OChem: Syn-Addition

Addition of Peroxide to Alkene

  • The reaction converts alkene to an epoxide
  • Peroxyacid is MCPBA (meta-chloroperoxybenzoic acid)
  • Solvent CHCl3 or CH2Cl2
  • It is a single step in which an oxygen from peroxyacid is transferred simultaniously tethering to the alkene concerted rxn
  • This leads to syn-addition of the epoxide
  • By-product is a carboxylic acid
  • Remember: double bonds in benzene never react

Hydrocarbon-oxidation Protocol

  • A two-step reaction that converts alkene to an alcohol by the addition of water across the double bond
  • H and OH are added in the syn-addition leading to cis stereochemistry (i.e., 1:1 ratio of enantiomers)
  • An anti-Markovnikov rxn, H adds to C with less Hs
  1. Hydroboration step: BH3 (borane) and THF (tetrahydrofuran)
  2. Oxidative Step: H2O2 (hydrogen peroxide) and OH

Hydrocarbon Oxidation

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Treatment of organoborane with basic hydrogen peroxide leads to formation of an alcohol

Stereochemistry is preserved.

In the 1st step:

  • H adds to more substituted alkene carbon; the less accessible C
  • BH2 (boron dihydride) adds to less substituted alkene carbon; the more accessible C
  • Concerted step

In 2nd step:

  • BH2 replaced by OH

Addition of Hydrogen to Alkene

  • Reaction involves metal catalyst:
    • Pt (platinum)
    • Pd/C (palladium on carbon)
    • Nickel
  • Reduces alkenes and alkynes
  • Syn-addition and form 1:1 ratio of enantiomers, so FFA and BFA
  • Double bonds of benzene don’t react

Hydroxylation of Alkene=Dihydroxylation

  • Alkene is converted to a vicinal diol (aka 1,2-diol or 2 OH groups added to alkene) by undergoing oxidation
  • Syn-addition so form 1:1 ratio of enantiomers (FFA and BFA)
Route 1

  • Large scale rxn
  • Reagents: cold basic KMnO4 (aq)
  • Pros: less expensive, less toxic
  • Cons: low yields
Route 2

  • Small scale rxn
  • Reagents: OsO4 then H2O2
  • Pros: high yields
  • Cons: expensive and toxic

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Oxidative Cleavage of Alkene

  • Alkene are reductively cleaved with ozone and then Zn/acetic acid or (CH3)2S by forming ozonide intermediate
  • Alkene replaced by 2 carbonyl groups, i.e. 2 O= are added in between the alkene double bond
  • E/Z isomers produce same products

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Answers Below….


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