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Chapter 27: Q 32. (page 1100)

Question: What cyclic product is formed when each decatetraene undergoes photochemical electrocyclic ring closure?

Short Answer

Expert verified

Answer

Formed cyclic product:

a.

b.

Step by step solution

01

Woodward-Hoffmann rule for photochemical electrocyclic reaction

A thermal reaction with an even number of electron pairs is conrotatory

鈥� If one of the reaction conditions changes 鈥� thermal to photochemical or even to odd 鈥� the stereochemistry of the reaction changes to disrotatory

鈥� If both reaction conditions change鈥� thermal to photochemical reaction and even to odd 鈥� the stereochemistry does not change

02

Product formation

  • It is an electrocyclic ring closure reaction of a compound with four conjugated double bonds, and the ring closure is dis-rotary.
  • A disrotatory process forms the product under photochemical conditions.
  • From the Woodward-Hoffmann rules, photochemical electrocyclic reactions involving (4n) electrons are dis-rotary.
  • Therefore, the product formed is of a 4+2 electron system arising from disrotatory cyclization.
  • Stereochemistry of products - Frontier orbital representation
  • The HOMO is in which the end orbitals are symmetrical.
  • The symmetry is correct for bond formation, so the orbitals rotate in opposite directions to form a new sigma bond.

a. The product formed is cis in configuration.

Cis product

b. The product formed is trans in configuration.

Trans product

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Most popular questions from this chapter

Question: Draw a stepwise, detailed mechanism for the following reaction.

Question: Classify each reaction as an electrocyclic reaction, a cycloaddition, or a sigmatropic rearrangement. Label the 蟽 bonds that are broken or formed in each reaction.

Question:(a) Draw the product of the following [4 + 2] cycloaddition, which was carried out in the early stages of the synthesis of the alkaloid reserpine (Problem 22.83). Indicate the stereochemistry at any newly formed stereogenic centers. (b) Draw the porbitals of the alkene and the terminal carbons of the conjugated diene, and show how the orientation of the reactants and orbital overlap lead to the observed stereochemistry.

Question: Using the Woodward鈥揌offmann rules in Table 27.4, predict the stereochemistry of each reaction.

a. a [6 + 4] thermal cycloaddition

b. photochemical electrocyclic ring closure of deca-1,3,5,7,9-pentaene

c. a [4 + 4] photochemical cycloaddition

d. a thermal [5,5] sigmatropic rearrangement

Question: What type of cycloaddition occurs in Reaction [1]? Draw the product of a similar process in Reaction [2]. Would you predict that these reactions occur under thermal or photochemical conditions?
See all solutions

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