Orbitals and Organic Chemistry: Pericyclic Reactions

Look at Figure 30-1, and tell which molecular orbital is the HOMO and whichis the LUMO for both ground and excited states of ethylene and 1,3-butadiene.

trans-3,4-Dimethylcyclobutene can open by two conrotatory paths to give either (2E,4E)-2,4-hexadiene or (2Z,4Z)-2,4-hexadiene. Explain why both products are symmetry-allowed, and then account for the fact that only the 2E,4Eisomer is obtained in practice.

What product would you expect to obtain from the photochemical cyclizationof (2E,4Z,6E)-2,4,6-octatriene? Of (2E,4Z,6Z)-2,4,6-octatriene?

What stereochemistry would you expect for the product of the Diels–Alder reaction between (2E,4E)-2,4-hexadiene and ethylene? What stereochemistry would you expect if (2E,4Z)-2,4-hexadiene were used instead?

1,3-Cyclopentadiene reacts with cycloheptatrienone to give the productshown. Tell what kind of reaction is involved, and explain the observed result.Is the reaction suprafacial or antarafacial?

When a 2,6-disubstituted allyl phenyl ether is heated in an attempted Claisen rearrangement, migration occurs to give the p-allyl product as the result oftwo sequential pericyclic reactions. Explain.

The cyclohexadecaoctaene shown isomerizes to two different isomers, depending on reaction conditions. Explain the observed results, and indicate whether each reaction is conrotatory or disrotatory.

Which of the following reactions is more likely to occur? Explain.

The following reaction takes place in two steps, one of which is a cycloaddition while the other is a reverse cycloaddition. Identify the two pericyclic reactions, and show how they occur.

Two sequential pericyclic reactions are involved in the following furan synthesis. Identify them, and propose a mechanism for the transformation.

Predict the product of the following pericyclic reaction. Is this [5,5] shift a suprafacial or an antarafacial process?

Propose a pericyclic mechanism to account for the following transformation.

Vinyl-substituted cyclopropanes undergo thermal rearrangement to yield cyclopentenes. Propose a mechanism for the reaction, and identify the pericyclic process involved.

The following thermal rearrangement involves two pericyclic reactions in sequence. Identify them, and propose a mechanism to account for the observed result.

Do the following electrocyclic reactions take place in a conrotatory or
 disrotatory manner? Under what conditions, thermal or photochemical, would you carry out each reaction?

The following thermal isomerization occurs under relatively mild conditions. Identify the pericyclic reactions involved, and show how the rearrangement occurs.

Heating (1Z,3Z,5Z)-1,3,5-cyclononatriene to 100°C causes cyclization and formation of a bicyclic product. Is the reaction conrotatory or disrotatory? What is the stereochemical relationship of the two hydrogens at the ring junctions, cis or trans?

(2E,4Z,6Z,8E)-2,4,6,8-Decatetraene has been cyclized to give 7,8-dimethyl-1,3,5-cyclooctatriene. Predict the manner of ring-closure- conrotatory or disrotatory for both thermal and photochemical reactions, and predict the stereochemistry of the product in each case.

Answer Problem 30-22 for the thermal and photochemical cyclizations of (2E,4Z,6Z,8Z)-2,4,6,8-decatetraene.

reaction involved.

Karahanaenone, a terpenoid isolated from oil of hops, has been synthesized by the thermal reaction shown. Identify the kind of pericyclic reaction, and explain how karahanaenone is formed.

What stereochemistry—antarafacial or suprafacial—would you expect to observe in the following reactions?

 (a) A photochemical [1,5] sigmatropic rearrangement

 (b) A thermal [4 + 6] cycloaddition 

(c) A thermal [1,7] sigmatropic rearrangement

 (d) A photochemical [2 + 6] cycloaddition

Bicyclohexadiene, also known as Dewar benzene, is extremely stable despite the fact that its rearrangement to benzene is energetically favored. Explain why the rearrangement is so slow.

Ring-opening of the trans-cyclobutene isomer shown takes place at much lower temperature than a similar ring-opening of the cis-cyclobutene isomer. Explain the temperature effect, and identify the stereochemistry of each reaction as either conrotatory or disrotatory.

Photolysis of the cis-cyclobutene isomer in Problem 30-35 yields cis-cyclododecaen-7-yne, but photolysis of the trans isomer yields trans-cyclododecaen-7-yne. Explain these results, and identify the type and stereochemistry of the pericyclic reaction.

The 1H NMR spectrum of bullvalene at 100 °C consists only of a single peak at 4.22 d. Explain.

The following rearrangement was devised and carried out to prove the stereochemistry of [1,5] sigmatropic hydrogen shifts. Explain how the observed result confirms the predictions of orbital symmetry.

The following reaction is an example of a [2,3] sigmatropic rearrangement. Would you expect the reaction to be suprafacial or antarafacial? Explain.