Química Orgânica 2 · Adição a aldeídos e cetonas
Wittig methylenation of acetone
Condições: Ph3P+CH3 Br− (1.1 equiv), n-BuLi (1.05 equiv), THF, 0 °C; then acetone (1.0 equiv), 0 °C to 25 °C
Desenhe este problema você mesmo
Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 6 passos escritos
Butyllithium makes the ylide
The methyl hydrogens next to the positively charged phosphorus are acidic (pKa about 22). The butyl carbanion of n-butyllithium removes one, giving the phosphorus ylide, a carbanion stabilised by the adjacent phosphonium.
As setas curvas
- The butyl carbanion lone pair takes a proton from the methyl group.
- The C–H pair stays on carbon, which becomes the ylide carbanion.
The ylide carbon adds to the carbonyl carbon
The nucleophilic ylide carbon attacks the carbonyl carbon of acetone, and the C=O π pair moves onto oxygen. The resulting species has an alkoxide and a phosphonium in the same molecule (a betaine).
As setas curvas
- The ylide carbon lone pair forms the new C–C bond.
- The C=O π pair moves onto oxygen.
Oxygen closes onto phosphorus
The alkoxide oxygen bonds to the positively charged phosphorus, forming a four-membered oxaphosphetane ring.
As setas curvas
- An oxygen lone pair forms the new O–P bond.
The oxaphosphetane collapses
The four-membered ring breaks in a concerted cycloreversion: the C–P bond becomes the new C=C π bond and the C–O bond becomes the P=O π bond. Formation of the very strong P=O bond drives the reaction.
As setas curvas
- The C–P pair becomes the new C=C π bond.
- The C–O pair becomes the P=O π bond.
Where does the double bond end up?
A Wittig reaction joins the ylide carbon to the former carbonyl carbon with a double bond, so the alkene position is fixed with no regiochemical ambiguity.
2-Methylpropene and triphenylphosphine oxide
The carbonyl oxygen of acetone has been exchanged for the CH2 group of the ylide. Triphenylphosphine oxide is the stoichiometric by-product, and butane is released from the deprotonation.
O que você vai praticar
Draw the deprotonation of methyltriphenylphosphonium by butyllithium, the addition of the ylide to acetone, ring closure to the oxaphosphetane and its collapse to 2-methylpropene and triphenylphosphine oxide.
Você acabou de desenhar cada seta deste problema.
Cada um dos 5349 problemas de Orgânica 1 e 2 funciona assim: feedback em cada seta, dicas quando você travar e um modo prova sem dicas. Grátis com uma conta, sem cartão.
Travou na sua própria lição?
Tire uma foto: o primeiro scan é grátis, sem cadastro
Você escaneia toda semana? Scan Standard: por cerca de problemas