Question

a. Circle the chiral centers in the following molecule and identify them as R or S Suggest a second component for synthesis of the molecule above via a Diels-Alder reaction (i.e. completer the reaction below) c. Is the configuration of your starting material in part b, E or Z? d. The concept of aromaticity (i.e. 4n + 2 pi electrons stable, aromatic; 4n electrons = unstable, antiaromatic) can be used to understand reaction like the Diels-Alder and related reactions. Use the concept of aromaticity/antiaromaticity to explain why the Diels-Alder reaction works, but the reaction below does not. e. Azulene is a deep blue hydrocarbon (structure shown below) that has a surprisingly large dipole moment. Use resonance and the concept of aromaticity to explain this observation.

Answer 1

5a. The given molecule is dimethyl dicyano cyclohexene compound has two chiral centres, both are attached to the cyano groups. As drawn one is exo and another is endo oriented, if the molecule is observed carefully one isomer is R and other is S because the molecule differs only at these positions and and are in opposite stereo oriented hence one of them is R and other is S.

5b. The second component for the synthesis of above molecule via Diels Alders reaction is trans 1,2 dicyano ethylene. This reaction is stereospecific with respect to both diene and dienophiles. A trans dienophile yields trans substitution products.

5c. The configuration of the starting material is E.

5d. The thermal [2+2] cycloaddition reactions are geometrically forbidden, as the HOMO and LUMO of the participating olefins would not be able to achieve the orbital overlap required for the sigma bond formation. Many processes that are Woodword Hoffman allowed can be forbidden to occur because of geometrical constraints of the system.

5e. Azulene is a deep blue hydrocarbon which has a surprisingly large dipole moment of 1.08 when compared with naphthalene which is zero. This polarity can be explained regarding azulene as the fusion of the 6 pi electrons of the cyclopentadienyl anion and 6 pi electrons of tropylium cation. One electron from the seven membered ring is transferred to five membered ring to give each ring the aromatic stability by Huckel's rule.

The reactivity studies have confirmed that seven membered ring is electrophilic and five membered ring is nucleophillic in nature.