AsymmetricCatalysisandAmplificationwithChiralLanthanideComplexes
JunjiInanaga,*HiroshiFuruno,andTetsujiHayano
InstituteforFundamentalResearchofOrganicChemistry,KyushuUniversity,Hakozaki,Fukuoka812-8581,Japan
ReceivedNovember9,2001
Contents
I.Introduction
II.ChiralComplexesandLigands
III.AsymmetricCatalysisandNonlinearEffects
A.Hetero-Diels−AlderReactionB.Diels−AlderReactionC.1,3-DipolarCycloadditionD.EneReaction
E.Aldol-TypeReaction
F.Hydrophosphonylationand
HydrophosphinationofAldehydesandIminesG.SilylationofAldehydes
H.ReductionofCarbonylGroupsI.Michael-TypeReaction
J.EpoxidationofConjugatedEnones
K.Ring-OpeningReactionofMesoEpoxidesL.Hydrogenation,Hydroamination,andHydrosilylationofOlefinsM.MiscellaneousIV.Conclusions
V.AcknowledgmentsVI.References
2211221122122212221622162217221722182218221822192220222222222223222322232223
I.Introduction
Theimportanceoflanthanidereagentsandcata-lystsinorganicsynthesishasbeenwell-recognizedsincethepioneeringworksbyKaganandco-workersontheSmI2-promotedreactions1andbyLucheandco-workersonthechemo-andstereoselectivereduc-tionofcarbonylcompoundswiththeCeCl3/NaBH4system.2Nonlineareffects(NLEs)inasymmetriccatalysis,oneofthemostimportantconceptualfindingsinasymmetricsynthesis,alsooriginatedintheworkofKaganandco-workers.3ThankstoKaganandotherresearchers,mechanistic,theoretical,andexperimentalstudiesonNLEshaveadvanced.4Thepositivenonlineareffect((+)-NLE),whichisalsocalled“asymmetricamplification”,5istheconvexdeviationfromtheusuallyassumedlinearrelation-shipbetweentheenantiopurityofthechiralligandandthatoftheproduct.Ontheotherhand,theconcavedeviationiscalledthenegativenonlineareffector(-)-NLE(Figure1).Suchphenomena(NLEs)havefrequentlybeenobservedinvariouscatalyticasymmetricreactions,especiallyusingchiralmetalcomplexes.6
ThenonlinearitiesshowninFigure1mayinprinciplearisebyautoassociationorassociation
aroundamatrixoftheinitialchiralspecies,whichproducediastereomericperturbations.MathematicaltreatmentsaswellasmechanisticstudiessuggestthattheNLEsobservedinsomecatalyticasymmetricreactionscomefromthediastereomericassociationsofchiralligandsinthecatalyticcycles.4Fromapracticalpointofview,suchreactionsinwhichasymmetricamplificationcanoperatearehighlyfavoredbecausetheymayprovideopticallyactiveproductswithhighenantiomericexcesses(ee’s)usingachiralligandthatisnotenantiomericallypure.Theuseofchirallanthanidecomplexesasnewcatalystsinasymmetricsynthesisiscurrentlyofintenseinterest.Lanthanideshavepartiallyfilled4forbitalsandalmostempty5dorbitals;inaddition,the4felectronsdonothaveasignificantradialextensionbeyondthefilled5s25p2orbitals.Therefore,theyaregenerallyquiteelectropositive.Inaddition,becauseoftheirlargeionicradii,ingeneraltheyexhibithighcoordinationnumberssuchas7,8,or9tothemaximum12.Thesepropertiesarehighlyadvantageousforassemblingvariouschiralandachiralligandsaroundthemetalions,thuscreatinganintegratedchiralspaceinwhichthestereochem-istryofthereactionmayeffectivelybecontrolled;theyalsomakeNLEslikelytooccurbecausetheyhaveahighcapacityforligandexchangeandag-gregation.
Thispaperbrieflyreviewsrecentprogress(uptomid-2001)intheasymmetricreactionscatalyzedbychirallanthanidecomplexes(scandium,yttrium,andlanthanumwillbeincludedaslanthanidesinthispaperforbrevity)sincesomeimportanttopicscon-cerningasymmetriccatalysiswithchirallanthanidemetallocenes,7lanthanidetriflates-basedchiralcom-plexes,8andchiralheterobimetalliclanthanidecom-plexes9aswellassomediscussionbasedonthecoordinationchemistry10willbemorepreciselycov-eredinothercontributionstothisissue,andspecialattentionisfocuseduponthenonlineareffects(es-peciallyasymmetricamplification)observedinthesereactions.
II.ChiralComplexesandLigands
InFigures2and3aresummarizedtheisolatedchirallanthanidecomplexeswhichhavebeenef-ficientlyusedascatalystsandthechiralligandsemployedfortheinsituformationofthelanthanidecomplexes,whichalsoworkedaseffectivecatalysts,respectively.
10.1021/cr010444tCCC:$39.75©2002AmericanChemicalSociety
PublishedonWeb05/17/2002
2212ChemicalReviews,2002,Vol.102,No.6JunjiInanaga,borninFukuokain1948,receivedhisPh.D.degree(ProfessorMasaruYamaguchi)fromKyushuUniversityin1975andthenwasappointedResearchAssociatethere,wherehehadbeenengagedinthesyntheticstudyofmacrolides.Beginningin1981hespenttwopostdoctoralyearsatIndianaUniversity(ProfessorPaulA.Grieco).Hestartedhissamariumchemistryin1985andwaspromotedtoAssociateProfessorin1989atKyushuUniversity.InthesameyearhemovedtotheInstituteforMolecularScienceatOkazakiandthenmovedbacktoKyushuUniversityin1993asastartingIFOCmember,wherehewaspromotedtoProfessorin2000.HeheldaVisitingProfessorshipatUniversiteParis-Sudin1994andatKyotoUniversityin1996.In1987hewasawardedtheLectureshipforYoungChemists,ChemicalSocietyofJapan,andin1988theProgressAwardinSyntheticOrganicChemistry,Japan.Hewasarecipientofthe1999ShiokawaAward,theRareEarthSocietyofJapan.Hiscurrentresearchinterestsincludenewsyntheticmethodsusinglanthanides,chiralrecognition,andasymmetriccatalysis.
HiroshiFurunowasborninNagasaki,Japan,in1971.HereceivedhisPh.D.degreein2000fromKyushuUniversityunderthesupervisionofProfessorJunjiInanaga.AfterspendingayearasaPostdoctoralFellowatKyotoInstituteofTechnology,hewasappointedResearchAssociateatIFOCin2001.Hisresearchinterestcentersonthedevelopmentofcatalyticasymmetricsynthesisusingchiralrare-earthmetalcomplexes.
III.AsymmetricCatalysisandNonlinearEffectsA.Hetero-Diels−AlderReaction
In1983,Danishefskyandco-workerspublishedamemorablepaperreportingachiraleuropiumcom-plex-catalyzedhetero-Diels-Alderreactionofben-zaldehydewiththeso-calledDanishefsky’sdieneasthefirstexampleofachirallanthanidecomplex-catalyzedreaction(Scheme1).11Thecatalyst(1),popularlyusedasaNMRshiftreagent,wasfoundtobeeffectiveforthereactionshowingconsiderable
Inanagaetal.
TetsujiHayanowasbornin1976inFukuoka,Japan.HegraduatedfromFukuokaUniversity,FacultyofScience,in1999andreceivedhisM.S.degreefromKyushuUniversityin2001underthedirectionofProfessorJunjiInanaga.InthesameyearhestartedhisPh.D.studyundertheguidanceofProfessorInanagaonthedevelopmentofnovelchirallanthanidecomplexesthatcanworkasreusableheterogeneouscatalystsinasymmetricsynthesis.
Figure1.
enantioselectivity(58%ee).Schuringandco-workerspreparedapolysiloxane-fixedeuropiumcomplex(3)asareusablepolymericcatalyst,whichshowedcomparableactivitytotheunsupportedcatalyst(1).12Theeuropiumcomplex(1)wasemployedalsofortheasymmetrichetero-Diels-AlderreactionofR-oxoesterswith1-methoxybutadiene,affordingreasonableenantioselectivities.13
Scheme1
Mikamietal.pointedoutthattheuseofalan-thanidetriflate/chiralligand/basesystemcouldef-fectivelypromotethereactionofbutylglyoxylatewithDanishefsky’sdieneinthepresenceofwatertogivethecycloadductwithupto66%ee(Scheme2).14Interestingly,theenantioselectivityishighlydepend-
AsymmetricCatalysisandAmplificationwithChiralLanthanideComplexes
Figure2.Isolatedchirallanthanidecomplexes.
entontheamountofwateradded.QianandWangreportedthatthesametransformationcouldbebestperformedbythecatalysiswithamixtureofYb(OTf)3andachiral2,6-bis[4′-isopropyloxazol-2-yl]pyridine(i-Pr-pybox)(31)inether-dichloromethanemixed-solventat-78°C,affordingupto77%eeoftheadduct.15
ChemicalReviews,2002,Vol.102,No.62213
Scheme2
In1995,wepreparedthe1:3complexesofaseriesoftrivalentlanthanideionswithachiralbinaphthylphosphateligand,Ln[(R)-BNP]3(4),anddemon-stratedthatsomeofthemeffectivelycatalyzedthehetero-Diels-Alderreactionatroomtemperature,affordingtheproductswithgoodee’s(upto70%ee).16Itwasalsoshownthattheadditionofanequimolaramountof2,6-lutidinetotheytterbiumcatalystYb-4furtherpromotedthereactionunderhomogeneousconditions,suggestingdeoligomerizationofthecom-plex,thusaffordingthecycloadditionproductswithenhancedenantioselectivities(upto93%ee)(Scheme3).17Theasymmetricinductionwashighlydepen-Scheme3
dentonthelanthanideionsused,eitherinthepresenceorabsenceof2,6-lutidine;theytterbiumcatalystaffordedthehighestenantioselectivityasshowninFigure4.16,18Onlyca.0.07Ådifference(fromGdtoYb)19broughtaboutalmostan85%eedifference.Carefulanalysisoftheytterbium-cata-lyzedreactionshowedthattheenantioselectivitydoesnotchangethroughoutthereaction,suggestingthethermodynamicstabilityoftheactivecatalyst(Figure4).
Inthisreactionaremarkablyhighasymmetricamplificationwasrealizedasthefirstexampleinthemetal/ligand)1:3system(Figure5).20CatalystspreparedeitherbymixingYb[(R)-BNP]3andYb[(S)-BNP]3(Yb-catalystA)orfromenantiomericallyim-pureNa-BNPandYbCl3(Yb-catalystB)gavesimi-larresults.
Onthebasisofsomeexperimentalresults,Furunoetal.explainedthisphenomenonintermsoftheautogeneticformationoftheenantiopurehomochiralytterbiumcomplex.FortheformationofYbL3,therearefourpossibilitiesinchoosingthreechiralligandsoutoffour:(LR)3,(LR)2LS,LR(LS)2,and(LS)3.Fromthesefourcomplexes,theheterochiralpairssuchasYb(LR)3andYb(LS)3and/orYb[(LR)2LS]andYb[LR-(LS)2]seemtoirreversiblyassembleformingthermo-dynamicallyverystablecomplexesthathavealmostnocatalyticactivityforthehetero-Diels-Alderreac-
2214ChemicalReviews,2002,Vol.102,No.6Figure3.Chiralligandsforlanthanidecomplexes.
Figure4.
Inanagaetal.
tion.Asaresult,theenantiopureYbcomplexbasedonanexcessamountoftheenantiomer,Yb(LR)3,wouldremaininsolutionastherealcatalyst(Figure6).Thisphenomenonturnedouttobequitegeneralwithinthelanthanide-metalionswithsimilarionicradiisuchasthatoftheytterbiumion.20
Hayanoetal.foundthattheCH2Cl2-solublechiralceriumcomplex(C)preparedfromcericammoniumnitrate(CAN)andNa-(R)-BNPalsoaffordshighasymmetricamplification(Figure7).21Spectral(ESCA,ICP-MS)dataofthecatalystindicatedittobethecerium(III)complex.Differentfromthecaseoftheytterbiumcatalyst,theoligomericandCH2Cl2-insolubleceriumcomplex(D)alsocatalyzedthereaction.Interestingly,alinearrelationshipwasobservedinthiscaseandtheobservedee’softheproductwerelinearlyhigherthanthoseofthecor-respondingligandsofthecatalysts(Table1).21
Recently,Kambaraetal.accomplishedalmostperfectenantioselectivity(99%ee)intheY[(R)-H8-BNP]3(5)catalyzedreactionwithoutusing2,6-lutidineasanadditive.22
AsymmetricCatalysisandAmplificationwithChiralLanthanideComplexesFigure5.
Figure6.
Table1.
eeof(R)-18,%
entryeeofcatalyst(CorD),%
forC
forD11052201034018
4509656098226
80
99
27
IshitaniandKobayashisucceededinthefirstcatalyticasymmetricaza-Diels-Alderreactionof2-hydroxyaniline-derivedaldimineswithelectron-richolefinsbyusingachiralytterbiumcomplexpre-paredinsitufromYb(OTf)3,(R)-BINOL(18),DBU,
ChemicalReviews,2002,Vol.102,No.62215
Figure7.
and2,6-di-tert-butyl-4-methylpyridine(DTBMP)inthepresenceofmolecularsieves4Aat0°C.23Thereactionswerecarriedoutat-15°Cinthepresenceofadditivestogivethe8-hydroxyquinolinederiva-tiveswithhighee’s(upto91%ee).Whitingandco-workersalsodemonstratedthatachiralytterbiumcomplexpreparedfromYb(OTf)3,(1R,2R)-diphenyl-ethylenediamine(26),and2,6-lutidinenicelypro-motedthereactionofthep-anisidine-derivedimineofmethylglyoxalatewithDanishefsky’sdieneafford-ing,afteracidicworkup,a2,3-dehydro-4-piperidonederivativewith87%ee.24(Scheme4)
Scheme4
2216ChemicalReviews,2002,Vol.102,No.6B.Diels−AlderReaction
Kobayashiandco-workersrealizedahighlyenan-tioselectiveDiels-Alderreactionbycatalysiswithin-situ-preparedchirallanthanidecomplexes.25Eitherenantiomercanbeselectivelysynthesizedbymodify-ingLn(OTf)3withasinglechiralsourceandachoiceofachiralligands.Forexample,thereactionof3-(2-butenoyl)-1,3-oxazolidin-2-onewithcyclopentadienepreferablygavetheendo-adduct(endo/exo)89:11)witha(2S,3R)configurationin93%eewhenacatalystpreparedinsitufromYb(OTf)3,(R)-BINOL(18),cis-1,2,6-trimethylpiperidine(TMP),and3-acetyl-1,3-oxazolidin-2-one(40)wasused.Ontheotherhand,the(2R,3S)enantiomerwasselectivelyob-tainedin81%eeusingadifferentcatalystpreparedfromYb(OTf)3,(R)-BINOL,1,2,2,6,6-pentamethylpi-peridine(41),and3-phenylacetylacetone(PAA)(Scheme5).Thereversaloftheenantiofacialselection
Scheme5
isaccountedforbythechangeinthecoordinationnumbers.Thecoordinationstructureofthecatalystisaffectedbytheligandsemployed;morethantwodifferentcomplexesseemtobeinvolvedinthereac-tionaseffectivecatalysts.Thiscomplexitywasfurtherascertainedbytheobserveddiversityoftherelationshipbetweentheenantiopurityofthechiralligandandthatoftheproductdependingontheachiralligandsaswellasmetalionsofthecatalystsused.Forexample,a(-)-NLEwasobservedinthecatalysiswiththechiralytterbiumcomplex(E)preparedinsitufromYb(OTf)3,(R)-BINOL,TMP,andPAA,whereastheytterbiumcatalyst(F)affordedanalmostlinearrelationship,andthescandiumcatalyst(G)preparedfromSc(OTf)3,(R)-BINOL,andTMPshoweda(+)-NLE(Figure8).25a,c
Nakagawaandco-workersusedanin-situ-pre-paredchiralcomplexcomposedofYb(OTf)3,2,2′-bis-(acylamino)-1,1′-binaphthyl(28),andN,N-diisopropyl-ethylamine(DIPEA)(1:1.2:2.4)forsimilarreactions
Inanagaetal.
Figure8.
andachievedexcellentenantioselectivities(upto>98%ee).26Fukuzawaetal.alsoexaminedthecombineduseofSc(OTf)3andi-Pr-pybox(31)undervariousconditionsandobtainedtheDiels-Alderadductswithfairlyhighenantioselectivities(upto90%ee).27Aoyamaandco-workerspreparedalan-thanum-involvedhelicalcoordinationpolymer(12)fromLa(O-i-Pr)3,(R)-BINOL,andanthracene-bis-resorcinolanddemonstratedthatitcatalyzestheDiels-Alderreactionwithmoderateenantioselectiv-ity(70%ee).28Shibasaki’slithium-containinghetero-bimetalliccomplex(R)-La-6wasfoundtoactasaneffectiveLewisacidcatalystfortheDiels-Alderreaction,thusaffordingtheproductwithupto86%ee.29
Marko´andco-workersreportedthatthechiralcatalystcomposedofYb(OTf)3,(R)-BINOL(18),andDIPEAfortheinverseelectron-demandDiels-Alderreactionsof2-pyronederivativesgaveadductsinupto95%ee(Scheme6).30Theyalsostatedthatanega-tivenonlineareffectwasobservedinthisreaction.
Scheme6
C.1,3-DipolarCycloaddition
In1997,Kobayashi’s31andJørgensen’s32groupsindependentlyfoundthatthecomplexespreparedfromYb(OTf)3andchiralligands[(R)-BINOL(18)+amineori-Pr-pybox(31),respectively]inducedenan-tioselectivityinthe1,3-dipolarcycloadditionofni-trones(42)withactivatedolefinssuchasR,-unsaturatedalkenoyloxazolidones.Thecorresponding
AsymmetricCatalysisandAmplificationwithChiralLanthanideComplexesisoxazolidinederivativeswereobtainedingoodee’s(upto78%ee).Kobayashiandco-workerssucceededinimprovingtheenantioselectivity(upto96%ee)aswellasdiastereoselectivity(endo/exo)99:1)byusinganadditionalchiralaminebase,N-methyl-bis[(R)-1-naphthylethyl]amine[(R)-MNEA].33Interestingly,thesenseofenantioselectivityisswitchablesimplybychangingtheadditivefrommolecularsieves4A(MS4A)toaNitron.Ohtaandco-workersproposedanewcomplexbetweenSc(OTf)3andachiral3,3′-bis(2-oxazolyl)-BINOL(24)asaneffectivecatalystforthesamereaction34(Scheme7).
Scheme7
D.EneReaction
Qianandco-workersusedtwotypesofchiralytterbiumcomplexesascatalystsoftheasymmetricglyoxylate-enereaction.OnecatalystisamixtureofYb(OTf)3anda6,6′-disubstituted(R)-BINOLde-rivative(19),35andtheotherisacomplexofYb(OTf)3withPh-pybox(33).36Moderateenantioselectivitieswereobservedinbothcases(Scheme8).
Scheme8
E.Aldol-TypeReaction
Greatsuccessinasymmetricaldol37,38(upto94%ee)andnitroaldolreactions37,39-42(upto98%ee)owes
ChemicalReviews,2002,Vol.102,No.62217
mainlytoShibasaki’soutstandingworkusingthechiralheterobimetalliclanthanidecomplexes,lan-thanide-alkali-metal-(R)-BINOL(LnMB),ascata-lysts.ThedetailswillbedisclosedbyShibasakietal.inthisissue.9AsymmetricamplificationwasreportedforthereactionofnitromethanewithR-naph-thoxyacetaldehydecatalyzedbythechiralcomplexbetweenLaCl3‚7H2O,dilithium(S)-binaphthoxide,andNaOH;thenitroaldolproductwith68%eewasobtainedusing56%eeofthechiralligand(Scheme9).39a
Scheme9
Thecatalyticasymmetricnitro-Mannich-typereac-tionwasbesteffectedbyanotherkindoflanthanideheterobimetalliccomplexcomposedofYb(O-i-Pr)3,KO-t-Bu,and(R)-BINOL(1:1:3)togivethedesiredproductwithupto91%ee(Scheme10).43
Scheme10
Thefirstcatalyticenantioselectivealdol-Tish-chenkoreactionbetweenaromaticaldehydesand2-methylpropanal(6equiv)wasrealizedbyMorkenandco-workersusingtheyttriumcomplexpreparedfromY5O(O-i-Pr)13andachiralsalenligand(36)togivethecorresponding-acyloxyalcoholswithupto74%ee(Scheme11).44
Scheme11
FortheMukaiyamaaldolreaction,useoflan-thanidetriflatesincombinationwithchiralligands
2218ChemicalReviews,2002,Vol.102,No.6wasfoundtobeeffective,producingupto82%ee(Scheme12).45,46
Scheme12
F.HydrophosphinationHydrophosphonylationofAldehydesand
andImines
Shibasaki-typechiralheterobimetalliclanthanidecomplexeswerefoundtobequiteeffectivealsofortheenantioselectivehydrophosphonylationandphos-phinationofaldehydesandimines.Thus,thecorre-spondingR-hydroxyphosphonatesandR-aminophos-phonates,whicharebiologicallyorpharmaceuticallyimportantcompounds,wereobtainedinhighenan-tioselectivities.Shibuyaetal.47andSpillingetal.48independentlyreportedthechirallanthanumcom-plex(composedof18‚Li2,NaO-t-Bu,andH2O)cata-lyzedreactionofaldehydesforwhichShibuyapointedoutasignificanteffectoftheelectron-donatingpara-substituentsofbenzaldehydeontheenantioselectiv-ity.49Thus,thehighestenantioselectivity(95%ee)wasattainedbyShibasakietal.inthereactionofp-dimethylaminobenzaldehyde.50Qianetal.alsoexaminedtheinfluenceofthesubstituentsatthe3,3′-and6,6′-positionsofBINOLasaconstituentofthechirallanthanidecomplex.51(Scheme13)
Scheme13
TheresearchchemistsoftheShibasaki’sandMartens’groupsactivelyinvestigatedtheenantiose-lectivehydrophosphonylation52,53andphosphination54ofacyclicandcycliciminesandfoundthatLnK3[(R)-BINOL]3(Ln-9)complexissuperiortothecorre-spondinglithium(Ln-7)orsodium(Ln-8)complexes
Inanagaetal.
asacatalyst.Thus,almostperfectenantioselectivitywasattainedinthereactionof2,2,5,5-tetramethyl-3-thiazolineusingLn-9asacatalyst(Schemes14and15.
Scheme14
Scheme15
G.SilylationofAldehydes
In1996,AbikoandWangreportedthefirstchiralyttriumcomplex-catalyzedasymmetricsilylcyanationofaldehydesthatrequiresonly0.2mol%ofthecatalystpreparedinsitufromY5O(O-i-Pr)13andaferrocene-derivedC2-chiral1,3-diketoneligand(38).55Aromaticaldehydeswereconvertedtothecorre-spondingcyanohydrinswithhighenantioselectivities(upto91%ee).Asaneffectivecatalystsystemforthesametransformation,Fangetal.proposedacombinationofSmCl3andachiralbis-phosphor-amidateligand(39)56andAspinallandGreevesetal.successfullyusedLnCl3incombinationwithPh-pybox(33).57Qianetal.reportedthatanin-situ-preparedcomplexfromLa(O-t-Bu)3andachiralBINOLderivative(21)ina2:3ratiowasalsoeffec-tive.58(Scheme16)
Scheme16
H.ReductionofCarbonylGroups
Thechirallanthanidecomplex-catalyzedenantio-selectivereductionofcarbonylcompoundshasbeenexaminedinthefollowingthreereactions:thereduc-tionwithNADHmodels(Scheme17),59theMeer-wein-Ponndorf-Verleyreduction(Scheme18),60-62andtheboranereduction(Scheme19).63ParticularlynoteworthyintermsofenantioselectivityistheworkofEvansetal.;asamariumcomplexpreparedinsitufromSmI3and(R)-styreneoxide-derivedC2-chiralaminodiol(30)catalyzedtheMeerwein-Ponndorf-Verleyreductionofarylmethylketonesinhigh
AsymmetricCatalysisandAmplificationwithChiralLanthanideComplexesScheme17
Scheme18
Scheme19
enantioselectivities(upto97%ee)(Scheme18).60Asignificanteffectofthelanthanide-metalsizeontheenantioselectivitywasobserved.Thus,thecomplexesofyttrium,terbium,samarium,andneodymiumshowedhigherselectivitiesthanthoseofscandium,lutetium,andlanthanum.Asymmetricamplificationwasalsoobserved:When80%eeofthechiralligandwasused,thereductionproductwasformedin95%ee,whichisthesameselectivityobtainedusingtheenantiopureligand.
I.Michael-TypeReaction
In1993,Scettriandco-workersappliedashiftreagent,(+)-Eu(tfc)3(2),asacatalysttotheMichaeladditionof1,3-dicarbonylcompoundstomethylvinylketoneobservingmodestenantioselectivities(Scheme20).64
Scheme20
ChemicalReviews,2002,Vol.102,No.62219
IntensivestudiesbyShibasakiandco-workersproducedexcellentresults.Thus,veryhighenantio-selectivities(upto99%ee)wererealizedintheMichaelreactionofvarious1,3-dicarbonylcompoundswithR,-unsaturatedcarbonylcompoundsbyusingLa(O-i-Pr)66,673/(S)-BINOL,65LaNa3[(R)-BINOL]3(La-8),orLa-linked-bisBINOL(10)68complexes.Theyalsosucceededinimmobilizingthelattercomplexbyattachingittoapolymerhavingapolystyrenebackbone(11).69Theeffectoftheimmobilizationontheenantioselectivitywasfoundtobesmall;goodenantioselectivities(over80%ee)wereensured.AnotherheterogeneouspolymericcatalystisAoya-ma’shelicalcoordinationpolymer(12),whichgavetheadditionproductofdimethylmalonateto2-cy-clohexenoneinca.70%ee28,70(Scheme21).
Scheme21
WefoundthatanovelcomplexcomposedofLa(O-i-Pr)3,(R)-BINOL,andDIPEA(1:3:3)isaseffectiveasthecorrespondingheterobimetalliccomplexesfortheMichaeladditionreactionofdibenzylmalonatetocyclohexenone.Furthermore,anotableasymmetricamplificationwasobservedinthereactioncatalyzedbythelanthanide-amine-BINOL(LAB)systemasshowninFigure9.71
TheMichaelreactionof2-(trimethylsilyloxy)furanswithoxazolidinoneenolateswasexaminedbyKat-sukiandco-workers.ThescandiumcomplexpreparedfromSc(OTf)3andaBINOLderivative(22)showedahighdiastereoselectivitybutwithamodestenan-tioselectivity(73%ee)(Scheme22).72
Scheme22
2220ChemicalReviews,2002,Vol.102,No.6Figure9.
Shibasaki’sLaK3[(R)-BINOL]3complex(La-9)wasfoundtocatalyzetheMichaeladditionofnitromethanetoconjugatedenoneswithhighenantioselectivities(upto97%ee)(Scheme23).73
Scheme23
EnantioselectiveconjugateadditionofO-benzyl-hydroxylaminetoR,-unsaturatedpyrazoleamidesusingaLn(OTf)3-bisoxazoline(34)complexwasre-portedbySibietal.togivethedesiredproductswithmoderateenantioselectivities(Scheme24).74
Scheme24
Recently,wefoundthattheSc[(R)-BNP]3complex(Sc-4)wasaquiteeffectivecatalystfortheadditionreactionofnitrogennucleophilestoconjugatedenones;theenantioselectiveadditionreactionofO-diphenyl-methylhydroxylaminetovariousconjugatedenoneswasaccomplishedatroomtemperaturewithalmostcompleteenantioselectivities(upto>99%ee)(Scheme25),andasshowninFigure10,asignificantasym-metricamplificationwasobservedinthisreaction.75TheMichaeladductsthusobtainedcanbecleanlyconvertedtothecorrespondingR-ketoaziridinesbycatalysiswithNaO-t-BuorLa(O-i-Pr)3.
J.EpoxidationofConjugatedEnones
Catalyticasymmetricepoxidationisanotherim-portantasymmetricprocess.Shibasakiandco-work-
Inanagaetal.
Figure10.Scheme25
erssuccessfullycarriedoutthehighlyenantioselec-tivecatalyticepoxidationofconjugatedenonesusinglanthanidecomplexespreparedfromLa(O-i-Pr)3and(R)-BINOL(18)or(R)-3-hydroxymethyl-BINOL(25)togivethecorrespondingepoxyketoneswithupto94%ee.76Thelattercatalystsystem,Yb(O-i-Pr)3/25,wasfoundtobequiteeffectivealsofortheepoxida-tionofcisenones(upto96%ee).77TheasymmetricamplificationobservedintheYb-(R)-BINOLcomplex-catalyzedepoxidationofbenzalacetoneisshowninFigure11,whichsuggeststhatsomeaggregationofthecatalystexists.76b
FromtheexperimentsdealingwiththeLn[(R)-BNP]3/2,6-lutidinecomplex-catalyzedhetero-Diels-Alderreaction(sectionA),wenoticedthattheadditionofanexternalligandtothechirallanthanidecomplexwasquiteeffectivenotonlyforsolubilizingthecatalystbutalsoforenhancingthestereoselec-tivity,whichsuggeststheimportanceofcoordinativesaturationofthelanthanidewithappropriateligandstodeoligomerizethepolymericcomplexes.Onthebasisoftheaboveidea,weexaminedtheeffectofavarietyofadditivesfortheLa-(R)-BINOLcomplex-catalyzedasymmetricepoxidationofchalconewithtert-butylhydroperoxide(TBHP)asanoxidant.78SomeselectedresultsareshowninTable2.Amongtheadditivestested,triphenylphosphineoxidegavethebestresultof96%eeandshowedanotableligand-accelerationofthereactionrate.
Itwasalsofoundthatcumenehydroperoxide(CMHP)istheoxidantofchoice.WhenCMHPwasusedinplaceofTBHP,theenantioselectivitywas
AsymmetricCatalysisandAmplificationwithChiralLanthanideComplexesFigure11.
Table2.
entryadditivetime,hee,%yield,%1none
3.073892lutidineN-oxide3.074964n-Bu3PdO3.073885Ph3PdO
0.596996(p-tolyl)3PddO1.094957(o-tolyl)3PddO1.573968(Me2N)3PdO
1.5
86
99
Table3.
entryR1R2time,hee,%yield,%1PhPh0.2>99992PhMe3.0>99903Phi-Pr4.0>99724Pht-Bu3.099935i-Pr
Ph3.098886
Ph(CH2)2
Me
1.0
86
60
furtherraisedtoover99%eeandtheamountofthecatalystcanbereducedto0.5mol%withoutaseriousdecreaseintheenantioselectivity.79Thismethodisapplicabletoawiderangeofsubstrates,ascanbeseenfromsomeexamplesinTable3.Inaddition,allofthereagentsrequiredforthisasymmetricepoxi-dationarecommerciallyavailableandthelowreac-tiontemperaturesusuallyrequiredtoattainahighenantioselectivityarenotnecessary,thusmakingtheprotocolhighlypractical.
Asexpectedfromtheaboveresults,averyhighasymmetricamplificationwasobservedintheepoxi-dationofchalconewithCMHPusingtheLa(O-i-Pr)3/(R)-BINOL/Ph3PO(1:1:3)system(Figure12).79Usingonly40%eeoftheligand,onecanattainmorethan99%enantioselectivity.
ChemicalReviews,2002,Vol.102,No.62221
Figure12.
Figure13.
Thisresultstronglysuggeststhattheactivecata-lystmaynotbemonomericandmayhaveaparticu-larstructurethathardlychangesduringthereactionbecauseofitsthermodynamicstability.Thiswasfurthersupportedbythefactthattheuseof(R)-3,3′-di(9-anthryl)-BINOL(23)inplaceof(R)-BINOL(18)inpreparingthecatalystsystemshowedtheoppositesenseofenantioselectionandthatneitherpositivenornegativenonlineareffectswereobservedintheepoxidation,suggestingamonomericstructureofthecatalystduetostericbulkinessofthe3,3′-anthrylgroups.80Thus,ahomochiraldimericcomplexlikeabinuclearµ-complex,composedofLa/(R)-BINOL/Ph3-PO/ROOH)1:1:1:1(Figure13),istentativelypro-posedasanactivecatalyticspeciesforthereaction;oneofthelanthanumionsmayworkasaLewisacidtoactivatethesubstrate,andtheperoxideattachedtotheotherlanthanumionmightbedeliveredasanactiveoxidant,thuscontrollingthestereochemistryoftheepoxidation.Actually,theepoxidationofchal-conewithCMHPusingthepreformedcatalystsys-tem,La(O-i-Pr)3/(R)-BINOL/Ph3PO/CMHP(1:1:1:1),affordedtheexpectedepoxidewith99%ee.79Thepresentprotocolissoconvenientandeffectivethatitcanbesuccessfullyappliedtolarge-scaleexperi-ments(e.g.,30kgscale;90%chemicalyield,>98%ee).81
Shibasakiandco-workersalsopointedoutthattheeffectivenessofadditivessuchaswater76bandtriphen-ylarsineoxide.82FromtheLDI-TOFMSspectrumofthecomplex,theLa(O-i-Pr)3/(R)-BINOL/Ph3AsO(1:1:1)structurewassuggested.Theysuccessfullyappliedthecatalysttothesynthesisofnaturalproducts82aandclaimedthattriphenylarsineoxideissuperiortotriphenylphosphineoxideasanadditivewithrespecttothereactionrateandthestoichiometry.82b
2222ChemicalReviews,2002,Vol.102,No.6Figure14.
Considerableasymmetricamplificationwasalsoobserved(Figure14),andtheformationofahetero-chiralcomplexasanineffectivecatalystwassug-gestedforthephenomenon.Furthermore,theydem-onstratedthattheprotocolisapplicabletotheepoxidationofR,-unsaturatedpyrazolamides,thusaffordingupto93%ee.83
K.Ring-OpeningReactionofMesoEpoxides
AsymmetricringopeningofcyclohexeneoxidewithanilinewasachievedbyHouandco-workers.84Theyusedacatalystsystem,Yb(OTf)3/(R)-BINOL/Ph2NBn,forthereactionandobtainedthecorrespondingtrans-1,2-aminoalcoholwith80%ee(Scheme26).
Scheme26
SchausandJacobsenreportedthattheasymmetricringopeningofmesoepoxideswithcyanotrimethyl-silane(TMSCN)canbecatalyzedbythechiralytterbiumcomplexpreparedinsitufromYbCl3andachiralPh-pybox(33),yieldingthe-trimethylsil-yloxynitrilering-openedproductswithgoodenanti-oselectivities(83-92%ee).85Thereactionexhibitsasecond-orderkineticdependenceonthecatalystconcentrationandafirst-orderdependenceontheepoxideconcentration,consistentwithabimetallicpathwayinvolvingsimultaneousactivationoftheepoxideandcyanide.Thiswasfurthersupportedbytheobservationofanotablepositivenonlineareffect(Figure15).
L.HydrosilylationHydrogenation,ofHydroamination,Olefins
andMarksandco-workerspreparedvariouschirallanthanidemetallocenes(13-16)bearing(-)-men-thylor(+)-neomenthylsubstituentsononeofthetwo
Inanagaetal.
Figure15.
cyclopentadienylringsconnectedwithdimethylsilylgroupandsuccessfullyusedthemforenantioselectiveolefinhydrogenation(upto96%ee)86andhydrosil-ylation(upto68%ee)87of1,1-disubstitutedethenesandalsoforintramolecularhydroamination(upto74%ee)86b,88(Scheme27).DetaileddiscussiononthissubjectisgivenbyMolanderetal.inthisissue.7
Scheme27
Recently,Tilleyetal.demonstratedthatthechiralyttriumhydridegeneratedinsitufrom[(S)-DADMB]-YMe(thf)2(17)(DADMB)2,2′-bis-(tert-butyldimeth-ylsilylamido)-6,6′-dimethylbiphenyl)worksasanef-ficientcatalystfortheenantioselectivehydrosilylationofnorbornene(90%ee)(Scheme28).89
Scheme28
AsymmetricCatalysisandAmplificationwithChiralLanthanideComplexesM.Miscellaneous
Anefficientenantiotopicdisplacementofpro-Rorpro-Schloridesonansp3carboncenterwithanalkylgroup,whichproceedsthroughMatteson’shomolo-gationprocess,wasattainedbyJadhavandManusingYb(OTf)3asamosteffectiveLewisacidincombinationwithachiralbisoxazolineligand(35)toaffordthecorrespondingalkylationproductwith88%ee(Scheme29).90
Scheme29
Naruseetal.reportedthattheeuropiumcomplex(1)isexceptionallyeffectivefortheenantiomericenrichmentofallenedicarboxylates.Forexample,diidopropylpenta-2,3-dienedioatewithover95%eewasrecoveredatthesacrificeoftheoppositeenan-tiomerwhenthesubstratewastreatedwithastoi-chiometricamountof(+)-1indeuteriochloroformat20°Cfor9days(Scheme30).91Theenantiomericenrichmentdoesnotseemtoproceedinacatalyticmanner.
Scheme30
IV.Conclusions
Chirallanthanidecomplexespreparedeitherinsituorasisolableformshavebeenshowntoef-fectivelycatalyzeavarietyofreactionswithhighenantioselectivities.Inallcases,lanthanideionsworkasaLewisacid,thusgatheringvariouschiralandachiralligands,substrates,and,insomecases,reagents.TheyhardlylosetheirLewisacidityeven,insomecases,inthepresenceofwater,amines,oraminoalcohols,althoughinsuchcircumstancesmosttraditionalLewisacidslosetheiractivitiesbyhy-drolysisorthroughthecompleteoccupationoftheircoordinationsites.Thelanthanidesprovidevariousionicradiicontinuouslywithintherangeof,e.g.,inthecaseofhexacoordinates,1.032(La3+)to0.861(Lu3+)orto0.745Å(Sc3+);therefore,fine-tuningoftransitionstructuresisoftenpossiblebychoosinganappropriatelanthanideionforthechirallanthanidecatalysts.However,inmanycasesexactcoordination
ChemicalReviews,2002,Vol.102,No.62223
numbersandstructuresoftheactivechirallan-thanidecatalystshavenotyetbeenfullydeterminedandtheissuesremaintobeclarifiedsoontoprovideaguidingprinciplefordesigningnewchirallan-thanidecomplexesasefficientcatalysts.
AlargenumberofexamplesofNLEshavebeenaccumulatedoverthepast15years.NLEsarenowrecognizedasaubiquitousphenomenoninasym-metricreactions,especiallyinenantioselectiveca-talysis.Thisisalsotrueforthecatalysiswithchirallanthanidecomplexes.Thus,notableasymmetricamplifications,(+)-NLEs,havebeenobservedinavarietyofreactions.SuchNLEscanbeexplainedintermsoftheself-organization,aggregation,and/orself-assembliesofthechiralcomplexesthatmayariseingivenreactions.Theremarkablyhighasymmetricamplificationsobservedinthelanthanidecomplex-catalyzedreactionsmayberesponsibleforthelargecoordinationnumberofthelanthanideions,whichisadvantageousfortherapidligandexchangeandalsofortheaggregationoftheheterochiralcomplexestogeneratecertainthermodynamicallystablecom-plexeswithverypoorcatalyticactivity.Asymmetricamplificationishighlyadvantageousfromapracticalviewpointbecauseitallowstheconvenientuseofenantiomericallyimpurechiralligandswithoutpu-rification.
V.Acknowledgments
OurworkinthisareawaspartlysupportedbytheMinistryofEducation,Culture,Sports,ScienceandTechnology,Japan,theJapanSocietyforthePromo-tionofScience,theAsahiGlassFoundationandtheNagaseScienceandTechnologyFoundation.
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