Sigmatropic Rearrangements Pt2

Question 1

What is a Cope rearrangement?

Answer 1

• is a sigmatropic rearrangement of 1,5-dienes, in which the position of the π-bonds and σ-bonds between them shift via a concerted, pericyclic reaction mechanism
• It is a [3,3]-sigmatropic rearrangment

Question 2

What is a core issue with the Cope rearrangement?

Answer 2

Which side of the equilibrium is favoured?
In practice, the simple Cope rearrangements are difficult to control, so we need to use biased starting materials (i.e. the rearrangement will proceed to a product that is obviously lower energy)
There is a need to pick a product which will have an obvious preference for one side of the equilibrium

Question 3

The following reaction should not occur due to the fact, an E-isomer is being formed and a non-conjugated 1,5-diene forming
Why does it and how do the electrons move?

Answer 3

• The Cope rearrangement is accelerated due to the relief of strain present in the 3-membered ring
• The rearrangment has a half-life of several minutes at ambient temperature (the reverse reaction is energetically unfavourable)

Question 4

BASF manufacture citral via a Clasien then two consective [3,3] sigmatropic rearrangements
What is the mechanism and product of the Claisen rearrangement

Answer 4

Question 5

What is the following mechanism and product of the first [3,3] Clasien rearrangement

Answer 5

Question 6

What is the mechanism and product of the second [3,3] Clasien rearrangement?

Answer 6

Question 7

The following reaction is an oxy-cope with the generation of a carbonyl is which used to drive the reaction forwards
The first step is a [3,3] sigmatropic rearrangement, what is the mechanism?

Answer 7

• Using the T.S. you can see that C=C must occupy equatorial position cuz H is in axial

Question 8

The second step of this oxy-cope reaction is a keto-enol tautomerism
What is the product

Answer 8

Question 9

The rate of the oxy-cope reaction can be increased through…

Answer 9

…deprotonate of an alcohol to form an alkoxide
This is known as the ‘anionic oxy-cope’

Question 10

This following reaction is the Anionic oxy-cope reaction
What is the mechanism and product

Answer 10

(Note 2,3 structure is the same as the 1,5 diene structure)

Question 11

The last step of the anionic oxy-cope reaction is a keto-enol tautomerism
What is the product

Answer 11

Question 12

Why is the anionic version of the oxy-cope reaction faster than the neutral version?

Answer 12

• In the anionic oxy-cope there is an important n→σ(star) interaction from the oxygen anion that leads to weakening of the σ-bond being broken in the transition state
• Dontation of electrons into the antibonding orbtial (σ-star) lowers the bond order and hence weakens the bond, thus making it more reactive in the rearrangement