Sigmatropic Rearrangements Pt3 Flashcards
what is a [2,3] sigmatropic rearrangement?
- Is a pericyclic reaction in which a σ-bond adjacent to a conjugated system shifts, accompanied by a reorganisation of π-electrons.
- Migration of a σ-bond from the position 2 to position 3 relative to a conjugated system
How would you number the following molecule used in a [2,3] Wittig Rearrangement
The first step of the [2,3] Wittig rearrangement is deprotonation using a Lewis base
Where does the deprotonation occur and why
- Adjacent to the benzylic group
- The phenyl rings pulls electron density from adjacent carbons
- This makes them more acidic then regular alkyl hydrogens because the phenyl group stabilises the resulting carbanion
Draw the curly arrows and the product of this [2,3] wittig rearrangement
- Start moving electron from the negative charge, as most e- density
What is the driver for this reaction?
- Although the carbanion is stablised by the EWG phenyl and oxygen
- It is not as stabilised if the negative charge is on the oxygen as it is more electronegative
where does this molecule deprotonate, in the first step of this [2,3] Wittig rearrangement
- Don’t need such a strong base for this reaction, as the anion is aromatic
What is the curly arrow mechanism and product of the [2,3] Wittig rearrangment
Briefly, what is the difference between a SN1 and SN2 reaction?
- SN1: Unimolecular Nucleophilic substitution - Two step process involving carbocation formation (RDS) and Nucleophilic attack
- SN2: Bimolecular Nucleophilic substitution - One-step concerted reaction - The nucleophile attacks the electrophilic carbon while the leaving group departs
Prof W. Clark Still developed a new version that generated the carbanion via tin-lithium exchange, where the precursors are readily available from allylic alcohol starting materials
What is the mechanism of the first step?
The next step of the Wittig Rearrangement using Tin-lithium exchange is deprotonation using BuLi
Where does it deprotonate?
- The oxygen will inductively stablise the negative charge on the carbanion
- This is due to the electronegativity of the oxygen atom
Draw the curly arrow mechanism and the product for this [2,3] Wittig rearrangement
Draw the mechanism and the product for this [2,3] Wittig Rearrangement
How can we use heteroatoms in [2,3] Wittig rearrangements?
To direct the reaction towards a certain isomer
i.e. using a methyl group to direct toward Z-trisubstituted product
Which sterochemistry is perferred of these two intermediates and why?
- The large the R group, the large the size difference it has from alkene
- Resulting in large sterics with the rest of the alkane chain
We can have nitrogen analogues of the same [2,3] Wittig reaction
What is the mechanism for the first SN2 reaction
What is the mechanism for the [2,3] Wittig rearrangement
Although S-chiral sulfoxides do not racemise very easily, enriched allyl sulfooxides more readily do. With equilibrium favouring…
The sulfoxide rather than the sulfenate ester
Draw the conformer and the enantiomers which occur due to [2,3] sigmatropic rearrangement
- The equilibrium favours the sulfoxide rather than the sulfenate ester
- The presence of this reversible [2,3] sigmatropic rearrangement explains why racemisation of enantiomerically enriched allyl sulfoxides is observed
