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Pericyclic Reaction Theory

I. DefinitionsA. CycloadditionB. Electrocyclic reactionC. Sigmatropic reaction

II. Conservation of Orbital SymmetryA. Symmetry correlation diagramsB. Frontier orbital approachC. Generalized Woodward-Hoffmann selection rules

III. Transition State AromaticityA. Hückel and Möbius aromaticityB. Interaction diagramsC. Aromatic and antiaromatic transition states

IV. Summary

2

hh

Ph

Ph

+

Ph

Ph

CO2CH3

CO2CH3

+

CO2CH3

CO2CH3

h

OAcHH

D

OAcHHD

thermally forbiddenphotochemically allowed

photochemically forbiddenthermally allowed

inversion of configuration

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I. Definitions

A. Cycloaddition

Pericyclic reaction: one in which concerted bond breaking and bond formation occur through a reorganization of electrons within a closed loop of interacting orbitals.

Ring formation by transfer of electrons from bonds to new bonds.Cycloreversion: reverse process.

+

+

+

“2 + 2” cycloaddition/cycloreversion

“4+2” “4+4”

“2+2”

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I. DefinitionsA. Cycloaddition

oror

suprafacial (s)process

antarafacial (a)process

4s + 2s(2s + 2s + 2s)(2a + 2a + 2s)

4a + 2s(2s + 2a + 2s)(2a + 2s + 2s)

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I. DefinitionsB. Electrocyclic reaction

Formation of a single bond between the termini of a linear system, and the reverse process.

electrocyclic ring closureelectrocyclic ring opening

conrotatory process

disrotatory process

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I. DefinitionsB. Electrocyclic reaction

disrotatory

conrotatory

6s(2s + 2s + 2s)

4s + 2s(2s + 2s + 2s)

6a(2s + 2s + 2a)

4s + 2a(2s + 2s + 2a)

4a + 2s(2s + 2a + 2s)

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I. DefinitionsC. Sigmatropic reaction

Bond migration over a system.

R R1

2

31' 1'

3

2

1

12

3

1'2'

3'

3

3'

[1,3] sigmatropic shift

[3,3] sigmatropic shift(Cope rearrangement)

[1,3]retention

a cba c

b

2s + 2ssupra migration

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I. DefinitionsC. Sigmatropic reaction

a cb

a cb

[1,5]inversion

[1,5]retention

acb

acb

[3,3]

4s + 2a2s + 2s + 2a)supra migration

4a + 2s2a + 2s + 2s)antara migration

2s + 2s + 2ssupra/supra migration

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II. Conservation of Orbital Symmetry

A. Symmetry correlation diagrams1. cycloaddition/cycloreversion

Woodward-Hoffmann RulesIn any concerted process, the starting material orbitals must be transformed into product orbitals of the same symmetry.

(There are other symmetry elements as well, but no additional information is gained; 1 and 2 are sufficient to differentiate orbital symmetries.)

1

2

1

2

+

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II. Conservation of Orbital SymmetryA. Symmetry correlation diagrams

1. cycloaddition/cycloreversion

1

2

mix orbitals

+1

2

1

2

1

2

mix orbitals

2

1 1

2

+

not symmetry correctwith respect to 1

not symmetry correctwith respect to 2

1 - 2

= SS

1 2

1 + 2

= AS

1 + 2

= SS

1 - 2

= SA

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II. Conservation of Orbital SymmetryA. Symmetry correlation diagrams

1. cycloaddition/cycloreversion

Orbital correlation diagram:

*AA

*SA

AS

SS

*AA

*AS

SA

SS

thermally (black):g.s. upper e.s. symmetry forbidden

photochemically (blue):1st e.s. 1st e.s. symmetry allowed

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II. Conservation of Orbital SymmetryA. Symmetry correlation diagrams

1. cycloaddition/cycloreversion

+

plane is only symmetry element

orbitals are correct, need to mix ’s

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II. Conservation of Orbital SymmetryA. Symmetry correlation diagrams

1. cycloaddition/cycloreversion

A

2A

S

A

S

S

4A

3S

2A

1S

2A

1S

thermallyallowed,

photochemicallyforbidden

avoided crossings-orbitals of the same symmetry do not cross

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II. Conservation of Orbital SymmetryA. Symmetry correlation diagrams

1. cycloaddition/cycloreversion

General for all-supra cycloadditions/cycloreversions:

allowed forbidden 4n+2 4n

h 4n 4n+2

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II. Conservation of Orbital SymmetryA. Symmetry correlation diagrams

2. electrocyclic reactions

conrotatory disrotatory

c2 axis

plane

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II. Conservation of Orbital SymmetryA. Symmetry correlation diagrams

2. electrocyclic reactions

S

A

S

A

A

S

A

S

A

S

A

S

A

A

S

S

conrotatory (c2) allowed

h forbidden

disrotatory () forbiddenh allowed

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II. Conservation of Orbital SymmetryA. Symmetry correlation diagrams

2. electrocyclic reactions

General for electrocyclic reactions:

allowed h allowedconrotatory 4n 4n+2disrotatory 4n+2 4n

4n conrotatory disrotatory4n+2 disrotatory conrotatory

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II. Conservation of Orbital SymmetryB. Frontier orbital approach: sigmatropic reactions

[1,3]-H:suprafacial

antarafacial

No symmetry elements; symmetry correlation diagrams not relevant.

Look at orbital symmetry conservation in HOMO/LUMO:

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II. Conservation of Orbital SymmetryB. Frontier orbital approach: sigmatropic reactions

mix LUMO

HOMO

isolatedorbitals

forbiddenh allowed

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II. Conservation of Orbital SymmetryB. Frontier orbital approach: sigmatropic reactions

General for sigmatropic reactions:

allowed h allowed[1,3]-H (4n) antara supra

[1,5]-H (4n+2) supra antara

[i,j]-alkyl:4n supra inversion retention

antara retention inversion4n+2 supra retention inversion

antara inversion retention

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II. Conservation of Orbital SymmetryC. Generalized Woodward-Hoffmann pericyclic selection rules

“A pericyclic reaction is thermally allowed if the total number of (4n+2) s and (4n) a components is odd.”

or - If broken down into two electron components, a pericyclic reaction is thermally allowed if the number of 2s components is odd.

+

+

H H

H

2s + 2s h allowed

2s + 2s + 2s allowed

2s + 2s h

2s + 2a

2s + 2s h

2s + 2a

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III. Transition State Aromaticity

A. Hückel and Möbius aromaticity

Dewar-Zimmerman Selection Rules

Hückel:

Möbius:

4n+2 e– = aromatic4n e– = antiaromatic

4n e– = aromatic4n+2 e– = antiaromatic

phaseinversion

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III. Transition State AromaticityB. Interaction diagrams

Fundamental topology of interacting orbitals:

• omit skeletal framework• disregard spatial orientations of orbitals• use p orbitals to represent all orbitals• assign algebraic signs to the orbitals to minimize the number of

phase inversions

= =

= =

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III. Transition State AromaticityB. Interaction diagrams

=

=Even number of phase inversions can always be reduced to zero.

Odd number of phase inversions can always be reduced to one.

No phase inversions = Hückel interactionOne phase inversion = Möbius interaction

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III. Transition State AromaticityC. Aromatic and antiaromatic transition states

Examine topology of interacting orbitals in TS:

Hückel Möbius4n antiaromatic aromatic

4n+2 aromatic antiaromatic

Aromatic TS allowed (h forbidden)Antiaromatic TS forbidden (h allowed)

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III. Transition State AromaticityC. Aromatic and antiaromatic transition states

‡ no phase inversions6 e– Hückel TS

aromatic allowed

= =

no phase inversions4 e– Hückel TS

antiaromatic forbidden

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III. Transition State AromaticityC. Aromatic and antiaromatic transition states

=

=

disrot:

conrot:

4 e– Hückel forbidden

4 e– Möbius allowed

R R

= 4 e– Möbius allowed

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IV. SummaryO

O

+

10 e– supra/supra cycloaddition2s + 2s + 2s + 2s + 2s10 e– Hückel TS

allowed

CH3

HH

H3C

HH

=

4 e– supra/antara cycloreversion2s + 2a4 e– Möbius TS

allowed

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IV. Summary

=

6 e– antara/antara cycloaddition2s + 2a + 2a6 e– Hückel TS

allowed

Ph

Ph

OO O

O

O

O

Ph

Ph

Ph

Ph

4 e– conrotatoryelectrocyclic ring opening2s + 2a4 e– Möbius TS

6 e– supra/supra cycloaddition2s + 2s + 2s6 e– Hückel TS