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Although most organic reactions take place by way of ionic or radical intermediates, a number of useful reactions occur in one-step processes that do not form reactive intermediates.
- A pericyclic reaction is a concerted reaction that proceeds through a cyclic transition state.
Pericyclic reactions require light or heat and are completely stereospecific; that is, a single stereoisomer of the reactant forms a single stereoisomer of the product. We will consider two categories of pericyclic reactions: electrocyclic reactions and cycloadditions. An electrocyclic reaction is a reversible reaction that can involve ring closure or ring opening.
- An electrocyclic ring closure is an intramolecular reaction that forms a cyclic product containing one more σ bond and one fewer π bond than the reactant.
∆
1,3,5-hexatriene 3 π bonds
1,3-cyclohexadiene 2 π bonds
new σ bond
- An electrocyclic ring-opening reaction is a reaction in which a σ bond of a cyclic reactant is cleaved to form a conjugated product with one more π bond.
∆
Cleave this σ bond.
cyclobutene 1 π bond
1,3-butadiene 2 π bonds
- A cycloaddition is a reaction between two compounds with π bonds to form a cyclic product with two new σ bonds.
∆ new π bond
new σ bond
new σ bond
The Diels–Alder reaction in Chapter 16 is one example of a cycloaddition. Two features determine the course of the reactions: the number of π bonds involved and whether the reaction occurs in the presence of heat (thermal conditions) or light (photochemical
C
Supplementary
Topic
Pericyclic Reactions
C-
Stereospecific reactions were first discussed in Chapter 10.
C-2 Topic C Pericyclic Reactions
conditions). These reactions follow a set of rules based on orbitals and symmetry first proposed by R. B. Woodward and Roald Hoffmann in 1965, and derived from theory described by Kenichi Fukui in 1954. To understand pericyclic reactions we must review and expand upon what we learned about the molecular orbitals of systems with π bonds in Chapter 17.
Problem C.1 Classify each reaction as an electrocyclic reaction or a cycloaddition. Label the σ bonds that are
broken or formed in each reaction.
a. c.
b. + H 2 C CH 2
O O
C.1 Background on Molecular Orbitals In Section 17.9 we learned that molecular orbital (MO) theory describes bonds as the math- ematical combination of atomic orbitals that forms a new set of orbitals called molecular orbit- als (MOs). The number of atomic orbitals used equals the number of molecular orbitals formed. Since pericyclic reactions involve π bonds, let’s examine the molecular orbitals that result from p orbital overlap in ethylene, 1,3-butadiene, and 1,3,5-hexatriene, molecules that contain one, two, and three π bonds, respectively. Keep in mind that the two lobes of a p orbital are opposite in phase, with a node of electron density at the nucleus.
Ethylene The π bond in ethylene (CH 2 – –^ CH 2 ) is formed by side-by-side overlap of two p orbitals on adja- cent carbons. Two p orbitals can combine in two different ways. As shown in Figure C.1, when two p orbitals of similar phase overlap, a π bonding molecular orbital (designated as ψ 1 ) results. Two electrons occupy this lower-energy bonding molecular orbital. When two p orbitals of oppo- site phase combine, a π* antibonding molecular orbital (designated as ψ 2 *) results. A destabiliz- ing node occurs when two orbitals of opposite phase combine.
Figure C.
The π and π* molecular orbitals of ethylene
p atomic orbital
energy of p atomic orbital
opposite phases interacting
π∗ antibonding MO
node
ψ 2 ∗
ψ 1
Energy
like phases interacting
π bonding MO
The electrons reside in the bonding MO.
C-4 Topic C Pericyclic Reactions
1,3,5-Hexatriene The three π bonds of 1,3,5-hexatriene (CH (^2) – –^ CH – CH (^) – –^ CH – CH (^) – –^ CH 2 ) are formed by overlap of six p orbitals on six adjacent carbons. As shown in Figure C.3, six p orbitals can combine in six different ways to form six molecular orbitals designated as ψ 1 – ψ 6. Three are bonding molec- ular orbitals (ψ 1 – ψ 3 ), and three are antibonding molecular orbitals (ψ 4 *–ψ 6 *). In the ground state electronic configuration, the six π electrons occupy the three bonding MOs, ψ 3 is the HOMO, and ψ 4 * is the LUMO. In the excited state, which results from electron promo- tion from ψ 3 to ψ 4 *, ψ 4 * is the HOMO and ψ 5 * is the LUMO.
Problem C.3 (a) Using Figure C.2 as a guide, draw the molecular orbitals for 2,4-hexadiene. (b) Label the HOMO
and the LUMO in the ground state. (c) Label the HOMO and the LUMO in the excited state.
Problem C.4 (a) How many π molecular orbitals are present in 1,3,5,7,9-decapentaene
(CH (^2) – –^ CH – CH (^) – –^ CH – CH (^) – –^ CH – CH (^) – –^ CH – CH (^) – –^ CH2 )? (b) How many are bonding MOs and how many are antibonding MOs? (c) How many nodes are present in ψ (^1)? (d) How many nodes are present in ψ 10 *?
p atomic orbital
Energy
(^) ground state LUMO
ground state HOMO
excited state LUMO
excited state HOMO
h
Ground state Excited state
ψ 6 ∗
ψ 5 ∗
ψ 4 ∗
ψ 3
ψ 2
ψ 1
ν
Figure C.3 The six π molecular orbitals of 1,3,5-hexatriene
C.2 Electrocyclic Reactions C-
C.2 Electrocyclic Reactions An electrocyclic reaction is a reversible reaction that involves ring closure of a conjugated polyene to a cycloalkene, or ring opening of a cycloalkene to a conjugated polyene. For example, ring closure of 1,3,5-hexatriene forms 1,3-cyclohexadiene, a product with one more σ bond and one fewer π bond than the reactant. Ring opening of cyclobutene forms 1,3-butadiene, a product with one fewer σ bond and one more π bond than the reactant.
Ring-closing reaction
Ring-opening reaction
∆
1,3,5-hexatriene 1,3-cyclohexadiene
new σ bond
∆
cyclobutene 1,3-butadiene To draw the product in each reaction, use curved arrows and begin at a π bond. Move the π electrons to an adjacent carbon–carbon bond and continue in a cyclic fashion. In a ring-forming reaction, this process forms a new σ bond that now joins the ends of the conjugated polyene. In a ring-opening reaction, this process breaks a σ bond to form a conjugated polyene with one more π bond. Whether the reactant or product predominates at equilibrium depends on the ring size of the cyclic compound. Generally, a six-membered ring is favored over an acyclic triene at equilib- rium. In contrast, an acyclic diene is favored over a strained four-membered ring.
Problem C.5 Use curved arrows and draw the product of each electrocyclic reaction.
a. ∆^ b. ∆^ c. ∆
Stereochemistry and Orbital Symmetry Electrocyclic reactions are completely stereospecific. For example, ring closure of (2 E, 4 Z, 6 E )-2,4,6-octatriene yields a single product with cis methyl groups on the ring. Ring opening of cis -3,4-dimethylcyclobutene forms a single conjugated diene with one Z alkene and one E alkene. CH 3
CH 3
CH 3
CH 3
CH
CH
∆
(2 E ,4 Z ,6 E )-2,4,6-octatriene cis -5,6-dimethyl-1,3-cyclohexadiene (^) NOT formed
cis product only
CH (^3)
CH 3
∆
cis -3,4-dimethylcyclobutene
CH 3
CH
(2 E ,4 Z )-2,4-hexadiene
(2 E ,4 Z ) diene only
CH (^3)
CH (^3)
NOT formed
C.2 Electrocyclic Reactions C-
tion, we consider the HOMO of the ground state electronic configuration. Rotation occurs in a disrotatory or conrotatory fashion so that like phases of the p orbitals on the terminal carbons of this molecular orbital combine.
- The number of double bonds in the conjugated polyene determines whether rotation is conrotatory or disrotatory.
Two examples illustrate different outcomes. Thermal electrocyclic ring closure of (2 E, 4 Z, 6 E )-2,4,6-octatriene yields a single product with cis methyl groups on the ring.
∆
counterclockwise clockwise (2 E ,4 Z ,6 E )-2,4,6-octatriene
cis -5,6-dimethyl-1,3-cyclohexadiene cis product
H
CH 3
H
CH 3
H CH 3
H CH (^3)
disrotatory
Cyclization occurs in a disrotatory fashion because the HOMO of a conjugated triene has like phases of the outermost p orbitals on the same side of the molecule (Figure C.3). A disrotatory ring closure is symmetry allowed because like phases of the p orbitals overlap to form the new σ bond of the ring. In the disrotatory ring closure, both methyl groups are pushed down (or up ), making them cis in the product. This is a specific example of the general process observed for conjugated polyenes with an odd number of π bonds. The HOMO of a conjugated polyene with an odd number of π bonds has like phases of the outermost p orbitals on the same side of the molecule. As a result:
- Thermal electrocyclic reactions occur in a disrotatory fashion for a conjugated polyene with an odd number of π bonds.
In contrast, thermal electrocyclic ring closure of (2 E, 4 E )-2,4-hexadiene forms a cyclobutene with trans methyl groups.
∆ H
H
conrotatory
trans -3,4-dimethylcyclobutene trans product
CH (^3)
CH
clockwise clockwise (2 E ,4 E )-2,4-hexadiene
H H CH 3 CH 3
Cyclization occurs in a conrotatory fashion because the HOMO of a conjugated diene has like phases of the outermost p orbitals on opposite sides of the molecule (Figure C.2). A conrotatory ring closure is symmetry allowed because like phases of the p orbitals overlap to form the new σ bond of the ring. In the conrotatory ring closure, one methyl group is pushed down and one methyl group is pushed up, making them trans in the product. This is a specific example of the general process observed for conjugated polyenes with an even number of π bonds. The HOMO of a conjugated polyene with an even number of π bonds has like phases of the outermost p orbitals on opposite sides of the molecule. As a result:
- Thermal electrocyclic reactions occur in a conrotatory fashion for a conjugated polyene with an even number of π bonds.
Since electrocyclic reactions are reversible, electrocyclic ring-opening reactions follow the same rules as electrocyclic ring closures. Thus, thermal ring opening of cis -3,4-dimethylcyclobutene—
The conrotatory ring closure of (2E, 4 E)-2,4-hexadiene is drawn with two clockwise rotations. The conrotatory ring closure could also be drawn with two counterclockwise rotations, leading to the enantiomer of the trans product drawn. Both enantiomers are formed in equal amounts.
C-8 Topic C Pericyclic Reactions
which ring opens to a diene with an even number of π bonds—occurs in a conrotatory fashion to form (2 E, 4 Z )-2,4-hexadiene as the only product.
H
conrotatory
counterclockwise counterclockwise cis -3,4-dimethylcyclobutene (2 E ,4 Z )-2,4-hexadiene
H H H
CH (^3) CH (^3)
CH (^3) CH (^3) ∆
Sample Problem C.1 Draw the product of each thermal electrocyclic ring closure.
a. ∆
(2 E ,4 Z ,6 Z )-2,4,6-octatriene
CH 3
CH 3
b.
B
CH 3 O2C CO 2 CH 3 ∆
Solution
Count the number of π bonds in the conjugated polyene to determine the mode of ring closure in a thermal electrocyclic reaction.
- A conjugated polyene with an odd number of π bonds undergoes disrotatory cyclization.
- A conjugated polyene with an even number of π bonds undergoes conrotatory cyclization. a. (2E, 4 Z, 6 Z)-2,4,6-Octatriene contains three π bonds. The HOMO of a conjugated polyene with an odd number of π bonds has like phases of the outermost p orbitals on the same side of the molecule, and this results in disrotatory cyclization.
∆
counterclockwise clockwise (2 E ,4 Z ,6 Z )-2,4,6-octatriene
trans -5,6-dimethyl-1,3-cyclohexadiene trans product
H
CH 3 H
H (^) CH 3
H CH 3
CH 3
disrotatory
b. Diene B contains two π bonds. The HOMO of a conjugated polyene with an even number of π bonds has like phases of the outermost p orbitals on opposite sides of the molecule, and this results in conrotatory cyclization.
∆ H
H
conrotatory
trans product
clockwise clockwise B
H H CO 2 CH CO2CH (^3)
CH 3 O 2 C
CH 3 O2C
Problem C.6 What product is formed when each compound undergoes thermal electrocyclic ring opening or ring
closure? Label each process as conrotatory or disrotatory and clearly indicate the stereochemistry around tetrahedral stereogenic centers and double bonds.
a.
C 6 H
C 6 H 5
b. c. d.
Problem C.7 What cyclic product is formed when each decatetraene undergoes thermal electrocyclic ring
closure?
a. b.
C-10 Topic C Pericyclic Reactions
Summary of Electrocyclic Reactions Table C.1 summarizes the rules, often called the Woodward–Hoffmann rules, for electro- cyclic reactions under thermal or photochemical reaction conditions. The number of π bonds refers to the acyclic conjugated polyene that is either the reactant or product of an electrocyclic reaction.
Table C.1 Woodward–Hoffmann rules for electrocyclic reactions
Number of π bonds Thermal reaction Photochemical reaction Even Conrotatory Disrotatory Odd Disrotatory Conrotatory
Sample Problem C.2 Identify A and B in the following reaction sequence. Label each process as conrotatory or
disrotatory. C 6 H 5 CH 3
C 6 H 5 CH 3
∆ h ν A B
Solution
Ring opening of a cyclohexadiene forms a hexatriene with three π bonds. A conjugated polyene with an odd number of π bonds undergoes a thermal electrocyclic reaction in a disrotatory fashion (Table C.1). The resulting hexatriene (A) then undergoes a photochemical electrocyclic reaction in a conrotatory fashion to form a cyclohexadiene with cis methyl groups (B).
CH
disrotatory
∆ conrotatory
h
C 6 H 5 C 6 H 5
trans CH 3 groups
H
H CH 3
CH 3
B
C6H5 C 6 H
cis CH3 groups
H
CH (^3)
H
CH 3
C6H5 C 6 H 5
A
H
H CH (^3)
CH 3
excited state HOMO
C6H 5 C 6 H
H
H CH 3
ν
Problem C.10 Draw the product formed when each triene undergoes electrocyclic reaction under [1] thermal
conditions; [2] photochemical conditions.
a. b.
Problem C.11 What product would be formed by the disrotatory cyclization of the given triene? Would this
reaction occur under photochemical or thermal conditions?
Problem C.12 Consider the following electrocyclic ring closure. Does the product form by a conrotatory or
disrotatory process? Would this reaction occur under photochemical or thermal conditions?
H
H
C.3 Cycloaddition Reactions C-
C.3 Cycloaddition Reactions A cycloaddition is a reaction between two compounds with π bonds to form a cyclic product with two new σ bonds. Like electrocyclic reactions, cycloadditions are concerted, stereospecific reactions, and the course of the reaction is determined by the symmetry of the molecular orbitals of the reactants. Cycloadditions can be initiated by heat (thermal conditions) or light (photochemical conditions). Cycloadditions are identified by the number of π electrons in the two reactants. The Diels–Alder reaction is a thermal [4 + 2] cycloaddition that occurs between a diene with four π electrons and an alkene (dienophile) with two π electrons (Sections 16.12–16.14).
∆
CO 2 CH3 CO2CH (^3)
CO2CH3 CO2CH (^3) diene dienophile (^) Diels–Alder product 4 π electrons
2 π electrons (new^ σ^ bonds shown in red)
[4 + 2] Cycloaddition
A photochemical [2 + 2] cycloaddition occurs between two alkenes, each with two π elec- trons, to form a cyclobutane. Thermal [2 + 2] cycloadditions do not take place.
∆
H
H h ν
2 π electrons
2 π electrons (new σ bonds shown in red)
O
O
O
O
O
O
CH 2
CH [2 + 2] Cycloaddition
Sample Problem C.3 What type of cycloaddition is shown in each equation?
a. + (^) CH 2 CH 2 b. + CH 2 CH 2
Solution
Count the number of π electrons involved in each reactant to classify the cycloaddition.
a. [2 + 2] Cycloaddition b. [4 + 2] Cycloaddition
2 π electrons
2 π electrons
CH 2
CH 2
4 π electrons
2 π electrons
CH 2
CH 2
Problem C.13 Consider cycloheptatrienone and ethylene, and draw a possible product formed from each type of
cycloaddition: (a) [2 + 2]; (b) [4 + 2]; (c) [6 + 2].
O + CH 2 CH
cycloheptatrienone
C.3 Cycloaddition Reactions C-
This is a specific example of a general cycloaddition involving an odd number of π bonds (three π bonds total, two from the diene and one from the alkene).
- Thermal cycloadditions involving an odd number of π bonds proceed by a suprafacial pathway.
Because a Diels–Alder reaction follows a concerted, suprafacial pathway, the stereochemistry of the diene is retained in the Diels–Alder product. As a result, reaction of (2 E, 4 E )-2,4-hexa- diene with ethylene forms a cyclohexene with cis substituents (Reaction [1]), whereas reaction of (2 E, 4 Z )-2,4-hexadiene with ethylene forms a cyclohexene with trans substituents (Reaction [2]).
cis product only
H CH 3 H CH 3 H CH 3 H CH (^3) [1]
CH 3 H H
CH 3
CH 3
H CH 3 H
CH (^3)
H
H
CH 3
suprafacial
∆
(2 E ,4 E )-2,4-hexadiene
trans product only
H
CH 3 CH (^3) H [2] suprafacial
∆
(2 E ,4 Z )-2,4-hexadiene
Problem C.14 Show that a thermal suprafacial addition is symmetry allowed in a [4 + 2] cycloaddition by using the
HOMO of the alkene and the LUMO of the diene.
Problem C.15 Draw the product (including stereochemistry) formed from each pair of reactants in a thermal [4 + 2]
cycloaddition reaction. a. + CH 2 CH2 b. +
CN
CN
[2 + 2] Cycloadditions In contrast to a [4 + 2] cycloaddition, a [2 + 2] cycloaddition does not occur under thermal condi- tions, but does take place photochemically. This result is explained by examining the symmetry of the HOMO and LUMO of the alkene reactants. In a thermal [2 + 2] cycloaddition, like phases of the p orbitals on only one set of terminal car- bons can overlap. For like phases to overlap on the other terminal carbon, the molecule must twist to allow for an antarafacial pathway. This process cannot occur to form small rings.
LUMO of the second alkene
HOMO of one alkene
Thermal [2 + 2] like phases on opposite sides cycloaddition
In a photochemical [2 + 2] cycloaddition, light energy promotes an electron from the ground state HOMO to form the excited state HOMO (designated as ψ 2 * in Figure C.1). Interaction of
In Section 16.13, we learned that the stereochemistry of the dienophile is retained in the Diels–Alder product.
C-14 Topic C Pericyclic Reactions
this excited state HOMO with the LUMO of the second alkene then allows for overlap of the like phases of both sets of p orbitals. Two bonding interactions result and the reaction occurs by a suprafacial pathway.
Photochemical [2 + 2] cycloaddition
LUMO of the second alkene
excited state HOMO of one alkene
suprafacial
two new σ bonds
h ν
This is a specifi c example of a general cycloaddition involving an even number of π bonds (two π bonds total, one from each alkene).
- Photochemical cycloadditions involving an even number of π bonds proceed by a suprafacial pathway.
Problem C.16 Draw the product formed in each cycloaddition.
a. (^) C 6 H5 C^6 H^5 + h ν b. +^ h ν
O
CN
CH 2 CH 2
Summary of Cycloaddition Reactions Table C.2 summarizes the Woodward–Hoffmann rules that govern cycloaddition reactions. The number of π bonds refers to the total number of π bonds from both components of the cycload- dition. For a given number of π bonds, the mode of cycloaddition is always opposite in thermal and photochemical reactions.
Table C.2 Woodward–Hoffmann rules for cycloaddition reactions
Number of π bonds Thermal reaction Photochemical reaction Even Antarafacial Suprafacial Odd Suprafacial Antarafacial
Problem C.17 Using the Woodward–Hoffmann rules, predict the stereochemistry for each cycloaddition:
(a) a [6 + 4] photochemical reaction; (b) an [8 + 2] thermal reaction.
Problem C.18 Using orbital symmetry, explain why a Diels–Alder reaction does not take place under
photochemical reaction conditions.
C-16 Topic C Pericyclic Reactions
Problem C.23 (a) What product is formed when triene N undergoes thermal electrocyclic ring closure? (b) What
product is formed when triene N undergoes photochemical ring closure? (c) Label each process as conrotatory or disrotatory.
N
Problem C.24 What type of cycloaddition occurs in Reaction [1]? Draw the product of a similar process in
Reaction [2]. Would you predict that these reactions occur under thermal or photochemical conditions?
C6H
C6H
[1]
O
O
[2]
O
Problem C.25 What cycloaddition products are formed in each reaction? Indicate the stereochemistry of each
product.
a. 2 h ν^ b. 2 h ν
Problem C.26 The bicyclic alkene P can be prepared by thermal electrocyclic ring closure from cyclodecadiene
Q or by photochemical electrocyclic ring closure from cyclodecadiene R. Draw the structures of Q and R, and indicate the stereochemistry of the process by which each reaction occurs. H
H P
