UV light can flip the script on electrocyclic reactions. It excites electrons, swapping HOMO and LUMO symmetries. This changes how molecules dance together, leading to different products than thermal reactions would.

Photochemical electrocyclic reactions follow a simple rule: even electron pairs go disrotatory, odd pairs go conrotatory. This flips the thermal reaction outcomes, giving us trans products where we'd expect cis, and vice versa.

Photochemical Electrocyclic Reactions

Effects of UV on orbital symmetries

• Ultraviolet (UV) irradiation promotes an electron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO) changes the symmetry of the frontier molecular orbitals involved in the reaction
  • The HOMO and LUMO switch symmetries upon UV irradiation alters the stereochemical outcome of the electrocyclic reaction
    • Thermal reactions involve the HOMO, while photochemical reactions involve the LUMO (1,3-butadiene, 1,3,5-hexatriene)
  • The HOMO and LUMO have opposite symmetry properties
    • If the HOMO is symmetric, the LUMO will be antisymmetric, and vice versa (cyclobutene, cyclohexadiene)
  • The excited state resulting from UV irradiation plays a crucial role in determining the reaction pathway

Stereochemistry in photochemical electrocyclizations

• For polyenes with an even number of electron pairs (4n electrons):
  • Photochemical electrocyclic reactions proceed in a disrotatory manner
    • Disrotatory: the terminal substituents rotate outward in opposite directions (1,3-butadiene)
  • The product will have a trans configuration at the newly formed sigma bond (cyclobutene)
• For polyenes with an odd number of electron pairs (4n+2 electrons):
  • Photochemical electrocyclic reactions proceed in a conrotatory manner
    • Conrotatory: the terminal substituents rotate in the same direction, either clockwise or counterclockwise (1,3,5-hexatriene)
  • The product will have a cis configuration at the newly formed sigma bond (cyclohexadiene)

Thermal vs photochemical electrocyclic paths

• For polyenes with 4n electrons (1,3-butadiene):
  1. Thermal electrocyclic reactions proceed in a conrotatory manner, leading to a cis product (cis-cyclobutene)
  2. Photochemical electrocyclic reactions proceed in a disrotatory manner, leading to a trans product (trans-cyclobutene)
• For polyenes with 4n+2 electrons (1,3,5-hexatriene):
  1. Thermal electrocyclic reactions proceed in a disrotatory manner, leading to a trans product (trans-cyclohexadiene)
  2. Photochemical electrocyclic reactions proceed in a conrotatory manner, leading to a cis product (cis-cyclohexadiene)
• The stereochemical outcomes of thermal and photochemical electrocyclic reactions are opposite for a given polyene system due to the involvement of the HOMO in thermal reactions and the LUMO in photochemical reactions
• The Woodward-Hoffmann rules can be used to predict the stereochemical outcome of electrocyclic reactions based on the number of electron pairs and the reaction conditions (thermal or photochemical)

Pericyclic reactions and related concepts

• Photochemical electrocyclic reactions are a subset of pericyclic reactions, which involve the concerted rearrangement of electrons in a cyclic transition state
• Photoisomerization can occur as a result of these reactions, leading to structural changes in the molecule
• The quantum yield of a photochemical reaction measures its efficiency in terms of the number of product molecules formed per photon absorbed