29.4: Photochemical Electrocyclic Reactions
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As discussed in Section 30.1 , irradiation of a conjugated polyene with ultraviolet light causes an electron from the groude-state HOMO to be excited to the ground state LUMO. This creates a new higher energy HOMO in an electron configuration called the excited state. Electron excitation changes the symmetry of the new HOMO which has a corresponding effect on the reaction stereochemistry. Under photochemical reaction conditions conjugated dienes undergo disrotatory cyclization whereas under thermal conditions they underwent conrotatory cyclization. Likewise, conjugated triene undergo conrotatory photochemical cyclization while undergoing disrotatory thermal cyclization. For example, trans,trans-2,4-hexadiene undergoes conrotatory photochemical cyclization to form cis -3,4-dimethylcyclobutene.
The conjugated tirene, trans , cis,trans -2,4,6-octatriene undergoes conrotation to form trans -5,6-dimethyl-1,3-cyclohexadiene during photochemical cyclization.
Generalized Statement of Woodward-Hoffmann Rules for Electrocyclic Reactions
Thermal and photochemical electrocyclic reactions always produce the opposite stereochemistry products due to the difference in symmetries in their HOMO frontier orbitals. This idea can be combined with the trend of even and odd polyenes to provide simple rules to predict the stereochemistry of electrocyclic reactions.
| Number of Double Bonds | Thermal | Photochemical |
|---|---|---|
| Odd | Disrotatory | Conrotatory |
| Even | Conrotatory | Disrotatory |
Exercise \(\PageIndex{1}\)
Would the following electrocyclic reaction be con or disrotatory? Please draw the expected product.
- Answer
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An even number of double bonds in photochemical conditions predicts a disrotatory reaction.