Lesson 11: Allylics, Conjugated Dienes, and Diels-Alder Reactions

Lecture 11- Allylics

Allylic Bromination:

• Mechanism:

  • Initiation: Bromine radicals are generated from NBS (N-Bromosuccinimide) under light or heat.

  • Propagation: A bromine radical abstracts a hydrogen atom from the allylic position, forming an allylic radical.

  • Termination: Another bromine molecule reacts with the allylic radical to produce the allylic bromide.

• Example: Propene reacts with NBS and light to form 3-bromopropene.

1. Allylic Oxidation:

   • Mechanism:

     • Formation of Allylic Cation: The alkene undergoes electrophilic attack, forming a resonance-stabilized allylic carbocation.

     • Oxidation: The allylic carbocation is oxidized by a suitable oxidizing agent, forming an allylic alcohol or ketone.

   • Example: Allylic oxidation of cyclohexene with KMnO4 leads to the formation of 1,2-cyclohexanediol.

Conjugated Dienes:

Conjugated dienes feature two adjacent carbon-carbon double bonds, allowing for resonance stabilization and unique reactivity patterns.

1. Electrophilic Addition to Conjugated Dienes:

   • Mechanism:

     • Diels-Alder Reaction: Involves a concerted cycloaddition of a conjugated diene (dienophile) and an alkene (dienophile), forming a cyclohexene ring.

     • Formation of σ-Bond: The HOMO (highest occupied molecular orbital) of the diene interacts with the LUMO (lowest unoccupied molecular orbital) of the dienophile.

   • Example: 1,3-butadiene reacts with ethene to form cyclohexene in a Diels-Alder reaction.

2. Oxidative Cleavage:

   • Mechanism:

     • Ozonolysis: Ozonolysis of a conjugated diene involves the cleavage of the double bonds by ozone (O3), followed by reductive work-up to yield carbonyl compounds.

     • Formation of Ozonide: Ozone attacks the double bonds, forming ozonide intermediates that decompose into carbonyl compounds.

   • Example: 1,3-butadiene reacts with ozone to form two molecules of formaldehyde upon reductive work-up.

Diels-Alder Reactions:

The Diels-Alder reaction is a powerful synthetic tool for forming six-membered rings with high stereoselectivity.

1. Mechanism:

   • Cycloaddition: The diene (conjugated) and the dienophile (alkene) undergo a cycloaddition reaction.

   • Stereoselectivity: The reaction occurs with retention of stereochemistry from the dienophile and diene precursors.

   • Example: 1,3-butadiene reacts with maleic anhydride to form cis-1,2,3,6-tetrahydrophthalic anhydride.