M.Periasamy,M.Thirumalaikumar/JournalofOrganometallicChemistry609(2000)137–151139

Veryrecently,ithasbeenreportedthatthetetrabutyl-ammoniumborohydrideliberatesdiboraneinsolventssuchasCH2Cl2,CHCl3andCCl4.Anumberofterminal,internalandcyclicalkeneswerehydroboratedusingthisborohydride(Eq.(16))[25].

(16)

The1-alkynesundergodihydroborationtoyieldthecorrespondingterminalalcoholsafteroxidation.Gener-ally,thedisubstitutedalkynesgivevinylboranesthatonoxidationoffordketonesasthemajorproduct(Eqs.(17)and(18))[25,26].However,thediphenylacetyleneyields1,2-diphenylethanolasthemajorproductthroughdihy-droborationundertheseconditions.

CNaBH4 Bu4N+Cl

6H5C CC6H5 CHClC6H5CH(OH)CH2C6H5

3

83%(17)CHNaBH4 Bu4N+Cl

3CH2C CCH2CH3

CHCl3

CH3CH2CO(CH90%

2)2CH3

(18)

ThePdCl2–NaBH4–polyethyleneglycol(PEG)–CH2Cl2systemiseffectiveforhydrogenationofcar-bon carbontriplebondstothecorrespondingcis-alkenes(Eq.(19))[27].Thisreagenthasadvantagesoffasterratesandhigherselectivity.

(19)

3.Reductionofcarboxylicacids

TheNaBH4givesacyloxyborohydridespeciesonreac-tionwithcarboxylicacidsinTHFthathydroborateole ns.Theacyloxymoietiesinsuchacyloxyborohydridesremainunchangedunderambientconditions.However,onehalfoftheacyloxymoietyundergoesreductionuponheatingtogivethecorrespondingalcohol(Eq.(20))[28].

(20)

AsimilarreactionwasalsoobservedusingNaBH4,RCOOHandcatecholat25°C(Eq.(21))[29].

(21)

AliphaticcarboxylicacidsarereducedbyNaBH4tothecorrespondingalcoholsingoodyieldswhenRCOOHandCF3COOHareusedin1:1ratiounderambientconditions(Eq.(22)).However,thearomaticacidsgivepooryields(e.g.benzoicacid20%).Also,theNaBH4–CF3COOHcombinationisgoodforthereductionofaliphaticcarboxylicacids(65–95%yields).Again,aro-maticacidsgivepoorresultsundertheseconditions(30%).

(22)

TheZnCl2–NaBH4reagentsystemreadilyreducesbothaliphaticandaromaticacidstothecorrespondingalcoholsinre uxingTHF(Eq.(23))[30].Thereactionrequiresonlystoichiometricquantitiesofhydrideforthisconversion.Also,dicarboxylicacidsarereducedtothecorrespondingdiolsundertheseconditions.RCOOH NaBH4 ZnCl2

THF,D

RCH2OH

(23)

R=alkyl/aryl

70 95%

ThereagentpreparedusingZrCl4andNaBH4reducesthecarboxylicacidsinexcellentyieldsundermildcondi-tions(Eq.(24))[31].

PhCOOH NaBH4 ZrCl4

THF,rt,5h

PhCH85%

2OH

(24)

CarboxylicacidsarereducedtothecorrespondingalcoholsunderambientconditionsbytheNaBH4–I2reagentsysteminverygoodyieldswithsomeselectivities(Eqs.(25)and(26))[32].

CHNaBH4–I2

3(CH2)8COOH THF

CH3(CH2)8CH2OH

(25)

0–25°C

95%

(26)