Pericyclic Reaction

Figure: 29-00-01UN Title: A polar reaction. Caption: A polar reaction is one in which a nucleophile reacts with an electrophile.
Figure: 29-00-02UN Title: A radical reaction. Caption: A radical reaction is one in which a new bond is formed using one electron from each of the reactants.
Figure: 29-00-03UN Title: An electrocyclic reaction. Caption: An electrocyclic reaction is an intramolecular reaction in which a new sigma bond is formed between the ends of a conjugated pi bond.
Figure: 29-00-04UN Title: Reversible electrocyclic reaction. Caption: A reverse electrocyclic reaction is one in which a sigma bond in a cyclic compound breaks, forming a conjugated pi system that has one more pi bond than the cyclic compound.
Figure: 29-00-05UN Title: A cycloaddition reaction. Caption: Two different pi bond-containing molecules react to form a cyclic compound.
Figure: 29-00-06UN Title: A sigmatropic rearrangement. Caption: In this example of a sigmatropic rearrangement, a sigma bond is broken in the middle of the pi system in the reactant, a new sigma bond is formed in the product, and the pi bonds rearrange.
Figure: 29-00-07UN Title: A sigmatropic rearrangement. Caption: In this example of a sigmatropic rearrangement, a sigma bond is broken at the end of the pi system in the reactant, a new sigma bond is formed in the product, and the pi bonds rearrange.
Figure: 29-00CO Title: Ball-and-stick model of vitamin D 3 . Caption: Vitamin D 3 is formed from 7-dehydrocholesterol by two pericyclic reactions.
Figure: 29-01 Title: Figure 29.1. The interaction of in-phase p atomic orbitals produces a bonding pi molecular orbital that is lower in energy than the p atomic orbitals. Caption: The interaction of out-of-phase p atomic orbitals produces an antibonding  * molecular orbital that is higher in energy than the p atomic orbitals.
Figure: 29-02 Title: Figure 29.2. Four p atomic orbitals interact to give the four  molecular orbitals of 1,3-butadiene. Caption: The normal electronic configuration of a molecule is known as its ground state.
Figure: 29-03 Title: Figure 29.3. Six p atomic orbitals interact to give the six  molecular orbitals of 1,3,5-hexatriene. Caption: The number of bonding interactions decreases and the number of nodes increases as the molecular orbitals increase in energy.
Figure: 29-03-01UN Title: Electrocyclic reactions. 2 E ,4 Z ,6 E -Octatriene cyclizes to cis -5,6-dimethyl-1,3-cyclohexadiene. 2 E ,4 Z ,6 Z -Octatriene cyclizes to t rans -5,6-dimethyl-1,3-cyclohexadiene. Caption: In the first reaction, a cis product is formed under thermal conditions. In the second reaction, a trans product is formed under thermal conditions.
Figure: 29-03-02UN Title: Electrocyclic reactions. 2 E ,4 Z ,6 E -Octatriene cyclizes to t rans -5,6-dimethyl-1,3-cyclohexadiene. 2 E ,4 Z ,6 Z -Octatriene cyclizes to cis -5,6-dimethyl-1,3-cyclohexadiene. Caption: In the first reaction, a trans product is formed under photochemical conditions. In the second reaction, a cis product is formed under photochemical conditions.
Figure: 29-03-03UN Title: Electrocyclic reactions. 2 E ,4 Z -Hexadiene cyclizes to cis -3,4-dimethylcyclobutene. Caption: A cis product is formed under thermal conditions.
Figure: 29-03-04UN Title: Electrocyclic reactions. 2 E ,4 E -Hexadiene cyclizes to trans -3,4-dimethylcyclobutene. Caption: A trans product is formed under thermal conditions.
Figure: 29-03-05UN Title: Electrocyclic reactions. 2 E ,4 Z -Hexadiene cyclizes to trans -3,4-dimethylcyclobutene. 2 E ,4 E -Hexadiene cyclizes to cis -3,4-dimethylcyclobutene. Caption: In the first reaction, a trans product is formed under photochemical conditions. In the second reaction, a cis product is formed under photochemical conditions.
Figure: 29-03-06UN Title: Conrotatory ring closure. Caption: Ring closure which occurs when both orbitals rotate in the same direction to achieve overlap is called conrotatory.
Figure: 29-03-07UN Title: Disrotatory ring closure. Caption: Ring closure which occurs when the orbitals rotate in opposite directions to achieve overlap is called disrotatory.
Figure: 29-03-08UN Title: Ring closure with symmetric HOMO. Caption: Molecules with symmetric HOMOs give disrotatory ring-closure products.
Figure: 29-03-09UN Title: Ring closure with antisymmetric HOMO. Caption: Molecules with antisymmetric HOMOs give conrotatory ring-closure products.
Figure: 29-03-10UN Title: (2 E ,4 Z ,6 E )-Octatriene ring closure. Caption: (2 E ,4 Z ,6 E )-Octatriene ring closure is disrotatory, yielding cis -5,6-dimethyl-1,3-cyclohexadiene.
Figure: 29-03-11UN Title: (2 E ,4 Z ,6 Z )-Octatriene ring closure. Caption: (2 E ,4 Z ,6 Z )-Octatriene ring closure is disrotatory, yielding trans -5,6-dimethyl-1,3-cyclohexadiene.
Figure: 29-03-12UN Title: Photochemically induced (2 E ,4 Z ,6 Z )-octatriene ring closure. Caption: Photochemically induced (2 E ,4 Z ,6 Z )-octatriene ring closure is conrotatory, yielding cis -5,6-dimethyl-1,3-cyclohexadiene.
Figure: 29-03-13UN Title: (2 E ,4 Z )-Hexadiene ring closure. Caption: (2 E ,4 Z )-Hexadiene undergoes conrotatory ring closure to yield cis -3,4-dimethylcyclobutene.
Figure: 29-03-14T01 Title: Table 29.1. Woodward-Hoffmann rules for electrocyclic reactions. Caption: The stereochemistry of an electrocyclic reaction depends on the mode of ring closure, which depends on the number of conjugated pi bonds in the reactant and on whether the reaction is carried out under thermal or photochemical conditions.
Figure: 29-03-14UN Title: (2 E ,4 E )-Hexadiene ring closure. Caption: (2 E ,4 E )-Hexadiene undergoes conrotatory ring closure to yield trans -3,4-dimethylcyclobutene.
Figure: 29-04-01T02 Title: Table 29.2. Configuration of the product of an electrocyclic reaction. Caption: If the bonds to the substituents in the reactant point in opposite directions, the substituents will be cis in the product if the ring closure is disrotatory and trans if ring closure is conrotatory. If the substituents point in the same direction, the substituents will be trans in the product if ring closure is disrotatory and cis if ring closure is conrotatory.
Figure: 29-04-03UN Title: [4 + 2] Cycloaddition and [2 + 2] cycloaddition. Caption: The first reaction is a Diels-Alder reaction.
Figure: 29-04-04UN Title: [8 + 2] Cycloaddition. Caption: Eight pi electrons participate in the reaction.
Figure: 29-04-05UN Title: Suprafacial versus antarafacial bond formation. Caption: In a cycloaddition reaction, bond formation is called suprafacial if both sigma bonds form on the same side of the pi system, and antarafacial if the sigma bonds form on the opposite side of the pi system.
Figure: 29-04a-c Title: Figure 29.4. Determining the stereochemistry of the product of an electrocyclic reaction. Caption: (a) A disrotatory ring closure causes the hydrogens to be cis in the ring-closed product. (b) Under photochemical conditions, the ring is opened. (c) The ring closure is under thermal conditions.
Figure: 29-05 Title: Figure 29.5. Frontier molecular orbital analysis of a [4 + 2] cycloaddition reaction. Caption: The HOMO of either of the reactants can be used with the LUMO of the other.
Figure: 29-05-01UN Title: A [2 + 2] cycloaddition does not occur under thermal conditions, but does take place under photochemical conditions. Caption: Under thermal conditions, suprafacial overlap is not symmetry-allowed. Antarafacial overlap is symmetry-allowed but is not possible because of the small size of the ring. Under photochemical conditions, the reaction can take place because the symmetry of the excited-state HOMO is opposite of that of the ground-state HOMO. Overlap of the excited-state HOMO of one alkene with the LUMO of the second alkene involves symmetry-allowed suprafacial bond formation.
Figure: 29-06 Title: Figure 29.6. Frontier molecular orbital analysis of a [2 + 2] cycloaddition reaction under thermal and photochemical conditions. Caption: Under thermal conditions, suprafacial overlap is not symmetry-allowed. Antarafacial overlap is symmetry-allowed but is not possible because of the small size of the ring. Under photochemical conditions, the reaction can take place because the symmetry of the excited-state HOMO is opposite of that of the ground-state HOMO. Overlap of the excited-state HOMO of one alkene with the LUMO of the second alkene involves symmetry-allowed suprafacial bond formation.
Figure: 29-06-03P09
Figure: 29-06-04P10 Title: Problem 10a -- solved. Why does 2,4,6-cycloheptatrienone use two pi electrons in this reaction?, I. Caption: Draw out the reaction.
Figure: 29-06-05P10 Title: Problem 10b -- solved. Why does 2,4,6-cycloheptatrienone use four pi electrons in this reaction?, I. Caption: Draw out the reaction.
Figure: 29-06-06P10Sol Title: Problem 10a -- solved. Why does 2,4,6-cycloheptatrienone use two pi electrons in this reaction?, II. Caption: The reaction is a [4 + 2] cycloaddition.
Figure: 29-06-07P10Sol Title: Problem 10b -- solved. Why does 2,4,6-cycloheptatrienone use four pi electrons in this reaction?, II. Caption: The reaction is a [4 + 2] cycloaddition.
Figure: 29-06-08UN Title: A [2,3] sigmatropic rearrangement. Caption: In a sigmatropic rearrangement, a sigma bond in the reactant is broken, a new sigma bond is formed, and the pi electrons rearrange.
Figure: 29-06-09UN Title: A [1,5] sigmatropic rearrangement. Caption: In a sigmatropic rearrangement, a sigma bond in the reactant is broken, a new sigma bond is formed, and the pi electrons rearrange.
Figure: 29-06-10UN Title: A [1,3] sigmatropic rearrangement. Caption: In a sigmatropic rearrangement, a sigma bond in the reactant is broken, a new sigma bond is formed, and the pi electrons rearrange.
Figure: 29-06-11UN Title: A [3,3] sigmatropic rearrangement. Caption: In a sigmatropic rearrangement, a sigma bond in the reactant is broken, a new sigma bond is formed, and the pi electrons rearrange.
Figure: 29-06-16T04 Title: Table 29.4. Woodward-Hoffmann rules for sigmatropic rearrangements. Caption: The selection rules for sigmatropic rearrangements are given.
Figure: 29-06-16UN Title: Suprafacial and antarafacial sigmatropic rearrangements. Caption: A sigmatropic rearrangement in which a migrating group remains on the same face of the pi system as it migrates is suprafacial. If the migrating group moves from one face of the pi system to the opposite face, the migration is antarafacial.
Figure: 29-06-17UN Title: Cope rearrangement. Caption: A Cope rearrangement is a [3,3] sigmatropic rearrangement of a 1,5-diene.
Figure: 29-06-18-1UN Title: The Ireland-Claisen rearrangement. Caption: The Ireland-Claisen rearrangement is a [3,3] sigmatropic rearrangement of an allyl ester.
Figure: 29-06-18UN Title: A Claisen rearrangement. Caption: A Claisen rearrangement is a [3,3] sigmatropic rearrangement of an allyl vinyl ether.
Figure: 29-06-20UN Title: Migration of hydrogen in a suprafacial and antarafacial rearrangement. Caption: A [1,3] sigmatropic migration of hydrogen has a four-membered-ring transition state.
Figure: 29-06-21UN Title: 1,3-Hydrogen shifts can take place if the reaction is carried out under photochemical conditions. Caption: The HOMO is symmetric under photochemical conditions, allowing hydrogen to migrate by a suprafacial pathway.
Figure: 29-06-22UN Title: 1,3-Hydrogen shifts can take place if the reaction is carried out under photochemical conditions. Caption: The HOMO is symmetric under photochemical conditions, allowing hydrogen to migrate by a suprafacial pathway.
Figure: 29-06-23UN Title: [1,5] Sigmatropic migrations of hydrogen. Caption: Three pairs of electrons are involved in [1,5] sigmatropic migrations of hydrogen.
Figure: 29-06-25UN Title: [1,7] Sigmatropic hydrogen migrations. Caption: Four pairs of electrons are involved in [1,7] sigmatropic migrations of hydrogens.
Figure: 29-06-26P16Sol Title: Problem 16 -- solved. Give the mechanism. Caption: Both equilibria involve [1,5] sigmatropic rearrangements.
Figure: 29-06-27UN Title: Migration of carbon using one lobe. Caption: Carbon can simultaneously interact with the migration origin and the migration terminus using one of its lobes.
Figure: 29-06-28UN Title: Migration of carbon using both lobes. Caption: Carbon can interact with the migration source and the migration terminus using both lobes of its p orbital.
Figure: 29-06-29UN Title: [1,3] Sigmatropic rearrangement. Caption: This [1,3] sigmatropic rearrangement has a four-membered-ring transition state that requires a suprafacial pathway.
Figure: 29-06-30UN Title: Two adjacent thymine residues in DNA, in the presence of light, results in a mutation-causing thymine dimer. Caption: The [2 + 2] cycloaddition takes place in the presence of ultraviolet light.
Figure: 29-06-32UN Title: Fireflies have an enzyme, luciferase, that catalyzes the reaction between ATP, luciferin, and molecular oxygen to form a compound with an unstable four-membered ring, which is broken and light is emitted. Caption: ATP activates the carboxylate group. Base removes a proton. The carbanion reacts with oxygen. Rearrangement occurs. A reverse 2 + 2 cycloaddition results in breaking the four-membered ring and the formation of oxyluciferin with an electron in oxyluciferin being promoted to the excited state. When the electron in the excited state drops down to the ground state, a photon of light is released.
Figure: 29-06-33UN Title: Synthesis of vitamin D. Caption: 7-Dehydrocholesterol undergoes an electrocyclic reaction, with light, to yield provitamin D 3 . A [1,7] sigmatropic rearrangement gives vitamin D 3 . Alternatively, ergosterol undergoes an electrocyclic reaction, with light, to yield provitamin D 2 . A [1,7] sigmatropic rearrangement gives vitamin D 2 .
Figure: 29-06T03 Title: Table 29.3. Woodward-Hoffmann rules for cycloaddition reactions. Caption: The selection rules for cycloaddition are summarized.

Pericyclic Reaction

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Pericyclic Reaction

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