AOR No.11 (2016.7.22)6.4 Explanations for the Woodward-Hoffmann Rules

6.4.1 The Aromatic Transition Structure

[4 + 2], [8 + 2], and [6 + 4] thermal cycloadditions are common [2 + 2], [4 + 4], and [6 + 6] thermal cycloadditions are rare

(4n+2)aromatictransitionstructuremodel----- suprafacial

(4n) Möbius aromatictransitionstructuremodel---antarafacial

6.4.2 Frontier Orbitals

“match”“unmatch” sign

Unimolecular--- treat a single molecule as having separate components

Hydroboration

6.4.3 Correlation Diagrams

An orbital in the starting material must feed intoan orbital of the same symmetry in the product,preserving the symmetry throughout the reaction.

Assumption

6.4.3.1 Orbital Correlation Diagrams Symmetry-allowedDiels-Alderreaction

Symmetry-forbidden[p2s +p2s]cycloadditionreaction

symmetric antisymmetric

Excitedstate

Groundstate

Excitedstate

Groundstate

6.4.3.2 State Correlation Diagrams.

barrier

Fig.6.18

Fig.6.20photochemically

allowed

6.4.3.3 Following Orbitals along the Reaction Coordinate

Fig.6.18

TSReactant Product

C2-C3:66%

C1-C2:22%

dienophile:32%

lateTSstructure

earlyTSstructure

Bond-order changes

s:C1-C1’:22%

p:HOMO(diene)/HOMO(dienophile)

HOMO(diene)/LUMO(dienophile)

6.5 Secondary Effects Substituenteffects6.5.1 The Energies and Coefficients of the Frontier Orbitals of Alkenes and Dienes

C-substituent (vinyl) on alkene HOMOup,LUMOdown HOMOupLUMOdown

C-substituent (phenyl) on alkene

Z-substituent on alkene

HOMOsimilar

LUMOdown

X-Substituent on alkeneStabilization by25 kJ/mol

HOMOup

LUMOup

C-substituents on C1-diene

HOMOup

LUMOdown

Substituenteffects

C2-substituent

Z-substituents on dienes

HOMOlowerorunaffected

LUMOdown Pentadienylcationvshexatriene

X-substituents on dienes Pentadienylanionvsunsubstituteddiene

HOMOup

LUMOslightlyup

C2-substituent SmallereffectthanC1-substituent

C1-substituent

Houk JACS1973,95,4092

6.5.2 Diels-Alder Reaction

6.5.2.1 The Rates 0f Diels-Alder Reactions~80%isolatedyield

9.1eV

11.9eV 8.5eV

Inverseelectrondemand

But,diene-diene adduct

caseFig.6.27(c)

Fujimoto,JACS 1976,98,2670

node

electronsareconcentratedinthecenterelectronsareconcentratedattheside

Risanelectron-withdrawingsubstituent,theLUMOofthedienophile islowered: closertotheHOMOofthediene.

Risanelectron-donatingsubstituent,theHOMOofthedienophile israised,closerinenergytotheLUMOofthediene.

6.5.2.2 The Regioselectivity 0f Diels-Alder Reactions

xy+(x+n)(y+m)>x(y+m)+(x+n)yifnm>0

Thewayoforganicchemist,,,

Butnotalways,

>

18 possible combinations of C-, Z-and X-substituted dienes and C-, Z-and X-substituted dienophiles.

The ‘ortho' adduct is predicted (and found) to be the major product foreight of the nine possible combinations with l-substituted dienes, and the‘para' adduct predicted (and found) for eight of the nine possiblecombinations of 2 substituted dienes.

Z-substituted dienophiles

X-substituted dienophiles

C-substituted dienophiles

meta-adduct----rarecases meta-adduct----aromatization

Diradical theoryAnalternativeexplanationforregioselectivity

stable

Z-substituenttype

meta

steric

ortho

6.5.2.3 The Regioselectivity of Hetero Diels-Alder Reactions

morepolarized

6.5.2.4 The Stereoselectivity of Diels-Alder Reaction

“endo”rule

Secondary orbital interaction

C2>C3

Secondary orbital interaction?

(74:26) (17:83)

Dimerization of cyclopentadieneendo 6.5.3 1,3-Dipolar Cycloaddition

1,3-diplole

dipolarophile

Huisgencycloaddition(Clickreaction)

6.5.3.1 The Rates of 1,3-Dipolar Cycloadditions.

LUMO(dipole)-HOMO(polarophile)HOMO(dipole) LUMO(polarophile)

6.5.3.2 The Regioselectivity of 1,3-Dipolar Cycloaddition

ambidentnucleophile tautomerism

dipole-HOcontrolled dipole-LUcontrolled large-large/small-small interaction

dipole-LUcontrolled

Z-Substituents, which lower both the HOMO and the LUMO energies of thedipole, speed up the normally slow reactions with X-substituted alkeneslike enamines. Furthermore, with a Z-substituent on the carbon atom ofthe diazomethane the coefficient on the carbon atom will be reduced inthe LUMO, just as it is in the LUMO of an alkene with a Z-substituent. Thereaction is now dipole-LU controlled and the regioselectivity change.

dipole-LUcontrolled

Z-substituted dipolarophiles and phenyl azide

dipole-HOcontrolled

+dipole-LUcontrolled

Perhaps the methyl group has raised the energy of both the HOMOand the LUMO of the dipolarophile, making the HOMO/LUMOseparations still more nearly equal.

nitrile oxides

dipole-LUcontrolled

dipole-LUcontrolled dipole-HOcontrolled

nitronesdipole-LUcontrolled

dipole-HOcontrolled

With Z-substituent, the HOMO of the dipole is not strongly polarised.

the third stage of the ozonolysis sequence

The Stereoselectivity of 1,3-Dipolar Cycloadditions

Noendorule

6.5.4 Other Cycloaddition

[4 + 6] Cycloadditions

exo---favour endo---repulsive

Ketene Cycloadditions Carbene Cycloadditions

Reaction takes place on the double bond with the highest coefficient in the HOMO.

Electrophilic carbene

Nucleophilic carbene

electrophilic carbene

Epoxidations and Related Cycloaddition

the highest adjacent pair of coefficients

the highest adjacent pair of coefficients

Electrophilic substitution

[3,3]-Sigmatropic Rearrangements

Other Pericyclic Reaction

unimolecularCoperearrangement

140 C->-15 C(I,Ag+)

oxoanion(-OM)---- X-substituent

carbocation---- Z-substituent

Slow down the rearrangement Claisen rearrangement

Report No9 (7/22)

1. The following reactions take place in three or four pericyclic steps. Suggest what pathway is followed, identify the intermediates A–G, and identify the classes of the pericyclic reactions.