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then the reaction is allowed. And if the new overlaps are unfavorable (antibonding over- laps), then the reaction is forbidden. Fukui’s method (the frontier orbital method ) is a little simpler, so it is used in this book. The examples presented in the following sec- tions illustrate how this method is applied.
22.3 Electrocyclic Reactions
Pericyclic reactions are commonly divided into three classes: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. An electrocyclic reaction forms a sigma bond between the end atoms of a series of conjugated pi bonds within a molecule. The 1,3-butadiene to cyclobutene conversion is an example, as is the similar reaction of 1,3,5-hexatriene to form 1,3-cyclohexadiene:
As can be seen from these examples, the product has one more sigma bond and one less pi bond than the reactant. Let’s begin by considering the simplest electrocyclic reaction, the thermally induced interconversion of a diene and a cyclobutene. As illustrated in the following example, the reaction is remarkably stereospecific, occurring only by a conrotatory motion:
In electrocyclic reactions the end carbons of the conjugated system must rotate for the p orbitals on these carbons to begin to overlap to form the new carbon– carbon sigma bond. The preference for the stereochemistry of the rotation in these reactions can be understood by examination of the new orbital overlap in the HOMO as the rotation occurs. For the formation of the new sigma bond to be favorable, ro- tation must occur so that the overlap of the orbitals forming this bond is bonding in the HOMO.
CH 3
CH 3
H
H
H
H
CH 3
CH 3
175 °C
1,3-Butadiene Cyclobutene
1,3,5-Hexatriene 1,3-Cyclohexadiene
22.3! ELECTROCYCLIC REACTIONS 965
For the ground-state reaction of a conjugated diene the HOMO is! 2. Conrotation of this MO causes the plus lobe of the p orbital on one end of the pi system to overlap with the plus lobe of the p orbital on the other end of the pi system:
This bonding overlap in the HOMO when the rotation occurs makes the formation of the new sigma bond favorable. The conversion of a diene to a cyclobutene by a conro- tatory motion is thermally allowed. In contrast, disrotation causes the plus lobe of the p orbital on one end of the pi system of the diene to overlap with the minus lobe of the p orbital on the other end in the HOMO:
The antibonding overlap in the HOMO when disrotation occurs makes the formation of the new sigma bond unfavorable. The disrotatory closure of a diene to a cyclobutene is thermally forbidden. The requirement for a favorable bonding overlap of the orbitals forming the new sigma bond in the HOMO allows the preferred rotation to be predicted for any electro- cyclic reaction. Let’s consider the photochemical conversion of a butadiene to a cy- clobutene. Because the reaction occurs through the excited state, the HOMO is! 3 *. As can be seen, disrotation is necessary here for the new overlap to be bonding:
In accord with this analysis, numerous experiments have shown that the disrotatory clo- sure of a diene to a cyclobutene is indeed the pathway that occurs when the compound is irradiated with UV light.
38951-22-U32-
H
H3C H H CH 3
H
HOMO of the excitedstate of the reactant π 3 *
H
H 3 C
H
CH 3
H
H
Plus lobe of onewith plus lobe of other p orbital overlaps p orbital
disrotation + +
Disrotation of a diene is photochemically allowed.
H
H
H
H
H 3 C CH 3
+ +^ +^ + + +
H
H 3 C H H CH 3
H
HOMO of the reactant
H
H 3 C
H
CH 3
H
H
Plus lobe of one p orbital overlaps with minus lobe of other p orbital
disrotation – +
Disrotation of a diene is thermally forbidden.
H
H
H
H
H3C CH 3
H
H3 C H H CH 3
H
HOMO ofthe reactant
H
H3C
H CH
H
H
Plus lobe of onewith plus lobe of other p orbital overlaps p orbital
conrotation –
Conrotation of a diene is thermally allowed.
H
H
H
H 3 C H
CH 3
966 CHAPTER 22! PERICYCLIC REACTIONS
Solution This anion has five p orbitals, so there are five MOs (see Figure 22.4). There are six electrons in these MOs, so the HOMO for the thermal (ground state) reaction is! 3 nb^. Conrotation is necessary to form the product with the methyl groups trans.
Because the overlap to form the new bond is antibonding, this reaction is thermally for- bidden. PROBLEM 22. Use orbital drawings to show that conrotation of a triene is thermally forbidden and that disrotation is photochemically forbidden. PROBLEM 22. Use orbital drawings to determine whether these reactions are allowed or forbidden:
It is possible to generalize the preferences for conrotation or disrotation based on the number of electron pairs in the pi MOs of the reacting molecule. A diene, with two pi electron pairs (two pi bonds), has! 2 as its HOMO in the ground state. Because this MO has one node, conrotation is favored for the thermal reaction. A triene, with three pi electron pairs (three pi bonds), has! 3 as its HOMO. This MO has two nodes (one more than! 2 of a diene), so the opposite rotation, disrotation, is preferred. It is appar- ent that the favored rotation alternates with the number of pi electron pairs. Therefore, a molecule with four pi bonds (! 4 is the HOMO) prefers conrotation. Furthermore, the preference for the excited state is just reversed from that of the ground state because the excited-state HOMO always has one more node than the ground-state HOMO. These preferences are summarized in the following chart: Number of Electron Pairs Disrotation Conrotation Odd Thermally Photochemically allowed allowed Even Photochemicallyallowed Thermallyallowed
h%
H 3 C CH 3 H 3 C
H 3 C CH 3
b)
a) "
CH 3
#^ #
H 3 C CH 3
H3C HH CH 3
CH 3
CH 3 H
H
CH 3
H3C
H
conrotation H..
968 CHAPTER 22! PERICYCLIC REACTIONS
Click Coached Tutorial Problems for more practice using MOs to predict the stereochemistry of Electrocyclic Reactions.
(Note that an odd number of electron pairs equals 4 n # 2 electrons, whereas an even number of electron pairs equals 4 n electrons, where n is any integer, including zero.) PROBLEM 22. Use the preceding chart to determine whether the reactions of problem 22.6 are allowed or forbidden.
PRACTICE PROBLEM 22.
Determine the number of electron pairs in this reaction, the type of rotation, and whether the reaction is allowed or forbidden:
Solution We have usually analyzed this type of reaction from the other direction—that is, the conversion of a diene to a cyclobutene. However, the same analysis works for either di- rection of a reaction. In this case an even number (two) of electron pairs are involved; the sigma bond and the pi bond of the reactant are converted to the two pi bonds of the product. The reaction occurs by a conrotation:
According to the chart, a conrotation involving an even number of electron pairs is ther- mally allowed. Therefore, the reaction shown is allowed. Note that the same reaction is forbidden under photochemical conditions. PROBLEM 22. Indicate the number of electron pairs and the type of rotation for these reactions and de- termine whether each is allowed or forbidden:
a) b) h% "
CH 2 CH 3
CH 2 CH 3
H
H
CH 2 CH 3
CH 2 CH 3
CH 3
CH 2 CH 3 CH 2 CH 3
CH 3
CH 3
CH 3
H
H
H
H
CH
CH3 conrotation
CH 3
CH 3
CH 3
CH 3
H
H
22.3! ELECTROCYCLIC REACTIONS 969
