An In Depth Look At The Versatility Of Diels-Alder Reactions

An In Depth Look at the Versatility of Diels-Alder Reactions

Introduction.

It all started in the 1920's, when a pair of skillful chemists discovered what would become one of the most useful experiments ever performed. The name of this fascinating discovery is the Diels-Alder reaction. This reaction was named after two chemists, Paul Otto Diels, and Kurt Alder. Diels was a German organic chemist who invented a dehydrogenation process involving selenium. His most important work was, of course, the Diels-Alder reaction. However, the Diels-Alder was a joint effort. The other chemist responsible for this reaction was Kurt Alder. Alder received degrees in chemistry from the University of Berlin. He moved to the Christian Albrecht University at Kiel where he worked under Diels. Together, they created a method to creating dienes to form cyclic rings, which became known as the "Diels-Alder synthesis". This joint effort led to nobel prizes for both of them.

The Diels-Alder Reaction is a one-step reaction that involves making a six membered ring, from a diene and dienophile. The Diels-Alder synthesis opened the way to synthesizing important biochemical substances such as insecticides, dieldrin and aldrin.

Experimental Section.

Synthesis of Dimethyl Tetraphenylphthalate.

A mixture of tetraphenylcyclopentadiene (0.10g, 0.26 mmol), dimethyl acetylene-dicarboxylate (0.1mL, 0.8 mmol), nitrobenzene (1.00 mL) and a boiling chip was placed in a small tube. The mixture was then heated to reflux. Upon refluxing, the solution had turned tan in color. The mixture was then allowed to cool until it was warm to the touch. Upon cooling, ethanol ( 3.00 mL) was added and the whole reaction was placed in an ice bath. The mixture was then filtered via vacuum filtration. The resulting solid was washed with ethanol to give a white product, dimethyl tetraphenylphthalate ( 0.1g, 0.2 mmol, 60 % yield)

Synthesis of Hexphenylbenzene.

A mixture of Tetraphenylcyclopentadienone (0.10g, 0.26 mmol) and diphenyl acetylene (0.50g, 3 mmol) was placed into a small reaction tube. The reaction was then heated to reflux. Upon refluxing, the solution turned a brown color. The solution was then cooled. Diphenyl ether (2.00 mL) was then added to the brown solution and the solution was heated until all of the solid was dissolved. The solution was allowed to cool which resulted in a product. Toluene (2.00 mL) was added to the product which was then placed in an ice bath. The whole mixture was then filtered via vacuum filtration. The resulting solid was washed with toluene to give a brown product, hexaphenylbenzene ( 0.93 g, 0.37 mmol, 100 % yield)

Synthesis of Tetraphenylnapthalene.

A mixture of tetraphenylcyclopentadienone ( 0.50 g, 1.3 mmol) and glyme (3.00 mL) and a boiling chip was added to a large reaction tube. Isoamyl nitrite ( 0.25 mL, 3.2 mmol) was added to the reaction tube. The mixture was then heated to a gentle reflux. While refluxing, a solution of anthranilic acid ( 0.25 g, 1.8 mmol) and glyme (2.00 mL) was added dropwise to the large reaction tube. The solution turned yellow in color and was allowed to reflux for an additional two minutes. After completion, the reaction tube was cooled and ethanol ( 10 mL) and saturated sodium bicarbonate (2 mL) were added. The reaction tube was then vigorously shook and placed in an ice bath. The mixture was filtered via vacuum filtration. The resulting white solid was recrystallized from a nitrobenze/ethanol solvent pair yielding a final white product, tetraphenyl naphthalene ( 0.26 g, 0.60 mmol, 46 % yield): m.p.195-199 oC

Synthesis of Triptycene

To a large reaction tube was added anthracene (0.40 g, 2.0 mmol), isoamyl nitrite (0.4 mmol), and 1,2 dimethoxyethane (0.4 mL). The reaction tube was then allowed to reflux via sand bath. In the meantime, to another tube, was added anthranilic acid (0.520 g, 3.8 mmol) and 1,2 dimethoxyethane (0.2 mL). The solution containing anthranilic acid was added to the anthracene mixture dropwise over a twenty minute period. The solution then turned brown in color and was allowed to reflux for an additional ten minutes. The solution was then cooled and ethanol (5 mL) followed by a 3 M sodium hydroxide solution (10 mL) was added. The brown solid was collected via vacuum filtration and then was then washed with a cold ethanol/water solution to remove any brown color. The washed solid was placed into a large reaction tube and maleic anhydride (0.20 g, 2.0 mmols) and triglyme (4 mL) was added. The tube was then refluxed for five minutes and then was cooled. Ethanol (2 mL) followed by a 3 M sodium hydroxide solution (6 mL) was again added. The brown solution was filtered via vacuum filtration and washed with a cold ethanol/water solution to give a white solid which was recrystallized from ethanol yielding a white product, triptycene (0.047 g, 0.2 mmols, 9.0 % yield)

Results and Discussion.

Dimethyl tetraphenylphthalate:

This reaction is typically favored because the aromatic ring that is formed is resonance stabilized. The product formed was a white solid. Before this reaction is further investigated, it must be mentioned that the diene is the tetraphenylcyclopentadienone and the dienophile is dimethylacetylene. The reaction proceeded, with the help of nitrobenzene, as the tetraphenylcyclopentadienone and the dimentylacetylene are combined together. The nitrobenzene served as a solvent to dissolve both compounds further driving the reaction. The reaction was also then driven by refluxing because heat drives the reaction further to form the resonance-stabilized intermediate. The solution then lost carbon dioxide, which was indicated by the color change to a tan color, forming a six-membered ring. The reaction gave a 60% yield of a white product dimethyl tetraphenyphtalate.

Hexaphenyl benzene.

In this reaction, the tetraphenylcyclopentadienone was the diene and the diphenylacetylene was the dienophile. This is a unique reaction because it requires no solvent to drive the reaction. The reason why this worked is because the dienophile was highly reactive because of strong electronegativity which drove the reaction because the diene was stable. The tetraphenylcyclopentadienone and the dephenyacetylene were reacted with each other. The reaction proceeded with the aid of heat which helped to drive