硼氢化钠还原(7)

M.Periasamy,M.Thirumalaikumar/JournalofOrganometallicChemistry609(2000)137–151143

TheNaBH4incombinationwithanhydrousAlCl3convenientlyreducesdiarylandarylalkylketonestomethylenichydrocarbons(Eq.(57))[64].

(57)

Althoughalkalimetalborohydrideshavereceivedmuchattentioninorganicsynthesis,thestudiesontheuseofalkalineearthmetalsarelimited.Forexample,a,b-unsaturatedketonesmorereadilyconvertedtoallylicalcoholsselectivelyusingNaBH4inthepresenceofCaCl2(Eq.(58))[65].Amongthealkalineearthmetalchloridesexamined,CaCl2givesthebestcombinationofgoodyieldsandselectivitiesintheNaBH4reductionof2-cyclo-hexen-1-one.Further,thismethodprovidesasimple,inexpensivealternativeprocedurefortheselective1,2-re-ductionofa,b-unsaturatedketones.

(58)

MetalsaltsmediatedNaBH4reductionofa-alkyl-b-ketoestersleadstodifferentstereochemicalcontroldependingonthenatureofthemetalatom.Forexample,thestronglychelatingTiCl4ledtothesynisomerwhilenon-chelatingCeCl3[66]gaveantiisomer[67].TheketoestersarealsoreducedusingtheZnCl2–NaBH4systemtoaffordthecorrespondinghydroxyester(Eq.(59))[68].

(59)

Stereoselectivereductionof3-keto-2-methylestersand3-keto-2-methylamideswithNaBH4andcatalyticamountofMnCl2leadstothecorrespondingerythro-al-cohols.Ofthemetalsaltsexamined,MnCl2provideshighselectivity(Eqs.(60)and(61))[69].

(60)

(61)

ThePdCl2–NaBH4reagentsystem[70]reducesarylketones,arylchloridesandbenzylicalcoholstothecorrespondinghydrocarbons(Eq.(62)).Also,certainhinderedsteroidalketonesarereducedtothecorrespond-ingalcoholsingoodyields.

(62)

Moreover,thisreagentisusefulforthepreparationofalkylbenzenesfromarylalkylketones.Thisreductionproceedsundermildconditionsatr.t.andneutralpHinashorttimewithhighselectivity.Also,thechlorineatomsattachedtoaromaticringsareremovedreduc-tivelybythisreagent.

Thelanthanoidion-promotedNaBH4reductionofenonestoallylicalcoholsisaprocessofimmenseinterest,sincetheregio-andchemoselectivityobservedareinsharpcontrasttothereductionpromotedusingotheradditives[71].Amongthelanthanoidchloridesexam-ined,theCeCl3isthebestreagentforselective1,2-reduc-tionofenonesbyNaBH4(Eq.(63)).Moreover,thisreducingsystemdoesnotaffectcarboxylicacids,esters,amides,halides,cyanoandnitrogroups.

(63)

Highchemo-andregioselective1,2-reductionsofun-saturatedcarbonylcompoundsusingNaBH4assistedbyorganolanthanidecomplexes,LnCpCl2(THF)3(Ln=SmandEr)inmethanolhavebeenreported(Eq.(64))[72].

(64)

Finally,theCeCl3–NaBH4system[66]reducesthea%-diphenylphosphinoylenonesandketonesstereoselec-tivelyingoodyields(Eq.(65))[73].