16. Conjugated Systems

A student found that heating any one of the isomers shown here resulted in scrambling of the deuterium to all three positions on the five-membered ring. Propose a mechanism to account for this observation.

Propose a mechanism for the following reaction:

b. Using arrows, show the electron rearrangement that takes place in each reaction.

3.

Draw the product of each of the following reactions:

e.

Choose the product most likely to form during the following sigmatropic rearrangement: A molecule of $1_3$-butadiene undergoes a ${[}_{\mathrm{3,3}}$] sigmatropic rearrangement (Claisen rearrangement). Which of the following is the major product?

a. Will thermal 1,3-migrations of carbon occur with retention or inversion of configuration?

Draw the product of each of the following sigmatropic rearrangements:

d.

Explain why [1,3] sigmatropic migrations of hydrogen cannot occur under thermal conditions, but [1,3] sigmatropic migrations of carbon can.

If the compounds shown here are heated, one will form one product from a [1,3] sigmatropic rearrangement and the other will form two products from two different [1,3] sigmatropic rearrangements. Draw the products of the reactions.

Provide the correct names and mechanisms for the following sigmatropic shifts

b. Will thermal 1,5-migrations of carbon occur with retention or inversion of configuration?

What is the product of the following [1,3] sigmatropic rearrangement, A or B?

Two products are formed in the following [1,7] sigmatropic rearrangement, one due to hydrogen migration and the other to deuterium migration. Show the configuration of the products by replacing A and B with the appropriate atoms (H or D).

b. Using arrows, show the electron rearrangement that takes place in each reaction.

1.

2.

Why was a deuterated compound used in the last reaction on the preceding page?