148M.Periasamy,M.Thirumalaikumar/JournalofOrganometallicChemistry609(2000)137–151

Acatalyticenantioselectiveborohydridereductionofiminesinthepresenceofopticallyactivecobalt(II)catalyst7wasachieved(Eq.(97))[84].(97)

TheNaBH4–I2reducingsystemwassuccessfullyusedforthereductionofazoarenesandazoxyarenestothecorrespondinghydrazobenzenesingoodyields(Eq.(98))[113].Thereactiondidnottakeplaceintheabsenceofiodine.

(98)

11.5.Reductionofalkylandarylhalides

Dechlorinationof4-chlorobiphenylwithNaBH4waspromotedbyLiClinhighboilinginertsolvents(Eq.(99))[114].

(99)

The2,2,2-trichloroethoxymoietyiswidelyusedasaprotectivegroupinorganicsynthesis.ItcanbeeasilydeprotectedbyNaBH4underthecatalysisofseleniuminDMF(Eq.(100))[115].

(100)

The(PhSe)2–NaBH4systemhasbeenusedasare-ducingagentforthereductionofa-bromosulfones[116].Thisreductiondidnotproceedwithout(PhSe)2.Whereasiodomethylsulfonealsoundergoesthisreduc-tion,thecorrespondingchloroderivativewasnotaf-fected(Eqs.(101)–(103)).

PhSO(PhSe)2–NaBH4

2CH2Br EtOH,r.t.

PhSO(quantitative)

2CH3

(101)PhSO2CH2I PhSO2CH3

(102)95%

PhSO2CH2Cl Noreaction

(103)

Thisreagentsystemwasalsoappliedfortheradicalcyclizationandintramolecularcouplingreactions(Eq.(104)).

(104)

ThereisnoappreciablereactionbetweenNaBH4andarylhalidesundermostconditions.Itwasdiscoveredthatinthepresenceofcatalyticamountoftris-(triphenylphosphine)nickel(0),[Ni(0)(Ph3P)3],NaBH4servesasapowerfulnucleophilicreagentinthereduc-tionofarylbromides(Eq.(105))[117].UseofDMFgivesgoodresultsandethanol,methanolandTHFwereprovedtobeineffective.Thearomaticchloridesalsoreact,butmuchmoreslowly.

(105)

ThereductionofarylhalidesbyNaBH4catalysedbytitanocenedichlorideishighlyeffective[118].Thead-ductofDMFandNaBH4reducessimplearylhalidestogivedechlorinatedanddimethylamino-substitutedproducts.Indimethyl–acetamideorinethersolventsonlydechlorinatedproductsareobtained(Eq.(106)).

(106)

Nickelborideonborohydrideexchangeresin(BER)[119]isagoodalternativereagenttotributyltinhydride[120]forthecouplingofalkyliodideswiththeelectronde cientalkenesinmethanol(Eq.(107)).

(107)

11.6.Clea6ageofethersusingNaBH4andadditi6es

AfacilecleavageofallylicethershasbeenachievedbyNaBH4usingcatalyticamountofPd(PPh3)4(Eq.(108))[121].ThePd(PPh3)2Cl2couldalsobeusedasacatalystinthisreaction.