Química Orgánica 2 · Enlaces carbono–carbono con organometálicos
trans-2,3-Dimethylcyclohexanone by conjugate addition and enolate trapping
Condiciones: (CH3)2CuLi, Et2O; then CH3I, HMPA
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 4 pasos escritos
Methyl adds to the β carbon
The soft cuprate transfers one methyl to the β carbon (C3). The C=C π pair shifts to form the enolate double bond and the C=O π pair moves onto oxygen, giving a lithium enolate that is regiospecifically placed between C1 and C2.
Las flechas curvas
- The C–Cu bond pair forms the new C–C bond at C3.
- The C=C π pair moves to form the enolate C=C bond.
- The C=O π pair moves onto oxygen.
The enolate attacks iodomethane
HMPA frees the lithium enolate and speeds its SN2 reaction. The enolate carbon C2 attacks the methyl carbon of iodomethane as the oxygen pair re-forms C=O, and iodide leaves. Iodomethane approaches the face opposite the C3 methyl group, so the two methyls end up trans.
Las flechas curvas
- An oxygen lone pair re-forms the C=O π bond.
- The enolate C=C pair forms the new C2–CH3 bond.
- The C–I pair leaves as iodide.
Where does the second methyl go?
The enolate formed by 1,4-addition has its C=C between C1 and C2.
trans-2,3-Dimethylcyclohexanone
Two new C–C bonds were made in one flask: the cuprate methyl at C3 and the iodomethane methyl at C2. The methyls are trans because alkylation occurs anti to the C3 substituent; the product is racemic.
Qué vas a practicar
Draw the 1,4-addition of lithium dimethylcuprate to cyclohex-2-enone and the trapping of the resulting enolate at C2 by iodomethane, trans to the new C3 methyl.
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