pericyclic reactions title cycloaddition electrocyclization

Pericyclic Reactions

+Cycloaddition

( )n( )n

Electrocyclization

π1-bonding HOMO

π∗2-antibonding LUMO

p-orbitals

S

A

Ethylene π-Molecular Orbitals

171 nm167 kcal/mol

400 nm800 nm 200 nmvisible UVIR

1,3-Butadiene from Ethylene

HOMO

LUMO

LUMO

HOMO

S

A

A

S

SS

A

S

A

Ethylene/Butadiene

214 nm133 kcal/mol

1,3-Butadiene π-Molecular Orbitals

400 nm800 nm 200 nmvisible UVIR

Butadiene

258 nm109 kcal/mol

1,3,5-Hexatriene π-Molecular Orbitals

LUMO

HOMO

S

S

S

A

A

A

400 nm800 nm 200 nmvisible UVIR

Hexatriene

FrontierMolecularOrbitalsFMOs

Butadiene: Orbital Coefficients

S

S

A

A

HOMO

LUMO

S

S

A

The Allylic System: Allyl Cation

+

The Allylic System: Allyl Radical

SOMO

S

S

A

LUMO

HOMO

The Allylic System: Allyl Anion

-

HOMO

LUMO

S

S

A

Cycloaddition: Diels-Alder ReactionAn Allowed [4+2] Cycloaddition

+

diene dienophile

HOMO π2

LUMO π*3

π*4

π1

π1-bonding HOMO

π∗2-antibonding LUMO

butadiene

ethylene

S

A

A

S

S

A

Diels-Alder Reaction:The Effect of Electron Withdrawing Groups

+

diene dienophile

EWG EWG

HOMO π2

LUMO π*3

π*4

π1

π1-bonding HOMO

π∗2-antibonding LUMO

butadiene deactivated ethylene

EWG

EWG

S

A

A

S

S

A

[4+2]-Cycloaddiitions

++ +

HOMO

LUMO

S

S

A

Diels-Alder Reaction: Mechanism

HOMO π2

LUMO π*3

π*4

π1

π1-bonding HOMO

π∗2-antibonding LUMO

butadiene

ethylene

S

A

A

S

S

A

Diels-Alder Reaction: The Endo Effect

O

Secondary Effect

What About a [2+2] Cycloaddition?

π1-bonding HOMO

π∗2-antibonding LUMO

S

A

π1-bonding HOMO

π∗2-antibondingLUMO (HOMO*)

S

A

light (h)

Thermally Allowed Cycloadditions4n+2 Rule

HOMOs

LUMOs

A

S

2

S

A

4

S

A

6

S

A

8

[4+2]=6 [6+4]=10

[8+6]=14

4n+2; n=1,2,3,4…..

Photochemically Allowed Cycloadditions4n Rule

HOMOs

LUMOs

A

S

2

S

A

4

S

A

6

S

A

8

4n; n=1,2,3,4…..

[2+2]=4

[4+4]=8

[6+6]=12

[8+8]=16

and [2+6]=8; [8+4]=12

Summary of Cycloadditions

2 4 6 8 10

2

4

6

8

10

P P P

P

P P

P P

P

T T

P

P P

TTT

T T T

TT

P

TT

Thermal 4n+2

Photochemical 4n

Electrocyclizations

1,3-Butadiene-Cyclobutene

thermal h

h thermal

trans

cis

E,E

E,Zdis

con dis

con

(E,E)-1,3-Butadiene-Cyclobutene

CH3

CH3

CH3

CH3

HH

CH3

CH3

HH

photochemical

disrotatory

A π2

thermal

conrotatory

S π3*

HOMO*

excited state

HOMOground state

h

S

A

CH3

CH3

(E,Z)-1,3-Butadiene-Cyclobutene

CH3

CH3

CH3

CH3

H

CH3

HCH3

H

CH3

HCH3

photochemical

disrotatory

A π2

thermal

conrotatory

S π3*

HOMO*

excited state

HOMOground state

h

S

A

(E,Z,E)-1,3,5-Hexatriene-Cyclohexadiene

thermal h

h thermal

trans

(E,Z,E) (E,Z,Z)

dis

con dis

con

(E,Z,E)-1,3,5-Hexatriene-Cyclohexadiene Orbitals

photochemical

conrotatory

A π4*

thermal

disrotatory

S π3

h

HOMOground state

HOMO*excited state

S cis

A trans

CH3

CH3

CH3

H3C

CH3

CH3

CH3

CH3

Electrocyclizations

n=1,2,3,4… thermal photochemical

4n con dis

4n+2 dis con

Violations

“There are none!”

Woodward and Hoffmann,The Conservation of Orbital Symmetry