www.elsevier.nl/locate/jorganchem

JournalofOrganometallicChemistry609(2000)137–151

Methodsofenhancementofreactivityandselectivityofsodium

borohydrideforapplicationsinorganicsynthesis

MariappanPeriasamy*,MuniappanThirumalaikumar

SchoolofChemistry,Uni6ersityofHyderabad,CentralUni6ersityPO,Hyderabad500046,India

Received29February2000;receivedinrevisedform16April2000

Abstract

NaBH4doesnotreducecarboxylicacids,esters,amidesandnitrilesunderambientconditions.However,thereactivityofNaBH4canbeenhancedbytheadditionofcertainadditives.Forexample,additionofiodinetoNaBH4inTHFprovidesH3B–THFthatisusefulforhydroborationsandreductionsofvariousfunctionalgroups.ThealdehydesandketonesarereducedinafastmannerbytheNaBH4reagent.Evenso,theselectivitiesrealisedinsuchreductionscanbeenhancedusingNaBH4alongwithanotheradditive.Inthisarticle,variousmethodsusedfortheenhancementofreactivityandselectivityofNaBH4inorganicsynthesisaredescribed.©2000ElsevierScienceS.A.Allrightsreserved.

Keywords:Sodiumborohydride;Enhancementofreactivity;Additives;Reductionoforganics

1.Introduction

Metalhydridesarevaluablereagentsinmodernor-ganicchemistry.ThemostfrequentlyusedhydrideistheNaBH4reagent.Itisamild,inexpensiveandinvalu-ablereagentforapplicationsinawiderangeofreduc-tionprocesses.Itisthereagentofchoiceforthereductionofaldehydesandketonestoalcohols[1]andimines[2]oriminiumsalts[3]toamineswithproticsolvents[4].Thecarboxylicacids,esters,amidesandnitrilesaremoreresistanttowardsNaBH4[1].How-ever,thereactivityofNaBH4canbeenhancedbycarryingoutthereactioninthepresenceofcertainadditives.Inthisarticle,variousmethodsofenhance-mentofreactivityandselectivityofNaBH4usingaddi-tivesforapplicationsinorganicsynthesisaredescribed.

stereospeci cities[5].Historically,BrownandSubbaRaodiscoveredthehydroborationreactionduringtheirinvestigationoftheactivationofNaBH4forthereduc-tionofestersusingAlCl3[6].TheuseofBF3intheplaceofAlCl3ledtomoreeffectiveutilisationofthehydrideforthegenerationofdiborane,B2H6andbo-raneLewisbasecomplexes(Eqs.1–3)[7].9RCH CH2+AlCl3

3(RCH2CH2)3B+AlH3+3NaCl12RCH CH2+3NaBH4+4BF3

diglymediglyme

(1)

4(RCH2CH2)3B+3NaBF4

(2)

(RCH2CH2)3B+3H2O2+NaOH 3RCH2CH2OH+NaB(OH)4

(3)

2.Hydroborationofalkenesandalkynes

Hydroborationofcarbon carbonmultiplebondsprovidesamethodforthesynthesisofthevaluableorganoboraneintermediateswithhighregio-and

*Correspondingauthor.Tel.:+91-40-3010904;fax:+91-40-3010120.

E-mailaddress:mpsc@uohyd.ernet.in(M.Periasamy)

Althoughseveraloftheseboranecomplexesarecom-merciallyavailable(e.g.H3B–THF,H3B–SMe2andH3B–NR3),therehavebeensustainedeffortstowardsthedevelopmentofalternative,simpleandconvenientmethodsofgenerationofboranesinsituforhydrobo-ration.In1963,itwasreportedthata1:1mixtureofNaBH4andCH3COOHhydroboratesalkenes[8a].Later,amodi edprocedureforhydroboranesusingNaBH4–CH3COOHwasreported(Eq.(4))[8b].

0022-328X/00/$-seefrontmatter©2000ElsevierScienceS.A.Allrightsreserved.PII:S0022-328X(00)00210-2