Epoxide Chemistry and Formation

Epoxide Chemistry

  • Epoxides are cyclic ethers with high ring strain due to non-ideal bond angles, making them more electrophilic than linear ethers.
  • Epoxides are valuable synthetic intermediates, more so than bromonium ions.
  • Reactions of epoxides provide a route to forming alcohols.
  • Epoxides are used in industry in epoxy resins for adhesives, paints, and in the oil, gas, and electronics industries.

Epoxide Formation

  • The simplest synthesis involves exposing an alkene to a peroxyacid (peracids), like meta-chloroperbenzoic acid (mCPBA), yielding a carboxylic acid byproduct.

  • Mechanism proceeds in a syn-periplanar manner.

  • All steps happen simultaneously.

    • Step 1: Nucleophilic ππ bond attacks the electrophilic oxygen of the peracid (σσ*).
    • Step 2: The O-O bond breaks and a new carbonyl bond is formed.
    • Step 3: The existing C=OC=O bond breaks and a new O-H bond is formed by proton abstraction.
    • Step 4: The original O-H bond breaks, and a new C-O bond is formed between the peracid and the alkene.

  • The nucleophile attacks, displacing carboxylate, which is a good leaving group due to resonance stabilization of the negative charge.

  • When the nucleophile is an alkene, an epoxide forms.

Ring Strain

  • Cyclopropane CCC Angles: 60∘60^{\circ}
  • Cyclobutane CCC Angles: ~85∘85^{\circ}
  • Cyclopentane CCC Angles: ~97−103∘97-103^{\circ}
  • Cyclohexane Angles lose to ideal angles.
  • Increasing ring strain from cyclohexane to cyclopropane.

Stereochemistry in Epoxide Formation

  • Trans-alkenes give trans-epoxides, and cis-alkenes give cis-epoxides.
  • Syn-periplanar addition results in two possible isomers, depending on which side of the alkene undergoes the reaction.

Stereoselective Epoxidation of Alkenes

  • To synthesize only one isomer, methods rely on using a chiral substrate, reagent, or catalyst.
  • One method uses a chiral allylic alcohol and mCPBA.
  • An allylic hydroxyl group, if on a chiral center, will direct mCPBA to the same face due to favorable hydrogen bonding interactions, leading to a major and minor product.