M.Periasamy,M.Thirumalaikumar/JournalofOrganometallicChemistry609(2000)137–151147

11.Miscellaneous

11.1.Reductionofcarbinolsandbenzylicalcohols

TheNaBH4incombinationwithanhydrousAlCl3convenientlyreducessomediaryl–arylalkylcarbinolstomethylenichydrocarbons(Eq.(88))[64].

(88)

Thereductionofdi-andtriarylmethanolsusingNaBH4andTFAisineffectiveinthecaseofnon-benzylicandmonobenzylicalcohols[103].Thesemethodsleadtorapidformationofaveryweakreducingagenti.e.tri uoroace-toxyborohydrides[2]whichareslowineffectinghydridecaptureofthecarbocationintermediate.Itwasfoundthatdropwiseadditionoftri uoroaceticacidtoamixtureofthesubstrateandNaBH4inTHFprovidesaprotonatingmediumrichinmorereactivereducingspecies(Eq.(89))[104].Thismethodwasfoundtobehighlyeffectiveforthereductionofmonobenzylicalcoholstogivethecorrespondinghydrocarbonsinmoderatetohighyields.

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11.2.Reductionofazides

Aroylazidesarereducedpredominantlytothecorre-spondingbenzylalcoholsusingNaBH4inmethanol[105].However,theyyieldthecorrespondingamidesusingtheNiCl2–NaBH4system(Eq.(90))[106].

p-X-CNaBH4–NiCl2·6H2O

6H4CON3 MeOH,0–10°C

p-X-C80–100%

6H4CONH2(90)

Afacilereductionofalkyl,arylandaroylazidesusingNaBH4–CuSO4systemhasalsobeenachieved(Eq.(91))[107].

PhCONNaBH4–CuSO4

3 MeOH,0–5°C

PhCONH90%

2

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AseriesofisoxazoleazideswerereducedselectivelytoisoxazoleaminesingoodyieldsbyNaBH4inthepresenceof1,3-propanedithiol(Eq.(92))[108].AsolventeffectonthisreactionwasobservedduetodifferencesintherateofdecompositionofNaBH4inthealcoholicmedia.Whenthereductionwascarriedoutinisopropanolunderthesameconditions,theazideswerereducedtoaminesinquantitativeyields.

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ItisofinteresttonotethatthereducingpowerofNaBH4increasesinTHF(ort-butylalcohol)whenmethanolisaddeddropwiseduringthereaction[109].

11.3.Reductionofoximesandoximeethers

AreagentsystemconsistingofNaBH4–LiCl–am-berlyst–15(H+)inTHFispowerfulforthereductionofoximes(Eq.(93))[110].

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ReducingagentspreparedusingZrCl4–NaBH4andchiralaminoalcohols9havebeensuccessfullyappliedtotheenantioselectivereductionofoximeethers[111].Opticallyactiveprimaryamineswereobtainedinhighenantiomericexcess(]95%ee)withgoodchemicalyields(Eq.(94)).

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ThereductionwasexaminedusingdifferentLewisacids.HighenantioselectivitywasobservedusingZnCl2andAlCl3(1:1)mixture,buttheremovalofthemixedsaltsposedadif cultyleadingtoloweringofthechemicalyield.ReductionsusingZrCl4–NaBH4systemgavehighchemicalandopticalyields.RemovalofthemetalsaltsafterthereactionandisolationoftheproductareeasyinthecaseofZrCl4.

O-AcylderivativesofaldoximesandketoximesarereducedingoodyieldstothecorrespondingaminesusingtheNaBH4–I2system(Eq.(95))[112].

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11.4.ReductionofC NandN Nfunctionalgroups

TheBiCl3–NaBH4systemcanbeusedeffectivelyforthereductionofazomethinestothecorrespondingamines(Eq.(96))[52].

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