Química Orgânica 2 · Adição e condensação aldólica
Diethyl 2-benzylidenemalonate by Knoevenagel condensation
Condições: piperidine (cat.), benzene, Δ (−H2O)
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Piperidine forms the malonate enolate
Piperidine (pKaH about 11) is basic enough to deprotonate diethyl malonate (pKa about 13) to a useful extent, because the anion is delocalized over two ester carbonyls. A weak amine base cannot enolize simple ketones or esters, so self-condensation does not compete.
As setas curvas
- The piperidine nitrogen lone pair takes a CH2 hydrogen.
- The C–H pair forms the enolate C=C bond.
- The ester C=O π pair moves onto oxygen.
Why is a weak base enough?
The acidity of the donor decides which base is needed.
The enolate adds to benzaldehyde
The stabilized enolate carbon attacks the benzaldehyde carbonyl carbon, forming the C–C bond and an alkoxide.
As setas curvas
- The oxygen lone pair re-forms the ester C=O.
- The enolate π pair forms the new C–C bond.
- The aldehyde π pair moves onto oxygen.
Piperidinium protonates the alkoxide
The piperidinium ion gives its proton to the alkoxide, giving the neutral aldol and regenerating piperidine.
As setas curvas
- The alkoxide takes the N–H proton.
- The N–H pair stays on nitrogen.
Second deprotonation
Piperidine removes the remaining acidic hydrogen between the two esters, giving the enolate that will expel hydroxide.
As setas curvas
- Piperidine takes the α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The ester C=O π pair moves onto oxygen.
E1cB loss of hydroxide
The enolate re-forms the ester C=O and expels hydroxide from the benzylic carbon. The new C=C is conjugated with the ring and both esters.
As setas curvas
- The oxygen lone pair re-forms the C=O.
- The enolate π pair forms the C=C bond.
- The C–O pair leaves as hydroxide.
Hydroxide is neutralized
The released hydroxide takes a proton from piperidinium, giving water and regenerating the piperidine catalyst. Removing water drives the condensation to completion.
As setas curvas
- Hydroxide takes the N–H proton.
- The N–H pair stays on nitrogen.
Diethyl 2-benzylidenemalonate
Diethyl 2-benzylidenemalonate forms with water; piperidine is regenerated as a catalyst.
O que você vai praticar
Draw the piperidine-catalysed Knoevenagel condensation of diethyl malonate with benzaldehyde through the enolate addition and E1cB dehydration to diethyl 2-benzylidenemalonate.
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