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The Diels-Alder Reaction

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 [4+2] cycloaddition pericyclic reaction

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The Diels-Alder Reaction

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  1. 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 • [4+2] 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. new s bond new s bond dienophile diene transition state

  7. 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. Typical EWGs http://www.brunel.ac.uk/depts/chem/ch241s/re_view/barry/diels2.htm

  8. O CH2 H2C CHCH CH benzene 100°C O CH Example + H2C CH (100%)

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

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

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

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

  13. Diels-Alder Reaction is Stereospecific* • syn addition to alkene • cis-trans relationship of substituents on alkene retained in cyclohexene product *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.

  14. Stereospecific, concerted, syn addition:

  15. Predict the reaction products: 1. Consider the alignment of the reactants

  16. 2. Consider the charge distribution in each of the reactants

  17. + CH2 H2C C CHCH C H C6H5 COH H O O Example C6H5 COH H H Only the s-cis conformation of the diene can participate in a Diels–Alder reaction Only product. But, is it enantiomeric?

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

  19. Cyclic dienes yield bridged bicyclicDiels-Alder adducts.

  20. Since only cis dienes can participate in Diels–Alder reactions: 5- & 6- membered rings are ideal

  21. O COCH3 H C C H CH3OC O H O COCH3 H COCH3 O +

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

  23. The p Molecular OrbitalsofEthylene and 1,3-Butadiene

  24. Reactants’ HOMO and LUMO

  25. 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.

  26. 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 p electrons of ethylene and 1,3-butadiene. We can ignore the framework of s bonds in each molecule.

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

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

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

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

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

  32. Two Possible Configurations of Bridged Bicyclic Compounds

  33. Secondary orbital overlap favors the endo product formation

  34. A p Molecular Orbital Analysisof theDiels-Alder Reaction

  35. MO Analysis of Diels-Alder Reaction • Inasmuch as 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.

  36. 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 s bond formation between the alkene and the diene LUMO of ethylene (dienophile)

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

  38. 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? +

  39. H2C CH2 H2C CH2 A "forbidden" reaction + HOMO of one ethylenemolecule HOMO-LUMOmismatch of twoethylene moleculesprecludes single-stepformation of two news bonds LUMO of other ethylenemolecule

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