Phosphorus based Olefination Flashcards
What is the strength of a P-O bond relative to the strength of the P-C or Si-O bond
Stronger than the P-C bond
But weaker than the Si-O bond
What is the strength of the P=O bond relative to the Si-O bond?
P=O bond is stronger than the Si-O bond
How does the following reaction occur under reflux
- LP on the phosphorus attacks the δ⁺ carbon
- New C-P bond is formed
How do the following reagents react together?
no reaction
The nitrogens on the phosphorus render it non nucleophilic due to large overlap in orbitals
How do you form the following phosphine salt
React triphenylphosphate with bromomethane under reflux
What happens if you react the phosphine salt with Butyllithium?
Deprotonates
Forming this phosphonium ylide
What is a Ylide?
A compound with anionic sites adjacent to a hetroatom with a positive formal charge
(stabilised through hyperconjugation of the anion by σ’ of the adjacent C-P bond)
What other resonance form exists of the following ylide
phosphorane
What is the mechanism of the following reaction
called oxophosphotane
What happens to oxophosphotane when it has formed?
- P=O bond forms breaking the C-O
- C=C bond forms breaking the P-C
Unstabilised phosphonium yields a … alkene
ciz (z) alkene
the phosphonium ylide is unstable due to no electron withdrawing group to stabilise it
Stabilised phosphonium ylides give …. alkene
trans (E) alkene
Why does a destabilised phosphonium ylide produce a cis alkene
- Due to the way the orbital overlap in the transition state, the phenyl group and the R group are within the same plane
- Meaning they are on the same side (cis) when the alkene is formed
Why does a stabilised phosphonium ylide produce a trans alkene
- The R group, which is the stabilising electron withdrawing group, is rotated upwards
- This means the phenyl are the R group are on opposite sides
How does the reaction occur between these two reagents?
(called the Arbuzov-Michaelis reaction)
- LP on phosphorus attacks δ⁺ carbon, breaking the C-Br bond
- The intemediate is not stable however, so Br- attacks the δ⁺ C of the ethoxy, breaking the C-O bond
- And the new LP on the oxgyen form a C=O bond
What is the transition state formed from the reaction of this phosphate compounds with NaOH
- base deprotonates, causing C=C bond to form and C=O to break
The phosphate transition state then reacts with PhCHO
How does this occur?
- New bond forms between P-O and C-O bond is broken
- New C=C bond is formed and P-C bond is broken
