16. Conjugated Systems

Would you expect the following nucleophiles to do 1,2- or 1,4-addition?

(g)

(h)

(i)

What products would be obtained from the reaction of 1,3,5-hexatriene with one equivalent of HBr? Disregard stereoisomers.

Whereas stabilized enolates do 1,4-addition, unstabilized (normal) enolates can do both 1,2- and 1,4-addition depending on the situation. Why might this be?

Predict the product of the following addition reactions to dienes.

(a)

What are the products of the following reactions, assuming that one equivalent of each reagent is used in each reaction?

c.

Would you expect the following nucleophiles to do 1,2- or 1,4-addition?

(a)

(b)

(c) HO^–

Would you expect the following nucleophiles to do 1,2- or 1,4-addition?

(d)

(e)

(f)

Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.

c. cyclopenta-1,3-diene + Br₂ → 3,4-dibromocyclopent-1-ene + 3,5-dibromocyclopent-1-ene

When Br₂ is added to buta-1,3-diene at –15 °C, the product mixture contains 60% of product A and 40% of product B. When the same reaction takes place at 60 °C, the product ratio is 10% A and 90% B.

d. If you had a solution of pure A, and its temperature were raised to 60 °C, what would you expect to happen? Propose a mechanism to support your prediction.

What are the products of the following reactions, assuming that one equivalent of each reagent is used in each reaction? Disregard stereoisomers.

a.

How many stereoisomers of each product could be obtained?

What are the products of the following reactions, assuming that one equivalent of each reagent is used in each reaction?

a.

a. How could each of the following compounds be prepared from a hydrocarbon in a single step?

b. What other organic compound would be obtained from each synthesis?

1.