29.3: Stereochemistry of Thermal Electrocyclic Reactions
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Frontier orbital theory can be used to predict the stereochemistry of electrocyclic reactions. Electrons in the HOMO are the highest energy and therefore the most easily moved during a reaction. A molecular orbital diagram can be used to determine the orbital symmetry of a conjugated polyene's HOMO. Thermal reactions utilize the HOMO from the ground-state electron configuration of the molecular orbital diagram while photochemical reactions utilize the HOMO in the excited-state electron configuration.
The molecular orbital of 1,3,5-hexatriene in its ground state electron configuration has psi three as its HOMO. The terminal molecular orbital lobes of the HOMO with the same sign are on the same side which predicts disrotatory ring closure under thermal conditions.
Disrotatory cyclization is observed during the electrocyclic reaction of 2,4,6-octatriene. The trans,cis, cis -2,4,6-octatriene isomer produces cis -5,6-dimethyl-1,3-cyclohexadiene as the product of thermal cyclization while the trans,cis, cis -2,4,6-octatriene isomer produces trans -5,6-dimethyl-1,3-cyclohexadiene.
The molecular orbital of a conjugated diene, such as 1,3-butadiene has psi two as the HOMO in its ground state electron configuration. The terminal molecular orbital lobes of the HOMO with the same sign are on opposite sides which predicts conrotatory ring closure under thermal conditions. However, the equilibrium of the electrocyclic reaction only allows for the ring opening to be observed.
Thus heating cis -3,4-dimethylcyclobutene causes the conrotatory ring opening to form cis , trans -2,4-hexadiene. Likewise, trans -3,4-dimethylcyclobutene forms trans , trans -2,4-hexadiene when heated.
A pattern begins to form revealing a relationship between the number of double bonds in the conjugated polyene and the rotation during electrocyclic reactions. For thermal electrocyclic reactions, polyenes with an odd number of double bonds undergo disrotation and those with an even number of double bond undergo conrotation.
Exercise \(\PageIndex{1}\)
1) The thermal electrocyclic ring opening of trans -3,4-dimethylcyclobutene could form trans , trans -2,4-hexadiene or cis , cis -2,4-hexadiene. However, the trans , trans -2,4-hexadiene is the isomer obtained from the reaction. Explain how it is possible to get both products and why the trans , trans -2,4-hexadiene is preferred.
2) If a conjugated tetraene were to undergo a thermal electrocylic reaction would the orbital nodes undergo con or disrotation?
- Answer
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1) Dienes undergo conrotation during thermal electrocyclic ring opening. Conrotation means the orbital nodes both rotate in the same direction either both clock with or both counter clockwise. If the nodes both rotate counter clockwise the trans , trans -2,4-hexadiene isomers forms. If they both rotate clockwise the cis , cis -2,4-hexadiene isomer is formed. Trans double bonds are more stable than cis due to steric strain. The trans , trans -2,4-hexadiene is perferably formed because it is more stable.
2) A tetraene has an even number of double bonds so it would be expected to undergo conrotation.