Cyclic Diene Dienophiles

Cyclic dienes added to alkene dienophiles also lead to two new form of stereoisomeric products, the endo and exo products (Kilway, “Lecture”). In cases where dienes and dienophiles align above one another, the endo product is formed (Hunt). The endo product will have two axial substituent groups (Kilway, “Lecture”). This formation typically forms the major product because it takes less kinetic energy to be formed due to secondary orbital interactions with the substituents in transition states (Hunt). The rule that guides this principle is typically referred to as the “Alder Endo Rule”. In instances where the diene and dienophile are staggered and they must reach for each other in order to form the product, the exo product is formed (Hunt).
This product will have equatorial substituent groups (Kilway, “Lecture”). However, endo and exo products are only formed through bridgehead molecules in which carbons 4 and 5 are sp2 hybridized. The variance in these formations is visualized below in Figure 3.

Formation of endo and exo products. Figure 3. Experimental Section
Dimethyl tetraphenylphthalate.
Dimethyl acetylenedicarboxylate (0.10 mL, 0.81 mmol) and nitrobenzene (1 mL) were added to to tetraphenylcyclopentadienone (0.103 g, 0.268 mmol) in a small reaction tube with a boiling stick using glass pipettes to form a dark purple soln. The rxn was then lowered into a sand bath and allowed to reflux until color change from dark purple to tan was observed. Upon cooling, ethanol (3 mL) was added and the soln was shaken to facilitate combination. The tan soln was then placed in an ice bath to cool for 10 min. The solvent was removed from the solid under vacuum filtration and washed with cold ethanol to retrieve a white precipitate. The final product, dimethyl tetraphenylphthalate was massed (0.055 g, 0.110 mmol,
41.2%).
Hexaphenylbenzene. Triphenylcyclopentadienone (0.102 g, 0.265 mmol) and diphenyl acetylene (0.498 g, 27.9 mmol) were combined in a small reaction tube and lightly capped. Then the rxn was heated to reflux using a sand bath, producing a mixture with a dark red hue. Once color change from dark red to brown was observed, the soln was shaken until a white precipitate was observed. Diphenyl ether (2 mL) was added, and the purple-colored soln was heated using a sand bath until the solid was dissolved. After cooling, toluene (2 mL) was added, and the soln was placed into an ice bath for 10 min. The solvent was removed from the product using vacuum filtration. The product was then rinsed with a pipette full of toluene to yield a shimmery pink final product of hexaphenylbenzene. The final product was then massed (0.123 g, 0.230 mmol, 86.8%).
Tetraphenylnapthalene.
A dark purple soln was created from tetraphenylcyclopentadienone (0.506 g, 1.32 mmol) and glyme (3 mL) in a large reaction tube with a boiling chip. Isoamyl nitrite (0.35 mL, 2.9 mmol) was added slowly via syringe to the soln, and the soln was allowed to gently reflux over a sand bath for 5 min. Another soln was made in a separate reaction tube by dissolving anthranilic acid (0.250 g, 1.82 mmol) in glyme (2 mL). This anthranilic acid/glyme soln was added to the refluxing soln dropwise via pipette to form a red soln. After 5 min, the reaction tube was gently shaken and a color change from red to dark brown was observed. After cooling, ethanol (10 mL) and sodium bicarbonate (5 mL) were added, forming a precipitate. The resulting solid was collected using vacuum filtration. The light yellow precipitate was washed twice with cold water (5 mL) and twice with cold ethanol (5 mL), then filtered to dryness. The resulting solid was then recrystallized using a nitrobenzene/ethanol solvent pair (1 mL). Vacuum filtration was used to yield a white mass of precipitate. The final product, tetraphenylnapthalene was massed (0.357 g, 0.825 mmol, 62.5%): mp 198.0-203.0
⁰C.
Triptycene. A tan colored soln was made by dissolving anthracene (0.400 g, 2.24 mmol) in isoamyl nitrite (0.40 mL, 2.97 mmol) and glyme (4 mL) in a large reaction tube with a boiling chip. Using a sand bath, the soln was heated to a gentle reflux, covering the tube with a septum. The needle to a syringe was placed into the septum to prevent too much pressure from forming during the reaction. A mixture of anthranilic acid (0.521 g, 3.88 mmol) and glyme (2 mL) was created in a separate test tube and added dropwise via syringe over 20 min through the septum. During this time, a color change to dark orange was observed. Isoamyl nitrite (0.40 mL, 2.97 mmol) was added to the mixture, and the soln was allowed to reflux for 10 min. After cooling, ethanol (5 mL) and 3 M sodium hydroxide (10 mL) was added, and a brown precipitate formed. The solvent was removed using vacuum filtration and rinsed with cold ethanol and water to yield a yellow precipitate (0.206 g, 0.809 mmol). The crude solid was then placed in a round bottom flask with maelic anhydride (0.204 g, 2.08 mmol) and triglyme (4 mL). This was then placed under an assembled reflux condenser and allowed to reflux for 5 min through the heat of a sand bath. After cooling, ethanol (2 mL) and sodium hydroxide (6 mL) were added. The resulting mixture was vacuum filtered with cold ethanol and water to yield a tan precipitate. The final product, triptycene was massed (0.032 g, 0.126 mmol, 6.06%).

Results and Discussion In our first three reactions, we synthesized dimethyl tetraphenylphthalate, hexaphenylbenzene, and tetraphenylnapthalene. In each reaction, we used