Química Orgánica 2 · Adición y condensación aldólica
4-Hydroxy-5-methylhexan-2-one by a directed aldol
Condiciones: LDA, THF, −78 °C; then 2-methylpropanal; then aq. NH4Cl
Dibuja este problema tú mismo
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 5 pasos escritos
LDA forms the enolate
LDA, a strong hindered base, removes an α-hydrogen of acetone completely and irreversibly at −78 °C (pKa of diisopropylamine about 36). No neutral acetone remains to compete as an electrophile.
Las flechas curvas
- The amide nitrogen lone pair takes the α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The C=O π pair moves onto oxygen.
Order of addition
Both partners have α-hydrogens, so a one-pot base-catalysed aldol would give a mixture.
The enolate adds to 2-methylpropanal
2 is added only after enolization is complete. The enolate carbon attacks its carbonyl carbon, forming the new C–C bond and a lithium alkoxide.-methylpropanal
Las flechas curvas
- The oxygen lone pair re-forms the C=O.
- The enolate π pair forms the new C–C bond.
- The aldehyde π pair moves onto oxygen.
Aqueous NH4Cl workup
Ammonium chloride protonates the alkoxide. It is acidic enough to do this but mild enough not to dehydrate the aldol.
Las flechas curvas
- The alkoxide takes a proton from ammonium.
- The N–H pair stays on nitrogen as ammonia.
4-hydroxy-5-methylhexan-2-one
The aldol 4-hydroxy-5-methylhexan-2-one is isolated; at −78 °C with a mild workup it does not dehydrate. The new stereocentre is racemic.
Qué vas a practicar
Draw the directed crossed aldol of acetone with 2-methylpropanal: complete LDA enolization of acetone, addition of the enolate to the aldehyde, and NH4Cl workup to 4-hydroxy-5-methylhexan-2-one.
Acabas de dibujar cada flecha de este problema.
Cada uno de los 5349 problemas de Orgánica 1 y 2 funciona así: comentarios en cada flecha, pistas cuando te atascas y un modo examen sin pistas. Gratis con una cuenta, sin tarjeta.
¿Atascado con tu propia tarea?
Tómale una foto: el primer escaneo es gratis, sin registrarte
¿Escaneas cada semana? Scan Standard: por unos problemas