Greener protocol for one pot synthesis of coumarin styryl dy(6)

110S.B.Phadtareetal./DyesandPigments97(2013)105e112

thereactionwith0.1mol%ofpiperidineascatalystat78 Cproducedonly60%yieldevenafter12hofreactiontimewith86.5%HPLCpurity(Table1.Entry7a*).

AlthoughtheroleofDESinthepresentworkhasnotbeencon rmedyet,weproposethefollowingmechanism(Scheme2)onthebasisoftheweaklyacidicnatureofDES.Inourcatalysissystem,thechlorideionmaybeapossibleparticipantinthereaction[19].Theco-existenceofchlorineandthehydrogenbondingdonorsisperhapsthemainreasonforthehighcatalyticactivityoftheDES.Cholinechloridehelpsmakethesolutionhomogeneousandservestheroleofphasetransfercatalystinboththereactions.

Scaleupandrecyclingstudiesweresuccessfulfortherepre-sentativereactionof4-(diethylamino)-2-hydroxybenzaldehyde1,ethylacetoactetate2,malononitrile4andbenzaldehyde(6a)inDES.Waterwasaddedafterthereaction,andthemixturewas lteredanddriedinanoventoisolatetheproduct7a.TheDESwasseparatedfromthereactionmixturebyphaseseparationbetweentheproductandtheaqueousphase.Followingthis,theDESwasrecoveredeasilyfromthe ltratebyevaporatingthewaterundervacuumat80 C.Thetargetedcompound7awasobtainedconsistentlyalong veconsecutiverecyclingswithoutanappre-ciabledecreaseintheactivityofDES(Table2).Thisindicatestheeaseofscalingupthereactiontoanindustrialscale.3.2.Structuralcharacterization

Structureofthedyemoleculeswascon rmedbyFT-IR,1HNMR,13

CNMR,correlatedspectroscopy(COSY),massspectrometryandelementalanalysis.InFT-IRanalysisofthedye7a(Fig.1),bandsat2800e2900cmÀ1indicatingthepresenceofaliphaticCeHwereobtained.Asharpbandat2219cmÀ1indicatedthepresenceofeCN.Bandsat1579cmÀ1and1697cmÀ1indicatedlactonecarbonylandaromaticCeH,respectively.BandscorrespondingtotheOeClactoneesterwereobservedat1132cmÀ1.IntheprotonNMRspectraofdye7atripletat1.28ppmobservedforsixprotons(Ha).Aquartetforfourprotonswasfoundat3.51ppm(Hb).Typicaldoubletwasobservedforoneprotonat6.57ppm(Hc)withJ¼2Hz.Cleardoubledoubletsat6.67ppmhavebeenobservedforoneproton(Hd).InCOSYitisclearlyseenthatHdcouplewithHeandHcwithJ¼7Hzand2Hzrespectively.Doubletat7.15ppmcorrespondstooneproton(Hg2),subsequentlysupportedbyCOSY,itcoupleswithHg1withJ¼15.8Hzthatcon rmsthetransgeometryofC]Cbond.Doubletforoneprotonhasbeenobservedat7.39ppm(He)couplewithHdbyJ¼7Hz,multipletforthreeprotonsofphenylring(Hi)wereobservedat7.44ppm.Doubletat7.54ppmwereobservedforoneproton(Hg1)thatcoupleswithHg2withJ¼15.8Hz,con rmstransgeometryofC]Cbondagain.At

Fig.2.UVeVisspectraof7ae7h(10À4M)inchloroform.

Fig.3.Emissionspectraof7ae7h(10À4M)in

chloroform.

7.57ppmdoubletfortwoprotons(Hh)wereobservedforphenylring,Singletat7.67ppmwereobservedforonevinylproton(Hf)ofcoumarinring.Inmassspectrometry,thepresenceofapeakatm/z394inM-1(negative)modewaswellinagreementwiththemolecularformulaweight(Mol.Wt.:395.45)of7a.Theelementalanalysisoftheproductagreedwiththemolecularformulaofdye7a,C25H21N3O2.

3.3.Spectralcharacteristics

TheUVevisibleand uorescenceemissionpropertiesofthecoumarin-baseddyes7ae7hinvarioussolventsarelistedinTable3.Theyshowabsorptionandemissionmaximabandsfrom300nmto550nmand400e700nmrespectively,asshowninFigs.2and3.TheStokesshiftsbetweentheabsorptionandtheemissionmaximaaresigni cant.Theef cientp-conjugationinthemoleculeisknowntoberesponsibleforthecharge-transfernatureoftheemissiveexcitedstateandfortheobservedrelativelylargeStokesshifts.Alldyesexhibitedbrilliant uorescence.Fig.4displaysthephotographsofthedyesindaylightaswellasunderUVlight(366nm).

Thesolventeffectsonthe uorescencecharacteristicsofallthecompoundswerestudied(Table3),indicatingredshiftintheemissionwavelengthofthecompoundswithanincreaseinsolventpolarity.Thespectralpropertiesofthemodeldye7awerecomparedwithestablisheddyes8,9and10.ThecomparativedataaresummarizedinTable4.Itisclearlyevidentthatthedye7adisplayedlargerStokesshiftrelativetotheestablisheddyes8,9,10[23].

Thevaluesofmolarextinctioncoef cientsinchloroformwereintherangeof13,900molÀ1dm3cmÀ1to61,700molÀ1dm3cmÀ1(Table5).Quantumyieldsofcompounds7ae7hweredeterminedbyacomparativemethodusingknownstandards( uoresceinandanthracene).Quantumyieldofthedyeswereobservedinthe

range

Fig.4.Photographofcoumarinintermediate5andstyryldyes7ae7hindaylightandinUVlight(366

nm).