羰基立体选择性加成亲核有时会导致异构体混合物nabh4 columbia有机化学课件16 html.pptx

17.14

StereoselectiveAdditionto

CarbonylGroupsNucleophilicadditiontocarbonyl

groupssometimesleadstoamixture

ofstereoisomericproducts.

NaBH480%20%ExampleCH3H3COOHHCH3H3COHHCH3H3C

H3B—H–thismethylgrouphinders

approachofnucleophile

fromtoppreferreddirectionofapproachistolesshindered(bottom)faceofcarbonylgroupStericHindrancetoApproachofReagent

Biologicalreductionsarehighlystereoselectivepyruvicacid?S-(+)-lacticacidOCH3CCO2HNADHH+enzymeislactatedehydrogenaseCO2HHOHCH3

carboxylate

bindingsiteH3COOCOC–carbonyl

bindingsiteNAD–HPyruvateisboundattheactivesiteoftheenzymeFigure17.11

carboxylate

bindingsiteH3COOCC–OHHNAD+carbonyl

bindingsitewhereitisreducedto(S)-(+)-lactate.Figure17.11

inaqueoussolutionRCHRCHRCOHOOHOHH2OO17.15

OxidationofAldehydes

K2Cr2O7H2SO4H2OOOCHOOCOH(75%)Example

K2Cr2O7H2SO4H2OOOCHOOCOH(75%)viaOOHCHOHExample

17.16

Baeyer-VilligerOxidation

ofKetonesOxidationofketoneswithperoxyacids

givesestersbyanovelrearrangement.

RCOOHORCRO+RCOHO+KetoneEsterROCROGeneral

C6H5COOHO(67%) Oxygeninsertionoccursbetweencarbonylcarbonandlargergroup. Methylketonesgiveacetateesters.CHCl3ExampleCCH3OOCCH3O

C6H5COOHO(66%) Reactionisstereospecific. Oxygeninsertionoccurswithretentionof

configuration.CHCl3StereochemistryOCCH3H3CHHOCCH3OH3CHH

RCOOHORCRO+ROCRORCOHO+Mechanism

RCOOHORCRO+ROCRORCOHO+OOCOHRROCRFirststepisnucleophilic

additionofperoxyacid

tothecarbonylgroupof

theketone.Mechanism

RCOOHORCRO+ROCRORCOHO+OOCOHRROCRSecondstepismigration

ofgroupRfromcarbon

tooxygen.TheweakO—Obondbreaksinthis

step.Mechanism

Certainbacteriausehydrocarbonsasasourceofcarbon.Oxidationproceedsviaketones,whichthenundergooxidationoftheBaeyer-Villigertype.BiologicalBaeyer-VillligerOxidationOOObacterial

oxidationO2.

cyclohexanone

monooxygenase,coenzymes