Expt 7. A Photocatalyzed Intramolecular 2+2 Cycloaddition

Relevant textbook readings - Klein - chapter 16.8 Mohrig

Literature References (1) Cookson, R. C.; Crundwell, E.; Hill, R. R.; Hudec, J. J. Chem. Soc. 1964, 3062– 3075 (2) Krauch, C. H.; Metzner, W. Chemische Berichte 1965, 98, 2106-2110.

Overview - You will carry out an intramolecular cycloaddition of the Diels-Alder from expt 6  to form the cool cage compound, 1-methyl-10-isopropylpentacyclo[6.4.0.0^2,7.0^4,12.0^5,9]dodeca-3,6-dione. Traditionally, this reaction requires a strong UV light source and special quartz glassware. However, research recently carried out at WSU has shown that the reaction can be carried out with visible light using 4,4'-dichlorodibenzalacetone (DCDBA) as a photocatalyst. Recall that DCDBA was prepared in expt 5. Also note that the formation of the intramolecular 2+2 adduct provides proof that the Diel-Alder adduct has the endo stereochemistry because the exo adduct the C-C double bonds are too far apart to be able to react with each other.

The proton NMR and IR spectra will verify the reaction product and a mp (lit mp = 92-94 ºC)² will also be obtained to verify purity.

Procedure 

Reaction:

1. Combine 50 mg of the Diels-Alder adduct with 12 mg 4,4’-dichlorodibenzalacetone in an NMR tube. Add 2 mL CH₂Cl₂ and cap and invert to dissolve. Place the tube in a test tube rack exposed to two 60-W LED lights for 1 hour. 

Purification:

1. Transfer the solution to a vial and evaporate the solvent under N₂ stream. Wash the obtained crude solid with 0.5 mL toluene and then collect it by vacuum filtration.
   

Analysis:

1. Obtain the mp, proton NMR, and IR spectrum of the product.