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2013-03-15 06:49:04

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  1. Classes of dienes
    • Cumulated: C=C=C-C
    • Conjugated: C=C-C=C
    • Isolated: C=C-C-C=C

    Conjugated dienes have interacting pi bonds
  2. Preparation of conjugated dienes
    2° Br --t-BuOK--> forms C=C toward less subs C (more H)
  3. Diene properties
    • Interaction between adjacent p orbitals and sp2-hybridized orbitals
    • Smaller C-C distance
    • Must be in trans form to get these properties
  4. MO theory re: dienes
    • # atomic orbitals = # molecular orbitals
    • wave property (+/-) can be additive or destructive addition
    • #MO = #energy levels, nodes increase from 0 to #MO-1
  5. Multiple configurations of dienes
    • two potential conformations (s-cis/s-trans) allow P orbital overlap
    • s-cis is higher in E due to steric hinderence
  6. 1,2 vs 1,4 addition in depth
    • C=C-C=C --HBr-> C-CBr-C=C (1,2 addition)
    • C=C-C=C --HBr-> C-C=C-CBr (1,4 addition)

    Both products are always seen, but the ratio can be manipulated with temperature

    • 0°C - 1,2 addition (71%); 1,4 addition (29%)
    • Kinetic control
    • Not a lot of E to get over trans state barrier
    • 1,2 has lower T state

    • 40°C - 1,2 addition (15%); 1,4 addition (85%)
    • Thermodynamic Control
    • More E present in system, so getting over trans state is not as important
    • More stable product is major product then
  7. Pericyclic reactions (general)
    • Concerted reactions (no ionic/radical intermediates)
    • Ring of e- moved in a closed loop
    • Solvent doesn't affect rate (no intermediate)
    • 1. Cycloadditions
    • 2. Electrocyclic reactions
    • 3. Sigmatopic rearrangement
  8. Diels-Alder Reaction
    • 4+2 cyclo addition
    • Diene + dienophile -> substituted cyclohexene

    Thermodynamic consideration: must be performed between 40°C and 200°C.  Anything above 200°C will cause ΔG to be +

    • Dienophile: e- withdrawing groups (carbonyl, O, etc) increase yield of reaction
    • Cis/trans of dienophile groups is retained in the final product
    • Alkynes can act as dienophiles, and leave a 2x bond afterward

    • Diene: The Diene exists in cis and trans (equilibrium), but rxn will only occur when diene is in s-cis
    • A compound locked into trans will not react
    • A compound locked into cis is HIGHLY reactive
  9. endo/exo
    1,3-pentadiene is locked into cis and is HIGHLY reactive

    there are two possible stereoisometric products: endo (syn to larger bridge), exo (anti to larger bridge)

    The trans state is much lower for endo product because of Pi-pi interactions, although exo would be expected due to steric hinderence
  10. Explanation of why two dienophiles can't react together
    Diels alder rxn occurs when HOMO (diene) reacts with LUMO (dienophile)

    ethylene + ethylene LUMO and HOMO do not "line up"

    with a photon of light an electron is excited to a higher energy level, which redefines LUMO and HOMO for that species AND CAUSES THEM TO LINE UP!

    2+2 and 4+4 will work in light! 2+4 will stop working!