CHM 420 PERICYCLIC REACTIONS &

NATURAL PRODUCT CHEMISTRY

OKE, D.G.

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INTRODUCTION

Pericyclic reactions are certain reactions which

proceed in a single concerted step via the formation

of a cyclic transition state involving π-electrons,

aided by heat in a thermally induced reaction or

ultraviolet light in a photo-induced reaction.

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Orbital Symmetry and the chemical reaction.

Woodward and Hoffmann formulated ‘roles’ and

described certain reaction path as symmetry allowed

(SA) and others symmetry forbidden (SF). These applied

to only concerted reactions and refers to the ease with

which they take place.

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A ‘SF’ reaction is one for which the concerted

mechanism is very difficult. If reaction is to occur

at all, it happens by a different concerted path

that is ‘SA’, otherwise by a stepwise and non-

concerted mechanism

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The stereochemistry of electrocyclic reactions canbe observed with a suitably substituted molecule.

E.g the inter-conversion of 3,4-dimethylcylobutene

and 2,4-hexadiene.CH3

CH3

H

H

CH3

CH3

H

Hcis-3,4- cis-2,4-

trans-3,4- trans-2,4-

hv

1.

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These reactions are completely stereoselective and stereospecific.

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The stereochemistry depends on two factor:

• The number of double bonds

• The types of reactions: thermally or photolytic.

HOMO are the least tightly held and are used during reactions.

Also, bond formation requires overlap of terminal lobes and for

this to happen, there must be rotation about two bonds C1-C2 and

C3-C4.

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These rotations can take place by:

•Conrotatory – rotation of bonds in the same direction or

•Disrotatory – rotation of bonds in opposite direction.

e.g. for a substituted buta1,3-diene where HOMO = ψ2

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Correlation diagram for buta1,3-diene

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Thermal cyclization of a 1,3-butadiene to a cyclobutanewhere HOMO = ψ2

conrotatory:bonding

disrotatory:antibonding

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With photochemical cyclization of a substituted buta1,3-diene where HOMO = ψ3

conrotatory:antibonding

disrotatory: bonding

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Woodward-Hoffmann Rules for electrocyclic reactions

Number of π electrons Reaction Motion

4n Thermal Conrotatory

4n Photochemical Disrotatory

4n + 2 Thermal Disrotatory

4n + 2 photochemical Conrotatory

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Assignment

1. Draw the correlation diagram for the ground and firstexcited state of 1,3,5-hexatriene.

2. Show that the thermal cyclization of substitutedhexatriene indicates disrotatory motion.

NOTE: To be submitted by email latest on Wednesday8th April, 2020 to my mail box.

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