中南大学有机合成第章还原

1Chapter7 Reduction

Ininorganicchemistry

lossofelectronsisoxidation gainofelectronsisreductionInorganicchemistry theoxidationstateofacarbonatomequalsthetotal numberofitsC–O,C–N,andC–Xbonds.23ReductionCatalytichydrogenationMetalhydridereductionsElectrontransferreactions(dissolvingmetalreduction)HeterogeneousHomogeneous48.1 Catalytichydrogenation

8.1.1HeterogeneouscatalysisLowpressure(0-100oC,1-4atm)Catalysts:Ra-Ni、Pt、Palladium、Ruthenium、Rhodium

Depositedonthesurfaceofaninertsupport

(carbon,alumina,BaSO4,CaCO3)

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catalystactivitydecreasesSolvent:EtOH,MeOH,EtOAc,H2O,Cyclohexane,Dioxane,THF,AceticacidCatalystactivityincreasesongoingtopolar,acidicsolventsHighpressure

(roomtemperature-300oC,100-300atm)Catalysts:Ra-Ni,CopperchromiteCuCr2O4,Rutheniumsupported oncarbonoralumina5LookatmechanismindetailHenceolefinscanbeisomerizedoverhydrogenationcatalysts.H68.1.2HomogeneouscatalysisIsomerizationmaybeminimizedWilkinson'sCatalyst:tris-(triphenylphosphine)rhodiumchloridesolubleinorganicsovlents(EtOH,etc.)7UsefulforthestereospecifictransfersofD2tounhinderedolefins:88.1.3TransferhydrogenationUsingahydrogendonorasthesourceofhydrogen,suchasCyclohexene,propan-2-ol,formicacid,hydrazine.Catalyst:heterogenousorhomogeneous.AvoidattendanthazardsOutofaccordwiththeeconomyrulesofatoms98.2MetalhydridereductionsLiAlH4mostreactiveLiAlH(OC(CH3)3)3orLiAlH4+3C(CH3)3OHNaAlH2(OCH2CH2OCH3)2RED-AlNaBH4lessreactiveC=O(type)reductions.LiAlH2(OCH2CH2OCH3)2

NaBH3(CN)Sodiumcyanoborohydride(onlyimines)LiBH4

AlH3Aluminumhydride[(CH3)2CHCH2]2AlHDiisobutylaluminumhydride(DlBAL-H)10Mechanisminvolvescoordinationofcation(Li+orNa+)withC=OandH-attack.11

ReactionConditions

Anhydrousconditions(exceptNaBH4)LAH(non-hydroxylicsolvents):ether,THF,1,2-dimethoxyethane, (MeO-CH2CH2-O-CH2CH2-O-Me)diglymeNaBH4

:H2O,MeOH,EtOH,mostcommonlyi-PrOH

Selectivity

Partialreduction

RCOOH----->RCHOReductionofonegroupinthepresenceofanother.12DIBAL-HallowstheadditionofoneequivalentofhydridetoanesterReplacingsomeofhydrogensofLiAlH4withORgroupsdecreasesthereactivityofthemetalhydridePartialreduction13NaBH4canbeusedtoselectivelyreduceanaldehydeoraketogroupinacompound148.3Electrontransferreactions(dissolvingmetalreduction)15ReactionConditions

Metalscommonlyused:lithium,sodium,potassium,zinc,magnesium,tin,ironSolvents

NH3(b.p.-33˚)foralkalimetals(andCalcium);"BirchReduction"

lowmolecularwt.amines

hexamethylphosphoramide(HMPA)

ether,THF,dimethoxyethane(DME)--dilutesolns.

crownethercomplexes

ether,toluene,xylene--suspensionsProtonSource

ethanol--Presentinreactionmedium

isopropanol--addedwithcompoundtobereduced

t-BuOH--addedduringisolation.

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