Diels-Alder reaction

Pericyclic reaction - reaction with no intermediates and no charges

Cycloaddition - two or more molecules form to create a cyclic structure

Diels-Alder reaction - Diene + Dienophile = Product

  • Geometry is retained through reaction

  • will create a racemic mixture

Diene - the 4 pi system

  • electron rich

  • electron donor groups

  • R groups must be cis

  • can be open chain or cyclic

Dienophile - the 2 pi system

  • electron poor

  • electron-withdrawing groups

    • the minimum for reaction is a phenyl group or a chlorine atom

  • R groups can be either cis or trans

  • Simple alkenes that work

    • conjugated carbonyl compounds

    • nitro compounds

    • nitriles, sulfones

    • aryl alkenes

    • vinyl ethers and esters

    • haloalkenes

    • dienes

Product - the final product

  • Six-member ring with a double bond inside the ring and a conjugating group outside of the ring, opposite to the alkene

  • Temperature should be around 100 - 150 °C to work

Sterioselectivity

  • Geometry is retained

    • Endo rule for Diels-Alder reaction

    • Exo product - more stable thermodynamic product

  • less sterics

    • Endo product - less stable kenetic product

  • more sterics

    • If reaction is reversible

  • Exo product formed

    • If reaction is irreversible

  • Endo product formed

    • Easiest way to figure out final sterochemistry

      1. Draw the overall mechanism

      2. Draw both molecules in the plane of the paper with the diene over the denophile

      3. Add all hydrogen atoms to the carbons with stereogenic centers

      4. Draw out the final product unfolded to show the geometry of the hydrogens

      5. Add the stereochemistry of the additional substituents

        1. This is the endo product

Dimerization of dienes

  • Due to high LUMO and low HOMO, dienes are susceptible to dimerization

Solvent - contrary to expectation for organic molecules, water works well as an “anti-solvent” due to it forcing organics to close proximity

Intermolecular Diels-Alder

  • Sterics are more important than endo rule

Regioselectivity

  • Attack the less substituted carbon first as it will create a more stable cation as an intermediate

  • The carbons with the largest coefficient of HOMO will attach first

Summary and tips

  • Diels-Alder reaction will lead to an aromatic which will direct ortho and para