ModernOxidationMethods1stEditionJanErlingB?Ckvall
https://ebookgate.com/product/modern-oxidationmethods-1st-edition-jan-erling-bckvall/
Download more ebook from https://ebookgate.com
More products digital (pdf, epub, mobi) instant download maybe you interests ...
Modern Oxidation Methods 2nd Edition Jan-Erling
Backvall
https://ebookgate.com/product/modern-oxidation-methods-2ndedition-jan-erling-backvall/
Tradition Translaion and Trauma The Classic and the Modern 1st Edition Jan Parker
https://ebookgate.com/product/tradition-translaion-and-traumathe-classic-and-the-modern-1st-edition-jan-parker/
RNA Sequence Structure and Function Computational and Bioinformatic Methods 1st Edition Jan Gorodkin
https://ebookgate.com/product/rna-sequence-structure-andfunction-computational-and-bioinformatic-methods-1st-edition-jangorodkin/
Medical Image Processing Reconstruction and Restoration Concepts and Methods 1st Edition Jiri Jan
https://ebookgate.com/product/medical-image-processingreconstruction-and-restoration-concepts-and-methods-1st-editionjiri-jan/
Methods for Exodus Methods in Biblical Interpretation
1st Edition Thomas B. Dozeman Phd
https://ebookgate.com/product/methods-for-exodus-methods-inbiblical-interpretation-1st-edition-thomas-b-dozeman-phd/
Materials characterization modern methods and applications 1st Edition Ranganathan
https://ebookgate.com/product/materials-characterization-modernmethods-and-applications-1st-edition-ranganathan/
Regulatory B Cells Methods and Protocols 1st Edition
Gaetano Vitale
https://ebookgate.com/product/regulatory-b-cells-methods-andprotocols-1st-edition-gaetano-vitale/
Plant Gravitropism Methods and Protocols 1st Edition
Elison B Blancaflor
https://ebookgate.com/product/plant-gravitropism-methods-andprotocols-1st-edition-elison-b-blancaflor/
Lipid Oxidation Pathways Volume Two 1st Edition Afaf
Kamal-Eldin
https://ebookgate.com/product/lipid-oxidation-pathways-volumetwo-1st-edition-afaf-kamal-eldin/
RecentDevelopmentsintheOsmium-catalyzedDihydroxylation ofOlefins
UtaSundermeier,ChristianDöbler, and MatthiasBeller
1.1
Introduction
Theoxidativefunctionalizationofolefinsisofmajorimportanceforbothorganic synthesisandtheindustrialproductionofbulkandfinechemicals[1].Amongthe differentoxidationproductsofolefins,1,2-diolsareusedinawidevarietyofapplications.Ethylene-andpropylene-glycolareproducedonamulti-milliontonscaleper annum,duetotheirimportanceaspolyestermonomersandanti-freezeagents[2]. Anumberof1,2-diolssuchas2,3-dimethyl-2,3-butanediol,1,2-octanediol,1,2-hexanediol,1,2-pentanediol,1,2-and2,3-butanediolareofinterestinthefinechemical industry.Inaddition,chiral1,2-diolsareemployedasintermediatesforpharmaceuticalsandagrochemicals.Atpresent1,2-diolsaremanufacturedindustriallybyatwo stepsequenceconsistingofepoxidationofanolefinwithahydroperoxideoraperacidfollowedbyhydrolysisoftheresultingepoxide[3].Comparedwiththisprocess thedihydroxylationofC=Cdoublebondsconstitutesamoreatom-efficientand shorterrouteto1,2-diols.Ingeneralthedihydroxylationofolefinsiscatalyzedbyosmium,rutheniumormanganeseoxospecies.Theosmium-catalyzedvariantisthe mostreliableandefficientmethodforthesynthesisof cis-1,2-diols[4].Usingosmiumincatalyticamountstogetherwithasecondaryoxidantinstoichiometric amountsvariousolefins,includingmono-,di-,andtrisubstitutedunfunctionalized, aswellasmanyfunctionalizedolefins,canbeconvertedintothecorresponding diols.OsO4 asanelectrophilicreagentreactsonlyslowlywithelectron-deficientolefins,andthereforehigheramountsofcatalystandligandarenecessaryinthese cases.Recentstudieshaverevealedthatthesesubstratesreactmuchmoreefficiently whenthepHofthereactionmediumismaintainedontheacidicside[5].Here,citric acidappearstobesuperiorformaintainingthepHinthedesiredrange.Onthe otherhand,inanotherstudyitwasfoundthatprovidingaconstantpHvalueof12.0 leadstoimprovedreactionratesforinternalolefins[6].
SinceitsdiscoverybySharplessandcoworkers,catalyticasymmetricdihydroxylation(AD)hassignificantlyenhancedtheutilityofosmium-catalyzeddihydroxylation (Scheme1.1)[7].Numerousapplicationsinorganicsynthesishaveappearedinrecentyears[8].
ModernOxidationMethods. EditedbyJan-ErlingBäckvall
Copyright 2004WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
ISBN:3-527-30642-0
Scheme1.1 Osmylationofolefins
Whiletheproblemofenantioselectivityhaslargelybeensolvedthroughextensive synthesisandscreeningofcinchonaalkaloidligandsbytheSharplessgroup,some featuresofthisgeneralmethodremainproblematicforlargerscaleapplications. Firstly,theuseoftheexpensiveosmiumcatalystmustbeminimizedandanefficient recyclingofthemetalshouldbedeveloped.Secondly,theappliedreoxidantsforOsVI speciesareexpensiveandleadtooverstoichiometricamountsofwaste.
Inthepastseveralreoxidationprocessesforosmium(VI) glycolatesorotherosmium(VI) specieshavebeendeveloped.Historically,chlorates[9]andhydrogenperoxide[10]werefirstappliedasstoichiometricoxidants,howeverinbothcasesthedihydroxylationoftenproceedswithlowchemoselectivity.Otherreoxidantsforosmium(VI) are tert-butylhydroperoxideinthepresenceofEt4NOH[11]andarangeof N-oxides,suchas N-methylmorpholine N-oxide(NMO)[12](theUpjohnprocess)and trimethylamine N-oxide.K3[Fe(CN)6]gaveasubstantialimprovementintheenantioselectivitiesinasymmetricdihydroxylationswhenitwasintroducedasareoxidantfor osmium(VI) speciesin1990[13].However,evenasearlyonas1975itwasalready beingdescribedasanoxidantforOs-catalyzedoxidationreactions[14].Todaythe“ADmix”,containingthecatalystprecursorK2[OsO2(OH)4],theco-oxidantK3[Fe(CN)6], thebaseK2CO3,andthechiralligand,iscommerciallyavailableandthedihydroxylationreactioniseasytocarryout.However,theproductionofoverstoichiometric amountsofwasteremainsasasignificantdisadvantageofthereactionprotocol.
Thischapterwillsummarizetherecentdevelopmentsintheareaofosmium-catalyzeddihydroxylations,whichbringthistransformationclosertoa“greenreaction”. Hence,specialemphasisisgiventotheuseofnewreoxidantsandrecyclingofthe osmiumcatalyst.
1.2
1.2.1 HydrogenPeroxide
EversincetheUpjohnprocedurewaspublishedin1976the N-methylmorpholine N-oxide-basedprocedurehasbecomeoneofthestandardmethodsforosmium-catalyzeddihydroxylations.However,intheasymmetricdihydroxylationNMOhasnot
beenfullyappreciatedsinceitwasdifficulttoobtainhigh ee withthisoxidant.Some yearsagoitwasdemonstratedthatNMOcouldbeemployedastheoxidantintheAD reactiontogivehigh ee inaqueous tert-BuOHwithslowadditionoftheolefin[15].
Inspiteofthefactthathydrogenperoxidewasoneofthefirststoichiometricoxidantstobeintroducedfortheosmium-catalyzeddihydroxylationitwasnotactually useduntilrecently.Whenusinghydrogenperoxideasthereoxidantfortransition metalcatalysts,veryoftenthereisthebigdisadvantagethatalargeexcessofH2O2 isrequired,implyingthattheunproductiveperoxidedecompositionisthemajor process.
RecentlyBäckvallandcoworkerswereabletoimprovetheH2O2 reoxidationprocesssignificantlybyusing N-methylmorpholinetogetherwithflavinasco-catalysts inthepresenceofhydrogenperoxide[16].ThusarenaissanceofbothNMOand H2O2 wasinduced.ThemechanismofthetriplecatalyticH2O2 oxidationisshown inScheme1.2.
Scheme1.2 Osmium-catalyzeddihydroxylationofolefinsusing H2O2 astheterminaloxidant
TheflavinhydroperoxidegeneratedfromflavinandH2O2 recyclesthe N-methylmorpholine(NMM)to N-methylmorpholine N-oxide(NMO),whichinturnreoxidizestheOsVI toOsVIII.Whiletheuseofhydrogenperoxideastheoxidantwithout theelectron-transfermediators(NMM,flavin)isinefficientandnonselective,various olefinswereoxidizedtodiolsingoodtoexcellentyieldsemployingthismildtriple catalyticsystem(Scheme1.3).
Scheme1.3 Osmium-catalyzeddihydroxylationof -methylstyrene usingH2O2
ByusingachiralSharplessligandhighenantioselectivitieswereobtained.Here, anincreaseintheadditiontimeforolefinandH2O2 canhaveapositiveeffectonthe enantioselectivity. 3 1.2EnvironmentallyFriendlyTerminalOxidants
4 1RecentDevelopmentsintheOsmium-catalyzedDihydroxylationofOlefins
Bäckvallandcoworkershaveshownthatothertertiaryaminescanassumetherole ofthe N-methylmorpholine.Theyreportedonthefirstexampleofanenantioselectivecatalyticredoxprocesswherethechiralligandhastwodifferentmodesofoperation:(1)toprovidestereocontrolintheadditionofthesubstrate,and(2)toberesponsibleforthereoxidationofthemetalthroughanoxidizedform[17].Theresultsobtainedwithhydroquinidine1,4-phthalazinediyldiether(DHQD)2PHALbothasan electron-transfermediatorandchiralligandintheosmium-catalyzeddihydroxylation arecomparabletothoseobtainedemployingNMMtogetherwith(DHQD)2PHAL. TheproposedcatalyticcycleforthereactionisdepictedinScheme1.4.
Theflavinisanefficientelectron-transfermediator,butratherunstable.Several transitionmetalcomplexes,forinstancevanadylacetylacetonate,canalsoactivatehydrogenperoxideandarecapableofreplacingtheflavininthedihydroxylationreaction[18].
MorerecentlyBäckvallandcoworkersdevelopedanovelandrobustsystemforosmium-catalyzedasymmetricdihydroxylationofolefinsbyH2O2 withmethyltrioxorhenium(MTO)astheelectrontransfermediator[19].Interestingly,hereMTOcatalyzesoxidationofthechiralligandtoitsmono-N-oxide,whichinturnreoxidizes OsVI toOsVIII.Thissystemgivesvicinaldiolsingoodyieldsandhighenantiomeric excessupto99%.
Scheme1.4 Catalyticcyclefortheenantioselectivedihydroxylation ofolefinsusing(DHQD)2PHALforoxygentransferandasasource ofchirality
1.2.2
Hypochlorite
Apartfromoxygenandhydrogenperoxide,bleachisthesimplestandcheapestoxidantthatcanbeusedinindustrywithoutproblems.Inthepastthisoxidanthasonly beenappliedinthepresenceofosmiumcomplexesintwopatentsintheearly1970s fortheoxidationoffattyacids[20].In2003thefirstgeneraldihydroxylationprocedureofvariousolefinsinthepresenceofsodiumhypochloriteasthereoxidantwas describedbyus[21].Using -methylstyreneasamodelcompound,100%conversion and98%yieldofthedesired1,2-diolwereobtained(Scheme1.5).
Interestingly,theyieldof2-phenyl-1,2-propanediolafter1hwassignificantly higherusinghypochloritecomparedwithliteratureprotocolsusingNMO(90%)[22] orK3[Fe(CN)6](90%)atthistemperature.Theturnoverfrequencywas242h–1, whichisareasonablelevel[23].UndertheconditionsshowninScheme1.5anenantioselectivityofonly77% ee isobtained,while94% ee isreportedusingK3[Fe(CN)6] asthereoxidant.Thelowerenantioselectivitycanbeexplainedbysomeinvolvement oftheso-calledsecondcatalyticcyclewiththeintermediateOsVI glycolatebeingoxidizedtoanOsVIII speciespriortohydrolysis(Scheme1.6)[24].
Nevertheless,theenantioselectivitywasimprovedbyapplyingahigherligandconcentration.Inthepresenceof5mol%(DHQD)2PHALagoodenantioselectivityof 91% ee isobservedfor -methylstyrene.Using tert-butylmethyletherastheorganic co-solventinsteadof tert-butanol,99%yieldand89% ee withonly1mol% (DHQD)2PHALarereportedforthesamesubstrate.Thisincreaseinenantioselectivitycanbeexplainedbyanincreaseintheconcentrationofthechiralligandintheorganicphase.Increasingthepolarityofthewaterphasebyusinga10%aqueous NaClsolutionshowedasimilarpositiveeffect.Table1.1showstheresultsofthe asymmetricdihydroxylationofvariousolefinswithNaOClastheterminaloxidant.
DespitetheslowhydrolysisofthecorrespondingstericallyhinderedOsVI glycolate, trans-5-decenereactedfastwithoutanyproblems.Thisresultisespeciallyinterestingsinceitisnecessarytoaddstoichiometricamountsofhydrolysisaidstothedihydroxylationofmostinternalolefinsinthepresenceofotheroxidants.
Withthisprotocolaveryfast,easytoperform,andcheapprocedurefortheasymmetricdihydroxylationispresented.
Scheme1.6 Thetwocatalyticcyclesintheasymmetricdihydroxylation
Tab.1.1 AsymmetricdihydroxylationofdifferentolefinsusingNaOClasterminaloxidant a
a Reactionconditions:2mmololefin,0.4mol%K2[OsO2(OH)4],5mol%(DHQD)2PHAL,10mLH2O, 10mL tBuOH,1.5equiv.NaOCl,2equiv.K2CO3,0 C.
Tab.1.1 (continued)
b 5mol%(DHQD)2PYRinsteadof(DHQD)2PHAL.
OxygenorAir
InthepastithasbeendemonstratedbyseveralgroupsthatinthepresenceofOsO4 andoxygenmainlynon-selectiveoxidationreactionstakeplace[25].However,in 1999Kriefetal.publishedareactionsystemconsistingofoxygen,catalyticamounts ofOsO4 andselenidesfortheasymmetricdihydroxylationof -methylstyreneunder irradiationwithvisiblelightinthepresenceofasensitizer(Scheme1.7)[26].Here, theselenidesareoxidizedtotheiroxidesbysingletoxygenandtheseleneoxidesare abletore-oxidizeosmium(VI) toosmium(VIII).Thereactionworkswithsimilar yieldsand ee valuestothoseoftheSharpless-AD.Potassiumcarbonateisalsoused, butonlyonetenthoftheamountpresentintheAD-mix.Aircanbeusedinsteadof pureoxygen.
Scheme1.7 Osmium-catalyzeddihydroxylationusing 1O2 and benzylphenylselenide
Thereactionwasextendedtoawiderangeofaromaticandaliphaticolefins[27].It wasshownthatbothyieldandenantioselectivityareinfluencedbythepHofthereactionmedium.Theprocedurewasalsoappliedtopracticalsynthesesofnaturalproductderivatives[28].ThisversionoftheADreactionnotonlyusesamoreecological co-oxidant,italsorequiresmuchlessmatter:87mgofmatter(catalyst,ligand,base,
1RecentDevelopmentsintheOsmium-catalyzedDihydroxylationofOlefins
reoxidant)arerequiredtooxidize1mmolofthesameolefininsteadof1400mg whentheAD-mixisused.
Alsoin1999therewasthefirstpublicationontheuseofmolecularoxygenwithoutanyadditivetoreoxidizeosmium(VI) toosmium(VIII).Wereportedthattheosmium-catalyzeddihydroxylationofaliphaticandaromaticolefinsproceedsefficiently inthepresenceofdioxygenunderambientconditions[29].AsshowninTable1.2 thenewdihydroxylationprocedureconstitutesasignificantadvancementcompared withotherreoxidationprocedures.Here,thedihydroxylationof -methylstyreneis comparedusingdifferentstoichiometricoxidants.Theyieldofthe1,2-diolremains goodtoverygood(87–96%),independentoftheoxidantused.Thebestenantioselectivities(94–96% ee )areobtainedwithhydroquinidine1,4-phthalazinediyldiether [(DHQD)2PHAL]astheligandat0–12 C(Table1.2,entries1and3).
Thedihydroxylationprocesswithoxygenisclearlythemostecologicallyfavorable procedure(Table1.2,entry5),whentheproductionofwastefromastoichiometric reoxidantisconsidered.WiththeuseofK3[Fe(CN)6]asoxidantapproximately8.1kg ofironsaltsperkgofproductareformed.However,inthecaseoftheKrief(Table1.2,entry3)andBäckvallprocedures(Table1.2,entry4)aswellasinthepresenceofNaOCl(Table1.2,entry6)somebyproductsalsoariseduetotheuseofcocatalystsandco-oxidants.Itshouldbenotedthatonlysaltsandbyproductsformed
Tab.1.2 Comparisonofthedihydroxylationof -methylstyreneinthepresenceofdifferentoxidants
EntryOxidantYieldReactionconditionseeTONWaste(oxidant)Ref. (%)(%)(kg/kgdiol)
1K3[Fe(CN)6]900 C94 a 4508.1 c [7b]
K2[OsO2(OH)4] tBuOH/H2O
2NMO900 C33b 2250.88d [22]
OsO4
acetone/H2O
3PhSeCH2Ph/O2 8912 C96a 2220.16e [26a]
PhSeCH2Ph/air87K2[OsO2(OH)4]93a 480.16e [26a] tBuOH/H2O
4NMM/flavin/H2O2 93RT–460.33f [16a]
OsO4 acetone/H2O
5O2 9650 C80a 192–[29]
K2[OsO2(OH)4] tBuOH/aq.buffer
6NaOCl990 C91a 2470.58g [21]
K2[OsO2(OH)4] tBuOH/H2O
a Ligand:Hydroquinidine1,4-phthalazinediyldiether. b Hydroquinidine p-chlorobenzoate.
c K4[Fe(CN)6]. d N-Methylmorpholine(NMM). e PhSe(O)CH2Ph. f NMO/flavin-OOH. g NaCl.
fromtheoxidanthavebeenincludedinthecalculation.Otherwasteproductshave notbeenconsidered.NeverthelessthecalculationspresentedinTable1.2givea roughestimationoftheenvironmentalimpactofthereaction.
Sincetheuseofpuremolecularoxygenonalargerscalemightleadtosafetyproblemsitisevenmoreadvantageoustouseairastheoxidizingagent.Hence,allcurrentbulkoxidationprocesses,e.g.,theoxidationofBTX(benzene, toluene, xylene) aromaticsoralkanestogivecarboxylicacids,andtheconversionofethyleneinto ethyleneoxide,useairandnotpureoxygenastheoxidant[30].InTable1.3theresultsofthedihydroxylationof -methylstyreneasamodelcompoundusingairas thestoichiometricoxidantareshownincontrasttothatwithpureoxygen(Scheme 1.8;Table1.3)[31].
Thedihydroxylationof -methylstyreneinthepresenceof1barofpureoxygenproceedssmoothly(Table1.3,entries1–2),withthebestresultsbeingobtainedat pH10.4.Inthepresenceof0.5mol%K2[OsO2(OH)4]/1.5mol%DABCOor1.5mol% (DHQD)2PHALatpH10.4and50 Ctotalconversionwasachievedafter16hor20h dependingontheligand.Whilethetotalyieldandselectivityofthereactionareexcellent(97%and96%,respectively),thetotalturnoverfrequencyofthecatalystiscomparativelylow(TOF=10–12h–1).Inthepresenceofthechiralcinchonaligand
Tab.1.3 Dihydroxylationof -methylstyrenewithair a
EntryPressureCat.LigandL/Os[L]TimeYieldSelectivityee (bar)c (mol%)(mmolL–1)(h)(%)(%)(%)
11(pureO2)0.5DABCOd 3:13.0169797–21(pureO2)0.5(DHQD)2PHALe 3:13.020969680
310.5DABCO3.13.0242485–410.5DABCO3.13.0685883–550.1DABCO3:10.6244193–690.1DABCO3:10.6247692–7200.5(DHQD)2PHAL3:13.017969682 8200.1(DHQD)2PHAL3:10.624959562 9200.1(DHQD)2PHAL15:13.024959583 10b 200.1(DHQD)2PHAL3:11.524949467 11b 200.1(DHQD)2PHAL6:13.024949478 12b 200.1(DHQD)2PHAL15:17.524609582
a Reactionconditions:K2[OsO2(OH)4],50 C,2mmololefin,25mLbuffersolution(pH10.4),10mL tBuOH. b 10mmol olefin,50mLbuffersolution(pH10.4),20mL tBuOH. c Theautoclavewaspurgedwithairandthenpressurizedtothe givenvalue. d 1,4-Diazabicyclo[2.2.2.]octane. e Hydroquinidine1,4-phthalazinediyldiether.
(DHQD)2PHALan ee of80%isobserved.Sharplessetal.reportedanenantioselectivityof94%forthedihydroxylationof -methylstyrenewith(DHQD)2PHALasthe ligandusingK3[Fe(CN)6]asthereoxidantat0 C[32].Studiesoftheceiling ee at50 C (88% ee )showthatthemaindifferenceintheenantioselectivitystemsfromthe higherreactiontemperature.Usingairinsteadofpureoxygengasgaveonly24%of thecorrespondingdiolafter24h(TOF=1h–1 ;Table1.3,entry3).Althoughthereactionisslow,itisimportanttonotethatthecatalyststaysactive,asshownbythefact that58%oftheproductisobtainedafter68h(Table1.3,entry4).Interestinglythe chemoselectivityofthedihydroxylationdoesnotsignificantlydecreaseafteraprolongedreactiontime.At5–20barairpressuretheturnoverfrequencyofthecatalyst isimproved(Table1.3,entries5–11).
Fullconversionofa -methylstyreneisachievedatanairpressureof20barinthe presenceof0.1mol%ofosmium,whichcorrespondstoaturnoverfrequencyof 40h–1 (Table1.3,entries8–11).Thus,byincreasingtheairpressureto20bar,it waspossibletoreducetheamountofosmiumcatalystbyafactorof5.Adecreaseof theosmiumcatalyst and theligandleadstoadecreaseintheenantioselectivityoffrom 82%to62% ee.Thisiseasilyexplainedbythefactthattheligandconcentrationdeterminesthestereoselectivityofthedihydroxylationreaction(Table1.3,entries7and9).
Whilethereactionathighersubstrateconcentration(10mmolinsteadof2mmol) proceedsonlysluggishlyat1barevenwithpureoxygen,fullconversionisachieved after24hat20barofair(Table1.3,entries10and11,andTable1.4,entries17and 18).Inallexperimentsperformedunderairpressurethechemoselectivityofthedihydroxylationremainedexcellent(92–96%).
Table1.4showstheresultsoftheosmium-catalyzeddihydroxylationofvariousolefinswithair.
AsdepictedinTable1.4allolefinsgavethecorrespondingdiolsinmoderateto goodyields(48–89%).Applyingstandardreactionconditions,thebestyieldsofdiols wereobtainedwith1-octene(97%),1-phenyl-1-cyclohexene(88%), trans-5-decene (85%),allylphenylether(77%)andstyrene(76%).Theenantioselectivitiesvaried from53to98% ee dependingonthesubstrate.Itisimportanttonotethatthechemoselectivityofthereactiondecreasesunderstandardconditionsinthefollowingsubstrateorder: -methylstyrene=1-octene>1-phenyl-1-cyclohexene> trans-5-decene> n-C6F13CH=CH2 >allylphenylether>styrene>> trans-stilbene.Acorrelationbetweenthechemoselectivityofthereactionandthesensitivityoftheproduceddioltowardsfurtheroxidationisevident,withthemainsidereactionbeingtheoxidativecleavageoftheC=Cdoublebond.Aromaticdiolswithbenzylichydrogenatomsareespeciallysensitivetothisoxidationreaction.Thus,thedihydroxylationof trans-stilbene gavenohydrobenzoininthebiphasicmixturewater/tert-butanolatpH10.4,50 C and20barairpressure(Table1.4,entry9).Insteadofdihydroxylationahighlyselectivecleavageofstilbenetogivebenzaldehyde(84–87%yield)wasobserved.Interestingly,changingthesolventtoisobutylmethylketone(Table1.4,entry12)makesit possibletoobtainhydrobenzoininhighyield(89%)andenantioselectivity(98%)at pH10.4.
Themechanismofthedihydroxylationreactionwithoxygenorairispresumedto besimilartothecatalyticcyclepresentedbySharplessetal.fortheosmium-cata-
Tab.1.4 Dihydroxylationofvariousolefinswithair a
EntryOlefinCat.LigandL/Os[L]TimeYieldSelectivityee (mol%)(mmolL–1)(h)(%) b (%) b (%)
10.5(DHQD)2PHAL3:13.024424287
2
4
0.5(DHQD)2PHAL3:13.016666686
30.5(DHQD)2PHAL3:13.014767687
0.5(DHQD)2PHAL3:13.024888889
50.5(DHQD)2PHAL3:13.024636367
6
0.5(DHQD)2PHAL3:13.018686868
70.5(DHQD)2PHAL3:13.014676766
80.5(DHQD)2PHAL3:13.09777768
90.5–––240(84)0(84)–10 c 1.0DABCO3:11.5244(77)5(87)–
a Reactionconditions:K2[OsO2(OH)4],50 C,2mmololefin,20barair,pH=10.4,25mLbuffersolution,10mL tBuOH; entries9–12:15mLbuffersolution,20mL tBuOH,entries17–18:50mLbuffersolution,20mL tBuOH. b Valuesinparenthesesareforbenzaldehyde. c 1mmololefin. d pH=12. e Isobutylmethylketoneinsteadof tBuOH. f 10mmololefin. g Hydroquinidine2,5-diphenyl-4,6-pyrimidinediyldiether.
lyzeddihydroxylationwithK3[Fe(CN)6]asthereoxidant(Scheme1.9).Theaddition oftheolefintoaligatedOsVIII speciesproceedsmainlyintheorganicphase.DependingonthehydrolyticstabilityoftheresultingOsVI glycolatecomplex,therate determiningstepofthereactioniseitherhydrolysisoftheOsVI glycolateorthereoxidationofOsVI hydroxyspecies.Theremustbeaminorinvolvementofasecondcatalyticcycle,assuggestedforthedihydroxylationwithNMO.Suchasecondcycle wouldleadtosignificantlylowerenantioselectivities,astheattackofasecondolefin moleculeontheOsVIII glycolatewouldoccurintheabsenceofthechiralligand.The observedenantioselectivitiesforthedihydroxylationwithairareonlyslightlylower thanthedatapreviouslypublishedbytheSharplessgroup,despitethehigherreactiontemperature(50 Cvs.0 C).ThereforethedirectoxidationoftheOsVI glycolate toanOsVIII glycolatedoesnotrepresentamajorreactionpathway. 11 1.2EnvironmentallyFriendlyTerminalOxidants
1.3
SupportedOsmiumCatalyst
Hazardoustoxicityandhighcostsarethechiefdrawbackstoreactionsusingosmiumtetroxide.Besidesthedevelopmentofprocedureswherecatalyticamountsof osmiumtetroxidearejoinedwithastoichiometricallyusedsecondaryoxidantcontinuouslyregeneratingthetetroxide,thesedisadvantagescanbeovercomebythe useofstableandnonvolatileadductsofosmiumtetroxidewithheterogeneoussupports[33].Theyoffertheadvantagesofeasyandsafehandling,simpleseparation fromthereactionmedium,andthepossibilitytoreusetheexpensivetransitionmetal.Unfortunately,problemswiththestabilityofthepolymersupportandleaching ofthemetalgenerallyoccur.
InthiscontextCainelliandcoworkershadalreadyreported,in1989,thepreparationofpolymer-supportedcatalysts:here,OsO4 wasimmobilizedonseveralamine typepolymers[34].SuchcatalystshavestructuresofthetypeOsO4 Lwiththe N-groupofthepolymer(=L)beingcoordinatedtotheLewisacidicosmiumcenter. Baseduponthisconcept,acatalyticenantioselectivedihydroxylationwasestablished byusingpolymerscontainingcinchonaalkaloidderivatives[35].However,sincethe amineligandscoordinatetoosmiumunderequilibriumconditions,recoveryofthe osmiumusingpolymersupportedligandswasdifficult.Os-diolatehydrolysisseems torequiredetachmentfromthepolymericligand,andhencecausesleaching.
HerrmannandcoworkersreportedonthepreparationofimmobilizedOsO4 on poly(4-vinylpyridine)anditsuseinthedihydroxylationofalkenesbymeansofhydrogenperoxide[36].However,theproblemsofgradualpolymerdecompositionand osmiumleachingwerenotsolved.
AnewstrategywaspublishedbyKobayashiandcoworkersin1998:theyusedmicroencapsulatedosmiumtetroxide.Herethemetalisimmobilizedontoapolymer onthebasisofphysicalenvelopmentbythepolymerandonelectroninteractions betweenthe -electronsofthebenzeneringsofthepolystyrenebasedpolymerand avacantorbitaloftheLewisacid[37].Usingcyclohexeneasamodelcompoundit wasshownthatthismicroencapsulatedosmiumtetroxide(MCOsO4)canbeused asacatalystinthedihydroxylation,withNMOasthestoichiometricoxidant (Scheme1.10).
Scheme1.10 Dihydroxylationofcyclohexeneusingmicroencapsulated osmiumtetroxide(MCOsO4)
IncontrasttoothertypicalOsO4-catalyzeddihydroxylations,whereH2O-tBuOHis usedasthesolventsystem,thebestyieldswereobtainedinH2O/acetone/CH3CN. WhilethereactionwassuccessfullycarriedoutusingNMO,moderateyieldswere obtainedusingtrimethylamine N-oxide,andmuchloweryieldswereobservedusing hydrogenperoxideorpotassiumferricyanide.Thecatalystwasrecoveredquantitativelybysimplefiltrationandreusedseveraltimes.Theactivityoftherecoveredcatalystdidnotdecreaseevenafterthefifthuse.
Astudyoftherateofconversionofthestartingmaterialshowedthatthereaction proceedsfasterusingOsO4 thanusingthemicroencapsulatedcatalyst.ThisisascribedtotheslowerreoxidationofthemicroencapsulatedosmiumesterwithNMO, comparedwithsimpleOsO4.
Subsequentlyacryronitrile/butadiene/polystyrenepolymerwasusedasasupport basedonthesamemicroencapsulationtechniqueandseveralolefins,including cyclicandacyclic,terminal,mono-,di-,tri-,andtetrasubstituted,gavethecorrespondingdiolsinhighyields[38].When(DHQD)2PHALasachiralsourcewas addedtothereactionmixtureenantioselectivitiesupto95% ee wereobtained. However,thisreactionrequiresslowadditionoftheolefin.Afterrunninga 100mmolexperiment,morethan95%oftheABS-MCOsO4 andthechiralligand wererecovered.
RecentlyKobayashiandcoworkersreportedonanewtypeofmicroencapsulated osmiumtetroxideusingphenoxyethoxymethyl-polystyreneasthesupport[39].With thiscatalyst,asymmetricdihydroxylationofolefinshasbeensuccessfullyperformed using(DHQD)2PHALasachiralligandandK3[Fe(CN)6]asacooxidantinH2O/acetone(Scheme1.11).
Scheme1.11 Asymmetricdihydroxylationofolefinsusing PEM-MCOsO4
Inthisinstancethedihydroxylationdoesnotrequireslowadditionoftheolefin, andthecatalystcanberecoveredquantitativelybysimplefiltrationandreusedwithoutlossofactivity.
Jacobsandcoworkerspublishedacompletelydifferenttypeofheterogeneousosmiumcatalyst.Theirapproachisbasedontwodetailsfromthemechanismofthe cisdihydroxylation:(1)tetrasubstitutedolefinsaresmoothlyosmylatedtoanosmate(VI) ester,buttheseestersarenothydrolyzedundermildconditions,and(2)anOsVI monodiolatecomplexcanbereoxidizedto cis-dioxoOsVIII withoutreleaseofthediol; subsequentadditionofasecondolefinresultsinanOsbisdiolatecomplex.These twopropertiesmakeitpossibletoimmobilizeacatalyticallyactiveosmiumcompoundbytheadditionofOsO4 toatetrasubstitutedolefinthatiscovalentlylinkedto asilicasupport.Thetetrasubstituteddiolateesterwhichisformedatonesideofthe Osatomisstable,andkeepsthecatalystfixedonthesupportmaterial.Thecatalytic reactioncantakeplaceatthefreecoordinationsitesofOs(Scheme1.12)[40].
Thedihydroxylationofmonosubstitutedanddisubstitutedaliphaticolefinsand cyclicolefinswassuccessfullyperformedusingthisheterogeneouscatalystand
Scheme1.12 ImmobilizationofOsinatertiarydiolatecomplex,and proposedcatalyticcyclefor cis-dihydroxylation
NMOasthecooxidant.Withrespecttotheolefin,0.25mol%Oswasneededandthe excellentchemoselectivityofthehomogeneousreactionwithNMOispreserved. However,somewhatincreasedreactiontimesarerequired.Thedevelopmentofan asymmetricvariantofthisprocessbyadditionofthetypicalchiralalkaloidligandsof theasymmetricdihydroxylationshouldbedifficultsincethereactionsperformed withtheseheterogeneouscatalystsaretakingplaceintheso-calledsecondcycle. Withalkaloidligandshigh ee valuesareonlyachievedindihydroxylationsoccurring inthefirstcycle.However,recentfindingsbythegroupsofSharplessandAdolfsson showthatevensecond-cycledihydroxylationsmaygivesubstantial ee results[41]. Althoughthisprocessmustbeoptimized,furtherdevelopmentoftheconceptofan enantioselectivesecond-cycleprocessoffersaperspectiveforafutureheterogeneous asymmetriccatalyst.
Choudaryandhisgroupreported,in2001,onthedesignofanion-exchangetechniqueforthedevelopmentofrecoverableandreusableosmiumcatalystsimmobilizedonlayereddoublehydroxides(LDH),modifiedsilica,andorganicresinfor asymmetricdihydroxylation[42].Anactivityprofileofthedihydroxylationof transstilbenewithvariousexchanger/OsO4 catalystsrevealedthatLDH/OsO4 displaysthe highestactivityandthattheheterogenizedcatalystsingeneralhavehigherreactivity thanK2[OsO2(OH)4].When trans-stilbenewasaddedtoamixtureofLDH/OsO4 , (DHQD)2PHALasthechiralligand(1mol%each),andNMOinH2O/tBuOH,the desireddiolisobtainedin96%yieldwith99% ee.Similarly,excellent ee resultsare obtainedwithresin/OsO4 andSiO2/OsO4 inthesamereaction.Alloftheprepared catalystsarerecoveredquantitativelybysimplefiltrationandreusedforfivecycles withconsistentactivity.Withthisprocedure,variousolefinsrangingfrommono-to trisubstitutedandfromactivatedtonon-activatedaretransformedintotheirdiols.In mostcases,thedesireddiolsareformedinhigheryields,albeitwithalmostsimilar ee valuesasreportedinhomogeneoussystems.Slowadditionoftheolefintothereactionmixtureiswarrantedtoachievehigher ee.ThisLDH/OsO4 systempresented byChoudaryandcoworkersissuperiorintermsofactivity,enantioselectivityand scopeofthereactionincomparisonwiththatofKobayashi.
AlthoughtheLDH/OsO4 showsexcellentactivitywithNMO,itisdeactivated whenK3[Fe(CN)6]ormolecularoxygenisusedastheco-oxidant[43].ThisdeactivationisattributedtothedisplacementofOsO42– bythecompetinganions,whichincludeferricyanide,ferrocyanide,andphosphateions(fromtheaqueousbuffersolution).Tosolvethisproblemresin/OsO4 andSiO2/OsO4 weredesignedandprepared bytheion-exchangeprocessonthequaternaryammonium-anchoredresinandsilica,respectively,astheseion-exchangersareexpectedtopreferbivalentanions ratherthantrivalentanions.Thesenewheterogeneouscatalystsshowconsistentperformanceinthedihydroxylationof -methylstyreneforanumberofrecyclesusing NMO,K3[Fe(CN)6]orO2 asreoxidant.Theresin/OsO4 catalyst,however,displays higheractivitythantheSiO2/OsO4 catalyst.InthepresenceofSharplessligandsvariousolefinswereoxidizedwithhighenantioselectivityusingtheseheterogeneous systems.Verygood ee resultswereobtainedwitheachofthethreeco-oxidants.Equimolarratiosofligandtoosmiumaresufficienttoachieveexcellent ee results.Thisis incontrasttothehomogeneousreactioninwhicha2–3molarexcessoftheexpen-
sivechiralligandtoosmiumisusuallyemployed.Thesestudiesindicatethatthe bindingabilityoftheseheterogeneousosmiumcatalystswiththechiralligandis greaterthanthehomogeneousanalogue.
Incidentally,thisformsthefirstreportofaheterogeneousosmium-catalyst mediatedADreactionofolefinsusingmolecularoxygenastheco-oxidant.Under identicalconditions,theturnovernumbersoftheheterogeneouscatalystarealmost similartothehomogeneoussystem.
Furthermore,Choudaryandcoworkerspresentedaprocedurefortheapplication ofaheterogeneouscatalyticsystemfortheADreactionincombinationwithhydrogenperoxideasco-oxidant[44].HereatriplecatalyticsystemcomposedofNMM andtwoheterogeneouscatalystswasdesigned.AtitaniumsilicaliteactsastheelectrontransfermediatortoperformoxidationofNMMthatisusedincatalytic amountswithhydrogenperoxidetoprovide insitu NMOcontinuouslyforADofolefins,whichiscatalyzedbyanotherheterogeneouscatalyst,silicagel-supportedcinchonaalkaloid[SGS-(DHQD)2PHAL]-OsO4 .Goodyieldswereobservedforvariousolefins.Againverygood ee resultshavebeenachievedwithanequimolarratioofligand toosmium,butslowadditionofolefinandH2O2 isnecessary.Unfortunately,recoveryandreuseoftheSGS-(DHQD)2PHAL-OsO4/TS-1revealedthatabout30%ofthe osmiumhadleachedduringthereaction.Thisamounthastobereplenishedineach additionalrun.
1.4
IonicLiquids
Recentlyionicliquidshavebecomepopularasnewsolventsinorganicsynthesis[45, 46].Theycandissolveawiderangeoforganometalliccompoundsandaremiscible withorganiccompounds.Theyarehighlypolarbutnon-coordinating.Ingeneral ionicliquidsexhibitexcellentchemicalandthermalstabilityalongwitheaseofreuse.Itispossibletovarytheirmiscibilitywithwaterandorganicsolventssimplyby changingthecounteranion.Advantageouslytheyhaveessentiallynegligiblevapor pressure.
In2002olefindihydroxylationbyrecoverableandreusableOsO4 inionicliquids waspublishedforthefirsttime[47].YanadaandcoworkersdescribedtheimmobilizationofOsO4 in1-ethyl-3-methylimidazoliumtetrafluoroborate[47a].Theychose 1,1-diphenylethyleneasamodelcompoundandfoundthattheuseof5mol%OsO4 in[emim]BF4 ,1.2equiv.ofNMO H2O,androomtemperaturewerethebestreactionconditionsforgoodyield.After18h100%ofthecorrespondingdiolwasobtained.OsO4-catalyzedreactionswithotherco-oxidantssuchashydrogenperoxide, sodiumpercarbonate,and tert-butylhydroperoxidegavepoorresults.WithanhydrousNMOonly6%diolwasfound.Afterthereactionthe1,2-diolcanbeextracted withethylacetateandtheionicliquidcontainingthecatalystcanbereusedfor furthercatalyticoxidationreaction.Itwasshownthateveninthefifthruntheobtainedyielddidnotchange.ThisnewmethodusingimmobilizedOsO4 inanionic liquidwasappliedtoseveralsubstrates,includingmono-,di-,andtrisubstitutedali-
phaticolefins,aswellastoaromaticolefins.Inallcases,thedesireddiolswereobtainedinhighyields.
ThegroupworkingwithYaodevelopedaslightlydifferentprocedure.Theyused [bmim]PF6 (bmim=1-n-butyl-3-methylimidazol)/water/tBuOH(1:1:2)asthesolventsystemandNMO(1.2equiv.)asthereoxidantfortheosmiumcatalyst[47b]. Here2mol%osmiumareneededforefficientdihydroxylationofvariousolefins. Afterthereaction,allvolatileswereremovedunderreducedpressureandtheproductwasextractedfromtheionicliquidlayerusingether.Theionicliquidlayercontainingthecatalystcanbeusedseveraltimeswithonlyaslightdropincatalystactivity.Inordertopreventosmiumleaching,1.2equiv.ofDMAPrelativetoOsO4 have tobeaddedtothereactionmixture.ThisamineformsstablecomplexeswithOsO4 , andthisstrongbindingtoapolaramineenhancesitspartitioninginthemorepolar ionicliquidlayer.Recently,SongandcoworkersreportedontheOs-catalyzeddihydroxylationusingNMOinmixturesofionicliquids(1-butyl-3-methylimidazolium hexafluorophosphateorhexafluoroantimonate)withacetone/H2O[48].Theyused 1,4-bis(9-O-quininyl)phthalazine[(QN)2PHAL]asthechiralligand.(QN)2PHALwill beconvertedintoanewligandbearinghighlypolarresidues(fourhydroxygroups inthe10,11-positionsofthequinineparts)duringADreactionsofolefins.Theuse of(QN)2PHALinsteadof(DHQD)2PHALaffordedthesameyieldsand ee results and,moreover,resultedindrasticimprovementinrecyclabilityofbothcatalytic components.InanotherrecentreportBrancoandcoworkersdescribedthe K2OsO2(OH)4/K3Fe(CN)6/(DHQD)2PHALor(DHQD)2PYRsystemfortheasymmetricdihydroxylationusingtwodifferentionicliquids[49].Bothofthesystems used,[bmim][PF6]/waterand[bmim][PF6]/water/tert-butanol(bmim=1-n-butyl-3methylimidazol),areeffectiveforaconsiderablenumberofruns(e.g.,run1,88%, ee 90%;run9,83%, ee 89%).Onlyafter11or12cycleswasasignificantdropinthe chemicalyieldandopticalpurityobserved.
Insummary,ithasbeendemonstratedthattheapplicationofanionicliquidprovidesasimpleapproachtotheimmobilizationofanosmiumcatalystforolefindihydroxylation.ItisimportanttonotethatthevolatilityandtoxicityofOsO4 aregreatly suppressedwhenionicliquidsareused.
References
[1] M.Beller,C.Bolm, TransitionMetals forOrganicSynthesis,Wiley-VCH,Weinheim, 1998
[2] Worldwideproductioncapacitiesfor ethyleneglycolin1995:9.7million tonsperannum;worldwideproduction of1,2-propyleneglycolin1994:1.1milliontonsperannum;K.Weissermel, H.J.Arpe, IndustrielleOrganischeChemie,5thedn.,Wiley-VCH,Weinheim, 1998,p.167andp.302.
[3] (a) H.H.Szmant, OrganicBuilding
BlocksoftheChemicalIndustry,Wiley, NewYork, 1989,p.347;(b) G.Pohl, H.Gaube in Ullmann’sEncyclopediaof IndustrialChemistry,Vol.A1,VCH, Weinheim, 1985,p.305.
[4] Reviews:(a) M.Schröder Chem.Rev 1980, 80,187;(b) H.C.Kolb,M.S. VanNieuwenhze,K.B.Sharpless, Chem.Rev. 1994, 94,2483;(c) M.Beller,K.B.Sharpless inB.Cornils, W.A.Herrmann(Eds.), AppliedHomogeneousCatalysis,VCH,Weinheim, 1996,
1RecentDevelopmentsintheOsmium-catalyzedDihydroxylationofOlefins
p.1009;(d) H.C.Kolb,K.B.Sharpless inM.Beller,C.Bolm(Eds.), TransitionMetalsforOrganicSynthesis Vol.2, VCH-Wiley,Weinheim, 1998,p.219; (e) I.E.Marko,J.S.Svendsen in E.N.Jacobsen,A.Pfaltz,H.Yamamoto (Eds.), ComprehensiveAsymmetricCatalysisII,Springer,Berlin, 1999,p.713.
[5] P.Dupau,R.Epple,A.A.Thomas, V.V.Fokin,K.B.Sharpless Adv. Synth.Catal. 2002, 344,421.
[6] G.M.Mehltretter,C.Döbler,U. Sundermeier,M.Beller Tetrahedron Lett. 2000, 41,8083.
[7] (a) S.G.Hentges,K.B.Sharpless J.Am.Chem.Soc 1980, 192,4263; (b) K.B.Sharpless,W.Amberg, Y.L.Bennani,G.A.Crispino,J.Hartung,K.-S.Jeong,H.-L.Kwong, K.Morikawa,Z.-M.Whang,D.Xu, X.-L.Zhang J.Org.Chem. 1992, 57, 2768.
[8] (a) Z.-M.Whang,H.C.Kolb, K.B.Sharpless J.Org.Chem. 1994, 59, 5104;(b) F.G.Fang,S.Xie,M.W.Lowery J.Org.Chem. 1994, 59,6142; (c) D.P.Curran,S.-B.Ko J Org. Chem. 1994, 59,6139;(d) E.J.Corey, A.Guzman-Perez,M.C.Noe J.Am. Chem.Soc 1994, 116, 12109;(e) E.J. Corey,A.Guzman-Perez,M.C.Noe J.Am.Chem.Soc 1995, 117,10805; (f) M.Nambu,J.D.White Chem. Commmun. 1996,1619;(g) E.J.Corey, M.C.Noe,A.Y.Ting TetrahedronLett. 1996, 37,1735;(h) G.Li,H.-T.Chang, K.B.Sharpless Angew.Chem.,Int.Ed. Engl. 1996, 35,451;(i) K.Mori,H.Takikawa,Y.Nishimura,H.Horikiri LiebigsAnn./Recueil 1997,327; (j) B.M.Trost,T.L.Calkins,C.G.Bochet Angew.Chem.,Int.Ed.Engl. 1997, 36,2632;(k) S.C.Sinha,A.Sinha, S.C.Sinha,E.Keinan J Am.Chem. Soc. 1998, 120,4017;(l) A.J.Fisher, F.Kerrigan Synth.Commun. 1998, 28, 2959;(m) J.M.Harris,M.D.Keranen,G.A.O’Doherty J.Org.Chem. 1999, 64,2982;(n) H.Takahata, M.Kubota,N.Ikota J.Org. Chem. 1999, 64,8594;(o) M.Quitschalle, M.Kalesse TetrahedronLett. 1999, 40, 7765;(p) F.J.Aladro,F.M.Guerra, F.J.Moreno-Dorado,J.M.Busta-
mante,Z.D.Jorge,G.M.Massanet TetrahedronLett. 2000, 41,3209; (q) J.Liang,E.D.Moher,R.E.Moore, D.W.Hoard J.Org.Chem. 2000, 65, 3143;(r) X.D.Zhou,F.Cai,W.S.Zhou TetrahedronLett. 2001, 42,2537; (s) D.P.G.Hamon,K.L.Tuck,H.S. Christie Tetrahedron 2001, 57,9499; (t) B.M.Choudary,N.S.Chowdari, K.Jyothi,N.S.Kumar,M.L.Kantam Chem.Commun. 2002,586;(u) P.Y. Hayes,W.Kitching J.Am.Chem.Soc. 2002, 124,9718;(v) S.Chandrasekhar, T.Ramachandar,M.V.Reddy Synthesis 2002,1867;(w) P.R.Andreana,J.S. McLellan,Y.C.Chen,P.G.Wang Org. Lett. 2002, 4,3875.
[9] K.A.Hofmann Chem.Ber. 1912, 45, 3329.
[10] (a) N.A.Milas,S.Sussmann J.Am. Chem.Soc. 1936, 58,1302;(b) N.A.Milas,J.-H.Trepagnier,J.T.Nolan,M.I. Iliopulos J.Am.Chem.Soc. 1959, 81, 4730.
[11] K.B.Sharpless,K.Akashi J.Am. Chem.Soc. 1976, 98,1986.
[12] (a) W.P.Schneider,A.V.McIntosh, (Upjohn)US-2.769.824(1956) Chem. Abstr 1957, 51,8822e;(b) V.VanRheenen,R.C.Kelly,D.Y.Cha Tetrahedron Lett. 1976, 17,1973;(c) R.Ray,D.S. Matteson TetrahedronLett 1980, 21, 449.
[13] M.Minamoto,K.Yamamoto,J.Tsuji J.Org.Chem. 1990, 55,766.
[14] M.P.Singh,H.S.Singh,A.K.Arya, A.K.Singh,A.K.Sisodia IndianJ. Chem. 1975, 13,112.
[15] L.Ahlgren,L.Sutin Org.ProcessRes. Dev 1997, 1,425.
[16] (a) K.Bergstad,S.Y.Jonsson,J.-E. Bäckvall J.Am.Chem.Soc. 1999, 121, 10424;(b) S.Y.Jonsson,K.Färnegardh,J.-E.Bäckvall J.Am.Chem. Soc. 2001, 123,1365.
[17] S.Y.Jonsson,H.Adolfsson,J.-E. Bäckvall Org.Lett. 2001, 3,3463.
[18] A.H.Ell,S.Y.Jonsson,A.Börje, H.Adolfsson,J.-E.Bäckvall TetrahedronLett. 2001, 42,2569.
[19] S.Y.Jonsson,H.Adolfsson,J.-E. Bäckvall Chem.Eur.J. 2003, 9,2783.
[20] (a) R.W.Cummins, FMC-Corporation NewYork,US-Patent3488394, 1970 ;
(b) R.W.Cummins, FMC-Corporation NewYork,US-Patent3846478, 1974
[21] G.M.Mehltretter,S.Bhor,M.Klawonn,C.Döbler,U.Sundermeier, M.Eckert,H.-C.Militzer,M.Beller Synthesis 2003, 2,295.
[22] E.N.Jacobsen,I.Marko,W.S.Mungall,G.Schröder,K.B.Sharpless J.Am.Chem Soc. 1988, 110,1968.
[23] M.Beller,A.Zapf,W.Mägerlein Chem.Eng.Technol. 2001, 24,575.
[24] J.S.M.Wai,I.Markó,J.S.Svendsen, M.G.Finn,E.N.Jacobsen,K.B. Sharpless J. Am.Chem.Soc. 1989, 111, 1123.
[25] (a) J.F.Cairns,H.L.Roberts,J.Chem. Soc.C 1968,640;(b) CelaneseCorp., GB-1,028,940(1966), Chem.Abstr. 1966, 65,3064f.;(c) R.S.Myers,R.C. Michaelson,R.G.Austin, (Exxon Corp.)US-4496779(1984), Chem.Abstr. 1984, 101,P191362k.
[26] (a) A.Krief,C.Colaux-Castillo TetrahedronLett. 1999, 40,4189;(b) A.Krief, C.Delmotte,C.Colaux-Castillo Pure Appl.Chem. 2000, 72,1709.
[27] (a) A.Krief,C.Colaux-Castillo, Synlett 2001,501; A.Krief,C.Colaux-Castillo PureAppl.Chem. 2002, 74,107.
[28] A.Krief,A.Destree,V.Durisotti, N.Moreau,C.Smal,C.Colaux-Castillo Chem Commun 2002,558.
[29] C.Döbler,G.Mehltretter,M.Beller Angew.Chem.,Int.Ed.Engl. 1999, 38, 3026;(b) C.Döbler,G.Mehltretter, U.Sundermeier,M.Beller J.Am. Chem.Soc. 2000, 122,10289–10297.
[30] K.Weissermel,H.-J.Arpe Industrielle OrganischeChemie,5thedn.,WileyVCH,Weinheim, 1998
[31] C.Döbler,G.Mehltretter,U.Sundermeier,M.Beller J.Organomet. Chem. 2001, 621,70.
[32] Y.L.Bennani,K.P.M.Vanhessche, K.B.Sharpless TetrahedronAsymmetry 1994, 5,1473.
[33] A.Severeyns,D.E.DeVos,P.A.Jacobs Top.Catal. 2002, 19,125.
[34] C.Cainelli,M.Contento,F.Manescalchi,L.Plessi Synthesis 1989,45.
[35] (a) B.M.Kim,K.B.Sharpless TetrahedronLett. 1990, 31,3003;(b) B.B.Lohray,A.Thomas,P.Chittari,J.R. Ahuja,P.K.Dhal TetrahedronLett.
1992, 33,5453;(c) H.Han,K.D.Janda J.Am.Chem.Soc. 1996, 118,7632; (d) D.J.Gravert,K.D.Janda Chem. Rev 1997, 97,489;(e) C.Bolm,A.Gerlach Angew.Chem.,Int.Ed. Engl. 1997, 36,741;(f) C.Bolm,A.Gerlach Eur. J.Org.Chem 1998,21;(g) P.Salvadori,D.Pini,A.Petri Synlett 1999, 1181;(h) A.Petri,D.Pini,S.Rapaccini,P.Salvadori Chirality 1999, 11, 745;(i) A.Mandoli,D.Pini,A.Agostini,P.Salvadori Tetrahedron:Asymmetry 2000, 11,4039;(j) C.Bolm, A.Maischak Synlett 2001,93;(k) I.Motorina,C.M.Crudden Org.Lett. 2001, 3,2325;(l) H.M.Lee,S.-W.Kim, T.Hyeon,B.M.Kim Tetrahedron:Asymmetry 2001, 12,1537;(m) Y.-Q.Kuang, S.-Y.Zhang,R.Jiang,L.-L.Wie TetrahedronLett. 2002, 43,3669.
[36] (a) W.A.Herrmann,G.Weichselbaumer EP0593425B1, 1994 ;(b) W.A. Herrmann,R.M.Kratzer,J.Blümel, H.B.Friedrich,R.W.Fischer,D.C. Apperley,J.Mink,O.Berkesi J.Mol. Catal.A 1997, 120,197.
[37] S.Nagayama,M.Endo,S.Kobayashi J.Org.Chem. 1998, 63,609.
[38] S.Kobayashi,M.Endo,S.Nagayama J.Am.Chem.Soc. 1999, 121,11229.
[39] S.Kobayashi,T.Ishida,R.Akiyama Org.Lett. 2001, 3,2649.
[40] A.Severeyns,D.E.DeVos,L.Fiermans,F.Verpoort,P.J.Grobet,P.A. Jacobs Angew.Chem. 2001, 113,606. [41] (a) M.A.Andersson,R.Epple,V.V.Fokin,K.B.Sharpless Angew.Chem. 2002, 114,490;(b) H.Adolfsson, F.Stalfors presentedatthe221st NationalAm.Chem.Soc.Meeting 2001
[42] B.M.Choudary,N.S.Chowdari, M.L.Kantam,K.V.Raghavan J.Am. Chem.Soc 2001, 123,9220.
[43] B.M.Choudary,N.S.Chowdari, K.Jyothi,M.L.Kantam J.Am.Chem. Soc. 2002, 124,5341.
[44] B.M.Choudary,N.S.Chowdari, K.Jyothi,S.Madhi,M.L.Kantam Adv.Synth.Catal 2002, 344,503.
[45] Reviews:(a) T.Welton Chem.Rev. 1999, 99,2071;(b) P.Wasserscheid, W.Keim Angew.Chem.,Int.Ed.Engl. 2000, 39,3772;(c) R.Sheldon Chem.
Commun 2001,2399;(d) S.T.Handy Chem.Eur.J 2003, 9,2938.
[46] (a) J.L.Reynolds,K.R.Erdner,P.B. Jones Org.Lett. 2002, 4,917;(b) I.A. Ansari,R.Gree Org.Lett. 2002, 4, 1507;(c) K.G.Mayo,E.H.Nearhoof, J.J.Kiddle Org.Lett. 2002, 4,567; (d) T.Fukuyama,M.Shinmen,S.Nishitani,M.Sato,I.Ryu Org.Lett. 2002, 4,1691;(e) D.Semeril,H.OlivierBourbigou,C.Bruneau,P.H.Dixneuf Chem.Commun 2002,146;
(f) C.S.Consorti,G.Ebeling,J.Dupont TetrahedronLett. 2002, 43 753; (g) S.J.Nara,J.R.Harjani,M.M.Salunkhe TetrahedronLett. 2002, 43, 2979.
[47] (a) R.Yanada,Y.Takemoto Tetrahedron Lett. 2002, 43,6849;(b) Q.Yao Org. Lett. 2002, 4,2197.
[48] C.E.Song,D.Jung,E.J.Roh,S.Lee, D.Y.Chi Chem.Commun 2002,3038.
[49] L.C.Branco,C.A.M.Afonso Chem. Commun 2002,3036.
