pericycli rxn

7/28/2019 pericycli rxn

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PERICYCLIC REACTIONS

Concerted reactions with cyclic transition states

1. Electrocyclic reactions

2. Cycloaddition reactions

3. Sigmatropic rearrangements

h

h

h

R R


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MOLECULAR ORBITALS(A review)

HOMO = Highest Occupied Molecular OrbitalLUMO = Lowest Unoccupied Molecula Orbital

SYMMETRIC = Symmetry operation does not changelobe signs

ANTISYMMETRIC = Symmetry operation inverts lobesigns

Generic energy level ordering: , , *, * (for-type

orbitals the more nodes, the higher the energy)

GS ES

GS ES

GS ES


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ELECTROCYCLIC REACTIONS

n

n

n

n

Disrotatory (one clockwise and one couterclockwise)

Conrotatory (both clockwise or both couterclockwise)


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1a. Disrotatory ring closure/opening (4 electrons)

thermally disallowed; photochemically allowed

S

A

S

A

S

S

A

A


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1b. Conrotatory ring closure/opening (4 electrons)

thermally allowed; photochemically disallowed

A

S

A

S

S

A

S

A


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2a. Disrotatory ring closure/opening (6 electrons)


A

S

S

S

A

A

S

A

S

A

S

A


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2b. Conrotatory ring closure/opening (6 electrons)


S

A

S

A

S

A

A

S

A

S

A

S


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Electrocyclic reactions are controlled by the symmetryof the HOMO of the polyene.

_______________________________________________

Electron pairs Thermal Photochemical______________________________________________

even number conrotatory disrotatoryodd number disrotatory conrotatory

Note:

If there is a choice, the sterically favored sense ofrotation is going to dominate, see for example:

Thermally allowed conrotatory opening

counterclockwiseclockwise


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CYCLOADDITION REACTIONS

Example: [2+4] thermal cycloaddition (Diels-Alder

reaction)

n

m

n

m

suprafacial/suprafacial antarafacial/suprafacial

suprafacial antarafacial

CO2Me

CO2Me

+

CO2Me

CO2Me

diene (cisoid) dienophile (cis) adduct (cis)


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1. Suprafacial [2+2] cycloaddition (4 electrons)


SS

AA

SS

AS

SA

AA

AS

SA

1

2

1

2


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2. Suprafacial [4+2] cycloaddition (6 electrons)


S

A

S

A

S

A

S

A

A

S

S

A


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Cycloadditon reactions are controlled by thesymmetries of the HOMO of one component and theLUMO of the other component (they must match).

_______________________________________________


even number antarafacial suprafacialodd number suprafacial antarafacial

Note:

Antarafacial additions may not be possible forgeometric reasons.

Stereochemical issues:

cis/trans products (stereochemistry of startingmaterials)

exo/endo products (secondary HOMO-LUMOinteractions)

regioselectivity (controlled by the relative size oforbital lobes)


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SIGMATROPIC REARRANGEMENTS

[1,n] shifts, for example [1,5]

[n,m] shifts, for example [3,3] (Cope rearrangement)

n n

n n

Suprafacial (suprafacial/suprafacial)

Antarafacial (antarafacial/suprafacial)

H

1

1'

23

45

H1

23

45

1'

1'

2'

21

3'

3

1'2'

21

3'

3


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Common examples of thermal shifts

1. [1,5] hydrogen shift

2. Cope rearrangement

3. Claisen rearrangement

H3C H H3CH

H

H3C

HH

25 oC 25 oC

OO

HHO

[3,3]


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The same symmetry rules as for cycloadditions_______________________________________________


even number antarafacial suprafacialodd number suprafacial antarafacial

Note:

Antarafacial sigmatropic rearrangements may not bepossible for geometric reasons, especially for "short"

systems

SUMMARY OF SYMMETRY RULES FOR

PERICYCLIC REACTIONS_______________________________________________

Electron state Electron Pairs Stereochemistry______________________________________________

ground state even number antara-conodd number supra-dis

excited state even number supra-disodd number antara-con

_______________________________________________


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A General Procedure for Determination of What'sAllowed

Note: The orbitals used here are "basis" orbitals (notany specific molecular orbitals).

1. Draw the reaction scheme and show with arrows the"electron flow".

2. Draw all basis orbitals participating in the reaction (

and ).

3. Connect with lines on the same face (side) all orbitallobes forming continuous networks or bonds.

4. Give the connected lobes the same sign (color).5. Indicate the newly made bonds with arrows, following

the stereochemistry you want to explore.6. Count the number of electrons, and the number of

sign inversions in the "cycle" of connected orbitals.7. Use the table below to determine whether the reaction

is allowed or disallowed.

For thermal reactions_______________________________________________

# of electrons odd # of sign even # of signin the cycle inversions inversions_______________________________________________

4n allowed disallowed4n + 2 disallowed allowed (aromatic)_______________________________________________

For photochemical reactions the rules are reversed

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