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LECTURE 3

Dr Ali El-Agamey

CHEM-405:

PERICYCLIC REACTIONS

DAMIETTA UNIVERSITY

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Rule

• For systems containing 4n + 2 electrons, the HOMO issymmetric.

• For systems containing 4n electrons, the HOMO isasymmetric.

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Woodward-Hoffmann rules for electrocyclic reactions

MotionMode of activation

No. of electrons

conrotatorythermal4n

disrotatoryphotochemical4n

disrotatorythermal4n + 2

conrotatoryphotochemical4n + 2

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Questions (a) Why Dewar-benzene (extraordinarily high-energy molecule) is not easily converted to benzene or compound 2 at room temperature, although isomerization of Dewar-benzene to benzene has been estimated to be exothermic by about 79 kcal/mol. (b) In your opinion, what are the reasons responsible for the high exothermicity of this reaction?

2

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It is not clear whether the isomerization of Dewar-benzene and other forbidden electrocyclic reactions, actually violate the Woodward-Hoffmann rules. We have assumed that both terminal carbons of a polyene must rotate simultaneously.

Forbidden electrocyclic reactions1

There is another possibility: initially, one end of a polyene might rotate breaking a pi bond to form a diradical. Rotation of the other end of the chain could then result in formation of the ring isomer. Such two-step mechanisms would not be expected to be stereospecific but should give rise to mixtures of stereoisomers.

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Electrocyclic opening and closing of some linearly conjugated ions.

Electrocyclic reactions of cations and anions1

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Electrocyclic reactions of cations and anions

Five-atom electrocyclizations

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Nazarov cyclization: electrocyclic ring closure of divinyl ketone, upon treatment with acid, to give a cyclopentenone.1,2

Electrocyclic reactions of cations and anions

Pentadienyl cation

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Nazarov cyclization:

Electrocyclic reactions of cations and anions

Pentadienyl cation

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Nazarov cyclization:

Electrocyclic reactions of cations and anions

Pentadienyl cation

In electrocyclic ring closure of the pentadienyl cation to the cyclopentenyl cation, the decrease in cation stabilization is compensated by the gain of a C-C sigma bond.2

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Electrocyclic reactions of cations and anions

Thermal electrocyclic ring closure of pentadienyl anions occursin a disrotatory fashion.1

Pentadienyl anion

Irradiation of the anion 4.96 gives the trans isomer of 4.97.1

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Electrocyclic reactions of cations and anions

Thermal electrocyclic ring closure of cyclooctadienyl anionoccurs in a disrotatory fashion.1

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Why is cyclooctadienyl anion stable for at least a week at -78 oCin broad daylight?1

Questions

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Electrocyclic reactions of cations and anions

• Allyl cations are conjugated systems containing 2 pi electrons.1

Three-atom electrocyclizations (2 electrons)

• Therefore, thermal electrocyclic ring closure occurs via a disrotatory manner.1

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Electrocyclic reactions of cations and anions

• The product would be cyclopropyl cation. In fact, it is the cyclopropyl cations that undergo this reaction very readily because of the relief of ring strain and the charge delocalization in the allyl cation.1,2

Three-atom electrocyclizations (2 electrons)

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Three-atom electrocyclizations (2 electrons)

• Reactions that would be expected to yield cyclopropyl cationsproceed with simultaneous ring opening to from allylic cationsi.e. case 2.1,3

• Cyclopropyl cations are very unstable (they are virtually unobservable) i.e. case 2.1,3

• In case 2, the cleavages of C-C bond and C-X bond take place in a concerted manner.2

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Electrocyclic reactions of cations and anions

• Solvolysis of 1-chloro-2,3-dimethylcyclopropane diastereomers(5a-5c).2

Three-atom electrocyclizations (2 electrons)

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scheme Miller p39

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Electrocyclic reactions of cations and anions

• Solvolysis of 1-chloro-2,3-dimethylcyclopropane diastereomers(5a-5c) clearly established the disrotatory ring opening.2

Three-atom electrocyclizations (2 electrons)

• Each of the isomers underwent only one of the two possible modes of disrotatory ring opening (torquoselectivity).2

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Electrocyclic reactions of cations and anions

Three-atom electrocyclizations (2 electrons)

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Electrocyclic reactions of cations and anions

Three-atom electrocyclizations (2 electrons)

• The observed stereospecificity and torquoselectivity suggest that the C-C bond (HOMO) and the C-X bond (LUMO) cleavages take place in a concerted manner and the electron density of the C-C bond breaking assists the cleavage of the C-X bond.2

• This is possible only if the disrotatory mode occurs in one direction so as to enable the electron density of the C-C bond to participate from the back side of the C-X bond (structure A and B) in a manner similar to the familiar SN2 mechanism.2

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Electrocyclic reactions of cations and anions

Three-atom electrocyclizations (2 electrons)

• Molecular orbital calculations show that of the two possible modes of disrotatory ring openings of cyclopropyl derivatives, the preferred path will be that in which the substituentstrans to the leaving group rotate apart.3

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Electrocyclic reactions of cations and anions

Three-atom electrocyclizations (2 electrons)

• During the inward motion of the two methyl groups there will be severe stericinteraction in the TS of 5a reaction, therefore its reaction is much slower than that of 5c.1,2

5a

5c

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Questions• Explain the following experimental rates observed during the solvolysis reactions of cyclopropyl tosylates?

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Questions• Explain the following experimental results?

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Electrocyclic reactions of cations and anions

• Cyclopropanones are in electrocyclic equilibrium with oxyallylcations. The cyclopropanone is generally lower in energy, butthe oxyallyl cation is not so much higher in energy that it is kinetically inaccessible. Oxyallyl cations can undergo cycloadditions as will be discussed later.1

Three-atom electrocyclizations (2 electrons)

• The ring closure involves the disrotatory pathway.2

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Electrocyclic reactions of cations and anions

Three-atom electrocyclizations (4 electrons)

Allyl anion-cyclopropyl anion is a 4 electron-system

Therefore, thermal electrocyclic ring closure (or ring openeing) occurs via a conrotatory manner.3

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Why is the opening of cylopropyl ring in anion 7 about 104

times as fast as the ring openings of anions 8?3

Questions

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Three-atom electrocyclizations (4 electrons)

Electrocyclic reactions of 1,3-dipoles

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Electrocyclic reactions of cations and anions

Three-atom electrocyclizations (4 electrons)

Generally, the 1,3-dipole, is a compound for which a relatively stable resonance structure can be drawn in which one terminus has a formal positive charge (and is electron-deficient) and the other terminus has a formal negative charge.2

All the common 1,3-dipoles, have a heteroatom (N or O) inthe central position in order to stabilize the electron-deficient terminus.2

1,3-Dipoles, are isoelectronic with allyl anion and they have 4 pi electons.1

The HOMO and LUMO of a 1,3-dipole are similar in symmetry to that in a diene.1

٣١Chapter 1 ٣١

Electronic Structure of the Atom

• An atom has a dense, positively charged nucleus surrounded by a cloud of electrons.

• The electron density is highest at the nucleus and drops off exponentially with increasing distance from the nucleus in any direction.

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Three-atom electrocyclizations (4 electrons)

Aziridines (three-membered ring amines) are isoelectronicwith cyclopropyl anions. Aziridines will open on heating to form azomethine ylids (1,3-dipoles).3,4

Electrocyclic reactions of 1,3-dipoles

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Three-atom electrocyclizations (4 electrons)

Azomethine ylids can be trapped by [3 + 2] cycloadditionreactions with dipolarophiles.4

Electrocyclic reactions of 1,3-dipoles

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Three-atom electrocyclizations (4 electrons)

Since cycloaddition is stereospecific (suprafacial on both components), the stereochemistry of the products can tell us thestereochemistry of the intermediate ylid (4 pi electron system), and confirms that the ring opening is conrotatory.4

Electrocyclic reactions of 1,3-dipoles

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Three-atom electrocyclizations (4 electrons)

Heating of oxiranes (epoxides) give the corresponding carbonyl ylids, which can be trapped by cycloaddition reactions.1,2

These ring openings proceed stereospecifically by conrotatorypaths.1,2

Electrocyclic reactions of 1,3-dipoles

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Questions

5a 5c5b

1- Provide an arrow pushing mechanism for the formation of 5a, 5b and 5c and show whether the reactions proceed via conrotatory or disrotatory.

2- Provide a molecular orbital representation for the transition state for the rearrangement of 5b to 5c. HINT: your answer requires you to identify the HOMO of the pi system, and indicate how the symmetry of this HOMO is reflected in the transition state.

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Woodward-Hoffmann rules for electrocyclic reactions1,2

PhotochemicalThermalNo. of electronsElectrocyclic reactions

conrotatorydisrotatory2e (4n + 2)allyl cation-cyclopropyl cation

allyl anion-cyclopropyl anion

butadiene-cyclobutene

pentadienyl cation-cyclopentenyl cation

pentadienyl anion-cyclopentenyl anion

hexatriene-cyclohexadiene

heptatrienyl cation-cycloheptadienyl cation

heptatrienyl anion-cycloheptadienyl anion

octatetraene-cyclooctatriene

nonatetraenyl cation-cyclononatrienyl cation

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Define the type of pericyclic reactions for the following reactions and determine if they are 4n + 2 or 4n systems.

Questions