Which of these two pathways is better for Robinson cycloaddition?....

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In summary, the conversation discusses the target molecule 4-phenyl-2-cyclohexenone and its possible reactions through retrosynthetic analysis. It is mentioned that there is one possible aldol condensation reaction and two possible Michael addition reactions. The speaker's guess is that the best reaction is 2-phenyl-2-propenal + acetone due to the delocalization of negative charge and potential for EAS. They also mention the need for a base to generate the active enolate and discuss the ease of generation and stability of the enolates. The conversation ends with the speaker mentioning that the same exercise was solved in a textbook, confirming their initial answer.
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Aristotelis1999
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Target molecule: 4-phenyl-2-cyclohexenone. 2-phenylacetaldehyde + 3-buten-2-one or 2-phenyl-2-propenal + acetone?
So this was a question on my orgII exam yesterday. The target molecule is 4-phenyl-2-cyclohexenone. By retrosynthetic analysis there is only one possible aldol condensation reaction, but two possible michael addition reactions afterwards. My "guess" was that the best one is 2-phenyl-2-propenal + acetone because in the other case the negative charge is delocalized throughout the benzene ring reducing the nucleophilic character of the a-carbon and also activating the ring in which case EAS could take place with the ring acting as the "Michael Donor". Please correct if I was supposed to post this question in another forum. Thank you a lot in advance.
 
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  • #2
You need a base to generate the active enolate. Which enolate is easier to generate? Which one is more stable?
 
  • #3
TeethWhitener said:
You need a base to generate the active enolate. Which enolate is easier to generate? Which one is more stable?
Well, there's an obvious answer for that. But anyway, I went and asked my teacher and it turns out the same exact exercise was solved in the textbook. The answer was what I said, but for very different reasons. Aldehydes undergo aldol condensation in base much faster than simple ketones. So, the issue with using the 2 - phenylacetaldehyde is aldol condensation with another same molecule as a side reaction.
 
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  • #4
Very interesting. Good to know.
 

1. What is the Robinson cycloaddition?

The Robinson cycloaddition is a chemical reaction in which two molecules, typically an enone and a diene, combine to form a new cyclohexene ring.

2. What are the two pathways for Robinson cycloaddition?

The two pathways for Robinson cycloaddition are the thermal pathway, which occurs at high temperatures, and the photochemical pathway, which occurs under UV light.

3. Which pathway is better for Robinson cycloaddition?

The answer to this question depends on the specific reactants and conditions. In general, the thermal pathway is more efficient and produces higher yields, but the photochemical pathway can be useful for specific reactions or when working with sensitive molecules.

4. What factors should be considered when choosing a pathway for Robinson cycloaddition?

Some factors to consider when choosing a pathway for Robinson cycloaddition include the stability and reactivity of the reactants, the desired product, and the availability of necessary equipment or conditions.

5. Are there any limitations to using the Robinson cycloaddition?

Yes, there are some limitations to using the Robinson cycloaddition. For example, the reaction may not work well with certain functional groups or in the presence of impurities. Additionally, the reaction may not proceed as desired if the temperature or light conditions are not carefully controlled.

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