Chapter 16 & 17 Discussion

Endo and exo are meaningless without substitutents to provide frame of reference

Meso = identical

1) Recognize that this is a Diels Alder reaction by the diene and dienophile (on the right)

2) Draw a cyclohexene ring

3) Build models of the possible isomers that could result from the cycloaddition

4) Use resonance to help predict regioisomer

5) Temperature for endo/exo preference.

diene dienophile

3) Build models of the possible isomers that could result from the cycloaddition

4) Use resonance to help predict regioisomer

5) Temperature for endo/exo preference.

endo

exo

Endo and exo

+ enantiomer + enantiomer

Exo (thermodynamic product)

Endo (kinetic product)

?

1) Recognize that this is a Diels Alder reaction by the diene and dienophile (on the right)

2) Draw a cyclohexene ring

3) Build models of the possible isomers that could result from the cycloaddition

4) Use resonance to help predict regioisomer

5) Temperature for endo/exo preference.

Four possible regioisomers and diastereomers (+ enantiomers)

Resonance contributors show which regioisomers will form

Regiochemistry is established. How about endo or exo?

?

No meta.

Problem:Diels-Alder Reactants?

Flat, cyclic, conjugated pi system and 4n + 2 pi electrons

Aromatic if 2, 6, 10, 14, 18 pi electrons…..Anti-aromatic if 4, 8, 12, 16, ….. pi electrons

If neither then not aromatic or anti-aromatic.

Anti-aromatic more stable than expected.

Anti-aromatic – less stable and likely will not form at all.

4 pi electrons. Antiaromatic

Cyclopropenyl anion

Aromatic?

6 pi electrons, aromatic

Cyclopentadienyl anion

Aromatic?

10 pi electrons aromatic

Lactarius azulene

chamomileYarrow

Azulene

aromatic : cyclic, 10 electrons, flat, conjugated

Naphthalene

Why is naphthalene nonpolar and azulene polar?

Azulenenaphthalene

Aromatic, anti-aromatic or neither?

Aromatic, anti-aromatic or neither?

Depends on conformation. If flat and fully conjugated, antiaromatic!!!!

Lower energy conformation (than anti-aromatic) is not flat

8 pi electrons