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10.15 The Diels-Alder Reaction

10.15 The Diels-Alder Reaction. Synthetic method for preparing compounds containing a cyclohexene ring. In General. +. conjugated diene. alkene (dienophile). cyclohexene. via. transition state. Mechanistic Features. Concerted mechanism Cycloaddition Pericyclic reaction

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10.15 The Diels-Alder Reaction

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  1. 10.15 The Diels-Alder Reaction • Synthetic method for preparing compounds containing a cyclohexene ring

  2. In General... + conjugated diene alkene (dienophile) cyclohexene

  3. via transition state

  4. Mechanistic Features • Concerted mechanism • Cycloaddition • Pericyclic reaction • A concerted reaction that proceeds through a cyclic transition state.

  5. Recall the General Reaction... + • The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile. alkene (dienophile) conjugated diene cyclohexene

  6. Typical EWGs EWG C O C C C N What Makes a Reactive Dienophile? • The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond.

  7. O CH2 H2C CHCH CH benzene 100°C O O via: CH CH (100%) Example + H2C CH

  8. O CH2 H2C CHC O CH3 O via: O O H3C H3C O O O O (100%) Example + benzene 100°C

  9. O O CH2 CCOCH2CH3 CH3CH2OCC H2C CHCH benzene 100°C O COCH2CH3 (98%) COCH2CH3 O Acetylenic Dienophile +

  10. Diels-Alder Reaction is Stereospecific* *A stereospecific reaction is one in which stereoisomeric starting materials give stereoisomeric products; characterized by terms like syn addition, anti elimination, inversion of configuration, etc. • Diels-Alder: syn addition to alkene • cis-trans relationship of substituents on alkene retained in cyclohexene product

  11. + CH2 H2C C CHCH C H C6H5 COH only product H O O Example C6H5 COH H H

  12. O COH H + CH2 H2C C CHCH C H C6H5 C6H5 H COH only product H O Example

  13. Cyclic Dienes Yield Bridged BicyclicDiels-Alder Adducts

  14. O COCH3 H C C H CH3OC O H O COCH3 H COCH3 O +

  15. O O H COCH3 COCH3 H H H COCH3 COCH3 O O • is thesame as

  16. 10.16 The  Molecular OrbitalsofEthylene and 1,3-Butadiene

  17. Orbitals and Chemical Reactions • A deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals of the reactants. • Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital (LUMO) of the other.

  18. Orbitals and Chemical Reactions • We can illustrate HOMO-LUMO interactions by way of the Diels-Alder reaction between ethylene and 1,3-butadiene. • We need only consider only the  electrons of ethylene and 1,3-butadiene. We can ignore the framework of  bonds in each molecule.

  19. The  MOs of Ethylene • Red and blue colors distinguish sign of wave function. • Bonding  MO is antisymmetric with respect to plane of molecule. Bonding  orbital of ethylene;two electrons in this orbital.

  20. The  MOs of Ethylene Antibonding  orbital of ethylene;no electrons in this orbital. • LUMOHOMO Bonding  orbital of ethylene;two electrons in this orbital.

  21. The  MOs of 1,3-Butadiene • Fourp orbitals contribute to the  system of 1,3-butadiene; therefore, there are four molecular orbitals. • Two of these orbitals are bonding; two are antibonding.

  22. The Two Bonding  MOs of 1,3-Butadiene HOMO 4  electrons; 2 ineach orbital. Lowest energy orbital

  23. The Two Antibonding  MOs of 1,3-Butadiene Highest energy orbital LUMO Both antibondingorbitals are vacant.

  24. 10.17A  Molecular Orbital Analysisof theDiels-Alder Reaction

  25. MO Analysis of Diels-Alder Reaction • Since electron-withdrawing groups increase the reactivity of a dienophile, we assume electrons flow from the HOMO of the diene to the LUMO of the dienophile.

  26. MO Analysis of Diels-Alder Reaction HOMO of 1,3-butadiene • HOMO of 1,3-butadiene and LUMO of ethylene are in phase with one another. • Allows  bond formation between the alkene and the diene. LUMO of ethylene (dienophile)

  27. MO Analysis of Diels-Alder Reaction HOMO of 1,3-butadiene LUMO of ethylene (dienophile)

  28. H2C CH2 H2C CH2 A “Forbidden" Reaction • The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? +

  29. H2C CH2 H2C CH2 A “Forbidden" Reaction + HOMO of one ethylenemolecule. HOMO-LUMOmismatch of twoethylene moleculesprecludes single-stepformation of two newbonds. LUMO of other ethylenemolecule.

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