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Pericyclic Reaction- reaction that occurs by a concerted process through a cyclic transition state- concerted means that all bonding changes occur at the same time and in a single step (no intermediates)

Example:

- usually involves a conjugated diene or polyene

[4+2]-π-electronsDiels-Alder Reaction

- two theories: 1) Woodward-Hoffman rules, 2) frontier orbital theory

+

diene dienophile

Molecular Orbitals in Conjugated Systems (Review)

2

Woodward-Hoffman Rules and Frontier Orbital Theory

Woodward-Hoffman Rules

- pericyclic reactions can only take place if the symmetries of the reactant molecular orbitals are the same symmetries as the product molecular orbitals (lobes of reactant MO’s must be of the correct algebraic sign for bonding to occur in the transition state leading to the product)

Frontier Orbital Theory

- we need to consider only two molecular orbitals, the HOMO and LUMO, to predict the structure of the product; called frontier MO’s

Three types of pericyclic reactions:

1) electrocyclic, 2) cycloaddition, 3) sigmatropic rearrangement

Electrocyclic Reaction- an electrocyclic reaction is a pericyclic process that involves the cyclization of a conjugated polyene

heat

heat

a conjugated triene a cyclohexadiene

a conjugated diene a cyclobutene

Stereochemisty of Electrocyclic Reactions

CH3

CH3

HH

CH3

CH3

H

H

H

H

CH3

CH3

H

CH3

CH3

H

CH3

H

CH3

H

CH3

H

H

CH3heat

hv

heat

hv(2E,4Z,6E)-octatriene

cis-5,6-dimethyl-1,3-cyclohexadiene

trans-5,6-dimethyl-1,3-cyclohexadiene

cis-3,4-dimethyl-cyclobutene

(2E,4Z)-hexadiene

(2E,4E)-hexadiene

- outermost π lobes must rotate so that bonding is achieved -

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Two Types of Rotation

disrotary

conrotary

- according to frontier orbital theory, the stereochemistry of an electrocyclic reaction is determined by the symmetry of the polyene HOMO

Thermal Electrocyclic Reactions

H

CH3

H

CH3

H

CH3

H

CH3

=disrotary

CH3H3C H H

H

CH3

H

CH3

H

CH3

H

CH3

HCH3

HCH3

=conrotary

(2E,4Z,6E)-octatriene cis-5,6-dimethyl-1,3-cyclohexadiene

cis-3,4-dimethylcyclobutene (2E,4Z)-hexadiene

- Note: conjugated diene and triene react in opposite stereochemical senses

Ground- and Excited-State Electronic Configurations

ψψψψ1

ψψψψ2

ψψψψ*3

ψψψψ*4

ψψψψ1

ψψψψ2

ψψψψ3

ψψψψ*4

ψψψψ*5

ψψψψ*6

LUMO

HOMOLUMO

HOMO

LUMO

HOMOLUMO

HOMOhv

hv

Groundstate

Groundstate

Excitedstate

Excitedstate

Conjugated diene Conjugated triene

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CH3H3C H H

H

CH3

H3C

H

H3CH

HCH3

H

CH3

H

CH3

disrotaryhv

conrotaryhv

(2E,4E)-hexadiene cis-3,4-dimethyl-cyclobutene

(2E,4Z,6E)-octatriene trans-5,6-dimethyl-1,3-cyclohexadiene

hv

hv

HOMO(ground)

HOMO(excited)

HOMO(excited)

HOMO(ground)

Photochemical Electrocyclic Reactions

Stereochemical Rules for Electrocyclic Reactions

Electron pairs(double bonds) Thermal Reaction

PhotochemicalReaction

even number

odd number

conrotary

disrotary

disrotary

conrotary

Cycloaddition Reactions

- a cycloaddition reaction is a reaction in which two unsaturated molecules add to one another, yielding a cyclic product

- controlled by orbital symmetry

- takes place when a bonding interaction occurs between the HOMO of one reactant and the LUMO of the other

- major examples:

1) Diels-Alder reaction (thermal)

2) [2+2] cycloaddition (photochemical)

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Diels-Alder Reaction

[2+2] Cycloaddition Reaction

C

C

C

C

C

C C

C

+

No reactionheat

hv

a cyclobutane

- how do reactants approach each other?

Suprafacial

- when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the same face of the other reactant

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Antarafacial- when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the opposite face of the other reactant

Stereochemistry of Cycloadditions- according to frontier molecular orbital theory, a cycloaddition reaction takes place when a bonding interaction occurs between the HOMO of one reactant and the LUMO of another reactant (i.e. one reactant donates electrons to the other)

- electrons in the HOMO of the first reactant are least tightly held and most likely to be donated

- electrons must be donated into a vacant orbital - LUMO

- selection of which reactant provides the HOMO and which reactant provides the LUMO is arbitrary

Stereochemistry of Diels-Alder Reaction

- only conditions to allow the Diels-Alder reaction to occur, accounting for the reaction being thermal

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Potential Thermal [2+2] Cycloaddition

Stereochemistry of the [2+2] Cycloaddition

C

C

C

C C

C C

C+ hv

Example:

O

+ hvH

H

O

2-cyclohexenone2-methylpropene

40%

8

hv2

a trans-stilbene

‘head-to-tail’ (45%)

Example:

OMeO2N

MeO

OMe

NO2

O2N

+

‘head-to-head’ (5%)

MeO

MeONO2

NO2

- control of reaction beyond the molecule?

Supramolecular Chemistry- supra means ‘beyond’, therefore supramolecular chemistry is chemistry ‘beyond the molecule’

- the chemistry is controlled using noncovalent forces (e.g. hydrogen bonds, van der Waals interactions)

- supramolecular chemistry is the chemistry of life (e.g. DNA, enzymes)

- can the [2+2] photodimerization be controlled supramolecularly?

OH

OH

NN

+?

1,3-benzenedioltrans-1,2-bis(4-pyridyl)ethylene

Template-Directed Organic SynthesisO

O

H

H

N N

NN

O

O

H

H

N N

NN

O

O

H

H

O

O

H

H

hv

reactants product

crystal

template template

solid

O

O

H

H

N N

NN

O

O

H

H

N N

NN

O

O

H

H

O

O

H

H

hv

reactants product

crystal

template template

solid

2(1,3-benzenediol)·2(trans-1,2-bis(4-pyridyl)ethylene)

- reaction proceeds in the solid state in quantitative yield (100% yield), where the diol functions as a linear template (compare to DNA)

- applications:

- stereo- and regiocontrol of reactivity

- construct molecules that cannot be made using ‘traditional’ synthesis

- Synthetic Chemistry, Green Chemistry, Materials Science