30.7 Sigmatropic Rearrangements

Sigmatropic rearrangements are like molecular acrobatics. These reactions involve a sigma bond doing a fancy flip across a chain of double bonds, changing the molecule's structure while keeping its overall connectivity intact.

These rearrangements come in different flavors, classified by how far the sigma bond moves. Understanding the rules behind them helps predict how molecules will change shape under heat or light, making them useful tools for chemists.

Sigmatropic Rearrangements

Concept of sigmatropic rearrangements

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• Involve concerted migration of a σ bond across a conjugated π system while π system remains intact allowing for σ bond migration
• σ bond breaks at original location and forms at new location resulting in structural change while π system adjusts to accommodate new bonding arrangement
• Number of π and σ bonds remains constant throughout rearrangement maintaining overall connectivity of molecule (1,3-pentadiene, 1,5-hexadiene)
• Sigmatropic rearrangements are a type of pericyclic reaction, characterized by a concerted mechanism

Notation for sigmatropic rearrangements

• Classified using [i,j] notation where i represents number of atoms σ bond migrates over from starting point and j represents number of atoms σ bond migrates over to reach final position
• [1,5] sigmatropic rearrangement: σ bond migrates over one atom from starting point and five atoms to reach final position (1,3-pentadiene)
• [3,3] sigmatropic rearrangement: σ bond migrates over three atoms from starting point and three atoms to reach final position (Cope rearrangement of 1,5-hexadiene)
• Sum of i and j determines number of atoms involved in rearrangement
  • For thermal reactions, i + j must be even number (Hückel topology)
  • For photochemical reactions, i + j must be odd number (Möbius topology)

Suprafacial vs antarafacial sigmatropic modes

• Suprafacial sigmatropic rearrangements: migrating σ bond remains on same face of π system throughout rearrangement
  1. Stereochemistry retained when i + j = $4n + 2$ (thermal reactions, Hückel topology)
  2. Stereochemistry inverted when i + j = $4n$ (photochemical reactions, Möbius topology)
• Antarafacial sigmatropic rearrangements: migrating σ bond starts on one face of π system and ends on opposite face
  1. Stereochemistry inverted when i + j = $4n + 2$ (thermal reactions, Hückel topology)
  2. Stereochemistry retained when i + j = $4n$ (photochemical reactions, Möbius topology)
• Woodward-Hoffmann rules predict stereochemical outcome of sigmatropic rearrangements based on number of atoms involved (i + j) and reaction conditions (thermal or photochemical)
  • Thermal reactions follow Hückel topology
  • Photochemical reactions follow Möbius topology

Orbital symmetry considerations

• Orbital symmetry plays a crucial role in determining the feasibility and stereochemical outcome of sigmatropic rearrangements
• The conservation of orbital symmetry governs the allowed pathways for these reactions
• Thermal and photochemical reactions differ in their orbital symmetry requirements, leading to distinct reaction outcomes
• Understanding orbital symmetry helps predict the stereochemistry of sigmatropic rearrangements and explains why certain reactions are favored under specific conditions