Química Orgânica 2 · Enóis, enolatos e reações em alfa
Methyl cyclopentanecarboxylate by Favorskii ring contraction of 2-chlorocyclohexanone
Condições: NaOMe, MeOH
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Methoxide forms the α′-enolate
Methoxide removes a proton from the CH2 on the side of the ketone away from chlorine. That enolate carbon is positioned to reach the C–Cl carbon on the other side of the carbonyl.
As setas curvas
- A methoxide lone pair takes the α′-proton.
- The C–H electrons form a C=C bond to the carbonyl carbon.
- The C=O π electrons move onto oxygen.
Intramolecular displacement gives a cyclopropanone
The enolate carbon attacks the carbon bearing chlorine and chloride leaves, closing a three-membered ring that contains the carbonyl carbon: a bicyclic cyclopropanone.
As setas curvas
- The oxygen lone pair re-forms the C=O bond.
- The C=C π electrons form the new C–C bond across the ring.
- Chloride leaves with the C–Cl electrons.
Methoxide adds to the strained ketone
The cyclopropanone carbonyl is very electrophilic because of ring strain. Methoxide adds to it, giving a tetrahedral alkoxide.
As setas curvas
- A methoxide lone pair attacks the carbonyl carbon.
- The C=O π electrons move onto oxygen.
Why the ring opens
The tetrahedral intermediate collapses by breaking a strained three-membered-ring bond instead of expelling methoxide.
The ring opens to a carbanion
Oxygen re-forms the C=O as an ester, and one cyclopropane bond breaks with its electrons going to carbon. The six-membered ring has become a five-membered ring carrying the ester.
As setas curvas
- The oxygen lone pair re-forms the C=O bond.
- The strained C–C bond breaks; its electrons stay on the ring carbon.
Methanol protonates the carbanion
The ring carbanion takes a proton from methanol, which regenerates methoxide.
As setas curvas
- The carbanion lone pair takes the methanol proton.
- The O–H electrons stay on oxygen as methoxide.
Methyl cyclopentanecarboxylate
The cyclohexanone ring has contracted by one carbon: the former carbonyl carbon is now the ester carbon outside a cyclopentane ring.
O que você vai praticar
Draw the Favorskii ring contraction: α′-enolate, cyclopropanone formation by displacement of chloride, methoxide addition, ring opening and protonation to methyl cyclopentanecarboxylate.
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