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Understanding the Diels-Alder Reaction: Mechanisms and Factors Influencing Reactivity

The Diels-Alder reaction is a powerful cycloaddition mechanism between a diene and a dienophile, enabling the formation of six-membered rings. Key factors in this process include the diene's s-cis conformation, the influence of substituents, and regioselectivity, which favors 1,2- or 1,4-substituted products. The reaction showcases the endo and exo product preference, influenced by temperature conditions, with the endo product often favored at lower temperatures due to secondary orbital interactions. Understanding these concepts is crucial for applying the Diels-Alder reaction in synthetic chemistry.

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Understanding the Diels-Alder Reaction: Mechanisms and Factors Influencing Reactivity

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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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