双语有机化学Special topic and pericyclic reaction

Chapter21PericyclicReactions•Apericyclicreaction•Apericyclicreactionoccurswhentheelectronsinoneormorereactantsarereorganizedinacyclicmanner.•Pericyclicreactionsareconcertedandhighlyselective.•Threemostcommontypesofpericyclicreactions:•Electrocyclicreaction•Cycloadditionreactions•SigmatropicrearrangementsPericyclicReactions–WhatAreThey?•Involvesseveralsimultaneousbond-makingbreakingprocesswithacyclictransitionstateinvolvingdelocalizedelectrons•ThecombinationofstepsiscalledaconcertedprocesswhereintermediatesareskippedMolecularOrbitalsandPericyclicReactionsofConjugatedSystems•Aconjugateddieneorpolyenehasalternatingdoubleandsinglebonds•BondingMOsarelowerinenergythantheisolatedpatomicorbitalsandhavethefewestnodes•AntibondingMOsarehigherinenergyMolecularorbitals1,3-butadieneS:SymmetricA:Antisym.MolecularOrbitalsandPericyclicReactionsofConjugatedSystemsIn-phaseorbitalsinteracttogiveabondinginteractionandout-of-phaseorbitalsinteracttocreateanode.Anodeisaplaceinwhichthereiszeroprobabilityoffindinganelectron•MolecularOrbitalsof1,3,5-HexatrieneAstheenergyofthemolecularorbitalincreases,thenumberofbondinginteractionsdecreasesandthenumberofnodesincreasesbetweenthenucleiincrease.Symmetricorbitalshaveamirrorplane,asymmetricorbitalsdonothave.Iftheporbitalsattheendsofthemolecularorbitalarein-phase,themolecularorbitalissymmetric.Ifthetwoendporbitalsareout-of-phase,themolecularorbitalisasymmetricorantisymmetric.1,3,5-Hexatriene•ThreedoublebondsandsixMOs•Onlybondingorbitals,1,2,and3,arefilledinthegroundstate•Onirradiationwithultravioletlightanelectronispromotedfrom3tothelowest-energyunfilledorbital(4*)•Thisisthefirst(lowestenergy)excitedstate•IfthesymmetriesofbothreactantandproductorbitalsmatchthereactionissaidtobesymmetryallowedundertheWoodward-HoffmannRules•Ifthesymmetriesofreactantandproductorbitalsdonotcorrelate,thereactionissymmetry-disallowedandtherenolowenergyconcertedpaths•Fukui’sapproach:weneedtoconsideronlythehighestoccupiedmolecularorbital(HOMO)andthelowestunoccupiedmolecularorbital(LUMO),calledthefrontierorbitalsMolecularOrbitalsandPericyclicReactions•Robertb.Woodward(1917-1979)wasborninBostonandfirstbecameacquaintedwithchemistryinhishomelaboratory.HeenteredMITattheageof16andreceivedaPh.D.thesameyearthatthosewhohadenteredMITwithhimreceivedtheirB.A.’s.HewenttoHarvardasapostdoctoralfellowandremainedthereforhisentirecareer.Cholesterol,cortisone,strychnine,reserpine(thefirsttranquilizingdrug),chlorophyll,tetracycline,andvitamineB12arejustsomeofthecomplicatedorganicmoleculesWoodwardsynthesized.HereceivedtheNobelPrizeinchemistryin1965forthetotalityofhisworkinthe“art”ofsyntheticchemistry,structuredetermination,andtheoreticalanalysis.•RoaldHoffmannwasborninPolandin1937andcametotheUnitedstateswhenhewas12.HereceivedaB.S.fromColumbiaandaPh.D.fromHarvard.WhenheandWoodwardproposedtheconservationoforbitalsymmetrytheory,theywerebothonthefacultyatHarward.HoffmanisatpresentaprofessorofchemistryatCornell.•KenichiFukai(1918-1998)wasborninJapan.HewasprofessoratKyotoUniversityuntil1982.whenhebecomepresidentofKyotoInstituteofTechnology.HewasthefirstJapanesecitizentoreceivetheNobelPrizeinChemistry.MolecularOrbitalsandPericyclicReactions•RoaldHoffmannandKenichiFukuisharedthe1981Nobelprizeinchemistryfortheconservationoforbitalsymmetrytheoryandthefrontierorbitaltheory.R.BWoodwarddidnotreceiveashareintheprizebecausehediedtwoyearsbeforeitwasawarded,andAlfredNobel’swillstipulatesthatprizecannotbeawardedposthumously.Woodward,however,hadreceivedthe1965NobelPrizeinchemistryforhisworkinorganicsynthesis.11ElectrocyclicReactions•Anelectrocyclicreactionisanintramolecularreactioninwhichtherearrangementofelectronsleadtoacyclicproductthathasfewerbondthanthereactant•Onebondisbroken,theotherbondschangeposition,anewσbondisformed,andacycliccompoundresults12ElectrocyclicInterconversionswithOctatriene•Anelectrocyclicreactioniscompletelystereoselective,preferentiallyformingonestereomer•Thecycliccompoundisfavoredforelectrocyclicreactionsthatformsix-memberedrings.UnderthermalconditionUnderthermalconditionUnderphotochemicalconditionAmesocompoundApairofenantiomersCH3HHH3C+hv13ElectrocyclicInterconversionswithDimethylcyclobuteneUnderthermalconditionUnderthermalconditionCH3HHH3C+•Theopenchaincompoundisfavoredforelectrocyclicreactionsthatformfour-memberedringsbecauseoftheanglestrainassociatedwithfour-memberedrings14TheSignsontheOutermostLobesMustMatchtoInteract•Thelobesoflikesigncanbeeitheronthesamesideoronoppositesidesofthemolecule.•Forabondtoform,theoutermostlobesmustrotatesothatfavorablebondinginteractionisachieved15DisrotatoryOrbitalRotation•Iftwoin-phaselobesareonthesamesideofthemolecule,thetwoorbitalsmustrotateinoppositedirections—oneclockwise,andonecounterclockwise•Woodwardcalledthisadisrotatoryopeningorclosure•Toformthenewσbond,theorbitalsmustrotatesothatin-phaseporbitalsoverlap,becausein-phaseoverlapisabondinginteraction.Out-of-phaseoverlapwouldbeanantibondinginteraction.16ConrotatoryOrbitalRotation•Iflobesoflikesignorin-phasesignareonoppositesid