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Ab-CarboneliminationstrategyforconvenientinsituaccesstocyclopentadienylmetalcomplexesG.
Smits,aB.
Audic,aM.
D.
Wodrich,bC.
Corminboeuf*bandN.
Cramer*aTheelectronicandstericpropertiesoftailoredcyclopentadienyl(Cp)ligandsarepowerfulhandlestomodulatethecatalyticpropertiesoftheirmetalcomplexes.
Thisrequirestheindividualpreparation,puricationandstorageofeachligand/metalcombination.
Alternative,ideallyinsitu,complexationprotocolswouldbeofhighutility.
WediscloseanewapproachtoaccessCpmetalcomplexes.
Commonmetalprecursorsrapidlyreactwithcyclopentadienylcarbinolsviab-carboneliminationstodirectlygivetheCp-metalcomplexes.
Anadvantageofthisisthedirectandexibleuseofstorablepre-ligands.
NoauxiliarybaseisrequiredandtheCpcomplexescanbepreparedinsituinthereactionvesselforsubsequentcatalytictransformations.
IntroductionCyclopentadienyl(Cp)-coordinatedtransition-metalcomplexesareubiquitousandmanyofthemareecientcatalystsforabroadrangeofversatile,atom-economictransformations.
1Manyofthesereactionshavehighlyoptimizedconditions,butusecommerciallyavailablecomplexeswithaconservedCp*orCpligand.
Onlyrecently,themodulationoftheelectronicandstericpropertiesoftailoredCpligandswasrecognizedasapowerfultooltoovercomesluggishreactivity,2toaddressregio-andpositionalissues,3andprovideentrytoenantiose-lectiveprocesses.
4Inrapidreactiondiscoveryandoptimization,theabilitytocombinealibraryofligandswithalibraryofmetalcomplexes,performinganinsitucomplexationtogivethedesiredcatalystspeciesisarelevantadvantage.
Whilethisapproachisverycommonforreactionsinvolving,forinstance,phosphineligands,itisstillelusiveforCpligands.
Inthiscase,eachligand/metalcombinationhastobesynthesizedindivid-ually,puriedandstockedpriortoanyuseincatalysis.
ThetypicalcomplexationofcommonCp*andCpcomplexesinvolvesthemetalasalimitingreagent.
5However,highlyelaborateCporchiralCpxligandsrequiretheuseoftheCpHderivativeasthelimitingreactant.
InparticularforchiralCpxligands,onehastorelyonundesirablereactionconditions,forexampleinvolvingthalliumalkoxideinbenzene.
4d,6Theseshortcomingsmakethedevelopmentofcomplementarycomplexationstrategiesapriority.
Ideally,suchtechnologyproceedsrapidlyandinaquantitativemanner,withoutthegenerationofinhibitingreactionbyproducts.
Itshouldalsobeuseablewitharangeofdierenttransition-metals.
b-Carboneliminationhasbeenreportedasacomplementarymethodtoaccessorganometallicspeciesincatalysis.
7Normallytheforwardreaction–theadditionacrossacarbonylgroup–isfavoredandreversingthispathwayrequiressomeadditionaldrivingforce.
Thiscouldbeacombinationofusingtherighttransition-metalandtheuseofsubstratesleadingtotheformationofstrongerCsp–[M]8orCsp2–[M]bonds.
9Thegener-ationofCsp3–[M]bondsrequiresenergy-richstartingmaterials,suchastert-cyclobutanols10,releasingstrainuponb-Celimi-nation.
11Aparticularclassofsubstrateisthatofhomoallylicalcoholsthatcangivep-boundallyl-metalspeciesbyretro-allylation.
12Alongthesamelines,acyclopentadienylcarbinolwouldgiveaCp–[M]speciesuponC–Cbondcleavage.
Theverystrongbondofthecyclopentadienylaniontothetransition-metalwouldbeaverystrongdrivingforce.
Sofar,theb-Celiminationmethodologyhasbeenusedexclusivelyasanelementarystepincatalytictransformations.
HereinweexploititspotentialforthepreparationofCp-metalcomplexes(Scheme1).
ResultsanddiscussionTherequiredcyclopentadienylcarbinolsubstratesforthepre-ligandsareaccessedinastraightforwardmannerbydeproto-nationandadditionacrossthedesiredaldehydeorketone(Scheme2,seeESIfordetails).
13IncontrasttomanylightlysubstitutedcyclopentadieneswhichfrequentlyundergoDiels–Alderdimerization,allpreparedCp-carbinolsarestableanddonotdimerizeuponstorage.
Exemplarilyfor[Rh(cod)OH]2,ligandexchangeofthehydroxyligandby2wouldbreakuptheaLaboratoryofAsymmetricCatalysisandSynthesis,EPFLSBISICLCSA,BCH4305,CH-1015Lausanne,Switzerland.
E-mail:nicolai.
cramer@ep.
chbLaboratoryforComputationalMolecularDesign,EPFLSBISICLCMD,BCH5312,CH-1015Lausanne,Switzerland.
E-mail:clemence.
corminboeuf@ep.
chElectronicsupplementaryinformation(ESI)available:Experimentalproceduresandcharacterisationofallnewcompounds.
SeeDOI:10.
1039/c7sc02986aPresentaddress:LaboratoryofCNSActiveCompounds,LatvianInstituteofOrganicSynthesis,Aizkraukles21,LV-1006,Riga,Latvia.
Citethis:Chem.
Sci.
,2017,8,7174Received7thJuly2017Accepted24thAugust2017DOI:10.
1039/c7sc02986arsc.
li/chemical-science7174|Chem.
Sci.
,2017,8,7174–7179ThisjournalisTheRoyalSocietyofChemistry2017ChemicalScienceEDGEARTICLEdimerandreleaseamoleculeofwatertogiveintermediate3.
Inturn,3ispredisposedforthenalb-CeliminationsteptoyieldtheCpmetalcomplex4aandketone1.
Aselectionofcarbinolswithstericallyandelectronicallydierentsubstituents,R1andR2,wereinitiallyevaluated(Table1).
Avarietyoffactorsinuencedthereactionperformance.
AnaromaticsubstituentR1orR2wasfoundtobebenecialforyieldandreactivity.
Atertiaryhydroxylgroupisbetterthanasecondaryone.
Strainedketonederived2fand2g(entries6–7)partiallyunderwentthealternativestrain-releasepathway,10openingthefour-memberedringinsteadofleadingtotheCp-metalspecies.
Mostsubstratesrequiredtheadditionofcesiumcarbonateforshorterreactiontimes.
Withouttherhodiumcomplex,carbinols2arestableandnoconversiontofreeCp*Hwasobserved.
Notably,somesubstratesdisplayedsignicantlyincreasedreactivityanddidnotrequiretheaddi-tionofanybase(entries8,10,13,and15).
Thetwofastestreactionswere2h(Ph/CO2Me)and2m(Ph/Me).
Amongthetestedsubstrates,thedimethyl-typecarbinolmoietywasfurtherinvestigated,despiteitslowerreactivity,asitreleasesonlyvolatileacetoneasabyproduct(Table2).
Itprovidesacleancomplexationgivingarangeofcomplexes4withdierentbulks,rangingfromMe3Cp(entry1)totBuMe4Cp(entry5).
Whilesubstrateswithbulkygroupsreactverysmoothly,lesssubstitutedoneslikethetrimethylCpprecursor5breactlessecientlyduetosomefulvenebyproductforma-tion.
Indenylcomplexescanbepreparedwithsimilareciency(entries6–7).
Methylphenylglyoxylateandacetophenonederivedpre-ligands5iand5jareeasiertoaccessandbettersuitedinthecomplexationgiving,asbefore,fastandcleantransformationswithouttherequirementofabasicadditive(entries8–9).
Importantly,thiscomplexationstrategyisequallysuccessfulforchiralCp*ligandsequippedwithouratropchiralbackbone.
4f–iImportantly,theb-Celiminativecomplexationisnotlimitedtorhodium.
Forinstance,using[Ir(cod)OH]2asthemetalsaltprovidedaccesstoindenylIr(cod)andcyclo-pentadienylcomplexes6g–6jincomparableyields(entries6–9).
ExposureofthechiralCpx*pre-ligand5ktothereactionconditionsinducedasmoothcomplexation,providingrhodiumcomplex4kin93%yield(entry10).
Thisisparticularlynote-worthyasclassicalcomplexationmethods4d,fforadi-substitutedchiralCpxligandfailedcompletelyforthischiralpenta-substitutedanalog.
Analogously,thechiraliridiumcomplex6kcouldbeobtainedin87%yield.
Moreover,wecouldextendthemethodforthepreparationofachiralCpx*CoIIIcomplex7k(entry11).
Forthersttime,chiralfullypenta-substitutedcyclopentadienylrhodium,iridiumandcobaltcomplexesarenowaccessible.
Thissetsthestageforfutureapplicationsinasymmetriccatalysis,especiallyfortransformationswherepreviouslyreportedcomplexeswiththedisubstitutedchiralvariants4a,bfailedtoprovideadequatereactivity.
Besidesthehydroxylbridgeddimer,other[Rh(cod)X]2complexes(XOMe,OAc,andCl)canbeused(Table3,entriesScheme1Generationoforganometallicsusingtheb-carbonelimi-nationstrategy.
Scheme2(a)SynthesisofCppre-ligandcarbinols2.
(b)Envisionedb-CeliminationtoaccessCp*Rh(I)complexes.
Table1Evaluationoftheinuenceofthecarbinolsubstituentsonb-CeliminationaEntry2R1R2AdditiveTime%yieldb12aHnPentylCs2CO33h3922bHPhCs2CO31h9532cHCO2EtCs2CO33h1042dMeMeCs2CO33h9952eCH2(CH2)3CH2Cs2CO33h9362fCH2CH2CH2Cs2CO31h3072gCH2OCH2Cs2CO33h2082hCO2MePh—15min7592iCO2MeMeCs2CO33h75102jCO2MetBu—1h45112kCO2MeCF3Cs2CO31hKosslerandN.
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ThisjournalisTheRoyalSocietyofChemistry2017Chem.
Sci.
,2017,8,7174–7179|7179EdgeArticleChemicalScience
Smits,aB.
Audic,aM.
D.
Wodrich,bC.
Corminboeuf*bandN.
Cramer*aTheelectronicandstericpropertiesoftailoredcyclopentadienyl(Cp)ligandsarepowerfulhandlestomodulatethecatalyticpropertiesoftheirmetalcomplexes.
Thisrequirestheindividualpreparation,puricationandstorageofeachligand/metalcombination.
Alternative,ideallyinsitu,complexationprotocolswouldbeofhighutility.
WediscloseanewapproachtoaccessCpmetalcomplexes.
Commonmetalprecursorsrapidlyreactwithcyclopentadienylcarbinolsviab-carboneliminationstodirectlygivetheCp-metalcomplexes.
Anadvantageofthisisthedirectandexibleuseofstorablepre-ligands.
NoauxiliarybaseisrequiredandtheCpcomplexescanbepreparedinsituinthereactionvesselforsubsequentcatalytictransformations.
IntroductionCyclopentadienyl(Cp)-coordinatedtransition-metalcomplexesareubiquitousandmanyofthemareecientcatalystsforabroadrangeofversatile,atom-economictransformations.
1Manyofthesereactionshavehighlyoptimizedconditions,butusecommerciallyavailablecomplexeswithaconservedCp*orCpligand.
Onlyrecently,themodulationoftheelectronicandstericpropertiesoftailoredCpligandswasrecognizedasapowerfultooltoovercomesluggishreactivity,2toaddressregio-andpositionalissues,3andprovideentrytoenantiose-lectiveprocesses.
4Inrapidreactiondiscoveryandoptimization,theabilitytocombinealibraryofligandswithalibraryofmetalcomplexes,performinganinsitucomplexationtogivethedesiredcatalystspeciesisarelevantadvantage.
Whilethisapproachisverycommonforreactionsinvolving,forinstance,phosphineligands,itisstillelusiveforCpligands.
Inthiscase,eachligand/metalcombinationhastobesynthesizedindivid-ually,puriedandstockedpriortoanyuseincatalysis.
ThetypicalcomplexationofcommonCp*andCpcomplexesinvolvesthemetalasalimitingreagent.
5However,highlyelaborateCporchiralCpxligandsrequiretheuseoftheCpHderivativeasthelimitingreactant.
InparticularforchiralCpxligands,onehastorelyonundesirablereactionconditions,forexampleinvolvingthalliumalkoxideinbenzene.
4d,6Theseshortcomingsmakethedevelopmentofcomplementarycomplexationstrategiesapriority.
Ideally,suchtechnologyproceedsrapidlyandinaquantitativemanner,withoutthegenerationofinhibitingreactionbyproducts.
Itshouldalsobeuseablewitharangeofdierenttransition-metals.
b-Carboneliminationhasbeenreportedasacomplementarymethodtoaccessorganometallicspeciesincatalysis.
7Normallytheforwardreaction–theadditionacrossacarbonylgroup–isfavoredandreversingthispathwayrequiressomeadditionaldrivingforce.
Thiscouldbeacombinationofusingtherighttransition-metalandtheuseofsubstratesleadingtotheformationofstrongerCsp–[M]8orCsp2–[M]bonds.
9Thegener-ationofCsp3–[M]bondsrequiresenergy-richstartingmaterials,suchastert-cyclobutanols10,releasingstrainuponb-Celimi-nation.
11Aparticularclassofsubstrateisthatofhomoallylicalcoholsthatcangivep-boundallyl-metalspeciesbyretro-allylation.
12Alongthesamelines,acyclopentadienylcarbinolwouldgiveaCp–[M]speciesuponC–Cbondcleavage.
Theverystrongbondofthecyclopentadienylaniontothetransition-metalwouldbeaverystrongdrivingforce.
Sofar,theb-Celiminationmethodologyhasbeenusedexclusivelyasanelementarystepincatalytictransformations.
HereinweexploititspotentialforthepreparationofCp-metalcomplexes(Scheme1).
ResultsanddiscussionTherequiredcyclopentadienylcarbinolsubstratesforthepre-ligandsareaccessedinastraightforwardmannerbydeproto-nationandadditionacrossthedesiredaldehydeorketone(Scheme2,seeESIfordetails).
13IncontrasttomanylightlysubstitutedcyclopentadieneswhichfrequentlyundergoDiels–Alderdimerization,allpreparedCp-carbinolsarestableanddonotdimerizeuponstorage.
Exemplarilyfor[Rh(cod)OH]2,ligandexchangeofthehydroxyligandby2wouldbreakuptheaLaboratoryofAsymmetricCatalysisandSynthesis,EPFLSBISICLCSA,BCH4305,CH-1015Lausanne,Switzerland.
E-mail:nicolai.
cramer@ep.
chbLaboratoryforComputationalMolecularDesign,EPFLSBISICLCMD,BCH5312,CH-1015Lausanne,Switzerland.
E-mail:clemence.
corminboeuf@ep.
chElectronicsupplementaryinformation(ESI)available:Experimentalproceduresandcharacterisationofallnewcompounds.
SeeDOI:10.
1039/c7sc02986aPresentaddress:LaboratoryofCNSActiveCompounds,LatvianInstituteofOrganicSynthesis,Aizkraukles21,LV-1006,Riga,Latvia.
Citethis:Chem.
Sci.
,2017,8,7174Received7thJuly2017Accepted24thAugust2017DOI:10.
1039/c7sc02986arsc.
li/chemical-science7174|Chem.
Sci.
,2017,8,7174–7179ThisjournalisTheRoyalSocietyofChemistry2017ChemicalScienceEDGEARTICLEdimerandreleaseamoleculeofwatertogiveintermediate3.
Inturn,3ispredisposedforthenalb-CeliminationsteptoyieldtheCpmetalcomplex4aandketone1.
Aselectionofcarbinolswithstericallyandelectronicallydierentsubstituents,R1andR2,wereinitiallyevaluated(Table1).
Avarietyoffactorsinuencedthereactionperformance.
AnaromaticsubstituentR1orR2wasfoundtobebenecialforyieldandreactivity.
Atertiaryhydroxylgroupisbetterthanasecondaryone.
Strainedketonederived2fand2g(entries6–7)partiallyunderwentthealternativestrain-releasepathway,10openingthefour-memberedringinsteadofleadingtotheCp-metalspecies.
Mostsubstratesrequiredtheadditionofcesiumcarbonateforshorterreactiontimes.
Withouttherhodiumcomplex,carbinols2arestableandnoconversiontofreeCp*Hwasobserved.
Notably,somesubstratesdisplayedsignicantlyincreasedreactivityanddidnotrequiretheaddi-tionofanybase(entries8,10,13,and15).
Thetwofastestreactionswere2h(Ph/CO2Me)and2m(Ph/Me).
Amongthetestedsubstrates,thedimethyl-typecarbinolmoietywasfurtherinvestigated,despiteitslowerreactivity,asitreleasesonlyvolatileacetoneasabyproduct(Table2).
Itprovidesacleancomplexationgivingarangeofcomplexes4withdierentbulks,rangingfromMe3Cp(entry1)totBuMe4Cp(entry5).
Whilesubstrateswithbulkygroupsreactverysmoothly,lesssubstitutedoneslikethetrimethylCpprecursor5breactlessecientlyduetosomefulvenebyproductforma-tion.
Indenylcomplexescanbepreparedwithsimilareciency(entries6–7).
Methylphenylglyoxylateandacetophenonederivedpre-ligands5iand5jareeasiertoaccessandbettersuitedinthecomplexationgiving,asbefore,fastandcleantransformationswithouttherequirementofabasicadditive(entries8–9).
Importantly,thiscomplexationstrategyisequallysuccessfulforchiralCp*ligandsequippedwithouratropchiralbackbone.
4f–iImportantly,theb-Celiminativecomplexationisnotlimitedtorhodium.
Forinstance,using[Ir(cod)OH]2asthemetalsaltprovidedaccesstoindenylIr(cod)andcyclo-pentadienylcomplexes6g–6jincomparableyields(entries6–9).
ExposureofthechiralCpx*pre-ligand5ktothereactionconditionsinducedasmoothcomplexation,providingrhodiumcomplex4kin93%yield(entry10).
Thisisparticularlynote-worthyasclassicalcomplexationmethods4d,fforadi-substitutedchiralCpxligandfailedcompletelyforthischiralpenta-substitutedanalog.
Analogously,thechiraliridiumcomplex6kcouldbeobtainedin87%yield.
Moreover,wecouldextendthemethodforthepreparationofachiralCpx*CoIIIcomplex7k(entry11).
Forthersttime,chiralfullypenta-substitutedcyclopentadienylrhodium,iridiumandcobaltcomplexesarenowaccessible.
Thissetsthestageforfutureapplicationsinasymmetriccatalysis,especiallyfortransformationswherepreviouslyreportedcomplexeswiththedisubstitutedchiralvariants4a,bfailedtoprovideadequatereactivity.
Besidesthehydroxylbridgeddimer,other[Rh(cod)X]2complexes(XOMe,OAc,andCl)canbeused(Table3,entriesScheme1Generationoforganometallicsusingtheb-carbonelimi-nationstrategy.
Scheme2(a)SynthesisofCppre-ligandcarbinols2.
(b)Envisionedb-CeliminationtoaccessCp*Rh(I)complexes.
Table1Evaluationoftheinuenceofthecarbinolsubstituentsonb-CeliminationaEntry2R1R2AdditiveTime%yieldb12aHnPentylCs2CO33h3922bHPhCs2CO31h9532cHCO2EtCs2CO33h1042dMeMeCs2CO33h9952eCH2(CH2)3CH2Cs2CO33h9362fCH2CH2CH2Cs2CO31h3072gCH2OCH2Cs2CO33h2082hCO2MePh—15min7592iCO2MeMeCs2CO33h75102jCO2MetBu—1h45112kCO2MeCF3Cs2CO31hKosslerandN.
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