Pericyclic Reactions!

Title

+Cycloaddition!

( )n ( )n

Electrocyclization!

π1-bonding HOMO

π∗2-antibonding LUMO

p-orbitals

S

A

Ethylene π-Molecular Orbitals!

171 nm!167 kcal/mol!

400 nm!800 nm! 200 nm!visible! UV!IR!

Ethylene

1,3-Butadiene from Ethylene !

HOMO

LUMO

LUMO

HOMO

S

A

A

S

SS

A

S

A

Ethylene/Butadiene

214 nm!133 kcal/mol!

1,3-Butadiene π-Molecular Orbitals!

400 nm!800 nm! 200 nm!visible! UV!IR!

Butadiene

258 nm!109 kcal/mol!

1,3,5-Hexatriene π-Molecular Orbitals!

LUMO

HOMO

S

S

S

A

A

A

400 nm!800 nm! 200 nm!visible! UV!IR!

Hexatriene

Butadiene Orbital Coeffients

Frontier!Molecular!Orbitals!FMOs!

!Butadiene: Orbital Coefficients!

S

S

A

A

Allyl Cation

HOMO

LUMO

S

S

A

!The Allylic System: Allyl Cation!

+

Allyl Radical !

The Allylic System: Allyl Radical!

HOMO

LUMO

S

S

A

Allyl Anion !

The Allylic System: Allyl Anion!

HOMO

LUMO

S

S

A

-

Cycloaddition: Diels-Alder Reaction!An 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

Diels-Alder 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

Allyl Cation DA !

[4+2]-Cycloadditions!

++ +

HOMO

LUMO

S

S

A

Diels-Alder Reaction: Mechanism!

Diels-Alder 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!!

Diels-Alder Endo Effect

O

Secondary Effect!

What About a [2+2] Cycloaddition?!!

2+2?

π1-bonding HOMO

π∗2-antibonding LUMO

S

A

π1-bonding HOMO

π∗2-antibondingLUMO (HOMO*)

S

A

light (hν)!

4n+2 Rule Thermally Allowed Cycloadditions!

4n+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…..!

4n Rule Photochemically Allowed Cycloadditions!

4n 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 !

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

!

Electrocyclizations!!

Butadiene-Cyclobutene 1,3-Butadiene-Cyclobutene!

!

thermal hν

hν thermal

trans

cis

E,E

E,Zdis

con dis

con

(E,E)-Butadiene-Cyclobutene (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

hν

S

A

CH3

CH3

(E,Z)-Butadiene-Cyclobutene (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

hν

S

A

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

!

thermal hν

hν thermal

trans

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

dis

con dis

con

(E,Z,E)-Hexatriene-Cyclohexadiene Orbitals

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

photochemical

conrotatory

A π4*

thermal

disrotatory

S π3

hν

HOMOground state

HOMO*excited state

S cis

A trans

CH3

CH3

CH3

H3C

CH3

CH3

CH3

CH3

Electrocyclization Summary

Electrocyclizations!!

!n=1,2,3,4…!

!thermal!

! photochemical!

!4n!

!con!

!dis!

!4n+2!

!dis!

!con!

Violations

Violations!

“There are none!”!

Woodward and Hoffmann,The Conservation of Orbital Symmetry!