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    Theoretical

ChineseChemicalLettersVo1.16,No.5,PP705708,2005

    ;http://www.imm.ac.cn/journal/cc1.html

    ;705

    ;TheoreticalStudyontheIsomerizationMechanismofEnolEster

    ;from2-Acyl1,3-cyclOhexanediOnes

    ;YanHuaWANG3,

    ;JianWeiZOU,BingZHANG2

    ;,

    ;MinZENG,

    ;GuiXiangHU’,

    ;KeWenZHENG,QingSenYU?.

    ;KeyLaboratoryforMolecularDesignandNutritionEngineeringofNingboCity,NingboInstitute ;ofTechnology,ZhejiangUniversity,Ningbo315100,

    ;Departmentofchemistry,ZhejiangUniversity,Hangzhou310027

    ;CollegeofBiologyandEnvironmentProtection,ZhejiangShurenUniversity,Hangzhou310015 ;Abstract:Thepresentpaperemployeddensityfunctiontheorytoinvestigatetworeaction ;pathwaysforisomerizationofenolesterproposedbyYang(patha)andthepresentauthors(patha), ;respectively.Thebasecatalyticeffectsofsolventtriethylamineonthesetworeactionswerealso ;evaluated.ItisdemonstratedthatpathBismorepreferablethanpathaduetolowbarrierheight ;Keywords:Densityfunctiontheory,reactionmechanism,solventeffect,1,3-Hshift,proton ;transify,1,5-Hshift.

    ;Thecompoundscontaining3-tricarbonylgrouphavebeenusedextensivelyintraditional ;andmodernmedicinefortheirvariousbiologicalactivities.InpastfeWyears,organic ;chemistshavepaidcloseattentiontotheirsynthesis.Ourgrouphaveinvestigatedthe ;tautomericbehaviorofbenzoylcyclohexane1.3-dionesandperformed3DOSARstudies

    ;ontheirinhibitionactivitieson4-hydroxyphenylpyruvatedioxygenase(HPPD1enzyme. ;Mostrecently.Yangeta1.discoveredanunprecedentedisomerizationofenolesters ;derivedfrom2.acyl1.3cvclohexanediOnesatambienttemperaturewhentheysynthe

    ;sizedanewtypeofpotentHPPDinhibitors.TheXraycrystallographicanalysisofthe

    ;structureofisomerizationproductiSunequivocalandthereactionmechanismwas ;proposedasdepictedaspathainScheme1.Thereactionwasthoughttobeginwithal, ;3Hshift.whichiStheintrinsicketoenoltautomerizationof2-acylgroup,thentheester

    ;carbonylgroupiSattackedbytheoxygenatomoftheenoltoforlTltheproduct. ;Tobestofourknowledge.’7thereactionbarrierfor1.

    ;3Hshiftinmodelcompound

    ;CHCH=0iSgenerallyupto250kJ?mol.Althoughsubstituentgroupsandsolvent

    ;effectsmaysignificantlyreducethebarrierheightastheoreticalcalculations ;demonstrated.thebarrieriShardlyfelundtobelowerthanl20kJ?mol.Thus,itseems

    ;impossiblefor1.3Hshiftofthetitlecompoundtooccur.Herewesuggestedan

;alternativeisomerizationmechanism.depictedaspathbinScheme1.inwhichthepro

    ;Email:jwzou@css.zju.edu.cn

    ;

    ;706YahHuaWANGeta

    ;Scheme1Reactionmechanismforpathaandpathb

    ;ReactantR

    ;TransitionState(TS1)Imermediate

    ;a:13-Hshift.b:1,5-acylshift,c:1,5-Hshift

    ;patha

    ;\

    ;R

    ;pathb

    ;Product

    ;R

    ;ductisobtainedviatwoconsecutivesteps,1,5acylshiftand1,5Hshift.Then,both

    ;mechanismswereinvestigatedtheoreticallyandtheresultrevealedthatthepathbis ;energeticallyfavorable.

    ;ComputationMethodology

    ;Allstablestructuresandtransitionstateswerefullyoptimizedandthenharmonic ;vibrationalfrequencieswereperformedatB3LYP/631Gleveloftheory.Intrinsic

    ;reactioncoordinate(IRC)analyseswerealsoconductedtovalidatethetransitionstates ;obtained.ComputationswerecarriedoutwiththeGaussian98programpackage. ;ResultsandDiscussion

    ;1.3Hshiftinpathaisnormallyconsideredasaratedeterminingstep,SOitsbarrier

    ;heightinthetitlecompoundiScomputed.theresultsofwhichiS280.0kJ?mol(all ;energiesreportedthroughoutthispaperrefertothezeropointenergycorrectedvalues)in

    ;vacuum.ButasshowninFigure1.thepotentialenergysurfaceforpathbinvacuum, ;theenergiesbarriersfor1.5acylshiftandl,5Hshiftarel6.7and99.5kJ?mol,

    ;respectively.whichmeansthat1.5HshiftiSratedeterminingstepinpathbandits

    ;energiesbarrieriSmuch1owerthanthatfor1,3Hshiftinpatha.sopathbiSmore

    ;favorablethanpathainvacuum.ThereasonforthisiSthatthetransitionstateof1.5H

    ;shiftwithstericfavorablesixmembercycleiSmorestablethanthatofl,3Hshiftwith

    ;stericunfavorablefour-membercycle.

    ;Besides.itiSofconsiderableinteresttostudythecatalyticroleoftriethylamineon ;Hshift.sinceexperimentallythereactantiSsynthesizedintriethylamine.TheHshift

    ;energybarriercanbe1oweredduetointeractionbetweentheshiftHofreactantandthe ;Noftriethylamine,whichcouldfacilitatetheHshiftprocess.Forsimplicity,I,3H

    ;shiftinaldehydeandl,5.HshiftinBut2enalwereexplored.Theactiveenergiesfor

    ;

    ;TheoreticalStudyontheIsomerizationMechanismofEnolEster707 ;thetwoHshiftsbothinvacuumandtriethylaminewerecalculated,whicharel24.1 ;kJ.mol’.,122.9kJ.mol.for1,

    ;5Hshiftand290.0kJ?mol,165.0kJ?molfor1,3H

    ;shift,respectively.Itshowedthatthesolventcatalyticeffectreducessubstantiallythe

;barrierheightfor1.3Hshift.buthaslittleinfluenceon1,5Hshift.Toshedmore

    ;lightonsolventcatalyticeffecton1.3Hshiftintitledcompound.thebarrierheightforl ;3Hshiftofpathaintriethylaminewasalsocalculated,whichis142.6kJ?mol,much ;lowerthan280.0kJ.tool..invacuum.Itindicatesthat.asthesameasinaldehyde.the ;solventcatalyticeffectisgreatontheactivateenergyfor1.3Hshiftinthetitlereaction.

    ;Itisunderstoodbythefactthatthereisaprotontransferbetweenprotondonor.methyl ;andstrongprotonacceptor,triethylamine,whichfoFinanionpairtransitionstate,whose

    ;harmonicvibrationalffequenciescomputationverifiedthatthesingleimagimwy ;frequencyexistactuallybetweenatomCofmethylandNoftriethylamine.Further, ;IRCcomputationconfirmedthattheionpairtransitionstateconnectthecorresponding ;reactantandproductindeed.ItisalsoreasonablethatNatomoftriethylaminecould ;captureprotoneasilyfromthestericunfavourablefourmembercycletransitionstateofl

    ;3Hshiftthanthestericfavourablesixmembercycletransitionstateof1.5Hshift.

    ;whichleadtoprominentdiminishmentofactivateenergyfor1,3.Hshift.However.the ;barrierheightfor1,3Hshiftofpathaisstillhigherthanthatfor1.5Hshift.

    ;alternatively,pathbispreferablereactionmechanism.

    ;Fromtheviewpointofthermodynamics,asshowninFigure1.theproductenolis ;24.8kJ?mol..thermodynamicallylessstablethanintermediateketoinvacuum. ;which

    ;conflictsnotablywiththefactthatisomerizatedenolcanbeeasilyobtained.Thereason ;maybeattributedtotwofactors.First,theonolcanbestabilizedbyintermolecular ;hydrogenbondbetweenNoftriethylamineandHofhydroxylinproduct.Whichis ;furtherverifiedbycalculatedintermediatebindingenergy.14.5ld.mol’..with

    ;counterpoisecorrectforBSSE.Second,theenolwithlowdipolemoment3.03Debye. ;ismorefavorabletoexistinnonpolartriethylaminethantheintermediateketowith ;dipolemoment3.27Debye.Additionally.theeffectofthesubstituentgroupon ;tautomericequilibriumbetweenketoandenolisundersystemicalconcerns. ;kJ.mol1

    ;Figure1Potentialenergysurfaceofpathbinvacuum

    ;8.1

    ;intermediate(keto)

    ;107.6

    ;32.9

    ;)

    ;

    ;

    ;708

    ;References

    ;YanHuaWANGPrf

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