CS10a pericyclic reactions.docx
4April2012
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I. Pericyclic Reactions: General Observations/Characteristics
A. isopolar - neither heterolytic or radical
B. concerted, no intermediates, one transition state
C. at least one reactant (or product) has a double bond
D.  bonds are broken or formed in exchange for double bonds
E. reactions involve orbitals in a cyclic array
F. controlled by symmetry of MO's participating in reaction: if a symmetry element is
retained during a reaction it must also be retained by MO's involved
II. electrocyclic reactions -  bonds formed at terminal atoms of  system
A. thermal reactions: 4n electrons and 4n+2 electrons
conrotatory
X
disrotatory
2,4-trans,trans
-hexadiene
2,4-cis,trans,
-hexadiene
X
disrotatory
2,4,6-trans,cis,trans-octatriene
conrotatory
X
disrotatory
disrotatory
conrotatory
conrotatory
2,4,4-cis,cis,trans-octatriene
X
B. the stereochemical course of photochemical reactions is reversed
h
conrotatory
h
conrotatory
X
disrotatory h
disrotatory
h
disrotatory
Xh
h
conrotatory
X h
disrotatory
h
conrotatory
X
III. Frontier Orbital method (acid-base theory)
A. HOMO of one system overlaps with LUMO of another
B. allowed reaction if no new nodes are created in transition state
CS10a pericyclic reactions.docx
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C. use HOMO for single  system
LUMO = 3
LUMO = 2
HOMO = 2
HOMO = 
HOMO = 3
HOMO = 
D. photolysis reverses stereochemistry since HOMO is next higher orbital
IV. Aromatic Transition State or Möbius-Hückel Method
A. construct basis set, connect forming odd or even nodes, calculate MO energies
B. Hückel transition state has even number of nodes
C. Möbius transition state has odd number of nodes
Hückel
disrotatory
Möbius
conrotatory
D. Möbius transition state allowed for 4n electrons
E. Hückel transition state allowed for 4n+2 electrons
V. Correlation Diagram Method
A. choose symmetry elements that are preserved during rearrangement and distinguish the two
pathways
CS10a pericyclic reactions.docx
4April2012
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B. label MO's antisymmetric or symmetric according to symmetry elements
C. connect orbitals of like symmetry, indicates where electrons end up in product
D. reaction is not symmetry allowed if reaction creates exicted state (and vice versa)
 plane
A
S
A
S
C2 axis
A
A
S
S
4
3
2
1
S
A
A
S
S
A
A
S



