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AfacileandconvenientmethodforsynthesisofalkylthiocyanatesunderhomogeneousphasetransfercatalystconditionsAliRezaKiasat*,RashidBadri,SoheilSayyahiChemistryDepartment,CollegeofScience,ShahidChamranUniversity,Ahvaz61357-4-3169,IranReceived15May2008AbstractAsimpleandenvironmentallyfriendlymethodisdescribedfortheefcientconversionofalkylhalidetoalkylthiocyanateusingtetrabutylammoniumbromide(TBAB)asaphasetransfercatalyst.
Thereactionsoccurinwaterandfurnishthecorrespondingalkylthiocyanateinhighyields.
Noevidencefortheformationofisothiocyanatesasby-productofthereactionwasobservedandtheproductswereobtainedinpureformwithoutfurtherpurication.
#2008AliRezaKiasat.
PublishedbyElsevierB.
V.
onbehalfofChineseChemicalSociety.
Allrightsreserved.
Keywords:Alkylthiocyanate;Alkylhalide;Tetrabutylammoniumbromide;Phasetransfercatalyst;SynthesisofalkylthiocyanatesItiswellknownthatthealkylthiocyanate[1]playsanimportantroleasanintermediateforthepreparationofsulfur-containingorganiccompounds[2].
Theyhavefoundawidevarietyofapplicationsasinsecticides,biocidal,antiasthmaticandstartingmaterialsforthepreparationofheterocycles.
Thiocyanatesarealsoconsideredtobeanimportantclassofcompoundsfoundinsomeanticancernaturalproductsformedbydeglycosylationofglucosinolatesderivedfromcruciferousvegetables[3].
Moreover,a-thiocyanatocarbonylcompoundsareintermediatesforapreferredsyntheticroutetoseveraltypesofthiazoles[4].
Thiocyanationisgenerallycarriedoutvianucleophilicsubstitutionusingthiocyanateanions.
Anumberofmethodsareavailableforthepreparationofalkylthiocyanates,suchas,displacementofleavinggroupswiththiocyanateions[5–7].
Thiocyanatescanalsobeobtainedfromalcohols[8],silylethers[9]oramines[10].
However,thisdisplacementfrequentlyrequiresratherseverereactionconditionsbecauseofthelownucleophilicityoftheNCSion.
Hencemanydrawbacksandlowyieldshavebeenobservedforthesethiocyanationmethodologies[2,11].
Inaddition,thethiocyanategroupispoorlystablewhenheatedorunderacidicconditions.
Chromatographyonsilicagelorprolongedheatingover508Ccancauseintramolecularrearrangementtothethermodynamicallyfavoredisothiocyanateisomers[12].
Consequently,thedevelopmentofnewmethodsthataremoreefcientandleadtoconvenientproceduresandbetteryieldsisdesirable.
Phasetransfercatalysts(PTCs)arepowerfulreagentsinchemicaltransformations[13,14],thecharacteristicsofwhichincludemildreactionconditions,safety,operationalsimplicityandselectivity.
PTCsareoftenusedinnucleophilicdisplacementreactionstofacilitatereactionsbetweenorganicreactantsandionicinorganicsalts[15].
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sciencedirect.
comChineseChemicalLetters19(2008)1301–1304*Correspondingauthor.
E-mailaddress:akiasat@scu.
ac.
ir(A.
R.
Kiasat).
1001-8417/$–seefrontmatter#2008AliRezaKiasat.
PublishedbyElsevierB.
V.
onbehalfofChineseChemicalSociety.
Allrightsreserved.
doi:10.
1016/j.
cclet.
2008.
07.
019Althoughmanyphasetransfercatalystsareknown,quaternarysaltsformedfromammoniaarepracticallyimportantandusedinmanyoforganicreactions.
Incontinuationofourinvestigationsonnewmethodologiesforthesynthesisoforgano-sulfurcompounds[16],wereportanovelandhighlyefcientprotocolthatallowstherapidsynthesisofalkylthiocyanatesusingTBABasaphasetransfercatalystundermildandhomogeneouscondition(Scheme1).
Itshighpolarityandabilitytosolubilizebothorganicandinorganiccompoundscanresultinenhancedreactionratesandcanprovidehigherselectivitycomparedtoconventionalmethods[17,18].
1.
ExperimentalAll1Hand13CNMRdatawererecordedonaBrukerAdvancedDPX400MHzinstrumentspectrometerusingMe4SiastheinternalstandardinCDCl3.
IRspectrawererecordedonaBOMEMMB-Series1998FT-IRspectrometer.
ThepuritydeterminationoftheproductsandreactionmonitoringwereaccomplishedbyTLConsilicagelpolygramSILG/UV254plates.
Generalprocedure:Inatypicalprocedure,amixtureofalkylhalide(1.
0mmol),TBAB(0.
5mmol),KSCN(1.
5mmol)andwater(3.
0mL)wasplacedinaaskandstirredatroomtemperatureorheated(408C)forthetimespeciedinTable1.
Oncompletionofthereaction,followedbyTLCexamination,themixtureallowedtobecoldandextractedintoether(35mL).
Thecombinedorganiclayerwashedwithcoldwater(310mL),driedoversodiumsulfateandltered.
Theltratewassubjectedtoavacuumgavethedesiredproduct(75–95%).
Itdidnotrequireanycolumnchromatographythusavoidingthepossibilityofrearrangement.
AllofthereactionsreportedherewerecleanasjudgedbyTLC,FTIRandNMRanalysisofthecrudereactionmixture.
2.
ResultsanddiscussionsInarstsetofexperiments,awell-stirredsolutionof4-bromobenzylbromide(1.
0mmol)inH2O(3.
0mL)wastreatedwithKSCN(1-2mmol)atroomtemperatureinthepresenceoftetrabutylammoniumbromide(0.
2–0.
5mmol)asaphasetransfercatalyst.
ThereactionwasmonitoredbyTLC.
Aftersomeexperiments,itwasfoundthattheuseof1.
5equivofKSCNperalkylhalideinthepresenceofTBAB(0.
5equiv)inwaterwerethebestconditionsandafterstirringfor45minatroomtemperature,thecleanformationofaproductwithlowerRfvaluewasobserved.
WeexaminedthecatalyticabilityofTBABforconversionofalkylhalidestoalkylthiocyanatewithKSCNinwateratroomtemperature.
Thiscatalystactedveryefcientlyanditconvertsdifferentalkylhalidestotheircorrespondingalkylthiocyanatesinhighisolatedyields.
TheobtainedresultsofthereactionaregiveninTable1.
ItisnoteworthythatnoevidencefortheformationofIsothiocyanatesasby-productofthereactionwasobservedandtheproductswereobtainedinpureformwithoutfurtherpurication.
13CresonanceoftheSCNandNCSgroupsat$111and$145ppm,respectively,areverycharacteristicforthiocyanateandisothiocyanatefunctionalities[19].
AsshowninTable1(entries7and8),thissimplemethodcanbeefcientlyusedforpreparationofa-thiocyanatocarbonylcompounds.
AsexpectedtheeffectsofreactantstructureontherateofBimolecularnucleo-philicsubstitution(SN2)reactionshaveappearedinentry12asnoproductwasobservedeven8hofvariouslystirring.
Inconclusion,wehavedevelopedanoptimizedfacilethiocyanationunderphasetransfercatalyst,whichrequiresonlyamoderatereactiontemperatureinwater.
Theadvantagesofpresentprotocol,suchascleanreactionproles,shortreactiontimes,simplicityinoperationandthelowcostofreagentsmakethisnewprocessanattractivealternativetocurrentmethodologies.
A.
R.
Kiasatetal.
/ChineseChemicalLetters19(2008)1301–13041302Scheme1.
A.
R.
Kiasatetal.
/ChineseChemicalLetters19(2008)1301–13041303Table1ConversionofalkylhalidetoalkylthiocyanateunderPTCinH2OEntryAlkylhalideProductTime(min)Yielda,b(%)1608526080330904458554580c64590c7458086080c94580c1018075113095c,d12–8e–aTheNMRandFTIRspectraofallsynthesizedalkylthiocyanatesareinaccordwiththeliterature[2,4,11].
bIsolatedyields.
cThereactionwasperformedat408C.
dGCanalysis.
e8h.
AcknowledgmentPartialsupportforthisworkbyChamranUniversityResearchCouncilisgratefullyacknowledged.
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