Chemistry 125: Lecture 55February 24, 2010

(4n+2) Aromaticity Cycloaddition

Electrocyclic Reactions This

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Furan

0 anti-bonding nodes

2 ABNs

4 ABNs

N

SHMo2 (Simple Hückel Molecular Orbital Program)

N

Benzene Pyridine

N

lower energy

node on N

identical shapeenergy

larger on N

lower energyhigh N density

Crude calculation shows heterocycle analogy.

Cyclodecapentaene Naphthalene (SHMo2)

Naphthalene

same ascyclodecapentaene

& butadiene

same asethylene

same asbutadiene

Another Criterion of Aromaticityis the PMR Chemical Shift

(coming soon, Chapter 15)

The Cyclodecapentaene SystemAngewandte Chemie

International Edition in EnglishVolume ﾊ 3, Issue ﾊ 3, Date: ﾊMarch 1964, Pages: ﾊ228-229E. Vogel, H. D. Roth

Generalized Aromaticity

pKa 15vs. 16 for H2O

H H

HH

H H

Sec. 13.6pp. 587, 592

cyclo-C7H8 cyclo-C7H7- pKa 39 (despite more resonance structures)

6 electrons (4n+2)

8 electrons (4n, antiaromatic)

R H

R R

+ Ph3C+

2 electrons (4n+2)

H

HH

H H

OH-

unusually stable cation (triply benzylic)

+ Ph3CH

R

R R

+

even more stable

Sec. 13.6 p. 591

Same for cyclo-C7H8 cyclo-C7H7+

(cycloheptatrienyl “tropylium”) 6 electrons (4n+2)

Electrocyclic Reactions

Pericyclic Reactions(in which transition states are “aromatic”)

Cycloadditions: Diels-Alder (Sec. 12.12, 14.3)

Cycloadditions: Diels-Alder (Sec. 15.3)

4 + 2 electrons

Ring

4 + 2 electrons

ene

diene

enediene

Cycloadditions: Diels-Alder

Stereochemistry (ene) (Sec. 12.12, p. 549)

Ene just “sits down” on Diene

Cycloadditions: Diels-Alder

Stereochemistry (diene)

Diene just “sits down” on Ene

Cycloadditions: Diels-Alder (Sec. 14.3, pp. 628-630)

LUMO

HOMO

Diels-Alder Reactioncyclic electrontransition state

HOMO

LUMO

TransitionState

Motion

front view side view

TransitionState

HOMO-1

TransitionState

HOMO

p. 1351

Diels-Alder Reaction

cyclicelectrontransition state

TransitionState

Motion

front view side view

?

HOMO () orthogonal to LUMO (*)

h

Shift electron from HOMO to LUMO

p. 1046

Pericyclic Reactions(in which transition states are “aromatic”)

Cycloadditions: Diels-Alder

Electrocyclic Reactions

13

conrotation disrotationMöbius

2

Hückelconnectivity

requires twistin 1 of 2 ways

Hückel

Transition StateMotion

1

2

3

4

21

3

4

5

6

21

3

4

5

6

1

2

3

4

top

touches

top(even #

of nodes)

top touches

bottom

(odd # of nodes)

! 21

3

4

5

6

Sec. 27.2pp. 1343-1346

(shown backwards in our text;same favored “aromatic”

conrotatory transition state)

CON4e

CON for 4nCON8e

DIS6e

DIS for 4n+2

2

Transition State HOMO-1

1

disrotation6e Hückel

bottom touches top(odd # of nodes)

top touches top(even # of nodes)

conrotation4e Möbius

End of Lecture 55Feb. 24, 2010

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