Sigmatropic Rearrangements

Question 1

What are Sigmatropic rearrangements?

Answer 1

Are the migration of a sigma bond from one end a of π-system to another

Question 2

What is the names of these two different types or arrangements

Answer 2

[3,3] sigmatropic rearrangment
[2,3] sigmatropic rearrangement
(the original C-C single bond is carbons 1, the two adjacent are carbon 2 etc)

Question 3

What is the product of this Claisen rearrangement

Answer 3

Question 4

What is the product of this Cope rearrangement

Answer 4

Question 5

Why does this particular reaction require quite a lot of energy (i.e. heat)?

Answer 5

As aromaticity of the starting material is initially lost
(can be regained via a simple tautomerisatio)

Question 6

What evidence is there that this is a concerted, sigmatropic rearrangement

Answer 6

Because of the regioselectivity observed in the substitute
(if not concerted the isomer would form)

Question 7

How does the sterochemistry change during this Claisen rearrangement?

Answer 7

Transferred the sterochemical information
(done using chair-like transition states)

Question 8

What is the mechanism and product of this oxymercuration via the Clasisen rearrangement

Answer 8

Question 9

What is the mechanism and product of this transacetalsation via the Claisen rearrangement

Answer 9

(intermediate 2 is a short-hand of steps 1 and 2)

Question 10

If you react these two reagents and sulfuric acid together you get?

Answer 10

An ester

Question 11

The first step of a ‘enolate-Clainse’ or ‘Ireland-Claisen’ is reacting this ester with a base
What is the mechanism and enolate product?

Answer 11

Question 12

What is the product of this [3,3] sigmatropic rearrangement?

Answer 12

Question 13

Could could also react the enolate with R₃SiCl to form

Answer 13

Question 14

The enolate- and Ireland-Claisen reactions allow you to use substituents on the ends of the alkenes to give new sterocentres in the products
What is the product of the frst step of this reaction

Answer 14

Question 15

Draw the chair of the following E-enolate

Answer 15

Question 16

Draw the [3,3] sigmatropic rearrangements of the following chair

Answer 16

Question 17

Unravel the following molecule to a straight chain compound

Answer 17

Question 18

For the following compound, draw the Z-Enolate and chair transtion state

Answer 18

(note O has to be equatorial and hence R1 is in a fixed position due to C=C)

Question 19

Draw the following mechanism and chair + planar product of the [3,3] sigmatropic rearrangement for this Z-enolate

Answer 19