Química Orgânica 2 · Adições à carbonila: revisão para a prova
Wittig synthesis of propylidenecyclopentane from 1-bromopropane
Condições: 1) Ph3P (1.0 equiv), 1-bromopropane, toluene, reflux; 2) n-BuLi (1.0 equiv), THF, 0 °C; 3) cyclopentanone (0.9 equiv), THF, 0 °C to 25 °C
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Triphenylphosphine displaces bromide (SN2)
The phosphorus lone pair attacks the primary carbon bearing bromine from the back side and bromide leaves, giving the phosphonium salt.
As setas curvas
- The phosphorus lone pair forms the new P–C bond.
- The C–Br pair leaves with bromine as bromide.
Butyllithium removes the proton next to phosphorus
The positive phosphorus makes the hydrogens on the adjacent CH2 acidic (pKa ≈ 22). The butyl carbanion removes one, giving the phosphorus ylide.
As setas curvas
- The butyl carbanion lone pair takes the proton.
- The C–H pair stays on carbon, which becomes the ylide carbanion.
Why butyllithium?
An alkyl phosphonium salt with no other activating group is only weakly acidic, so a very strong base in a dry solvent is needed.
The ylide and cyclopentanone form an oxaphosphetane
Cyclopentanone is added. The ylide carbon bonds to the carbonyl carbon while the C=O π pair forms an O–P bond, closing the four-membered oxaphosphetane ring.
As setas curvas
- The ylide carbon lone pair forms the new C–C bond.
- The C=O π pair forms the new O–P bond.
The oxaphosphetane collapses
The C–P bond becomes the new C=C π bond and the C–O bond becomes the P=O π bond, releasing the alkene and triphenylphosphine oxide.
As setas curvas
- The C–P pair becomes the new C=C π bond.
- The C–O pair becomes the P=O π bond.
Propylidenecyclopentane and triphenylphosphine oxide
The three carbons of the propyl group are joined to the former carbonyl carbon by an exocyclic C=C bond. Because the two ring carbons attached to that carbon are equivalent, no E/Z isomers exist.
O que você vai praticar
Draw the SN2 formation of propyltriphenylphosphonium bromide, its deprotonation by butyllithium to the ylide, and the Wittig reaction with cyclopentanone that gives propylidenecyclopentane and triphenylphosphine oxide.
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