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. dienedienophile

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 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?

Depends on conformation. If flat and fully conjugated, antiaromatic!!!! Lower energy conformation (than anti-aromatic) is not flat 8 pi electrons