M.Periasamy,M.Thirumalaikumar/JournalofOrganometallicChemistry609(2000)137–151149

(108)

Also,afacilecleavageofallylethershasbeenachievedusingtheNaBH4–I2reagentsystem(Eq.(109))[122].

(109)

11.7.DeoxygenationofphenolsandenolsusingNaBHandadditi6es

4Deoxygenationreactionofphenoland1,3-dicarbonylcompoundswasinvestigated[123].Phenols,enolizable1,3-diketonesand3-ketoestersareconvertedtothetoluene–p-sulfonatesthatarereducedbytheNiCl2–NaBH4systemtoobtainthedeoxygenatedaromaticcompounds,alcoholsandesters,respectively(Eqs.(110)and(111)).

(110)

(111)

11.8.Reductionofsul desusingNaBH4andadditi6es

Itwasfoundthatthearylalkyl,diarylanddialkylsulfoxideswerereducedtothecorrespondingsul desbyFeCl3–NaBH4inexcellentyieldundermildcondi-tions(Eq.(112))[124].Thesulfur oxygenbondisweakenedbythecoordinationofthesulfoxideoxygenwiththemetalion,renderingitmoreliabletoborohy-dridereduction[125].

(112)

11.9.ReactionsusingBu3SnCl–AIBNincombinationwithNaBH4

Thereactionof(Z)-3-iodoacrylicacidwithelec-trophilicalkenesusingtributyltinhydride,NaBH4andacatalyticamountofAIBNfollowedbyNaFtreat-mentproducedthe(E)-a,b-unsaturatedcarboxylicacidsinastereoselectivemanner(Eq.(113))[126].

(113)

Also,ithasbeenreportedthatreplacementofhy-drideinborohydridebyanelectrondonatingalkyl-aminogroupgreatlyenhancesthereducingabilityoftheresultingreagents.Thus,sodium(dimethyl-amino)borohydride(NaDMAB)and(tert-butylamino)-borohydride(NaTBAB)aremoreeffectiveforthecon-versionofaldehydes,ketonesandesterstoalcohols(Eqs.(114)and(115))andprimaryamidestoaminesingoodyields[127].Arylhalidesareslowlyconvertedtoarenes,butalkylhalidesandepoxidesundergounusualreactionswiththeaminoportionofthereagents.PhCHO NaDMAB–THF

PhCH90%

2OH

(114)PhCOPh NaDMAB–THF

PhCH(OH)99%

Ph

(115)

BesidestheuseofLewisacidsandotheradditivesforenhancingthereactionandselectivityofNaBH4,differ-entsolventscanbealsousedtorealisesimilarobjec-tives.Forexample,therateofreductionofaketoneissolventdependentwiththeorderofreactivityamongalcoholsbeing,MeOH\EtOH\i-PrOH\t-BuOH.Thus,theNaBH4reactivitymaybeattenuatedbythenatureofthehydroxylicsolventanditsconcentration[128].

12.Conclusions

Therehavebeensustainedeffortsonthedevelop-mentofmethodsofactivationofNaBH4usingLewisacids,carboxylicacids,metalsaltsandsomeotheradditivesforsyntheticapplications.Metalhalides,LewisandproticacidsandoxidisingagentsenhancethereactivityofNaBH4towardsthereductionofvariousfunctionalgroups,includingcarbon carbondoublebonds,carbon carbontriplebonds,carboxylicacids,acylchlorides,carboxamides,oximes,sulfoxides,ni-trilesandnitrocompounds.Also,certainadditivesenhancetheselectivitiesinthereductionofaldehydesandketones.Further,severalrecipeswerereportedfor