Pericyclic reactions for Undergraduate Courses

By definition, pericyclic reactions proceed through a concerted mechanism involving a single, cyclic transition state. Because of this, prior to a systematic understanding of pericyclic processes through the principle of orbital symmetry conservation, they were facetiously referred to as 'no-mechanism reactions'.

1. Pericyclic reactions
Dr. Satyajit Dey
Department of Chemistry
Tamralipta Mahavidyalaya
Tamluk
Midnapore (East)
Under Graduate
Organic Chemistry Course (Vth SEMESTER)

2. Syllabus
PericyclicReactions:
Mechanism,stereochemistry,regio-selectivityincaseof:
Electrocyclicreactions:FMOapproachinvolving4π-and6π-electrons
(thermalandphotochemical)andcorrespondingcycloreversionreactions.
Cycloadditionreactions:FMO approach,Diels-Alderreactions,
photochemical[2+2]cycloadditions.
Sigmatropic reactions: FMO approach, sigmatropic shifts and their order;
[1,3]-and [1,5]-H shifts and [3,3]-shifts with reference to Claisen and Cope
rearrangements.
Paper –C12T (Organic)

3. Pericyclicreactionsarecharacterizedbyaconcertedbondingchanges
takingplacethroughthereorganizationofπandσbondingelectronina
cyclicT.S.whichisassociatedwithcyclicarrayofinteractingorbitals.
Cyclic Transition State
Concerted PericyclicReaction
Concerted ionic Reaction
Transition State

4. 1.Bondingchangesmustbeconcertedi.e.;allbondsbreakingandbond
formingstepsaresimultaneousbutnotnecessarilysynchronous.Thatmeans
thatallbondbreakingandmakingmaynotoccurtothesameextentatall
stagesduringtheformationoftransitionstate.
2.Pericyclicreactionsarereversibleinnatureandfollowthemicroscopic
reversibilitypath.
3.Thereactioninvolvesnodistinctpolarintermediatesduringthecourseof
thereaction.
4.Reactionproceedsinasinglestepthroughtheformationofnonpolar
transitionstate.
5.Thereislittlesolventeffectontherateofpericyclicreactions(unlessthe
reactantsthemselveshappentobechargedspecies)andnormallyoccurinthe
gasphasewithnosolvent.
6.Thereisnonucleophilicorelectrophiliccomponent.

5. Characteristic Features , cont….
7.Normallynocatalystisneededtopromotethereactions.However,
manytransitionmetalcomplexescancatalyzepericyclicreactionsby
virtueoftheird-orbitalparticipation.Lewisacidsalsocatalyzemany
pericyclicreactions,eitherdirectly,orbychangingthemechanismof
thereactionsothatitbecomesastepwiseprocess.
8.Pericyclicreactionsnormallyshowveryhighstereospecificityand
regieselectivity.
9.No.ofσandorπelectronsareimportantinpredictingthe
stereochemistryoftheproducts.
10.Pericyclicreactionscanbepromotedbylightaswellasheat.The
stereochemistryofproductsundertwosetsofconditionsisdifferent
anditisjustopposite.

6. Classifications

7. Classification , cont….

8. Classification , cont….

9. Symmetry properties of molecular orbitals
Theknowledgeofmolecularorbitalsofconjugatedpolyenesystemsandtheir
symmetrypropertiesisessentialinunderstandingthetheoryofpericyclic
reactionsthroughtheconstructionoforbitalmodelforbonding.Beforethat
letmeexplainthesymmetrypropertiesofmolecularorbitals.
Asymmetryoperationisageometricoperation(rotation,reflectionetc)
whichwhenperformedonanobjectproducestheindistinguishableformof
theoriginalstructureoftheobject.Therespectivesymmetryelementsof
differentsymmetryoperationsarei)Cn(simpleaxisofsymmetry)ii)m
(mirrorplaneofsymmetry)iii)i(centreofsymmetry)iv)Sn(alternating
axisofsymmetry).Itisnotnecessarytoconsiderallthesymmetryelements
presentinthereactingmoleculesortheproductmolecules.Inorderto
explainthepericyclicreactionsonlytwo-foldaxisofsymmetry(C
2)and
mirrorplaneofsymmetry(m)areconsidered.

10. Molecular orbitals and their symmetry
Pi-MO’s of Ethylene

15. 1.Frontier orbital interaction method (FOI)
(Woodward, Hoffmann, Longuet -Higgins and Abrahamson)
2. Orbital symmetry correlation method (Conservation of
orbital symmetry)
(Woodward, Hoffmann, and Fukui)
3.Hückel-Möbius transition state (aromatic and antiaromatic
TS) theory
(Dewar and Zimmerman)

16. Frontier Molecular Orbital Theory (FMO Theory)
Thecovalentbondformationoccursbytheinteractionsbetweenafilledorbitalofone
moleculeandavacantorbitaloftheother,havingappropriatesymmetry.Thoughthe
interactionsaresmall,theyarestabilizing.Theenergyseparationbetweenthe
interactingoccupiedandvacantorbitalisameasureofthemagnitudeofcovalent
force.Thesmallertheenergydifference,thegreateristhestabilizationduetotheir
interaction.

17. Therefore,Stabilizationwillbegreaterwhenfilledandemptyorbitalsofequal
(degenerateorbitals)ornearlyequalenergyinteract(Figure2a)thanthe
orbitalsofunequalenergy(nondegenerateorbitals)interact(Figure2b).So,
molecularorbitalshavingsimilarenergiesaretheessentialcriteriafora
strongestinteractionbetweentwointeractingorbitals.
Frontier Molecular Orbital Theory, cont…..

18. Frontier Molecular Orbital Theory, cont…..
SincetheenergydifferencebetweenHOMOandLUMOisverysmall,the
interactionbetweenthemwillbeverylargeandhenceastrongercovalentbond
willbeformedbytheirinteraction.TheseHOMOandLUMOarecalledFrontier
MolecularOrbitals(FMO).TheinteractionsbetweenHOMO-LUMOarecalled
frontiermolecularorbitalinteractions(FMOinteraction).

21. Stereochemistry of electrocyclic reactions
4np-Electron Systems:
They are highly stereospecific. The stereochemistry of the product depends upon the
following three factors:
1.The Number of electrons (4n or 4n+2) involved.
2.The stereochemistry of the reactant.
3.Mode of activation (thermal or photochemical).
SS
RRMeMe MeMe MeMe MeMe
Enantiomers
thermal
E,E-isomer
thermal
RSMe
Me MeMe MeMe
Meso
Z,E-isomer

22. (4n+ 2) p-Electron Systems:
MesoMeMe MeMe MeMe
RS
thermal
E,E-isomerMe
Me MeMe MeMe MeMe
SS
RR
Enantiomers
thermal
Z,E-isomer

23. Prediction of stereochemistry of electrocyclic reactions:
Frontier molecular orbital theory

24. FMO description of electrocyclic reactions:
InFMOapproach,WoodwardandHoffmannproposedthatthestereochemistryandthe
allowednessofelectrocyclicreactionsiscontrolledbythesymmetrypropertiesofthehighest
energyoccupiedmolecularorbital(HOMO)oftheopenchainformofthereactant-product
pair.IfthisHOMOissymmetricwithrespecttotwo-foldaxis(C
2)symmetry,thereaction
proceedsbyconrotatorypathwayandifitissymmetricwithrespecttomirrorplane(m)of
symmetry,thedirectionofrotationisdisrotatory.
Thermal ring closure of 4n
p
-
Electron Systems :

25. Photochemical ring closure of 4np-Electron Systems :

26. Thermal ring closure of (4n+2) p-Electron Systems :

27. Photochemical ring closure of (4n+2) p-Electron Systems :

28. Woodward-Hoffmann selection rules:
“allconjugatedpolyenes(4nor4n+2πelectronssystem)withHOMO’s
havingC
2symmetrywillundergoelectrocyclicringclosureorring
openingreactionbyconrotatorymotionandwithHOMO’shavingplaneof
symmetry(m)willundergoelectrocyclicringclosureorringopening
reactionbydisrotatorymotion.”
π-System Mode of activation Symmetry of
HOMO
Allowed pathway Stereochemistry of
the reactant
Stereochemistry of
the product
4n π-System
Thermal (Δ) C
2 Conrotation E,ZorZ,E Cis
Z,ZorE,E Trans
Photochemical
(hγ)
m Disrotation E,ZorZ,E Trans
Z,ZorE,E Cis
(4n+2) π-System
Thermal (Δ) m Disrotation E,ZorZ,E Trans
Z,ZorE,E Cis
Photochemical
(hγ)
C
2 Conrotation E,ZorZ,E Cis
Z,ZorE,E Trans

29. Equilibrium and microscopic reversibility in electrocyclic
reactions and its deviation:

30. Deviations ………
Other examples:
AlthoughDewarbenzeneislessstablethanitsisomerbenzeneby71Kcal/mole,itsconversionintobenzene
isextremelyslowwithanE
act.ofabout37Kcal/mole.

31. Examples of Electrocylic reactions

32. Superimposable Mirror Images
Exactly the same compound
MESO Compound
RS RS
DISROTATORY
y3–HOMO
Examples of Electrocylicreactions, cont………
Thefollowingtrieneundergoesathermalelectrocycliccyclisation.UsingFMOs
identifyalltheproducts.

33. Examples of Electrocylicreactions, cont………
Thefollowingtrieneundergoesaphotochemicalelectrocycliccyclisation.Using
FMOsidentifyalltheproducts.
Nonsuperimposable Mirror Images
Active Compound
CONROTATORY

34. Thetwodiastereoismerictrienes1and2undergothermalelectrocycliccyclisationreactionseach
affordingapairofdisubstitutedconjugatedcyclicdienes.Identifyallfourproductsbyconstructing
thetransitionstategeometries,andstatethestereochemicalrelationshipsthatexistbetweenthepairs
ofstereoisimersformedfromeachreactionandthestereochemicalrelationshipoftheproducts
betweenthepairofreactions
y3–HOMO
Enantiomers
RS RS
DISROTATORY
Enantiomers
RR SS
DISROTATORY
Examples of Electrocylicreactions, cont………

35. Examples of Electrocylicreactions, cont………
Thetwodiastereoismerictrienes1and2undergophotochemicalelectrocycliccyclisationreactions
eachaffordingapairofdisubstitutedconjugatedcyclicdienes.Identifyallfourproductsby
constructingthetransitionstategeometries,andstatethestereochemicalrelationshipsthatexist
betweenthepairsofstereoisimersformedfromeachreactionandthestereochemicalrelationshipof
theproductsbetweenthepairofreactions
Enantiomers
CONROTATORY
Enantiomers
RS RS
CONROTATORY

36. HH H H
0% 100% 4pElectron Process
H H
CONROTATORY
HH
y2
HOMO
Butadiene
6pElectron Process
H
H
H H
DISROTATORY
y3
HOMO
1,3,5-Hexatriene
Examples of Electrocylicreactions, cont………

37. HH
H
H H
H
H
H
GEOMETRICALLY
IMPOSSIBLE: Hydrogen placed
inside a six-membered ring
CON HH
H
H
HH
HH
CON HH
HH UsingFMOswecanrationalisewhythetwodiastereoisomershavesuch
differentreactivities.
Examples of Electrocylicreactions, cont………

38. Cascade Electrocyclic reactionsMeMe
HH
MeMe
Me Me
HH
MeMe
y4 (3 nodes 9/4)
of 1, 3, 5, 7-octatetraene
4n -CONROTATORY
Me Me
HH
y
3 (2 nodes)
of 1, 3, 5-hexatriene
(4n + 2) -DISROTATORY
MeMe
HH
MeMe
Me Me
HH
Me Me
HH
HH
MeMe

39. Cascade reactions, cont ………

40. Tandem Electrocyclic Reaction
Thearrowpushingmechanismrevealsthatthereactioninvolvestheringclosureoftwo
1,3,5-hexatrienesystems.Thus,needtoconsidery3HOMOof1,3,5-hexatriene.
UseFMOstopredictthestereochemical
outcomesinthereactionschemeright.
Inprinciple,therearetwopossible
products.Whichwillbeformedin
highestyield.Justifyyouranswer.H
H
H
H H
H
H
H
Thermodynamic
Product. Least
sterically hindered
Disrotatory
of both
triene systems
H
H
H
H
H
H
H
H

41. Electrocyclisation of cation (Nazarov cyclisation)