16.27 : 伍德沃德-霍夫曼选择规则和微观可逆性

Woodward–Hoffmann rules are a set of generalizations used to predict the stereochemistry of pericyclic reactions based on orbital symmetry.

The rule states that thermal pericyclic reactions are symmetry-allowed when the sum of (4q + 2)_s and (4r)_a components is odd and photochemically allowed when the sum is even.

Here, q and r are integers. (4q + 2)_s and (4r)_a designate the number of electrons in the suprafacial and antarafacial components.

Recall that suprafacial and antarafacial refer to the two distinct ways new bonds develop.

Let's apply this to the electrocyclization of octatriene.

First, identify the components. A triene is a _π6 component belonging to the (4q + 2) category.

Next, label the components as suprafacial or antarafacial. The ground state HOMO  has symmetric terminal lobes; bond formation occurs through a suprafacial, disrotatory pathway.

Finally, add the components. There is one (4q + 2)_s component and no (4r)_a components. The sum is one, and the reaction is thermally allowed.

Pericyclic reactions are reversible. The selection rules apply equally to the forward and reverse reactions as they proceed through the same transition state.