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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
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Preparation of conjugated dienes
2° Br --t-BuOK--> forms C=C toward less subs C (more H)
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Diene properties
- Interaction between adjacent p orbitals and sp2-hybridized orbitals
- Smaller C-C distance
- Must be in trans form to get these properties
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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
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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
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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
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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
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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
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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
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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!
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