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Lecture 9b

Lecture 9b. Diels-Alder Reaction II. Diene Conformation. A diene and dienophile undergo a cycloaddition Prototype reaction: butadiene and ethylene [ 4n+2 ] p -addition In order for the aromatic transition state to form, the diene has to be s-cis conformation ( cisoid ).

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Lecture 9b

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  1. Lecture 9b Diels-Alder Reaction II

  2. Diene Conformation • A diene and dienophile undergo a cycloaddition • Prototype reaction: butadiene and ethylene [4n+2]p-addition • In order for the aromatic transition state to form, the diene has to be s-cis conformation (cisoid) diene dienophile “aromatic TS” cyclo-adduct DHf= 73.16 kJ/mol 72.54 kJ/mol DHf= 79.12 kJ/mol 40.53 kJ/mol

  3. Substituent Effect I • Substituents on the diene and the dienophile can have a significant effect on their reactivity and therefore the conditions required to carry out the reaction • Strategically placed donor and acceptor groups both seem to facilitate the reaction

  4. Substituent Effect II • Explanation (simplified version) • Acceptor groups lower the orbital energies for the HUMO and the LUMO orbital because they reduce the electron-density in the p-system • Donor groups raise the orbital energies for the HUMO and the LUMO orbital because they increase the electron-density in the p-system • Thus, placing a donor in the diene and an acceptor in the dienophile (or vice versa  inverse electron-demand) is most effective in decreasing the HUMO-LUMO gap, which directly relates to the activation energy for the reaction

  5. Alder-Stein Rule • Diels-Alder reactions are stereoselective because they are concerted for most parts (=all bonds are broken and formed at the same time) • The stereochemistry of the diene and the dienophile are retained in the cyclo-adduct • Cis to cis • Trans to trans

  6. Regioselectivity • For many Diels-Alder reactions, a high degree of regioselectivity is observed favoring six-membered rings with 1,2- or 1,4-substitution • Example: Reaction of 1-methoxybutadiene (R=OMe) and methyl vinyl ketone (X=COCH3)

  7. Endo Rule I • If a bicycle is formed in the reaction, an endo or an exo product can be formed in the reaction depending on the temperature • Endo product • Exo product • Which product is preferentially formed highly depends on the reaction conditions • Low temperature: endo product • High temperature: exo product

  8. Endo Rule II • Example:Maleic anhydride and cyclopentadiene • Exo approach • This product is usually thermodynamically more stable • Endo approach • This product is formed at lower temperatures because of the secondary orbital interaction (in red) which lower the activation energy for the endo pathway

  9. Endo Rule III • The activation energies for the endo and the exo pathway are different resulting different rates of reaction • The endo pathway has the lower activation energy and is therefore favored at low temperatures (=kinetic control) • The endo-product can be converted to the exo-product by heating (T=190 oC, 1.5 hrs) because the reaction is reversible (=thermodynamic control) • At high temperatures (T=206 oC), an almost equimolar mixture of the endo and the exo-product is obtained from the reaction of dicyclopentadiene with maleic anhydride DHf= -293.3 kJ/mol (AM1) DHf= -300.7 kJ/mol (AM1)

  10. Which Reactions are allowed? • The simplest reaction would be the reaction of ethylene with itself • Experience tells us that this reaction does not take place • Ethylene has only two p-electrons • The combination of the HUMO and LUMO of ethylene leads to one bondingand one anti-bondinginteraction, which cancel each out other energetically

  11. Which Reactions are allowed? • The next case would be the reaction of ethylene with butadiene • This reaction seems to proceed with low yields (~20 %) • Butadiene has four p-electrons, which means that the two lowest p-orbitals are filled making p2theHOMO and p3* the LUMO anti-bonding orbitals bonding orbitals

  12. Which reactions are allowed? • Either combination leads to the formation of two new bonding interactions and no anti-bonding interactions • The cyclo-adduct is formed in this reaction • Reactions that involve [4n+2]p-electrons are allowed thermodynamically speaking (D) • Reactions that involve [4n]p-electrons often require photochemically activation (hn)

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