Stereoisomerism in Biphenyl Compounds (Atropisomerism)

Stereoisomerism in Biphenyl Compounds (Atropisomerism)

Stereoisomerism in Biphenyl Compounds (Atropisomerism) occurs when restricted rotation around the biphenyl bond creates stable, isolable isomers with distinct properties.

What is Biphenyl?

• Biphenyl is a molecule made of two benzene rings joined by a single bond:
  • The two rings can rotate around the central C–C bond.
  • However, rotation can be restricted if large groups are present at the ortho positions (positions 2 and 2′, next to the connecting bond).

Atropisomerism

• Atropisomerism is a form of axial stereoisomerism caused by restricted rotation around a single bond due to steric hindrance.
• Results in stable enantiomers (non-superimposable mirror images).
• The term comes from Greek: a-tropos = “without turning”.
• Commonly observed in ortho-substituted biphenyls.

Conditions for Atropisomerism in Biphenyls

1. Bulky Ortho Substituents

   • Sterically hinder rotation.
   • Examples: –NO₂, –COOH, –Br, –t-butyl, –OH, –NH₂

2. High Rotational Barrier

   • Barrier ≥ 100 kJ/mol allows isolation of enantiomers.
   • Lower barriers result in rapid interconversion.

3. Lack of Internal Symmetry

   • Must lack a plane of symmetry to be chiral.

Example

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