Safety: Consider all compounds to be irritants and toxic. If any
of the compounds contact your skin, you should rinse with cool
water for fifteen minutes. CO is very poisonous and since it is
being released in the reaction the reaction must be carried out in
the fume hood and the reaction must be allowed to stay in the
fume hood for an adequate period following the reaction. Do not
breath the fumes. Dump all residues and do all washing in the
fume hood. Generally do all work in the fume hood with the
door down at an effective level. Wear gloves, goggles, aprons
and appropriate clothing, including closed shoes while carrying
out all procedures.
Experiment 23: Synthesis of Tetraphenyl-1,2-Dihydrophthalic
Anhydride
In this experiment tetraphenyl-1,2-dihydrophthalic anhydride will be prepared via a
Diels-Alder reaction followed by loss of carbon monoxide. The reaction and mechanism are
outlined below. You can use this as a model for your total synthesis project. Think about
how you would have to modify the Diels-Alder to obtain hexaphenylbenzene using
tetraphenylcyclopentadienone as one of the reagents.
This reaction is a good model for
the formation of the central benzene ring of hexaphenylbenzene because the second
electrocyclic reaction is driven by the loss of carbon monoxide (a leaving group that leaves
as a gas) and the formation of the diene in the central ring which is highly conjugated.
Obviously you will not be doing this exact reaction, but you will be doing something very
similar. Do a retro-electrocyclic reaction to starting with hexaphenylbenzene to the
intermediate still containing the strained carbonyl. Then you should be able to see the retro
Diels-Alder. Again, your synthesis can be completed using this sort of reaction. This will
give you an idea about how you can use a known reaction to predict a related reaction.
The intermediate product, I, can exist in two possible stereochemistries, exo or endo
shown below.
The endo configuration is favored in this reaction.
Now as far as your modeling for your total synthesis goes, consider the following…….
Procedure
1. Mix 3.5 grams of tetraphenylcyclopentadienone (prepared by student previously via aldol
condensation reactions) and 0.95 g of dry maleic anhydride thoroughly in a 100 mL
round bottom flask. Add 2.5 mL of bromobenzene to the round bottom. This will result in a
thick mixture of solids. A reflux condenser is then attached to the flask and is attached as
normal to water hoses (red tubing, water going in the bottom of the condenser and out the
top of the condenser into the cup sink in the hood).
2. Since CO (carbon monoxide) will be produced in the reaction, it is necessary to set up
the reaction in a fume hood or attach a CO trap. Please see your TA or instructor if for some
reason you are not working in a fume hood.
3. When the apparatus is assembled, the mixture is refluxed for a total of three hours
until the dark brown color of the solution disappears and a light, yellow solid mass
appears. Agitate the mixture occasionally during the reflux period.
4. When the reaction appears to be complete, cool the flask to room temperature and add
25 mL of hexanes. Using a spatula, break up the solid into a finely divided solid in the
hexanes and vacuum filter the mixture. Wash the solid with two 5 mL portions of hexanes
and let it dry. Take the melting point and then recrystallize the solid from an appropriate
solvent system, followed by vacuum filtration and drying. Weigh the purified product and
determine its melting point. Measure the IR spectrum for the product as a film or a KBr
pellet.