Chapter 14 Conjugation, Resonance, and Dienes

Overview of Dienes and Conjugation

• Conjugation and Dienes in Organic Chemistry

  • Differentiate types of dienes

  • Predict behaviors of dienes in addition reactions

  • Assess reactivity of dienes and dienophiles in Diels-Alder reaction

  • Predict outcomes (molecular, stereochemical, regio) of the Diels-Alder reaction

14.1 Conjugation

• 1,3-Diene Characterization

  • Electron delocalization in 1,3-dienes

  • Example: 1,3-butadiene (buta-1,3-diene)

• Molecular Orbital (MO) Theory

  • Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO)

  • Characteristics:

    • 1.54 Å for double bonds, shorter bonds indicate stability

    • Importance of overlap of p orbitals

14.8 and 14.9 Dienes: Stability

• Conformations of 1,3-butadiene

  • s-trans conformer stability: 2.3 kcal more stable than s-cis conformer

  • At room temperature, easy interconversion between conformers

  • Conjugated dienes exhibit enhanced stability

14.10 and 14.11 Electrophilic Addition to Conjugated Dienes

• Addition Mechanism

  • Addition to isolated dienes behaves like alkenes

  • Addition to conjugated dienes can lead to allylic cations

  • Product interconversion occurs with temperature changes:

    • At lower temperatures: 1,2-addition products

    • At higher temperatures: 1,4-addition products favored

Mechanism Considerations

• Kinetic vs. Thermodynamic Products

  • Kinetic product: formed rapidly, smaller activation energy

  • Thermodynamic product: most stable product, lower Gibbs free energy

  • Note: In most organic reactions, the kinetic product = thermodynamic product

14.12 – 14.14 The Diels-Alder Reaction

• Reagents Required

  • Diene (with H or electron-donating group)

  • Alkene or alkyne with electron-withdrawing group (dienophile)

• Outcome

  • Produces cyclohexene ring

Stereochemical Requirements/Outcomes

• Conformation Requirements

  • Diene must be in s-cis conformation for the reaction

  • Overlap of C1 and C4 p orbitals with dienophile’s p orbitals leads to new sigma bond formation

  • Involves syn stereochemistry due to concerted mechanism

Reactivity and Stereospecificity

• Endo vs. Exo Products

  • Usually occurs in endo fashion when kinetically controlled

• Stereospecific Nature of Diels-Alder Reaction

  • Configuration of both dienophile and diene is retained in the product

Regioselectivity in Diels-Alder Reaction

• Examples of Selectivity

  • Organic groups influence regioselectivity of the addition process

Retrosynthetic Analysis of Diels-Alder Reaction

• Retrosynthesis

  • Illustrates how to synthesize or analyze products and reactants in synthetic pathways

  • Significant in designing reactions for different organic transformations.