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10.1.ReductionofketonesusingNaBH4supportedreagents

AnovelreagentsystemconsistingofNaBH4andamberlyst-15(H+)inTHFisapowerfulreducingagentforthereductionofunreactiveketones(Eq.(66))[74].Thereductionisfast,highyieldingandtheworkupisverysimple.Ketals,silylethers,acetates,allylicacetates,allylicg-lactones,carboxylicesters,halidesandisolateddoublebondsarenotdisturbedduringthereduction.

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TheNaBH4impregnatedonneutralaluminareducesawidevarietyofcarbonylcompoundstothecorrespond-inghydroxyderivativesinsolutionphase[75].ThesolidstatereductionhasalsobeenachievedbymixingketoneswithNaBH4andstoringthemixtureinadryboxfor5days[76].Themajordisadvantageofthesesolidstatemethodsisthattheyrequirelongreactiontime.Accord-ingly,afacilemicrowaveassistedreductionofaldehydesandketonesbyaluminasupportedNaBH4hasbeendeveloped(Eq.(67))[77].

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Signi cantchemoselectivityisobservedinthereduc-tionoftrans-cinnamaldehydeandtheole nicmoietyremainsintactundertheseconditions(Eq.(68)).

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AnovelsolidstatereductionoforganicfunctionalgroupsusingNaBH4impregnatedonMerri eld’sresinhasbeenreported[78].Severalaldehydesandketoneswerealsoreducedusingthisreagentsystem.

10.2.Reductionofaldehydesandketonesunderphasetransfercatalysis

Asimple,ef cientandeconomicalmethodforchemoselectivereductionofaldehydesinthepresenceof

ketonesusingstoichiometricamountsofNaBH4underphase-transfercatalysiswasreported(Eq.(69))[79].Aliphatic,aromaticandunsaturatedaldehydesarere-ducedrapidlyinhighyieldswhiledialkyl,aralkyl,diarylandcyclicketoneswerenotaffectedunderthesecondi-tions.TolueneandCH2Cl2havealsobeenusedassolventswhilePhCH2NEt3ClandAliquat-336workedwellascatalysts.

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10.3.AsymmetricreductionofketonesusingNaBH4andachiralauxiliary

TheuseofNaBH4–Me3SiClreagentsystemintheb-hydroxysulfoximinecatalysedasymmetricreductionofketonesaffordsthecorrespondingsecondaryalcoholsinhighyieldswithgoodenantioselectivities(Eq.(70))[80].

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Thereareafewreportsintheliteratureonasymmetricreductioninnon-aqueoussolutionsusingNaBH4modi edbychiralligands[81].Forexample,asymmetricreductionofacetophenone,propiophenoneand2-acetyl-naphthaleneusingNaBH4andopticallyactive(S)-lacticacidderivativesproducedthecorrespondingopticallyactive(R)-alcoholsinupto38.3%ee(Eq.(71))[82].

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Also,successfulasymmetricreductionoffunctional-izedketonesusingaNaBH4–(L)-tartaricacidsystemwasreported(Eq.(72))[83].Thissystemiseffectiveforthereductionofketoesterswhichareexpectedtochelatetoareductantthroughboththecarbonylandalkoxycar-bonylgroups.

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Unfortunately,lowerreactivitywasobservedinthecaseofsimpleketonesandtheproductswerealmost