Pericyclic reaction - Classification, stereochemistry

Definition , transition state, Classification Electrocyclic Reaction Cycloadditions Reaction, Cheletropic Reaction , Sigmatropic Reaction, Transfer Reaction ,
stereochemistry of pericyclic reactions , photochemical reactions, Ene reactions , cope elimination reaction ,Pyrolysis of Amine oxide ,various applications , Chugaev reactions/ xanthine pyrolysis

1. Pericyclic reactions
Dr.AnitaGoswami-Giri
ProfessorandHead
DepartmentofChemistry
B.NBandodkarCollegeofScience(Autonomous),
Thane-1
transferring electrons around a circle
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2. Definition:
The organic reactions involve one step concerted reactions featuring a
cyclic flow of electrons via a single transition state without forming any
intermediateare referred to as “pericyclic.”
Transition states are cyclic and the reactions are attended by a high
degree of stereo selectivity.
Unaffected by polar reagents, solvent change radical initiator
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3. Characteristics: The reaction -
ØReversiblereactionsinvolvingringclosureorringopening
ØSingletransitionstructure(TS)withnointermediates
ØForwardorreversereactionsprovidethesameanalysis
ØMechanismsexplainedthroughanalysisoffrontiermolecularorbitals(FMOs)
Øinitiatedbytheactionoflight(photolysis)&heat(thermolysis).
ØNotsensitivetothepresenceorabsenceofsolventsandcatalysts
ØBondbreakingandformationtakeplacesimultaneously(concerted)
ØStereospecificdependingonwhetherinitiatedbyheatorlight.Ifthermolysisgives
acis-product,thenphotolysisgivestrans-productandviceversa.
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5. Classification of Pericyclic Reaction
Therearefourmajorclassesofpericyclicreactions:
1. Electrocyclic Reaction:
involve rearrangement of two or more Pi-bonds in the system
2. Cycloadditions Reaction:
involve addition of two Pi-electrons systems forming a cyclic product.
3. Cheletropic Reaction: They are a special group of cycloadditions.
4. SigmatropicReaction:
•involve rearrangement of σ-–bonds.(intramolecular reactions involving the migration of a sigma bond across a pi system.where one atom on one of the reagents gets two new bonds.)
5. Group Transfer Reaction:
They are a special group of Sigmatropicrearrangement
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7. An electrocyclicring-openingreactionisareactioninwhichaσbondof
a cyclic reactantiscleavedtoformaconjugatedproductwithonemoreπ
bond.
Astrikingfeatureofthermalelectrocyclicreactionsthatproceedbyconcertedmechanisms
istheirhighdegreeofstereospecificity.Thuswhencis-3,4-dimethylcyclobuteneisheated,it
affordsonlyoneofthethreepossiblecis-transisomersof2,4-hexadiene,namely,cis,trans-
2,4-hexadiene:
An electrocyclicringclosureisanintramolecularreactionthatformsa cyclicproduct
containingonemoreσ bondandonefewerπbondthanthe reactant.
Electrocyclicreactionare completely stereospecific :
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19. Pi and sigma bonds remain constant and sigma
bonds migrates across the atom
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23. Reactiontakeplaceatlower
temperature(120-150oC)
comparetoformationof
olefinbythedecomposition
withesterandcarboxylicacid.
Thusdecompositionof
sensitiveolefinsoften
avoided.
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24. PyrolysisofAmineoxideisregioselectiveandtakeplaceundermild
conditionsandallowsformationofnewC=Cwithoutmigrationinto
conjugationwithotherunsaturatedsystemsinthemolecule.E.gsynthesiof
1,4-pentadiene
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25. Xanthates,aliphaticesterandamineoxidegivesZ/Ealkenesonpyrolysis
E-isomerismorestable,
Itissynelimination,thereforethethreoisomeroftheamineoxidegivesZ-
alkenewhileerythroisomergivesE-alkene. 25

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