What is the mechanism for synthesis of tetraphenylcyclopentadienone?
Looks like this can be formed via TWO aldol condensations. Nice! Let's see...
Alright, let's start with these two compounds, then:
The mechanism will involve a base (we'll use
Note that the explicitly shown protons are the most acidic on dibenzyl ketone. There are two, so that's how there can be TWO aldol condensations.
DISCLAIMER: This mechanism is very advanced and will be long. It is possible that this may be on an exam, but more likely you should expect something similar that takes less time to draw. Still, this is a good practice problem.
Also, before I draw it out, I will abbreviate the phenyl groups as
Typical enolate formation via a base grabbing a proton from the
#alpha#-carbon on a ketone.
Enolate nucleophilic attack of the target, the 1,2-diphenylethanedione.
Proton transfer to form the
#beta#-hydroxyketone intermediate. This step concludes the aldol addition portion of the overall first aldol condensation.
Another base comes in to grab an
#alpha#-proton and form another enolate. At this point this is done under high heat.
#pi#electron conjugation, heat, and the antiperiplanar conformation are enough to make the hydroxide leave without protonating it. This is the "condensation" step in the overall first aldol condensation.
Repeat step 1.
Repeat step 2, except for the fact that the nucleophilic carbon on the enolate is exactly four carbons away from the other carbonyl carbon on the compound. If a bond forms via the nucleophilic attack, it forms a five-membered ring, which is stable. Thus, intramolecular aldol condensation can occur here.
The bond has formed. This technically unnecessary step is to make it easier to visualize.
The oxide grabs a proton to form another
#beta#-hydroxyketone intermediate, as in step 3. This step concludes the second aldol addition that occurs during the second aldol condensation.
A base comes in yet again to grab a proton, as in step 4. At this point this is done under high heat.
The same dehydration-elimination of a hydroxide occurs, as in step 5. This is the "condensation" step in the overall second aldol condensation.