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9.Reductionofnitrocompounds

NitrocompoundsarereducedtothecorrespondingaminocompoundseffectivelyusingtheNaBH4–CuSO4system(Eq.(48))[51].Itwasreportedthatthissystemalsoreducesketones,aliphaticesters,ole nsandnitriles.

(48)

Anovelreagentsystem,preparedusingBiCl3andNaBH4,reducesaromaticnitrocompoundstothecor-respondingaminesingoodyields(Eq.(49))[52].ThefunctionalgroupssuchasMe,OH,NH2,OMe,Clinthearomaticringdonothaveanymarkedeffectontherateofthereaction.Moreover,thesefunctionalgroupssurviveduringthereduction,makingthisprocessfairlygeneralandselective.RNONaBH4–BiCl3

2

THFRNH(49)

R=alkyl/aryl2

35–90%

ThearomaticnitrocompoundsarealsoreducedingoodyieldstothecorrespondingN-arylhydroxyl-aminesusingNaBH4inthepresenceofcatalyticamountsofmetallicselenium(Eq.(50))[53].

(50)

Theseseleniumcatalysedreductionswereacceleratedbyelectronwithdrawinggroups.Thealiphaticnitrocompoundsgavethecorrespondingoximesundertheseconditions.Theactivespeciesinthissystemisthehydrogenselenideanion[54].Hence,thismethodopensupanewwaytotheuseofseleniumasaredoxcatalystinorganicsynthesis.

TheN-arylhydroxylamineshavebeenalsopreparedthroughantimonycatalysedNaBH4reductionofni-troarenes(Eq.(51))[55].

(51)

ThenovelNaBH4–(NH4)2SO4reagentsystemhasbeenusedfortheselective,rapidreductionofnitrocompoundstothecorrespondingaminoderivativesingoodyields(Eq.(52))[56].RNO4–(NH4)2SO4

2

NaBHEtOH,r.t.,30–120minR=arylRNH(70–90%)

2

(52)

10.Reductionofaldehydesandketones

TheNaBH4isareagentofchoiceforthereductionofaldehydesandketones.ThereactivityofNaBH4canbereadilymodi edthroughitsreactionwithaceticacid(Eq.53)[2,57].Withexcessofaceticacid,triacyloxyborohydrideisformed.TheNaBH4–CH3COOHreagentsystemhasbeenusedforreductionsofenam-ines,imines,vinylogouscarbamates,aromaticandaliphatica,b-unsaturatedtosylhydrazones,pyryliumsalts[58].IthasbeenalsousedforthereductionorreductiveN-alkylationofamines,oximes[59]andni-trogencontainingheterocycles[2].

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TheNaBH(OAc)3reagenthasbeenusedforthechemoselectivereductionofaldehydesinthepresenceofketones[2].Also,selectivereductionofketonesusingNaBH4inglacialaceticacidsolventorNaBH4inTHFusing3equivalentsofaceticacidwerereported[60].Further,highlystereoselectivereductionofketoneswasalsoachievedusingNaBH(OAc)3inaceticacid(Eq.54)[61].

(54)

InthepresenceofZrCl4,NaBH4reducesthealde-hydesselectivelyinhighyields(Eq.(55))[31].PhCHO NaBH4 ZrCl4

THF,r.t.,5h

PhCH95%

2OH

(55)

Aromaticaldehydesaremoreselectivelyreducedthantherelatedketonesbythereducingsystemconsist-ingofNaBH4andSnCl2inTHF(Eq.(56))[62].Itmayofinteresttonotethatselectivereductionofaldehydesinthepresenceofketonesisordinarilyimpracticableusingthereducingagentssuchasalkalimetalborohy-drides,aluminohydridesanddiborane[63].PhCHO NaBH4–SnCl2

PhCH96.8%

2OH+PhCOPh

92%

(56)