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Chapter 24: Problem 14

The aryl diene undergoes sequential Heck reactions to give a product with the molecular formula \(\mathrm{C}_{15} \mathrm{H}_{18}\). Propose a structural formula for this product.

Short Answer

Expert verified
Answer: The structural formula of the final product is \(\mathrm{C}_{6} \mathrm{H}_{5} - (\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{7} \mathrm{H}_{11})\). This structure represents a triphenylene ring system with a seven-carbon linear side chain.

Step by step solution

01

Understanding the Heck Reaction

The Heck reaction is a palladium-catalyzed carbon-carbon coupling reaction between an alkene and an aryl, vinyl, or benzyl halide. The reaction results in the formation of a new C-C double bond between the alkene and the partner molecule.
02

Determine the Molecular Formula of the Aryl Diene

We know that the final product has a molecular formula of \(\mathrm{C}_{15} \mathrm{H}_{18}\). The aryl diene must have a molecular formula of \(\mathrm{C}_{n} \mathrm{H}_{m}\), where the values of n and m can be determined by analyzing the difference in the molecular formula after the Heck reaction steps. In each Heck reaction step, one hydrogen atom is replaced by a carbon atom, increasing the carbon count by one. Therefore, n must equal 13 and m must equal 16. So, the molecular formula of the aryl diene is \(\mathrm{C}_{13} \mathrm{H}_{16}\).
03

Identify the Minimum Structure of Aryl Diene

An aryl diene molecule contains a phenyl ring and two double bonds. To fulfill the molecular formula and keep the aryl diene structure intact, the phenyl ring must have one side chain containing two double bonds. Therefore, the aryl diene structure must be \(\mathrm{C}_{6} \mathrm{H}_{5} - \mathrm{C}_{7} \mathrm{H}_{11}\). The side chain must have two double bonds and five single bonds or a conjugated diene system.
04

Perform First Heck Reaction

In the first Heck reaction, one of the double bonds in the side chain reacts with an aryl halide and the palladium catalyst to form a new C-C bond, increasing the carbon count by one but keeping the hydrogen count constant. For example, the reaction could be with phenyl bromide (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\)) to give a new product: \(\mathrm{C}_{6} \mathrm{H}_{5} - (\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{7} \mathrm{H}_{11})\)
05

Perform Second Heck Reaction

In the second Heck reaction, the remaining double bond in the side chain reacts with another equivalent of the aryl halide and the palladium catalyst, forming a new C-C bond and completing the transformation to the final product, which has a molecular formula of \(\mathrm{C}_{15} \mathrm{H}_{18}\). The product will be: \(\mathrm{C}_{6} \mathrm{H}_{5} - (\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{7} \mathrm{H}_{11})\)
06

Write the Resulting Structural Formula

After going through two Heck reaction steps, the resulting structure for the given product with the molecular formula \(\mathrm{C}_{15} \mathrm{H}_{18}\) is \(\mathrm{C}_{6} \mathrm{H}_{5} - (\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C}_{7} \mathrm{H}_{11})\). This structure represents a triphenylene ring system with a seven-carbon linear side chain.

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Most popular questions from this chapter

Show how you might prepare each compound by a Heck reaction using methyl 2-propenoate as the starting alkene.

Treatment of cyclohexene with iodobenzene under the conditions of the Heck reaction might be expected to give 1-phenylcyclohexene. The exclusive product, however, is 3-phenylcyclohexene. Account for the formation of this product.

What sequence of reactions will produce the following product if starting with trimethylsilyl acetylene and the appropriate two aryl iodides?

Show the sequence of Heck reactions by which the following conversion takes place. Note from the molecular formula given under each structural formula that this conversion corresponds to a loss of \(\mathrm{H}\) and I from the starting material.

When the \(\mathrm{Pd}(0)\)-catalyzed reactions covered in this chapter are run with a slight pressure of carbon monoxide, it is often found that a ketone is created as the product. For example, the following Stille coupling conditions with added CO give the product shown. Write a mechanism for how this reaction could occur using the organometallic mechanistic steps introduced in this chapter, along with new steps that would be required in this transformation.
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