amine attacks the carbonyl carbon of cyclopentanone, and the C=O π pair moves onto oxygen, giving a zwitterion.
As setas curvas
- The nitrogen lone pair forms the new C–N bond.
- The C=O π pair moves onto oxygen.
Proton transfer from N⁺–H to O⁻
The negatively charged oxygen takes the proton from the positively charged nitrogen, giving the neutral carbinolamine. No acid is needed for this shuttle: the N⁺–H (pKa ≈ 10.6) is far more acidic than the O–H it forms (pKa ≈ 16).
As setas curvas
- An alkoxide lone pair takes the N–H proton.
- The N–H pair stays on nitrogen, which becomes neutral.
TsOH protonates the OH
To lose the oxygen, the OH must become water. TsOH protonates the hydroxyl oxygen.
As setas curvas
- A hydroxyl oxygen lone pair takes the proton from TsOH.
- The O–H pair stays on the sulfonate oxygen.
The nitrogen lone pair expels water
The nitrogen lone pair forms a C=N π bond while the C–O bond breaks and water leaves, giving an iminium ion with no hydrogen on nitrogen.
As setas curvas
- The nitrogen lone pair forms the C=N π bond.
- The C–O bond breaks and water leaves with the pair.
What can the iminium ion lose?
With no N–H left, the only proton that can be removed to give a neutral product is on a carbon next to the iminium carbon.
Water removes an α-hydrogen
The water molecule released when the iminium ion formed takes a hydrogen from the ring carbon next to the iminium carbon. The C–H pair forms the C=C bond and the C=N π pair returns to nitrogen as its lone pair.
As setas curvas
- A water lone pair takes the α-hydrogen.
- The C–H pair forms the new C=C π bond.
- The C=N π pair returns to nitrogen as a lone pair.
1-(Cyclopent-1-en-1-yl)pyrrolidine
The product is an enamine: the nitrogen is attached to a ring C=C carbon, and its lone pair is conjugated with the C=C bond. The carbonyl oxygen left as water, and H₃O⁺ with tosylate is the TsOH catalyst again.
O que você vai praticar
Draw the acid-catalysed condensation of cyclopentanone with the secondary amine pyrrolidine through a carbinolamine and an iminium ion to the enamine, and explain why an enamine rather than an imine forms.
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