Nanocrystalline and Reusable ZnO Catalyst for the Assembly

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NanocrystallineandReusableZnOCatalystfortheAssemblyofDenselyPotThreeFunctionalizedComponentReactions:4H ChromenesAGreenerinAqueous“NOSEMedium”Approach

viaOne-ParthaPratimGhoshandAsishR.Das*

DepartmentofChemistry,UniversityofCalcutta,Kolkata-700009,India

*

SupportingInformationINTRODUCTION

e cient,andmoreecofriendly,whichinstitutes“greenInrecentyears,emergentawarenessaboutecologicalsafetyandchemistry”andproducesmostdesirableproducts.Theglobalwarminghascausedworldwideconcernabouttheuseofemerginginterestonthecatalyticpropertiesoftransitionrenewablesourcesandreductionofwaste.Thishasshiftedthemetalnanoparticlesisduetotheirlargesurfacearea,distinctiveparadigmtowardtheuseofecofriendlyandgreenprotocolsinelectronic,magnetic,optical,thermalandchemicalproperties.4allphasesofchemicalconstructionandcanbeappreciatedbyThede niteintentiontoworkwithnanoparticlesistheirhighcreativeresearchthatwidelyaddressestheissuesofatomcatalyticactivity,recoverability,improvedselectivity,criteriaofeconomy,economyofsteps,andavoidanceofhazardousevolutionandroleingreenchemistry.5Hence,organicchemicals.1Developmentofe cientandenvironmentallysynthesiscatalyzedbymetal/metaloxidenanoparticleshasfriendlysyntheticmethodologiesforthesynthesisofcompoundreceivedremarkableimportanceinrecentyears.

librariesofmedicinalsca oldsisasanattractiveareaofresearchChromenesandtheirstructuralanaloguesareofgreatinbothacademicandpharmaceuticalindustry.2Multicompo-interestbecausetheyarefrequentlyfoundinanumberofnentreactions(MCR)inwaterwillbeoneofthemostaptnaturalproductsandbiologicallyactivemoleculeslikeantibioticapproaches,whichwillmeettherequirementsofgreenrhodomyrtone,aglycosidaseinhibitormyrtucommulone-EandchemistryaswellasfordevelopinglibrariesofmedicinalHA14-1,apoptosisinducer(Figure1).6Theirsyntheseshavesca olds.3

attractedwideattentionfortheirbiologicalproperties,suchasNanotechnologyandrelatedscienceso ertheopportunityanticonvulsant,7antimicrobial,8antitumor,9anticoagulant,tomakeproductsandprocessesgreenfromthebeginning.Thediuretic,spasmolytic,andantianaphylacticactivities.10Onthedevelopmentofgreenprocessesandpollutant-freecatalystshasotherhand,3-substitutedcoumarin,3-substitutedindole,

integratedgreatimportance.Catalysisliesattheheartofinnumerablechemicalprotocols.PioneeringcatalyticprocessesReceived:April18,2013basedonsuchnanocatalystswillbesimpler,economically

Published:May23,2013

Figure1.Somebiologicallyactivechromenes,coumarins,indoleandpyrazolones.

pyrazolone,β-naphthol,uracilderivativesareofmuchanalysiscon rmedthepresenceofzincandoxygenelementsigni canceduetotheirpresenceinplenty11ofnaturalandmedicinalsca olds,namely,coumatetralyl,bromadialone,12only(Figure2)inthenanosample.Theweight%ofZnandOmonatin,13phenazone,14propyphenazone,15etc.(Figure1),is73.80and26.20,andatomic%is40.81and59.19,possessingantibacterial,anti-HIV,anticancer,antioxidantandrespectively.

antitubercularactivities.

Amolecularsca oldthataccumulateschromeneaswellasvariousbioactivenucleophileslike3-substitutedcoumarin,3-substitutedindole,pyrazolone,β-naphthol,and6-amonouracilmoietiesmightintegratepropertiesofboth,andthesynergismofboththeheterocyclicmoietiesinasinglenucleusmayresultintheformationofsomeworthwhilemoleculesfromthebiologicalpointofview.Aliteraturesurveysuggestedthatthereareveryfewreportswherechromene18assimilateswithindole,16pyrazolone17and4-hydroxycoumarinmoieties.However,thereportedmethodssu erfrommanydrawbackslikeutilizationofhazardouschemicals,longerreactiontime,unsatisfactoryyieldsandburdensomeproductisolationprocedure.Moreover,thestrikingdisadvantageofalmostallreportedmethodsisthatFigure2.EDXanalysisof

nano-ZnO.

thecatalystsareconsumedinthereaction.Toovercomethoseshortcomings,ourinterestgoesinthegrowthofnanoparticle-Fortheidenti cationoffunctionalgroups,bondingcatalyzedorganicsynthesisenhancement(NOSE)chemistry,19information,studyofstrengthandfractionofhydrogenwhichenablesustoreportZnOnanoparticlescatalyzedbonding,comparativeFT-IRspectra(Figure3)wererecordedsynthesisofhighlyfunctionalizedchromenederivativesviabetweenbulkZnOandnano-ZnO.Broadbandat3408andmulticomponentreactionapproach,commencingwith2-3377cm 1areassignedtotheO Hstretchingmodeofhydroxybenzaldehyde,anactivemethylenecompoundandahydroxylgroupsofatmosphericmoistureforbothnano-ZnObioactivecarbon-basednucleophileinwater(Scheme1).TheandbulkZnO,respectively.Thepeaksat1631cm 1ofnano-ZnOindicatesthepresenceofCOresidues,probablydue

to

Scheme1.Synthesisof4H-Chromene

Derivatives

presentworkmaterializesasapartofourongoingresearchprograminvolvingwaterasthesolventandnano-ZnOasthekeycatalystinthesynthesisofvariousbiologicallyactivemolecules.■

20

RESULTSANDDISCUSSION

ZnOnanoparticles21werepreparedthroughthe“bottomup”method.TocharacterizeZnOnanoparticles,EDXanalysiswasperformedtodeterminetheelementalcomposition.EDX

Figure3.FT-IRspectrumofnano-ZnO.

atmosphericCOTheTGAandSDTAcurvemeasuresthecompositionalsizeofthe2,whichwasnotobservedinbulkZnO.Hence,asthenanoparticlesincreases,thecontentofCOresiduesinthesampledecreases.

changesassociatedwiththecalcinationsprocessesandisshownFigure4showstheXRDpatternofZnOnanoparticles.TheinFigure6.Becauseofhighthermalstabilityofnano-ZnO,noobservedpeakscorrespondtotheBraggangleforthe(100),

changehasbeenobservedupto600°C.

http://wendang.chazidian.comparativeXRDpatternsof(a)bulkZnO,(b)nano-ZnO,(c)recoveredaftersixth

cycle.

Figure6.TGAandSDTAcurveofnano-ZnO.

(002),(101),(102),(110),(103)and(112)planesoftheInordertocon rmZnOnanoparticlestobethecrucialforcrystallineZnO,whichareconsistentwithstandardJCPDSrenderingthereactionspossibleinaqueousmedia,wecarriedreportedvalues.ApplyingSherrer’sformula,Doutalongscreeningtestemployingaseriesofcatalystsandλ)=1.5406Åp=0.941λ/βzcosθ;whereX-raywavelength(andβisthesolvents(Table1).Initially,salicylaldehyde(1.0mmol),correspondingfullwidthathalf-maximum(fwhm)valueofdimedone(1.0mmol),and4-hydroxycoumarin(1.0mmol)characteristicpeaks(100)and(101),andaccordingly,(a)thewerere uxedinpresenceofHparticlesizesofbulkZnOisabout68nm;(b)thesynthesized2Oandethanolasthesolventwithoutanycatalyst;however,thereactions,evenafter24h,ZnOwerefoundtoliebetween10and12nm;and(c)afterfailedtogenerateanyproduct(Table1,entries1and2).Thensixthrunofrecyclingitisabout13 14nm,conformingthethereactionwascarriedoutinwaterinthepresenceofp-unchangedmorphology.Thesesizesareconsistentwiththosetoluenesulphonicacid(PTSA)at55°C,andtheproductwasmeasuredfromtheTEMimages,indicatingthesinglecrystalisolatedin41%yield(Table1,entry3).Thereactionswerenatureofnanoparticles.

alsorestrainedinaqueousmediabyusingL-prolineastheFigure5a cshowstheTEMimagesofsynthesizednano-catalyst(Table1,entries4),whiletheuseofzeoliteasacatalystZnO.Thesizeoftheparticleofnano-ZnOisestimatedthroughinaqueousmediaprovidedatraceamountofthedesiredTEMandfoundtobenearabout10nm.Noimpuritieswereproduct(Table1,entry5).AmongtheLewisacidcatalysts,involvedinthesynthesizedZnOnanoparticlessample.TheInClsynthesisofZnOnanoparticleswascarriedoutbyaqueous3wastested,butitdidnotpromotethereactionwell(Table1,entries6).However,animprovedyieldwasobtainedchemicalmethodusingzincacetatedihydrateandpotassiumwhenweappliedmetaloxidelikenano-MgO,nano-Alhydroxideassourcematerials.TheentireprocesswascarriedbulkZnOascatalystinwater(Table1,entries7,8and2O3and9).outindeionizedwaterforitsintrinsicadvantages,asitisInterestingly,weobservedthatZnOwasmoste ectiveforthesimple,cost-e ective,environment-friendlyandcanbeeasilyselectiveformationofdesiredproduct.AgainZnOnano-scaledupforlargescalesynthesis;moreover,inthismethodparticlesshowedoutstandingactivityintheformationofthereisnoneedtousehighpressureandtoxicchemicals.desiredproductthanbulkZnOintermsofreactiontime

and

Figure5.TEMimagesofnano-ZnOat(a)50,(b)20,and(c)10nmscale,respectively,inaqueous

media.

Table1.Screeninga

ofCatalyst,Solvents,andReaction4H-chromenederivatives(4)ingoodtoexcellentyields(85 Conditions93%)(Table2).

Toestimatethescopeandgeneralityoftheprotocol,2-entrycatalystssolventsconditions(°C)time(h)

yieldsb(%)hydroxyaromaticaldehydes,havingbothelectron-withdrawing1 Handelectron-donatinggroups,werereactedwithanactive2Ore ux24 c2

EtOHre ux24 cmethylenecompoundlikedimedone,1,3-cyclohexanedione,3TsOH(10mol%)HmalononitrileandN,N-dimethylbarbituricacidandacarbon-2O558414L-prolinebasednucleophilelike4-hydroxycoumarin,4-aminocoumarin,mol%)

(10H2O558466-aminouracil,β-naphthol,indoleandpyrazoloneunder5zeolite(10mol%)H2O558traceoptimizedreactionconditions,andtheresultsareexhibitedin6InCl3(10mol%)H2O55829Table2.Thereactionswereconsistentlycarriedoutatthe17nano-MgOmmolscale,andnochangeofproductyieldwasobservedwhenmol%)

(10H2O55851scaleduptothe10mmolscale.Fromtheperspectiveofgreen8nano-Almol%)2O3(10H2O55855chemistry,itwaspositiveto ndthatthe nalproductscould9bulkbeisolatedby ltrationbecauseoftheirsolubilitydi erenceofmolZnO%)

(10H2O55868theproductfromthestartingmaterials,leadingtoseparationof10nano-ZnOproductfromthereactionmixtureuponcompletion,therebymol%)(10H2O5545min92facilitatingeasyisolationofsolidproductfromthereaction11nano-ZnOmol%)

(5H2O5545min81mixturesimplyby ltration.SinceZnOnanoparticlesareoften12nano-ZnOrecoveredeasilybysimpleworkup,whichpreventscontami-mol%)

(15H2O5545min87nationofproducts,theymaybeconsideredaspromisingsafe13nano-ZnOandreusablecatalystsaswellasbeinggreenercomparedtomol%)

(10EtOH5545min77traditionalcatalysts.Furthermore,thepurityoftheproducts14nano-ZnOwastoohightorequiretheirpreparationinananalyticallypuremol%)

(10CH3CN5545min56formbysinglerecrystallization,thusavoidingextractionsteps15

nano-ZnOmol%)

(10CHCl3

55

45min

31

andchromatographicseparations.Therefore,wepreferreda

waterasthereactionmediumoverunsafeorganicsolvents,dimedoneAllreactionswhichdecreasesthechemicalimpurity,featureseasyworkupsolvent.bYield(1.0mmol),werecarriedoutwithsalicylaldehyde(1.0mmol),ofisolatedand4-hydroxycoumarinproduct.cReaction(1.0failedmmol)toprovidein5mLanyofprocedure,andminimizeslargevolumesofwastefromtheproduct.

discarded chromatographicstaticphases.Thestructure13

nalproductswereHRMS)wellcharacterizedandelementalbyusinganalysisspectraldata(IR,ofthe1H,CNMRand.ThestructuralmotifwasfullyestablishedbysingleX-raycrystallo-temperature,andeventuallyweachievedsatisfactionbecausegraphicanalysisofonesigni edcompound4e(CCDCthereactionproceededwell,a ordingthedesiredproductin926789)(Figure7).22Thecompound4ecrystallizesinthe92%yieldwithin45min(Table1,entry10).ThereactionwastriclinicP1spacegroupwithonemoleculeintheasymmetricverycleanbecauseoftheamphotericnatureofnano-ZnO;unit(Z=2).Whilethemoleculeintheasymmetricunitassuchbesides,uponapplicationofacidsorbases,manyunwantedhadnoconventionalfunctionalgroupstoformH-bonding,theproductsinthereaction askhavebeengeneratedbecauseofcoumaringroupspresent,however,wereresponsibleforthethehighacid basesensitivityofstartingmaterials,speci callyformationofweakintermolecularH-bondingalongc-direction.carbon-basednucleophiles.

TheinversionrelatedmoleculeintheasymmetricunitformingAscertainingnano-ZnOastherightcatalystforthetheheterochiraldimersynthonthroughC H···Ointeractionisexperiment,wethenconcentratedourattentionondesigningshowninFigure8.Theyformedchannelswithonemoleculeandalsogeneralizingthefavorableconditionsforthereaction.aboveandonemoleculebelowviaC H···OinteractionandareWe rstattemptedsomescreeningtestswithnano-ZnO.ThestabilizedbyaromaticC H···πinteractions.

quantityofthecatalysthadalargee ectontheformationofPresumablythereactionseemstoproceedthroughfollowingthedesiredproduct.Theuseof5mol%nano-ZnOdiminishedmechanisticpathwayaspresentedinScheme2.TheZnOthequantityoftheyield,whereastheyieldoftheproductalsonanoparticlesfacilitatetheKnoevenagel-typecouplingthroughdecreasedwhenweused15mol%nano-ZnO(Table1,entriesLewisacidsites(Zn2+)coordinatedtotheoxygenofcarbonyl11and12).Water(Table1,entry10)hasshownitssuperioritygroupsof2-hydroxybenzaldehyde.Ontheotherhand,ZnOtoothersolventstested[ethanol(Table1,entry13),nanoparticlescanactivateactivemethylenecompoundssothatacetonitrile(Table1,entry14)andchloroform(Table1,deprotonationoftheC Hbondoccursinthepresenceofentry15)].AsapartofourprogramaimedindevelopingnewLewisbasicsites(O2 )andformintermediate(I).andenvironmentallybenignsyntheticmethodologieswithZnOSubsequently,duringtheMichaeladditionstep,nucleophilicnanoparticles,theseoptimizedconditionswerethenappliedforattackonintermediate(I)bythecarbon-basednucleophileallexperiments:takingequimolaramountsof2-hydroxya ordedthedesiredproduct(4)viaintermediate(II)pavingbenzaldehyde(1),activemethylenecompound(2)andathewayforitsringclosure.Alternatively,thereisanothercarbon-basednucleophile(3)at55°Cinthepresenceof10possiblereactionpathwayforthereaction,viatheformationofmol%nano-ZnOinaqueousmedia(Scheme1).Typically,aintermediate(III)followedbyitsreactionwithdimedonetomixtureof2-hydroxybenzaldehyde(1.0mmol),activea ord4.Oure ortstoisolateintermediate(III)fromthetwo-methylenecompound(1.0mmol)andacarbon-basedcomponentreactionof2-hydroxybenzaldehyde(1equiv)andnucleophile(1.0mmol)and10mol%nano-ZnOin3mLof4-hydroxycoumarin(1equiv)inwaterinpresenceofnano-waterwasre uxedfor40 65min,whicha ordedalibraryof

ZnOdidnotfructify.Thesereactions,instead,ledto(i)

Table2.SubstrateScopesfortheSynthesisof4H-Chromenes

a