Chem 336 - Spring 2000
Organic Chemistry III
Dr. Carl C. Wamser

Chapter 22 - Conjugated Dienes

Molecular Orbitals (MOs) of Conjugated Pi Systems

• n atomic orbitals (AOs) combine to make n molecular orbitals (MOs)
• ethene - pi bond and pi* antibond, with two electrons in the pi bond
• allyl pi system (could be cation, radical, or anion) - 3 pi MOs (bonding, nonbonding, antibonding)
  • allyl cation (2 pi electrons), allyl radical (3 pi electrons), allyl anion (4 pi electrons)
• 1,3-butadiene - 4 pi MOs (two bonding and two antibonding) with 4 pi electrons
• frontier MOs are the most important for electron pushing
  • highest occupied MO - HOMO
  • lowest unoccupied MO - LUMO

Conjugate Addition to Dienes

• conjugated dienes are very reactive towards electrophiles
  • addition gives a relatively stable allylic cation
• conjugated dienes can give 1,2 or 1,4 addition
  • the two ends of the allyl cation may be nonequivalent

• addition at low temperature favors the 1,2-addition product (kinetically controlled)
• addition at high temperature favors the 1,4-addition product (thermodynamically controlled)
• heating the kinetic mixture of products gives the thermodynamic mixture (i.e., mostly 1,4)
  • kinetic control (lower Ea) - the more substituted cation has more + charge, reacts faster
  • thermodynamic control - the more substituted alkene is the more stable product
  • thermodynamic control requires equilibration to form the more stable product, since it has a higher barrier
• Note - this situation of the more stable product having a higher activation barrier is unusual
  • in most situations, a more stable product means a lower barrier (The Hammond Postulate)

The Diels-Alder Reaction

• cycloaddition of a diene to an alkene (usually called a dienophile in this reaction)
• overall, two pi bonds are converted to two sigma bonds while making the ring

• note that the transition state is a 6-membered ring of delocalized pi electrons
  • such transition states are particularly stable when they have (4n+2) electrons
• the diene must be in an s-cis conformation for this reaction
  • s-cis means the single bond between the two double bonds is cis
  • cyclic dienes are much more reactive than acyclic dienes
  • cyclopentadiene undergoes reaction with itself

• the reaction goes best with alkenes having electron-withdrawing substituents (dienophiles)

• this allows good electron flow from the diene HOMO to the alkene LUMO
• Lewis acids also enhance the Diels-Alder reaction by complexing the dienophile
• the combination of electron-deficient diene with electron-rich alkene also works
• stereochemistry of the diene and the alkene is retained

• endo product is usually the major product, due to secondary interactions between the pi systems

Other Pericyclic Reactions

• pericyclic reactions have cyclic delocalized transition states and are more stable when the transition stste unvolves (4n+2) electrons (aromatic)
• cycloadditions (e.g., the Diels-Alder reaction) are just one type of reaction termed "pericyclic"
  • cycloadditions make two new sigma bonds in a new ring
• electrocyclic reactions make one new sigma bond in a new ring, by joining the ends of a pi system
• sigmatropic reactions shift the position of a sigma bond with respect to a pi system
  • Claisen rearrangement and Cope rearrangement are examples
• Cope rearrangement

• Claisen rearrangement - special case of an oxy-Cope rearrangement
  • allyl phenyl ether -> o-allylphenol