Diels-alder reaction

• Diels-Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene and a dienophile (an alkene or alkyne) to form a six-membered cyclic compound.
• It proceeds via a concerted mechanism, meaning the bonds are formed simultaneously without intermediates.
• This reaction is stereospecific and regioselective, typically occurring under mild conditions and often accelerated by heat or electron-withdrawing groups on the dienophile.

Basic Principles of Diels-alder reaction

1. Reaction Type

   • A [4+2] cycloaddition between a conjugated diene (4π electrons) and a dienophile (2π electrons).
   • Named after Otto Diels and Kurt Alder.

2. Conformation Requirement

   • The diene must be in the s-cis conformation to align properly with the dienophile.
   • The s-trans conformation does not allow proper orbital overlap, preventing the reaction.

Reaction Mechanism and Example of Diels-alder reaction

1. Mechanism

   • The reaction proceeds in a single concerted step, where π bonds break and new σ bonds form, creating a cyclohexene ring.
   • The reaction is stereospecific, maintaining the relative stereochemistry of reactants in the product.

2. Example Reaction

   • 1,3-Butadiene reacts with ethene to form cyclohexene.
   • Reaction equation: $H2C=CH-CH=CH2 + H2C=CH2 -> Cyclohexene$

Significance

• This reaction is valued for its high regioselectivity, stereoselectivity, and atom economy.
• These features make it a powerful tool in the synthesis of complex natural products, pharmaceuticals, polymers, and materials.

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