SYNTHESIS 7: BENZYNE FORMATION AND A DIELS-ALDER REACTION

[Pavia, D.L.; Lampman, G.M.; Kriz, G.S. Organic Laboratory Techniques; Saunders: New York, NY, 1995; pp 286-289, 293-296, 429-434]

Introduction This reaction involves a very interesting unstable intermediate called benzyne. Because unstable species are hard to observe by their very nature, they are often trapped in known reactions. In this case, benzyne is generated from anthranilic acid and trapped as a dienophile in a Diels-Alder reaction. The diene for the Diels-Alder reaction is 1,2,3,4-tetraphenylcyclopentadienone, which is synthesized from benzil and dibenzyl ketone in an Aldol reaction.

Experimental

Benzil: Place 0.30 g of benzoin in a 5-mL conical vial and add 2.0 mL of the copper(II) acetate/ammonium nitrate solution. Add a spin vane and attach an air condenser. Heat the mixture to reflux at about 135C until the solids dissolve and the mixture acquires a green color. Continue heating at 135C for ~60 min (the mixture should boil gently). Cool the mixture and transfer it by pipet to a beaker containing 4 mL cold water. Rinse the conical vial with a little more water. Stir the mixture vigorously with a stirring rod to induce crystallization and break up any solid mass that may have formed. Vacuum filter the crude benzil and then recrystallize it from 95% ethanol. Characterize the recrystallized benzil.

1,2,3,4-Tetraphenylcyclopentadienone: Add 0.100 g of benzil, 0.100 g of dibenzyl ketone (1,3-diphenyl-2-propanone), and 1 mL ethanol to a 3-mL conical vial. Add a spin vane and attach a water-cooled condenser. Heat the mixture at about 80C until the solids dissolve. Raise the temperature of the mixture until it is just below its boiling point. With a syringe, carefully add dropwise 0.15 mL of ethanolic potassium hydroxide solution through the condenser. After the addition, heat the mixture at a gentle boil for 15 min. Cool the mixture to room temperature and then place the vial in an ice bath for 5 min. Collect the product by vacuum filtration. Wash the crystals with three 0.5-mL portions of cold 95% ethanol. Characterize the crude tetraphenylcyclopentadienone (you dont need to purify it).

1,2,3,4-Tetraphenylnaphthalene: Place 0.100 g dry tetraphenylcyclopentadienone, 0.045 g anthranilic acid, and 1.2 mL of 1,2-dimethoxyethane in a 5-mL conical vial. Add a spin vane and attach a water-cooled condenser. Place 0.06 mL of isopentyl nitrite (isoamyl nitrite) in a 3-mL conical vial. Quickly dissolve the isopentyl nitrite in 0.5 mL 1,2-dimethoxyethane and cap the vial. Heat the mixture containing the tetraphenylcyclopentadienone to about 140C. When the solution starts to boil, add the solution of isopentyl nitrite through the top of the condenser with a pipet over a period of about 30 seconds. Make sure you insert the pipet deeply to add directly to the solution. Use a few drops of 1,2-dimethoxyethane to rinse the vial and add that to the reaction. Continue boiling the solution until it turns orange. If this color change does not occur after 10 min, add a drop of pure isopentyl nitrite and continue heating. If no color change occurs, repeat the addition of 1 drop of pure isopentyl nitrite and heat for another 10 min. After the color change, cool the reaction mixture to room temperature and use a pipet to transfer the mixture to a beaker containing 5 mL water and 2 mL methanol. Stir the mixture well to break up the precipitate. Collect the solid by vacuum filtration and wash it with ice-cold methanol. Additional solid may crystallize in the filtrate and you should add this to the rest of the solid. Recrystallize the solid from 2-propanol. Characterize the 1,2,3,4-tetraphenylnaphthalene.