金属偶联反应

Cobalt-CatalyzedCross-CouplingReactions

Ge´rardCahiez*andAlbanMoyeux

DepartmentofChemistry(FRE3043),CNRS-Universite´deParis13,74RueMarcelCachin,F-93017Bobigny,France

ReceivedFebruary28,2009

Contents

1.Introduction

2.HomocouplingReactions

3.Csp2-Csp2Cross-couplingReactions3.1.Alkenylation

3.1.1.FromAromaticOrganomagnesium

Reagents

3.1.2.FromVinylicGrignardReagents

3.1.3.FromArylHalidesandAlkenylAcetates3.2.Aryl-ArylCross-coupling

3.2.1.FromAromaticOrganometallicReagents3.2.2.FromTwoAromaticHalides4.Csp2-Csp3Cross-couplingReactions4.1.Alkenylation

4.1.1.FromAliphaticOrganometallicReagents4.1.2.FromAliphaticHalides4.2.Arylation

4.2.1.FromAliphaticHalides4.2.2.FromAromaticHalides

4.3.AllylationofAromaticOrganometallics5.Alkynylation

5.1.PioneeringWorks

5.2.BenzylationofAcetylenicGrignardReagents5.3.AlkylationofAcetylenicGrignardReagents5.4.AlkenylationofAcetylenicGrignardReagents6.Csp3-Csp3Cross-coupling6.1.Allylation

6.1.1.AllylationofAliphaticOrganozinc

Compounds

6.1.2.AlkylationofAllylicGrignardReagents6.1.3.AllylationofTrimethylsilylmethylmagnesium

Chloride

6.1.4.AllylationofAllylicGrignardReagents6.2.Benzylation6.3.Alkylation7.Acylation

7.1.FromOrganometallicCompounds

7.2.Cobalt-MediatedAcylationofArylBromides8.ReductiveCyclizationandHeck-TypeReactions8.1.RadicalCyclization

8.1.1.IntramolecularRadicalCyclization

8.1.2.CyclizationviaanIntermolecularRadical

Addition

8.2.Cobalt-CatalyzedHeck-TypeReactions9.MiscellaneousReactions

9.1.Cobalt-MediatedMichaelAddition

*E-mail:gerard.cahiez@univ-paris13.fr.

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9.2.Allylationof1,3-DicarbonylCompounds9.3.ActivationofC-HBonds:C-NCoupling

Reactions

9.4.Three-ComponentReactions

9.4.1.Synthesisof󰀁-AcetamidoKetones9.4.2.SynthesisofHomoallylsilanes9.5.CouplingofAlkenesandAlkynes9.6.VariousReactions10.Conclusion

11.Acknowledgments12.References

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1.Introduction

SincethepioneeringworkofKharasch1onthemetal-catalyzedhomocouplingreactionofaromaticGrignardreagents(see2)inthemiddleofthe20thcentury,cobalt-catalyzedcarbon-carbonbond-formingreactionshavere-ceivedparticularattention.2Thescopeofthesereactionsisdifferentfromthatofthepalladium-andnickel-mediatedprocedures.Thus,cobalt-catalyzedcross-couplingreactionsareveryefficientfortheelaborationofCsp2-Csp2bonds(seesection2),buttheyareespeciallyinterestingforcouplingsinvolvingalkylhalides,sincethedecompositionby󰀁-hy-drogeneliminationofalkyl-cobaltintermediatesisnotalimitationasinthecaseofpalladiumornickelcatalyzedreactions(seesections3,4,and5).Cobalt-mediatedacyla-tions,radicalcyclizations,andHeck-typereactionshavealsobeendescribed(seesections6and7).Allthesereactionswillbediscussedhereafter.

ItisworthyofnotethatcobaltsaltshavealsobeenextensivelyusedascatalystsforPauson-Khand3andhydro-formylation4reactionsbutalsoforthecyclopropanation5ofolefinsandfor[2+2+2]6and[2+2]7cycloadditionreactions.Thesereactionswillnotbediscussedherein.

2.HomocouplingReactions

Thefirstreportsoncobalt-catalyzedcross-couplingreac-tionsdescribethehomocouplingreactionofGrignardreagents.In1939,GilmanandLichtenwalter8obtainednearlyquantitativeyieldsofhomocouplingproductbytreatingaromaticGrignardreagentswithastoichiometricamountofcobaltchloride.In1941,Kharasch1discoveredthatgoodyieldsofhomocouplingproductsareobtainedbyusingonlycatalyticamounts(i.e.,3mol%)ofcobaltchlorideandastoichiometricamountofanoxidantsuchasaromaticoraliphatichalides(Scheme1).TheroleofthislatteristooxidizetheCobalt(0)speciesintoacobalt(II)speciesafterthereductiveeliminationstep.

Twentyyearslater,in1962,MorizurappliedtheKharaschreactiontothesynthesisofvariousbiaryls.9aTheyieldsare

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Ge´rardCahiezreceivedhisPh.D.in1973,attheUniversityPierre&MarieCurie(ParisVI),underthesupervisionofProfessorJeanFranc¸oisNormant,onthecarbocuprationofterminalalkynes(vinylcopperreagents).Then,hejoinedtheCNRS.AfterapostdoctoralyearintheRousselUclafLaboratories(nowSanofiAventis)onthechemistryofsteroid,hecamebacktotheUniversityPierreandMarieCurie,andin1980,hebecameDirectorofResearchattheCNRS.Then,hemovedtotheEcoleSupe´rieuredeChimieOrganiqueetMine´rale(ESCOM,Cergy-Pontoise)in1993.From1993to2008hewasDirectorofResearchattheCNRSandProfessorofChemistryatESCOM.From2000to2009,hewasalsoDirectoroftheUMR8123,ajointresearchunitCNRS-UniversityofCergyPontoise-ESCOM.In2009,hemovedtotheUniversityofParis13,asDirectorofResearchattheCNRS,tocreateanewresearchgroupinorganometallicchemistry.Theresearchdevelopedsince1973dealtwiththeuseoforganometallicreagentsinorganicsynthesisandespeciallywiththedevelopmentofthechemistryoforganomanganesereagents.Hiscurrentinterestisalwaysfocusedonorganomanganesechemistrybutmoregenerallyonthesearchfornewhighlyselectiveorganometallicreactions,i.e.,Mn-,Co-,andFe-catalyzedcross-couplingreactions,involvingnotoxicandexpensivemetaloradditive.

AlbanMoyeuxwasborninLaBasse´e(France)in1982.HeobtainedhisB.Sc.DegreefromtheUniversityofLensin2004.HeobtainedhisM.Sc.DegreefromtheUniversityofCergy-Pontoisein2005.Hethenworkedoniron-andcobalt-catalyzedcross-couplingreactionsunderthesupervi-sionofProfessorGe´rardCahiezandwasawardedhisPh.D.DegreeinNovember2008.HefinallyjoinedthegroupofProfessorAloisFu¨rstnerasapostdoctoralfellowattheMax-PlanckInstituteofMu¨lheim(Germany),whereheiscurrentlyworkingontotalsynthesis.

significantlylowerthanthosepreviouslyreportedbyKhara-sch.Almostidenticalresultswereobtainedbyusingbutylbromide(Scheme2)orbromobenzene(Table1)asanoxidant.

Itshouldbeunderlinedthataryllithiumandarylmagnesiumcompoundsgivesimilaryields9b(Table1).

Interestingly,benzylicorhomobenzylicGrignardreagentscanalsobeused(Table1,entries6-10).

CahiezandMoyeux

Scheme1.TheKharaschReaction

Scheme2.Cobalt-CatalyzedHomocouplingReactionofAromaticGrignardReagents

3.Csp2-Csp2Cross-couplingReactions3.1.Alkenylation

3.1.1.FromAromaticOrganomagnesiumReagents

In1943,Kharaschdescribedthefirstcobalt-catalyzedalkenylationofaromaticGrignardreagents10(Table2).Theveryreactivevinylor1-propenylhalidesgivemoder-ateyields(Table2,entries1-3)whereasstericallyhinderedormoresubstitutedalkenylhalidesonlyaffordverypoorresults.LargeamountsofhomocouplingproductsarethenobtainedfrombothalkenylhalideandarylGrignardreagent(Table2,entries4-6).Thesyntheticinterestofthisreactionislimited,since,eveninthebestcases,2equivofalkenylhalidesareused.

In1982,Uemurareportedthecross-couplingreactionbetweenarylGrignardreagentsandalkenyltelluridesundercobaltcatalysis11(Scheme3).

Yieldsarequantitative,butalargeexcessofGrignardreagentisused(2.5equiv).Itshouldbeunderlinedthatasubstantialamountofhomocouplingproduct(biaryl)isformedasasideproduct.Thisisacleardrawback,sincethepurificationofthefinalproductisoftentricky.

In1998,Cahiezreinvestigatedthecobalt-catalyzedalk-enylationofaromaticGrignardreagents.12Aclearimprove-mentwasobservedbyusingaTHF/NMPmixtureasasolvent(Table3).Thus,variousalkenylhalideshavebeencoupledstereospecificallyingoodtoexcellentyields(Table3,entries1-5).

Veryrecently,Hayashi13describedthecobalt-catalyzedcouplingbetweenalkenyltriflatesandarylGrignardreagents.

Cobalt-CatalyzedCross-CouplingReactions

Table1.Cobalt-CatalyzedHomocouplingReactionofAryllithiumandArylmagnesiumHalides

Table2.Cobalt-CatalyzedAlkenylationofAromaticGrignardReagents

Thecouplingtakesplaceundermildconditions(Scheme4).ThepresenceofNMPisnotnecessary,butPPh3isusedasaligand.

Thismethodcanbecomplementarytotheprecedentmethodusingalkenylbromides,sincevinyltriflatescanbeeasilypreparedfromthecorrespondingketones.

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Scheme3.Cross-couplingbetweenAlkenylTelluridesandAromaticGrignardReagents

Table3.Cobalt-CatalyzedAlkenylationofAromaticGrignardReagentsinthePresenceofNMP

Scheme4.Cobalt-CatalyzedCross-couplingbetweenAlkenylTriflatesandAromaticGrignardReagents

3.1.2.FromVinylicGrignardReagents

Untilnow,therehaveonlybeenveryfewcobalt-catalyzedalkenyl-alkenylcross-couplingreactions.In1998,Cahiez12describedoneexampleofcobalt-catalyzedcross-couplingbetweenanalkenylhalideandanalkenylGrignardreagent(Scheme5)inmoderateyield.Thereactionisstereospecificandgivesonlythepuretrans-diene.

Recently,Hayashi13describedthecobalt-catalyzedcross-couplingbetweenalkenyltriflatesandalkenylGrignardreagents.GoodyieldsareobtainedfromR-monosubstituted

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Scheme5.Cobalt-CatalyzedCouplingbetween

StyrylmagnesiumBromideand2-Methyl-1-bromopropane

Scheme6.Cobalt-CatalyzedCouplingbetweenAlkenylmagnesiumHalidesandAlkenylTriflates

Table4.Cobalt-CatalyzedCouplingbetweenFunctionalizedArylHalidesandAlkenylAcetates

orR,󰀁-and󰀁,󰀁-disubstitutedalkenylGrignardreagents(Scheme6).

3.1.3.FromArylHalidesandAlkenylAcetates

In2005,Pe´richonreportedthecobalt-catalyzedcouplingbetweenfunctionalizedarylhalidesandalkenylacetates.Itistheonlyexampleofacobalt-catalyzedarylationofenolesters.Thereactionisperformedbyusingacatalyticamountofcobaltbromide(5%)inthepresenceof10equivofmanganese(Table4).14

Moderatetogoodyieldsareobtained.Asarule,arylchloridesorbromidesgivesimilarresults(Table4,entries

CahiezandMoyeux

Scheme7.Cross-couplingbetweenChloropyridinesandIsopropenylAcetate

Scheme8.MechanismProposalfortheCobalt-CatalyzedAlkenylationofArylHalidesbyVinylAcetates

1-6).However,insomecases,arylchloridesleadtobetteryields(2-and4-halobenzonitriles,entries7-8and13-14).Aryliodidessuchas4-iodoanisoleandethyl4-iodoben-zoatearenotsuitablesubstrates,sincetheyleadessentiallytoamixtureofreducedandhomocouplingproducts.

Thereactionwasextendedtohalopyridines(Scheme7).Amechanisticpathwaywasproposed(Scheme8).Inthefirststep,CoIIisreducedbymanganesepowderinthepresenceofpyridineeitherinCoIorinCo0.Inaprecedentwork,Pe´richondemonstratedthatCoIisstabilizedbyvinylicacetateinanacetonitrile/pyridinemixture.15Thelow-valentcobaltspeciesformedisprobablycoordinatedbythevinylacetate.Itthenundergoesanoxidativeadditionofthearylhalidetogivean“arylcobalt”species.Thisspeciescouldthenreactwithvinylicacetateaccordingtoasix-memberedtransitionstate[A].Afterreductiveelimination,cobaltisreducedbytheexcessofMn0toregeneratetheactivecobaltspecies.

3.2.Aryl-ArylCross-coupling

3.2.1.FromAromaticOrganometallicReagents

In1983,Uemuradescribedthecross-couplingbetweendiaryltellurides(Ar2Te)andaromaticorganomagnesiumreagentsinthepresenceofcobaltsalts11(Scheme9).

Thereactionaffordspooryields,andasubstantialamountofhomocouplingproductaccruingfromtheGrignardreagentisproduced.Thepurificationofthefinalproductisthusoftendelicate.In2003,KnochelandCahiezreportedthecross-couplingbetweenheteroarylchloridesandaryl-orheteroarylmagne-siumhalides16(Scheme10).

Cobalt-CatalyzedCross-CouplingReactions

Scheme9.Cross-couplingbetweenDiaryltelluridesandAromaticGrignardReagents

Scheme10.CoCl2-CatalyzedCross-couplingbetweenHeteroarylChloridesandAromaticorHeteroaromaticGrignardReagents

Scheme11.CobaltorIronPowder-MediatedAryl-ArylCouplingReactions

Yieldsareexcellentinmostcases.Noteworthy,theinfluenceofsterichindranceisnotveryimportant.Thus,thereactionbetweenmesitylmagnesiumchlorideand1-chlor-oisoquinolineleadsto85%ofcouplingproduct.TwoequivalentsofGrignardreagentarerequired.

Interestingly,cobaltorironpowdersaresuitablecatalystsforthereaction16(Scheme11).

Oneyearlater,Oshima16breportedaverysimilarreaction.Thecouplingisachievedindioxaneinthepresenceofcobalt(II)acetylacetonatewhereasdiethyletherandcobalt(II)chloridewereusedinthepreviousmethod16a(Scheme10).Yieldsarecomparable,but3equivofphenylmagnesiumbromideareusedinsteadof2andtheamountofcatalystistwicehigher.

Theaboveexampleswereperformedbyusingheteroaromaticoractivedaromatichalides.Veryrecently,Nakamura17de-scribedacobalt-catalyzedcross-couplingreactionfromnonactivatedarylchloridesorheteroarylbromidesandaromaticGrignardreagents(Scheme12).Thereactionisperformedinthepresenceofcobalt(II)fluorideandNHCligand.

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Scheme12.Cobalt-CatalyzedCross-couplingbetweenNonactivatedArylHalidesorHeteroarylBromidesandAromaticGrignardReagents

Scheme13.Cobalt-CatalyzedCross-couplingbetweenFunctionalizedArylcyanocupratesandortho-Bromoorortho-ChloroAromaticKetones,Esters,orAldehydes

Then,Knochelhasshownthatthechemoselectivecouplingbetweenpolyfunctionalaryl-orheteroarylcyanocupratesArCu(CN)MgClandortho-bromoorortho-chloroaromaticketones,esters,oraldehydescanbeperformedundercobaltcatalysis.18Numerouspolyfunctionalbiarylsweresuccess-fullysynthesizedaccordingtothismethodology.SelectedexamplesarepresentedinScheme13.

Toreactthestartinghalidecompletely,itisessentialtoperformthecouplinginthepresenceofBu4NI(1equiv)and4-fluorostyrene(20mol%)byusingalargeexcessofcyanoarylcuprate(3equiv).19TheuseofaTHF/DME/DMPUmixtureinsteadofTHFasasolventclearlyacceleratesthereaction(15mininsteadof21h).

1440ChemicalReviews,2010,Vol.110,No.3Scheme14.Cobalt-CatalyzedReactionofp-Anisylcyanocuprateswitho-,m-,andp-Bromobenzophenones

Scheme15.Cobalt-CatalyzedCross-couplingbetweenHeteroarylcyanocupratesandActivatedArylBromides

Scheme16.Cobalt-CatalyzedCross-couplingbetweenActivatedArylFluoridesorTosylatesandArylcyanocuprates

Theyieldsaresignificantlylowerwhentheactivatingcarbonylgroupisinthemetaortheparaposition(Scheme14).

Heteroarylcyanocupratescanalsobeusedsuccessfully(Scheme15).

Thereactionhasbeenextendedtoortho-fluoroaromaticketones,esters,oraldehydes20(Scheme16).Thecorrespond-ingtosylatesalsoreactsuccessfully.Thisisworthyofnote,sincetheuseofarylfluorides21andtosylates22inmetal-catalyzedcross-couplingreactionsisnotusual.Asarule,goodtoexcellentyieldsareobtained.

Withpentafluorobenzophenone,thetwofluorineatomsintheortho-positionscanbeselectivelysubstituted(Scheme17).However,yieldsaremoderateand6equivofarylcy-anocupratearerequired.

3.2.2.FromTwoAromaticHalides

Veryrecently,Gosminireportedthesynthesisofunsym-metricalbiarylcompoundsbycouplingtwoaromatichalidesArXandAr′Xundercobaltcatalysis23(Scheme18).Theuseof2equivofthemorereactivearylhalideisgenerallyrequiredtoobtainsatisfactoryyieldsofcross-couplingproducts.

Similarresultswereobtainedfromaryliodides,bromides,orchlorides(Table5).Interestingly,aryltriflatesalsoreact

CahiezandMoyeux

Scheme17.Cobalt-CatalyzedCouplingReactionbetweenArylcyanocupratesandPentafluorobenzophenone

Scheme18.SynthesisofUnsymmetricalBiarylCompoundsbyCouplingTwoArylHalidesunderCobaltCatalysis

Table5.Cobalt-CatalyzedCouplingofTwoArylHalides:InfluenceoftheNatureoftheArylHalides

a

Isolatedyield.bYieldsinparenthesesrefertoGCyield.

Scheme19.Cobalt-CatalyzedCouplingReactionbetween󰀁-BromostyreneandMethylmagnesiumIodide

Scheme20.InfluenceofNMPontheCobalt-CatalyzedAlkenylationofGrignardReagents

successfully(Table5,entry7).Itshouldbeunderlinedthatasignificantamountofhomocouplingproductisproducedduringthereaction.Thisisapracticaldrawback,sincetheexpectedproductcouldthenbeverytrickytoisolate.

4.Csp2-Csp3Cross-couplingReactions4.1.Alkenylation

4.1.1.FromAliphaticOrganometallicReagents

In1945,Kharasch24showedthatthecouplingbetweenmethylmagnesiumiodideand󰀁-bromostyrenetakesplaceingoodyieldinthepresenceofcobaltchloride(Scheme19).

Cobalt-CatalyzedCross-CouplingReactions

Scheme21.Cobalt-CatalyzedAlkenylationofGrignardReagents:StereospecificityoftheReaction

Table6.Cobalt-CatalyzedCouplingbetweenAlkenylHalidesandAliphaticGrignardReagents

However,themethodislimitedtotheveryreactive󰀁-bromostyrene,and2equivofGrignardreagentarenecessary.

In1998,CahiezdiscoveredthattheuseofN-methylpyr-rolidone(NMP)asacosolventallowsadramaticimprove-ment(Scheme20;seealsosection3.1).Theyieldsareclearlybetter,andonly1.1equivofGrignardreagentisrequired.12Alkenyliodides,bromides,andchloridesgivegoodyieldsofcross-couplingproduct(Scheme21).

Itisimportanttonotethatthereactionisstereospecific(Scheme21).

Theprocedurehasbeenappliedtothesynthesisofvariousolefins(Table6).

Goodyieldsareobtainedbycouplingprimary(Table6,entries1-3)orsecondary(entries4-5)aliphaticGrignardreagentswithvariousalkenylbromides.ItisnoteworthythattertiaryaliphaticGrignardreagentsgivepoorresults(entry6).

Thereactionishighlychemoselective.Thus,thepresenceofanesterorevenaketogroupistolerated(Scheme22).Finally,itshouldbeunderlinedthat(E)-1,2-dichloroeth-ylenereactstogiveonlythecorresponding(E)-alkenylchlorides.Theformationoftheproductresultingfromasubstitutionofbothchlorineatomsisnotobserved(Scheme23).Thisresultisinsharpcontrastwiththatobtainedunder

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Scheme22.ChemoselectiveCobalt-CatalyzedAlkenylationofGrignardReagents

Scheme23.Synthesisof(E)-1-AlkenylChloridesfrom(E)-Dichloroethylene

Scheme24.Cobalt-CatalyzedAlkenylationofFunctionalizedOrganozincHalides

Scheme25.Cobalt-CatalyzedAlkenylationofBenzylzincBromide

ironcatalysis,since,inthiscase,thedoublesubstitutionproductisformedexclusively.25

CahiezandKnochelthenextendedthereactiontoalkylzinchalides.26Thesereagentsarelessreactivethanthecorre-spondingorganomagnesiumhalides,andthecouplingtakesplaceslowlyatahighertemperature(55°C,4-8h).Inaddition,20-30mol%ofcatalystandalargeexcessoforganozinchalide(3equiv)arenecessary.AswithGrignardreagents,12thereactionischemoselectiveandstereospecific.Goodyieldsofcross-couplingproductareobtained(Scheme24).

Itisworthyofnotethatbenzylicorganozinchalidescanbecoupledwithalkenyliodides(Scheme25).

ThecouplingofvarioussilylatedGrignardreagentswith1,2-dihalogenoethyleneswasalsoreported(Table7).27

Excellentyieldsareobtainedfromtrimethylsilylmethyl-magnesiumchloride(Table7,entries1and3)orphenyldim-ethylsilylmethylmagnesiumchloride(Table7,entry2).AspreviouslyreportedbyCahiez,12theformationofthedisubstitutedproductswasnotobserved.Itisnoteworthythatstereodifferentiationbetween(E)-and(Z)-1,2-dihaloethylenesispossible.Thus,byreactingamixtureof(E)-and(Z)-1,2-dibromoethylenes(5equiv)withtrimethylsilylmethylmag-nesiumchloride,theonlyproductisthe(E)-1-bromoalkene(Table7,entry3).

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Table7.Cobalt-CatalyzedSelectiveCouplingbetween1,2-DihalogenoethylenesandPhMe2SiCH2MgClorMe3SiCH2MgCl

a

5equivof1,2-dibromoethylenewasused.

Table8.Cobalt-CatalyzedCross-couplingbetweenAlkenylHalidesandSilylatedGrignardReagents

a

Thereactionwasperformedat35°C.

Scheme26.Cobalt-CatalyzedAlkylationofStyrylmagnesiumBromide

Later,thecross-couplingbetweenalkenylhalidesanddimethylalkylsilylmethylmagnesium28chlorideswasreportedbyOshima.Goodtoexcellentyieldsareobtained(Table8).

R-or󰀁-monosubstitutedaswellas󰀁,󰀁-bisubstitutedalkenyliodides,bromides,orchloridesaffordgoodyields(entries1-6).Ontheotherhand,R,󰀁-bisubstitutedalkenylhalidesonlyleadtopoorresults(entry7).

4.1.2.FromAliphaticHalides

In2006,Oshima29describedthereactionbetweenoctylbromideandstyrylmagnesiumbromideinthepresenceof5mol%cobaltchlorideandN,N,N′,N′-tetramethyl-1,2-cyclo-hexanediamineasaligand(Scheme26).

Thesameyear,hereportedthecobalt-catalyzedcross-couplingbetweenvariousalkylhalidesand1-(trimethylsil-yl)ethenylmagnesiumbromidebyusingTMEDAasasol-

CahiezandMoyeux

Table9.Cross-couplingbetweenPrimaryorSecondaryAlkylHalidesand1-(Trimethylsilyl)ethenylmagnesiumBromide

a

Thetransisomerwasformedexclusively.

Table10.TandemRadicalCyclizationandCross-couplingReaction

a

Diastereomericratio.bTheproductwasisolatedasalactoneafteroxidationofthecyclicacetal.

vent.30Goodtoexcellentyieldsareobtainedfromeitherprimaryorsecondaryalkylhalides,butalargeexcessoforganometallicreagent(4equiv)hastobeusedandthereactionseemslimitedtothisspecificGrignardreagent(Table9).

Fromε-unsaturatedalkyliodides1,acyclicproductisformedviaaradicalcyclizationfollowedbyacross-couplingreaction(Table10).ThemechanismproposedbyOshimaisdepictedinScheme27.Thus,variousheterocycliccom-poundshavebeensynthesizedingoodtoexcellentyields.

4.2.Arylation

4.2.1.FromAliphaticHalides

Thefirstexampleofcobalt-catalyzedarylationofaCsp3centerwasdescribedin1969.Inthisreport,Hey31notedthattheyieldofthereactionbetween2-bromopyridineandmethylmagnesiumiodideisclearlyimprovedinthepresenceofcatalyticamountsofcobalt(II)chloride(Scheme28).

Cobalt-CatalyzedCross-CouplingReactions

Scheme27.MechanismforTandemRadicalCyclizationandCross-couplingReaction

Scheme28.FirstCobalt-CatalyzedArylationofAliphaticGrignardReagents

However,thereactionisnotgeneralandverypooryieldsareobtainedinmostothercases.

In2001,Oshima32describedthecobalt-catalyzedtandemradicalcyclizationandarylationreactionfromethylenichaloacetal2(Table11).Thismethodologyhasbeenappliedtothesynthesisofheterocyclic(entries1-7)andcarbocyclic(entry8)compounds.Itshouldbenotedthattheyieldclearlydependsonboththepositionandthenumberofsubstituentsonthesubstrate(entries4and6).ThemechanismproposedinthiscaseissimilartotheonedescribedinScheme27.Itisnoteworthythatwhenthedoublebondofthestartingproductisnotterminal(haloacetals3and4,Scheme29)thearylationproductisnotobtained.Inthiscase,thereactionleadstoaHeck-typeproduct(seesection9).

In2006,thisreactionwassuccessfullyappliedtothesynthesisofoxasilacyclopentanes(Scheme30).33

Thesecompoundscaneasilybeconvertedinto4-aryl-1,3-diolsbyTamao-Flemingoxidation.34ThisapproachhasbeenusedinasynthesisofanantagonistofhumanCCR5receptor(Scheme31).

Thesameyear,Oshimadescribedthecross-couplingbetweenprimaryalkylhalidesandaromaticGrignardreagentsinthepresenceofCoCl2(dppp).35SelectedexamplesarepresentedinTable12.ItmustbeunderlinedthatalargeexcessofGrignardreagentisrequired(3equiv).Asarule,moderateyieldsareobtainedfromprimaryalkylbromideswhereasthecorrespondingiodidesgivelowyields(entries1and2).Moreover,secondarycyclicalkylbromidessuchascyclohexylbromideleadtopoorresults(entry4)andalkylchloridesdonotreact.

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Table11.TandemRadicalCyclizationandCross-couplingReactionfromUnsaturatedHaloacetal2

a

Isolatedyield.Thediastereomericratioisgiveninparentheses.

Scheme29.Heck-TypeReactionfromUnsaturatedHaloacetals3and4

Scheme30.SynthesisofOxasilacyclopentanes

Heteroaromaticorganomagnesiumcompoundscanbeusedsuccessfully(entry5).Thereactionissensitivetosterichindrance;thus,ortho-substitutedarylGrignardreagentsdonotaffordtheexpectedcross-couplingproduct(entry6).Interestingly,somefunctionalgroupssuchasanesteroranacetalaretolerated(entries7and8).

Oshimahasfinallyshown29thatbetterresultsareobtainedbyusingadiamine(N,N,N′,N′-tetramethyl-1,2-cyclohex-anediamine)asaligandinsteadofdppp(Table13).Moreover,onlyaslightexcess(1.2equiv)ofGrignard

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Scheme31.SynthesisofanAntagonistofHumanCCR5Receptor

Table12.Cobalt-CatalyzedCross-couplingbetweenPrimaryAlkylHalidesandAromaticGrignardReagents

a

Only3-chloropropylbenzeneisformed.

reagentisthenused.Withthiscatalyticsystem,theyieldsaresignificantlyhigherfromprimaryalkyliodidesthanfromthecorrespondingbromides(entries3and4).Inaddition,secondarycyclicandacyclicalkylhalides(entries1and2)canalsobeusedsuccessfully.Finally,thereactionislesssensitivetosterichindrance;asanexample,ortho-tolylmag-nesiumbromideleadstoanexcellentyield(entry7).

Thereactionishighlychemoselective,andvarioushalo-genoesterswerecoupledsuccessfully(entries9-11).ThismethodologyhasbeenappliedtothesynthesisofAH13205,asyntheticprostaglandinusedasanEP2-receptoragonistthatlowersintraocularpressure(Scheme32).36

Thereactionoffive-andsix-memberedcyclichaloacetals5havingastereogeniccenterintheRpositionoftheC-Brbondhasbeenexamined(Scheme33).Thestereoselectivityofthisreactionishighlydependentonthesizeofthering.Thus,goodenantioselectivitywasonlyobtainedwiththefive-memberedcyclicacetal.

Veryrecently,Cahiez37describedthecobalt-catalyzedcross-couplingbetweenprimaryorsecondaryalkylbromides

CahiezandMoyeux

Table13.Cobalt-CatalyzedCross-couplingbetweenPrimaryorSecondaryAlkylHalidesandAromaticGrignardReagents

Scheme32.SynthesisofAH13205

andaromaticorganomagnesiumcompounds.Acheapandsimplecatalyticsystem,cobalt(III)acetylacetonate/TMEDA(1:1),isused.Thisisveryinteresting,sincecommercialTMEDAisaverysimpleandinexpensivestartingmaterialcomparedtotheN,N,N′,N′-tetramethyl-1,2-cyclohexanedi-aminepreviouslyemployedbyOshima.29

Itispossibletocouplesecondaryalkylbromideswithexcellentyields,whereas,undertheconditionsdescribedbyOshima,29alkylbromidesleadtoclearlyloweryieldsthanalkyliodides.Thispointisveryinteresting,sincealkyl

Cobalt-CatalyzedCross-CouplingReactions

Scheme33.Cobalt-CatalyzedStereoselectiveArylationofCyclicHaloacetals5

Table14.Cobalt-CatalyzedCross-couplingbetweenAliphaticBromidesandAromaticGrignardReagents

bromidesarecheaperandmorestablethanthecorrespondingiodides.ResultsaresummarizedinTable14.

Asarule,excellentyieldsareobtainedfromprimaryandsecondaryalkylGrignardreagents(entries1-8).Ontheotherhand,tertiaryalkylbromidescannotbeused(entry9).Itshouldbeunderlinedthatonlyaslightexcess(1.1equiv)ofGrignardreagentwasused.Alkyliodidesandbromidesleadtosimilarresults(entries3-4)whereasalkylchloridesortosylatesonlyaffordpooryields(entry5).

Thereactionisnotverysensitivetosterichindrance,anditispossibletocoupleanortho-substitutedaromaticGrignardreagentwithasecondaryalkylbromideinhighyields(entry8).

Thereactionishighlychemoselective(Table15).Thus,1-bromo-5-chloropentanecouplesselectivelytogivethecorrespondingalkylchlorideingoodyield(entry1).Inaddition,thepresenceofvarioussensitivegroupssuchasanacetate(entry2),anester(entry3),anamide(entry4),orevenaketone(entry5)istolerated.

Thereactionisalsohighlychemoselectiveinthecaseofsecondaryalkylbromides(Scheme34).Itisthefirstreportofametal-catalyzedcross-couplingbetweenanaromaticGrignardreagentandafunctionalizedsecondaryalkylbromide.

ChemicalReviews,2010,Vol.110,No.31445

Table15.Cross-couplingofArylGrignardReagentswithFunctionalizedPrimaryAlkylBromides

Scheme34.Cobalt-CatalyzedChemoselectiveArylationofFunctionalizedSecondaryAlkylBromides

Scheme35.Cobalt-CatalyzedCross-couplingbetweenArylBromidesandAlkylGrignardReagents

4.2.2.FromAromaticHalides

In2008,Oshima38describedthecobalt-catalyzedcouplingbetweenarylbromidesandaliphaticGrignardreagents(Scheme35).Itistheonlyreportinthisfield.Thisreactionisaninterestingalternativetotherelatedpalladiumornickelprocedures.

Yieldsaregenerallygoodtoexcellent;however,thisreactioncanonlybeappliedtoprimaryalkylGrignardreagents.

1446ChemicalReviews,2010,Vol.110,No.3

Scheme36.Cobalt-CatalyzedCouplingbetweenAllylAcetateandFunctionalizedArylBromides

Scheme37.Cobalt-CatalyzedCouplingbetweenAllylAcetateandFunctionalizedArylChlorides

4.3.AllylationofAromaticOrganometallics

In2003,Pe´richonreportedthecouplingofarylhalideswithallylacetatesinthepresenceofacatalyticamountofcobaltbromide;zincisusedasareductant.39

Moderateyieldswereobtainedfromvariousarylbromides(Scheme36).Itshouldbenotedthatthepresenceofanesteroranitrileistolerated.Thismethodiscomplementarytothecobalt-catalyzedelectrochemicalmethodspreviouslyreported.40

Thereactioncanalsobeachievedwitharylchlorides(Scheme37),butthereactionconditionsareverydifferent.Itisnecessarytoincreasetheamountofcobaltbromide(40%insteadof20%),zincdusthastobereplacedbymanganesedustasareductant,andthepresenceofpyridineaswellasastoichiometricamountofironbromideisrequired.More-over,thecouplingisperformedatahighertemperature(50°C),andthereactiontimeislonger(24h).However,yieldsaregenerallybetterthanthosepreviouslyobtainedfromthecorrespondingarylbromides.

In2004,Oshima41describedthereactionbetweenallylicethersandphenylmagnesiumbromideinthepresenceofcobaltsalts.Goodyieldswereobtained,butthereactionisnotregioselective,exceptedwith1-and3-phenyl-2-propenylmethylethers(Scheme38).

Undersimilarconditions,allylicacetalsleadtothemonosubstitutionproduct(Scheme39).

5.Alkynylation5.1.PioneeringWorks

In1945,Kharasch42

reportedthecouplingbetweenme-thylmagnesiumbromideand2-bromo-1-phenylacetylenein

CahiezandMoyeux

Scheme38.Cobalt-CatalyzedCross-couplingbetween1-and3-Phenyl-2-propenylMethylEthersandPhenylmagnesiumBromide

Scheme39.Cobalt-CatalyzedArylationofAllylicAcetals

Scheme40.Cobalt-CatalyzedCouplingbetween

2-Bromo-1-phenylacetyleneandMethylmagnesiumBromide

Scheme41.Cobalt-CatalyzedCouplingbetween

PhenylethynylmagnesiumBromideandPhenylethynylIodide

Table16.Cobalt-CatalyzedAlkynylationofVariousGrignardReagents

thepresenceofcobaltchloride(Scheme40).Itisthefirstexampleofcobalt-catalyzedalkynylationofGrignardreagents.Afewyearslater,thecobalt-catalyzedcouplingbetweenphenylethynylmagnesiumbromide43andphenylethynyliodidewasreported(Scheme41).Asatisfactoryyieldofdiphe-nylbutadiynewasobtained.

In19,severalexamplesofcouplingbetweenaliphatic,aromatic,oracetylenicGrignardreagentsand1-haloalkyneswerereportedbyWeedon.44Asarule,onlypooryieldswereobtained(Table16).

5.2.BenzylationofAcetylenicGrignardReagents

In2006,OshimareportedthebenzylationofacetylenicGrignardreagentsundercobaltcatalysis.45Moderatetoexcellentyieldsareobtained(Table17).

Cobalt-CatalyzedCross-CouplingReactions

Table17.Cobalt-CatalyzedBenzylationofAlkynylmagnesiumHalides

Scheme42.PreparationofDiynesfrom

1,2-Bis(bromomethyl)-or1,3-Bis(chloromethyl)benzene

Scheme43.Cobalt-CatalyzedCross-couplingbetweenPrimaryorSecondaryAlkylBromidesand1-TrimethylsilylethynylmagnesiumBromide

Trimethylsilylethynylmagnesiumbromidereactswithben-zylicchloridesorbromidestogivegoodyieldsofcross-couplingproducts(entries1-4,R)Me3Si).Ontheotherhand,onlybenzylicbromidesleadtogoodresultsfrom1-hexynylmagnesiumbromide(entry1,R)n-Bu).

With1,2-bis(bromomethyl)-or1,3-bis(chloromethyl)ben-zene,thedoublesubstitutionispossibleandthereactionaffordsdiynes(Scheme42).

5.3.AlkylationofAcetylenicGrignardReagents

Recently,Oshimareportedthereactionbetweenalkylhalidesandtrimethylsilylethynylmagnesiumbromideinthepresenceofcobaltchloride.29Satisfactoryyieldsareobtainedfromprimaryorsecondaryalkylhalides(Scheme43),butalargeexcessofGrignardreagentisused.

ε-Haloalkenes6reactwithtrimethylsilylethynylmagnesiumbromideaccordingtoatandemradicalcyclizationandcross-

ChemicalReviews,2010,Vol.110,No.31447

Table18.Cobalt-CatalyzedTandemRadicalCyclizationandAlkynylationReaction

a

Diastereomericratio.bTheproductwasisolatedasalactoneafteroxidationofthecyclicacetal.

Scheme44.Cobalt-CatalyzedTandemRadicalCyclizationandAlkynylationReaction

couplingreaction.Variousheterocycliccompoundshavethusbeensynthesizedinmoderatetogoodyields(Table18).Oshima46showedthatsimpleacetylenicGrignardreagentscanalsobeused(Scheme44).

However,itisimportanttonotethat5equivofGrignardreagenthavetobeusedinthiscase.

5.4.AlkenylationofAcetylenicGrignardReagents

Thecobalt-catalyzedcouplingbetweenalkynylmagnesiumhalidesandalkenyltriflateswasdescribedbyHayashiin2007.47Goodtoexcellentyieldsareobtainedfromawiderangeofsubstrates(Scheme45).

Someexamplesofchemoselectivecouplingsinthepres-enceofanalkenyl,analkyl,oranarylbromidearedescribed(Table19).Itisnoteworthythatnoadditionalligandwasusedinthiscase.

1448ChemicalReviews,2010,Vol.110,No.3

Scheme45.Cobalt-CatalyzedCouplingbetweenAlkenylTriflatesandAlkynylGrignardReagents

Table19.ChemoselectiveCobalt-CatalyzedAlkenylationofAcetylenicGrignardReagents

Scheme46.Cobalt-CatalyzedAllylationofDiorganozincCompounds

6.Csp3-Csp3Cross-coupling6.1.Allylation

6.1.1.AllylationofAliphaticOrganozincCompounds

In1996,Knochel48describedtheallylationoforganozinccompounds(R2ZnorRZnX)undercobaltcatalysis(Scheme46).

ItshouldbepointedoutthatbothRgroupsfromRaretransferred.Moreover,onlytheSNsubstitution2Znproductsareobtained.Dialkylzincsandalkylzinc2halidesgivesimilaryields,butthelatterrequirelongerreactiontimes(about5hat-10°C).

Allylicphosphatescanalsobeusedsuccessfully(Scheme47).

6.1.2.AlkylationofAllylicGrignardReagents

ThecouplingbetweenunactivatedalkylhalidesandallylicGrignardreagentsinthepresenceofcobaltchloride

CahiezandMoyeux

Scheme47.Cobalt-CatalyzedCouplingbetweenDiethylCinnamylPhosphateandDipentylzinc

Table20.Cobalt-CatalyzedCouplingbetweenUnactivatedAlkylHalidesandAllylmagnesiumChloride

a

trans/cis)86:14.bdppewasusedasaligand.

Scheme48.Cobalt-CatalyzedAlkylationofCrotyl-andPrenylmagnesiumChlorides

anddpppwasdescribedbyOshimain2002(Table20).49Goodyieldsareobtainedundermildconditionsfromprimary(entry1),secondary(entries2and3),ortertiary(entries4and5)alkylhalides.Benzylichalidescanalsobeused(entry6).

WithsubstitutedallylicGrignardreagents,theregiose-lectivityisverydependentonthestructureoftheallylicmoiety.Amixtureofproductsresultingfromanalkylationonthelessandthemoresubstitutedsidesoftheallylicsystemisgenerallyobtained(Scheme48).

Atandemradicalcyclizationandcross-couplingreactionwasusedtopreparevariouscycliccompoundsingoodtoexcellentyields(Scheme49).Forthemechanismofthereaction,seeScheme27.

Unexpectedly,byusingetherasasolvent,acyclopropanederivativeisformed(Scheme50).

Thefollowingmechanismisproposed.Atfirst,cobaltchlorideisconvertedtoanactivereducedspecies[Con]byactionoftheGrignardreagent.Afterasingleelectrontransfertothehalide(formationofaradicalanion)andthencleavageoftheC-Ibond,aradicalisobtained.Sequential5-exoradicalcyclizationandtrappingoftheresultingradicalbythecatalyticcobaltspeciesthenaffordthecobaltintermediate10.Indiethylether,10undergoes

Cobalt-CatalyzedCross-CouplingReactions

Scheme49.Cobalt-CatalyzedTandemRadicalCyclizationandAlkylationofAllylicGrignardReagents

Scheme50.MechanismoftheCobalt-CatalyzedReactionofδ-IodoallylicAcetal9withAllylmagnesiumChloride

Scheme51.Cobalt-CatalyzedReactionofδ-IodoallylicAcetal9withCrotyl-andPrenylmagnesiumChlorides

arapideliminationofBuO-Metal(Metal)CoorMg)togivethecyclopropanederivative11.Suchaneliminationprobablyinvolvesthecomplexationofthemetalliccenterof10(CoorMg)withtheoxygenatomofthebutoxygroup(inTHF,thiscomplexationismuchmoredifficultandtheeliminationdoesnotoccur).Theresultingalde-hyde11thenreactswithallylmagnesiumchloridetoaffordthealcohol12.

Thereactionwasextendedtoprenyl-orcrotylmagnesiumchlorides(Scheme51).

6.1.3.AllylationofTrimethylsilylmethylmagnesiumChloride

Thecouplingbetweenallylicethersandtrimethylsilylm-ethylmagnesiumchlorideundercobaltcatalysiswasreportedin2004.41Aswithphenylmagnesiumbromide(seesection4.3),goodyieldsandselectivityareobtainedfrom1-and3-phenyl-2-propenylmethylethers(Scheme52).However,thereactionisgenerallynotregioselectivewiththeotherallylicethers.

Itisnoteworthythat,fromallylicacetals13(Scheme53),itispossibletosubstituteoneortwomethoxygroupsbyusingrespectively1.5or3equivofGrignardreagent.

ChemicalReviews,2010,Vol.110,No.31449

Scheme52.Cobalt-CatalyzedCross-couplingbetween1-and3-Phenyl-2-propenylMethylEthersandTrimethylsilylmethylmagnesiumChloride

Scheme53.Cobalt-CatalyzedCouplingofAllylicAcetalswithTrimethylsilylmethylmagnesiumChloride

Scheme.Cobalt-CatalyzedCouplingbetween

AllylmagnesiumBromideandCinnamylMethylEther

Scheme55.Cobalt-CatalyzedBenzylationof4-AcetoxybutylzincIodide

6.1.4.AllylationofAllylicGrignardReagents

Acobalt-catalyzedcouplingreactionbetweenallylicGrignardreagentsandallylicethershasbeendescribedbyOshima(Scheme).41

Goodyieldsareobtained.Interestingly,theregioselectivityofthereactioncanbeeasilycontrolledbychangingthenatureofthecatalyst.Thus,onlythelinearsubstitutionproduct14isformedinthepresenceofuncomplexedcobaltchloridewhereasthebranchedsubstitutionproduct15isthemainisomerwhenthecomplexCoCl2•dpppisused.

6.2.Benzylation

Anexampleofcobalt-catalyzedbenzylationofanorga-nozinccompound(Scheme55)wasreportedbyKnochelin1996.48

In2004,OshimaachievedthecouplingbetweensecondaryalkylhalidesandbenzylicGrignardreagentsinthepresenceofCoCl2•dppp.Asarule,yieldsaremoderate(Table21).49Tertiaryalkyliodidesalsoreactbutleadtopooryields(entry7).

1450ChemicalReviews,2010,Vol.110,No.3

Table21.Cobalt-CatalyzedAlkylationofBenzylicGrignardReagents

a

3equivofGrignardreagentwasused.

Table22.Cobalt-CatalyzedTandemRadicalCyclizationandAlkylationReactionofVariousδ-IodoEthylenicCompoundswithRMe2SiCH2MgCl

a

Diastereomericratio.

6.3.Alkylation

In2007,Oshimareportedthetandemradicalcyclizationandcross-couplingreactionofvariousδ-halogenoethyleniccompoundswithRMe-Heterocycliccarbenes2SiCHare2MgClundercobaltcatalysis.46Nemployedasligands.Asarule,goodyieldsareobtainedundermildconditions.SelectedexamplesareshowninTable22.

Veryrecently,Cahiezdescribedthecross-couplingbe-tweenfunctionalizedalkylhalidesandaliphaticGrignard

CahiezandMoyeux

Table23.Cobalt-CatalyzedAlkyl-AlkylCouplingReaction:EffectofIodideAnionontheCourseoftheReaction

EntryCatalyticsystem(5mol%)Yield(%)1CoCltraces2CoCl2

3532;4TMEDA

CoCl274CoCl2•2LiCl;4TMEDA5

2•2LiBr;4TMEDA30CoCl2•2LiI;4TMEDA

79

Table24.Cobalt-CatalyzedCouplingbetweenUnactivatedAlkylHalidesandAliphaticGrignardReagents

a

Amixtureofdecaneand1-decenewasmainlyobtained.

reagentsinthepresenceoftheatecomplexCoClaligand.

2•2LiI.50TMEDAisusedasByusingcobaltchlorideasacatalyst,onlytracesofcross-couplingproductareobtained(Table23,entry1).InthepresenceofTMEDA,theyieldisbetterbutremainsunsatisfactory(entry2).Finally,thebestresultsareobtainedbyaddingbothTMEDAandlithiumiodide(entry5).Itisthefirstexampleofsuchabeneficialinfluenceofiodideaniononthecourseofacobalt-catalyzedcross-couplingreaction.

Variousalkyl-alkylcouplingswereperformedundertheseconditions(Table24).Acyclicorcyclicsecondaryalkylbromides(entries1-8)gavemoderatetogoodyieldsofcross-couplingproduct.Itisimportanttonotethattheinfluenceofstericeffectsisdeterminant.Thus,2-bromopen-tanegaveabetteryieldthan3-bromopentane(entries5and6).Similarly,theresultsobtainedwithvarious2-bromoal-kanesclearlydependonthelengthofthealkylchain(entries1,3,and7).Itispossibletousealkyliodidesinplaceofbromides,butthecorrespondingchloridesdonotreact(entries2-4).

Thereactionhasbeensuccessfullyextendedtoprimaryalkylbromides(entries9and10).Ontheotherhand,tertiaryalkylbromidesdonotreact(entry11).Finally,itshouldbenotedthatsecondaryortertiaryalkylGrignardreagentscannotbeused(entries12and13).

Thereactionischemoselective(Table25),thus,anester(entries1and2),anitrile(entry3),orevenaketogroup(entry4)aretolerated.Itshouldbeunderlinedthatfunc-

Cobalt-CatalyzedCross-CouplingReactions

Table25.Cobalt-CatalyzedCouplingbetweenFunctionalizedAlkylHalidesandAliphaticGrignardReagents

Scheme56.Cobalt-CatalyzedChemoselectiveAlkyl-AlkylCouplingReactionfromFunctionalizedSecondaryAlkylHalides

Scheme57.Cobalt-CatalyzedAcylationofMethyl-andPhenylmagnesiumBromides

tionalizedsecondaryalkylhalidescanalsobeusedsuccess-fully(Scheme56).

Itisthefirstexampleofchemoselectivealkyl-alkylcouplingfromafunctionalizedsecondaryalkylbromide.

7.Acylation

7.1.FromOrganometallicCompounds

In1943,Kharaschshowedthatmethyl-orphenylmagne-siumbromidesreactwithmesitoylchlorideinthepresenceofcobaltchloridetogivetheexpectedketonesinmoderateyields(Scheme57).51Itisthefirstreportofacobalt-catalyzedacylationoforganometalliccompounds.

Fiftyyearslater,KnochelshowedthatdiorganozincsreactwithcarboxylicacidchloridesinthepresenceofcobaltbromideinaTHF/NMPmixtureasasolvent(Scheme58).48Excellentyieldsareobtainedwitharomaticandaliphaticcarboxylicacidchloridesaswellaswithoxalylchlorideandtrifluoroaceticanhydride.Itishoweverimportanttonotethatthreeequivalentsofdiorganozinccompoundhavetobeused(i.e.,6equivoforganometallicreagent).

7.2.Cobalt-MediatedAcylationofArylBromides

Thecobalt-mediatedsynthesisofaromaticketonesfromarylbromidesandcarboxylicacidanhydrideswasreportedbyPe´richon52in2004(Table26).Thereactionisperformedinthepresenceofacatalyticamountofcobaltbromidebyusingastoichiometricamountofzincasareductant.

ChemicalReviews,2010,Vol.110,No.31451

Scheme58.Cobalt-CatalyzedAcylationofDiorganozincs

Table26.Cobalt-MediatedReactionbetweenArylBromidesandCarboxylicAcidAnhydrides

EntryArylbromideRYield(%)1p-MeO-CMe672p-MeO-C6H4Brn-Bu793-MeO-C6H4BrpPh4-C6H4Brp-NCMe56HH4Brp-NC-Cn-Bu766p-NC-CBrPh30a7-EtO6H4Brp6282C-C6H4BrMep-EtOn-Bu7-EtO2C-C-C6H4Brp33b102CPhH6H4Brp-F-CMe69a11p-FC-6C4Br61a1236H4BrMem-MeO-CMe7213-C6H4Brm-NCMe34b14

m-F6H4Br3C-C6H4Br

Me

71a

a

10mol%CoBr2wasused.b7.5mol%CoBr2wasused.

Variouspara-(entries1-11)ormeta-substituted(entries12-14)functionalizedarylbromidesreactsuccessfully.Asarule,aliphaticacidanhydridesgivegoodresultswhereasaromaticacidanhydridesonlyleadtomoderateyields(entries3,6,and9).

8.Reactions

ReductiveCyclizationandHeck-Type8.1.RadicalCyclization

Itiswell-knownthatvariouscobalt(II)complexesreactwithalkylorarylhalidesviaahalogenatomtransfertoleadtothecorrespondingalkylorarylradicals.53,Cyclizationviaaninter-oranintramoleculartrappingoftheradicalthusgeneratedprovidesaneasyaccesstovariouscycliccompounds.

8.1.1.IntramolecularRadicalCyclization

8.1.1.1.Radical-MediatedAryl-ArylCoupling.Cobalt-catalyzedreductivecyclizationhasbeenreportedforthefirsttimebyTiecco55in1965(Scheme59).

8.1.1.2.Fromδ-HalogenoAcetylenicCompounds.In1982,Tada56reportedthereductivecyclizationof2-prop-argyloxyalkylbromidesinthepresenceofbis(dimethylgly-

1452ChemicalReviews,2010,Vol.110,No.3Scheme59.Cobalt-CatalyzedCyclizationviaaRadical-MediatedAryl-ArylCoupling

Table27.Cobaloxime(I)-CatalyzedReductiveCyclizationof2-PropargyloxyalkylBromides

EntryRR1R2Yield(%)

1PhPhH852PhMeH733PhH

H78

4H-(CH5

H

-(CH2)4-2)3-

48

*Cobaloxime(I))[bis(dimethylglyoximato)(pyridine)cobalt(I)].

Scheme60.Cobaloxime(I)-CatalyzedReductiveCyclizationofHalogenoPropargylAcetals16

oximato)(pyridine)cobalt(I),generallycalledcobaloxime(I),byusingastoichiometricamountofsodiumborohydrideasareductant.ResultsarereportedinTable27.

Goodyieldsofexomethylenefuranesareobtainedfromprimaryalkylhalides(entries1-3).Cyclicalkylhalidesleadtobicycliccompoundsinsatisfactoryyields(entries4and5).

In19,Hoffmann57appliedthereactiontothesynthesisofvariousheterocycliccompounds(Scheme60).8.1.1.3.Fromδ-HalogenoEthylenicCompounds.In1984,Pattenden58describedtheradicalcyclizationof2-al-lyloxyalkylbromidesusingcobaloximeorVitaminB1259asacatalystundertheconditionsdescribedbyTada(Table27)for2-propargyloxyalkylbromides.56Theorganocobaltspeciesresultingfromthecyclizationundergoesa󰀁-hydro-geneliminationtogiveabicyclicproductingoodyields(Scheme61).

Afewyearslater,Jones60reportedasimilarreactioncatalyzedbyacobalt-salencomplex.Amixtureofsaturatedandunsaturatedproducts17and18isobtainedinallcases(Scheme62).

Thereafter,Giese61showedthatitispossibletoformselectivelythesaturatedortheunsaturatedproducts19and20bychangingthereactionconditions(Table28).

CahiezandMoyeux

Scheme61.Cobaloxime(I)-orVitaminB12-CatalyzedRadicalCyclizationof2-AllyloxyalkylBromides

Scheme62.RadicalCyclizationCatalyzedbyCobalt(I)-Salen

Table28.RadicalCyclizationCatalyzedbyCobalt(I)-Salen:InfluenceoftheNatureoftheReducingSystemontheOutcomeoftheReaction

Ratio19/20(yield%)Reactionconditions

20mol%catalyst,1mol%catalyst,Entry

RZn,DMF,hυ

Zn/NH4Cl,DMF

1H81/19(80)1/99(80)2

Ph

99/1(90)

3/97(88)

Scheme63.TandemRadicalCyclizationandMichaelAdditionReaction

Interestingly,theradical(ortheorganocobaltspecies)obtainedafterthecyclizationstepcanbetrappedbyusingareactiveMichaelacceptorsuchasanacrylicester(Scheme63).61

In1994,Jones62performedthereductivecyclizationofortho-amidoiodobenzenes21inmoderatetogoodyieldsinthepresenceofcobaltchloridebyusingmethylmagnesiumiodideasareductant(Scheme).Alargeamountofcatalystisrequired(50mol%).

Cobalt-CatalyzedCross-CouplingReactions

Scheme.Cobalt-CatalyzedRadicalCyclizationofortho-Amidoiodobenzenes21

Scheme65.CoCl2•dppb-CatalyzedRadicalCyclizationofδ-HalogenoEthylenicCompounds

ItispossibletolowertheamountofcatalystbyusingacomplexcobaltCoCl2(dppb).Thus,Oshima63achievedtheradicalcyclizationofδ-halogenoethyleniccompoundsinthepresenceofCoClmagnesiumchloride2(dppb)byusingtrimethylsilylmethyl-asareductant(Scheme65).Variouscyclopentanederivativeswerepreparedingoodyields.

8.1.2.CyclizationviaanIntermolecularRadicalAddition

In2003,Chengreportedthereactionofvariouso-iodoarylketonesandaldehydeswithalkynesorconjugatedalkenesinthepresenceofCoIamountofzincasareductant.2•dppeandastoichiometricThecyclizationreactionproceedsviaatandemcarbometalationandintramolecular1,2-additiontotheketone.

Sucha65reactionhad66beenpreviouslydescribedunderpalladiumornickelcatalysis.Nevertheless,thecobalt-catalyzedreactiongiveshigheryieldsandbetterregioselec-tivitiesundermilderconditions.Inaddition,thereactiontimesareshorter(Table29).

Goodtoexcellentyieldsareobtainedfromo-iodoarylketones(Table29,entries1-5)oraldehydes(entries6-9).Whenunsymmetricalalkynesareused,amixtureoftworegioisomers22and23isobtained(entries2and7).Interestingly,withtrimethylsilylpropyneorethyltrimethyl-silylpropiolate,theregioselectivityisexcellent,sinceonlycompound22isformed(entries3-5and8).

Asimilarreactiontakesplacebyusingvariousacrylicestersinsteadofalkynes(Table30).

Itisnoteworthythatitispossibletoperformathree-componentreactionbetweeno-iodobenzaldehyde,p-tolui-dine,andalkynestoformheteroatom-substitutedderivatives67(Scheme66).

8.2.Cobalt-CatalyzedHeck-TypeReactions

TheHeckreaction68,69isapowerfultoolfortheelaborationofcarbon-carbonbonds.However,thereac-tionstillsufferssomelimitations.Thus,theuseofalkylhalideshavinghydrogen(s)atom(s)inthe󰀁-positiontothehalideatomleadstopoorresults,sincetheintermediatecatalyticalkylpalladiumspeciesundergoesaveryfast

ChemicalReviews,2010,Vol.110,No.31453

Table29.Cobalt-CatalyzedCarbocyclizationofortho-IodoarylKetonesandAldehydeswithAlkynes

a

Ratio22/23.

Table30.IntermolecularCyclizationofortho-IodoarylKetonesandAldehydeswithAcrylicEsters

󰀁-hydrogenelimination.Interestingly,thecobalt-catalyzedHecktypereactioniscomplementarytotheclassicalpalladium-catalyzedreaction,sinceitallowsavoidanceofthisdrawback.

In1991,Branchaud70reportedthefirstcobalt-catalyzedHeck-typereactionbetweenstyreneandvariousalkylhalides(Table31)inthepresenceofcobaloxime(II)andastoichio-metricamountofzincasareductant.Itshouldbenotedthatexposuretodaylightisnecessary.

14ChemicalReviews,2010,Vol.110,No.3

Scheme66.Cobalt-CatalyzedThree-ComponentCyclizationReaction

Table31.Cobaloxime-CatalyzedHeck-TypeReaction

Primary(Table31,entries1-3and6)andsecondary(entry4)alkylbromidesleadtomoderateyieldswhereasthereactionfailedwithtertiaryalkylbromides(entry5).Later,OshimareportedthatexcellentyieldswereobtainedbyusingCoCl2•dpphasacatalystandtrimethylsilylmethyl-magnesiumchlorideasareducingagent(Table32).71a

Primary(Table32,entries1-3),secondary(entries4-5and8-10),andeventertiary(entry6)alkylhalidesreacttogivegoodtoexcellentyields.Asarule,alkylbromidesaffordbetteryieldsthanthecorrespondingiodidesorchlorides(entries1-3).Functionalizedstyrenederivativescanbecoupledefficiently(entries7-10).Itisinterestingtonotethatthereactioncanbeachievedstereoselectively(Scheme67).

Adetailedmechanisticstudywasperformed(Scheme68).

Atfirst,0CoCl•dpph2•dpph[A]reactswithMe[B],whichistherealcatalyst.3SiCHA2MgCltogiveCosinglemonoelectronictransfertothealkylhalideleadstotheCoIspecies[C]thatreactswithMeThisonereacts3SiCHwith2MgCltogivetheintermediate[D].thebenzylicradicalaccruingfromRXtoproducethediorganocobalt

CahiezandMoyeux

Table32.CoCl2(dpph)-CatalyzedHeck-TypeReaction

Scheme67.Cobalt-CatalyzedDiastereoselectiveHeck-TypeReaction

Scheme68.MechanismoftheCoCl2(dpph)-CatalyzedHeck-TypeReaction

[E].TheHeckproductRCHdCHPhisthenobtainfrom[E]by󰀁-hydrogenelimination.Finally,Co0•dpph[B]isregeneratedfromthehydridocobaltspecies[F]byreduc-tiveelimination.

ItisIIIimportanttonotethatamechanisminvolvingaCoI/CocouplelikethisoneproposedforthevitaminB12-orthecobaloxime-mediatedreaction60,61cannotbediscarded.

Thecobalt-catalyzedHeck-typereactionbetweenanepoxideandstyrenehasalsobeendescribed(Table33).72

Cobalt-CatalyzedCross-CouplingReactions

Table33.Cobalt-CatalyzedHeck-TypeReactionbetweenEpoxidesandStyrene

EntryRYield(%)

24/251Me5779:212n-Bu5857:433c-C4863:37a4

6Ht-Bu

1126

>99:1

a

Tolueneisusedassolvent.

Scheme69.VitaminB12-CatalyzedMichael-likeAddition

Scheme70.EnantioselectiveCobalt-MediatedMichaelAdditiontoChalcone

Thereactionleadstomoderateyields,andamixtureofisomersisobtained.Thus,exceptinthecaseofahinderedepoxide,thestereoselectivityofthereactionisverypoor(Table33,entry4).

9.MiscellaneousReactions

9.1.Cobalt-MediatedMichaelAddition

Asmentionedabove,alkylandarylradicalsaregeneratedbyreactinganalkyloranarylhalidewithvariouscobalt(II)complexes(seesection8).

DiastereoselectiveconjugateadditionoftheradicalsthusobtainedtoactivatedalkeneshasbeenstudiedforthefirsttimebyGiese73in1992(Scheme69).ThereactionisperformedbyusingacatalyticamountofvitaminB12andzincasareductant.

In1998,Pfaltz74obtainedapooryield,butapromisinghighenantiomericexcess,byreactingtert-butylmalonatewithchalconeinthepresenceofcobaltacetateandthechiralmacroheterocyclicligand28(Scheme70).

Abetteryield,butamoderate75enantiomericexcess,wasobtainedbyWuandZhoubyreactingethylmalonateinthepresenceofasimilarcatalyticsystem(Scheme71).

ChemicalReviews,2010,Vol.110,No.31455

Scheme71.EnantioselectiveCobalt-CatalyzedMichaelAdditiontoChalcone

Scheme72.Cobalt-CatalyzedMichaelAdditiontoVariousActivatedAlkenes

Table34.Cobalt-MediatedMichaelReactionbetweenArylHalidesorTriflatesandActivatedAlkenes

EntryFGXEWGYield(%)1p-COBrCO802p-CO2EtBr2EtCN853p-CO2EtOTfCO72a4p-CN2EtBr2EtCO70a5p-CNBrCN2Et77a6p-CNClCO667

p-CN

OTf

CO2Et2Et

69a

a

20mol%ofcatalystwasused.

In2006,acobalt-mediatedMichaeladditionwasper-formedwithalkylbromidesandactivatedalkenesinthepresenceofCoI2(dppe)andzincasareductant.76Goodtoexcellentyieldsareobtained(Scheme72).

Finally,acobalt-mediatedMichaeladditionfromarylhalidesortriflatesandactivatedalkeneshasalsobeendescribedbyPe´richon(Table34).77Arylbromides(entries1,2,4,and5),chlorides(entry6),ortriflates(entries3and7)givesimilaryields.Thereactionischemoselective;thus,esterandcyanogroupsaretolerated.

9.2.Allylationof1,3-DicarbonylCompounds

Theallylationof1,3-dicarbonylcompoundsundercobaltcatalysishasbeenreportedbyIqbal.78Thereactioncanbeperformedunderneutralconditions,contrarytothepal-ladium-79orthemolybdenum-catalyzed80reactions.There-fore,theuseofbase-sensitivesubstrateslike30ispossible.SelectedexamplesaregiveninTable35.

1456ChemicalReviews,2010,Vol.110,No.3

Table35.Cobalt-CatalyzedAllylationof1,3-DicarbonylCompoundswithAllylicAcetates

Table36.Cobalt-CatalyzedAllylationof1,3-DicarbonylCompoundswithAllylicAlcohol

EntryZRR1R2Catalyst33/34Yield(%)

1HMeMePrCoIII

DMG1/8612AcMeMePrCoCl1/3683HOMePhEtCoIIIDMG2

1/94AcOMePhEtCoCl1/3555HOMeMeBuCoIIIDMG2

100/0536

Ac

OMe

Me

Bu

CoCl2

1/1.5

24

Moderatetogoodyieldsareobtained.Regio-andstereo-selectivityheavilydependonthenatureofthesubstrate;amixtureofisomersisoftenobtained.

Thisreactionhasthenbeenextendedtoallylicalcohols81(Table36).

9.3.Reactions

ActivationofC-HBonds:C-NCouplingTransitionmetal-mediatedactivationandfunctionalizationofC-Hbondsiscurrentlyaverychallengingresearcharea.82Cobalt-porphyrin-catalyzedC-HbondaminationshavebeenwidelyinvestigatedbyCenini.83In1999,theaminationofanallylicC-Hbondwasdescribed84a(Scheme73).However,onlymoderateyieldsareobtainedandthescopeofthereactionislimitedtocyclohexene.

ThereactionhasthenbeenextendedtobenzylicC-Hbonds.84b,cAccordingtothenatureofthesubstrate,anamineoraniminecanbeobtainedinmoderateyields(Scheme74andTable37).

9.4.Three-ComponentReactions

Multicomponentreactionshaveemergedasausefulsynthetictoolinthepastdecade.84Asillustratedabove,cobaltsaltscanbeemployedascatalyststoachievesuchreactions.

CahiezandMoyeux

Scheme73.Cobalt-CatalyzedAllylicC-HBondAmination

Scheme74.Cobalt-CatalyzedBenzylicC-HBondAmination

Table37.BenzylicC-HAmination(seeScheme74)

Yield(%)

EntrySubstrateCatalysta

AmineImine1Toluene

A252IsopropylbenzeneA533CyclohexylbenzeneB284EthylbenzeneA25195DiphenylmethaneB55

116

Fluorene

C

41

a

CoIIporphyrinA:RC:R)H.

2)H,R3)p-Cl-C6H4;B:R2)H,R3)Ph;2)Et,R39.4.1.Synthesisof󰀁-AcetamidoKetones

In1994,Iqbal85describedathreecomponentreactionbetweenaketone,analdehyde,andacetonitrile.Various󰀁-acetamidoketoneswerepreparedinmoderatetogoodyields(Scheme75).

Similarly,thereactionbetweena1,3-dicarbonylcom-pound,analdehyde,andacetonitrile86affords󰀁-acetami-dodiketonesinmoderatetogoodyields(Scheme76).

ItshouldbenotedthataromaticaldehydesleadtoacetamidodiketoneswhenthereactionisperformedunderaninertatmospherewhereastheKnoevenageladductisformedinthepresenceofatmosphericoxygen(Scheme77).Ontheotherhand,aliphaticaldehydesbehavedifferently,sincetheacetamidodiketonesareobtainedonlyinthepresenceofatmosphericoxygen.Acomplexmixtureofproductsisformedwhenthereactiontakesplaceunderaninertatmosphere(Scheme77).

Cobalt-CatalyzedCross-CouplingReactions

Scheme75.Synthesisof󰀁-AcetamidoKetonesviaaCobalt-CatalyzedThree-ComponentReaction

Scheme76.Synthesisof󰀁-AcetamidoDiketonesviaaCobalt-CatalyzedThree-ComponentReaction

Scheme77.Cobalt-CatalyzedReactionbetween

1,3-DicarbonylCompounds,Aldehydes,andAcetonitrile:InfluenceofthePresenceofOxygenontheCourseoftheReaction

Thismethodologyhasbeenappliedtothesynthesisoffuransandγ-lactams(Scheme78).

9.4.2.SynthesisofHomoallylsilanes

In2003,Oshima87describedacobalt-catalyzedthree-componentreactionbetweenanalkylhalide,a1,3-diene,andtrimethylsilylmethylmagnesiumchloride.Varioushomoal-lylsilaneshavebeensynthesizedingoodtoexcellentyieldsfromprimary,secondary,ortertiaryalkylhalides(Scheme79).

Thefollowingcatalyticcyclewasproposed(Scheme80).

ChemicalReviews,2010,Vol.110,No.31457

Scheme78.SynthesisofFuransandγ-LactamsviaaCobalt-CatalyzedThree-ComponentReaction

Scheme79.SynthesisofHomoallylsilanesviaaCobalt-CatalyzedThree-ComponentReaction

Scheme80.TentativeMechanismfortheFormationofHomoallylsilanesviaaCobalt-CatalyzedThree-ComponentReaction

Atfirst,Me3SiCH2MgClreducescobalt(II)chlorideintoanactiveCo0species[A].ThisonereactswiththealkylhalideviaasingleelectronItransfertoformthecorrespondingalkylradicalandtheCocomplex[B].Afterward,thealkylradicaladdstothedienetogiveanewradical,whichcombineswith[B]toformtheCoIIcomplex[C].Me3SiCH2MgClthenreactswith[C]toaffordthediorga-nocobalt[D],whichundergoesareductiveelimination,leadingtothehomosilane[E]andtothecomplex[A].

9.5.CouplingofAlkenesandAlkynes

Copper-mediatedreactionshavebeenextensivelyusedforthepreparationofconjugateddiynes.88TheGlaser90andEglingtonproceduresforthehomocouplingofterminalalkynes,ortheCadiot-Chodkiewicz91procedureforthe

1458ChemicalReviews,2010,Vol.110,No.3

Scheme81.Cobalt-CatalyzedDimerizationofTerminalAlkynes

Scheme82.Cobalt-CatalyzedReductiveDimerizationofActivatedAlkenes

couplingofaterminalalkynewitha1-halo-1-alkyne,areprobablythemostpopularcouplingreactionsforthesynthesisof1,3-diynes.Acobalt-catalyzedreactioncanalsobeusedtopreparediynes.92Thus,in2001,Krafft93reportedthecobalt-catalyzeddimerizationofterminalalkynesinexcellentyields(Scheme81).Itisnoteworthythat,inthecaseofconjugatedenynes,notraceofthePauson-Khand94productisdetected.

Thecobalt-catalyzedreductivedimerization95ofconjugatedalkeneswasdescribedbyChengin2004.Excellentresultsareobtainedfromvariousactivatedalkenesorstyrenes(Scheme82).Itshouldbenotedthatsuchreactionshavebeenpreviouslyperformedinthepresenceofstoichiometric96orcatalytic97amountsofcobaltsalts,buttheyieldsareverypoor.

Thesamemethodologycanbesuccessfullyappliedtothereductivecouplingofinternalalkyneswithconjugatedalkenes.98Interestingly,thereactioncanbehighlyregio-andstereoselective.Thus,onlyoneproductisobtainedfromconjugatedacetylenicestersandphenylacetylenederivatives(Scheme83).

Nonactivatedterminalolefinswerealsoused.99Asarule,goodyieldsandsatisfactorystereoselectivitiescanbeobtainedundermildconditions(Scheme84).

Veryrecently,Cheng100extendedthereactiontotheintramolecularreductivecouplingofactivatedalkeneswithalkynes(Scheme85).Thisreactionallowsthesynthesisof

CahiezandMoyeux

Scheme83.Cobalt-CatalyzedReductiveCouplingofInternalAlkyneswithConjugatedAlkenes

Scheme84.Cobalt-CatalyzedAlder-EneReaction

Scheme85.Cobalt-CatalyzedIntramolecularReductiveCouplingofActivatedAlkenesandAlkynes

variousexomethylenecyclopentanederivativesingoodtoexcellentyields.

Itisnoteworthythatitispossibletoperformacobalt-catalyzedintramolecularreductivecoupling/lactonizationofacrylateswithpropargylalcohols(Scheme86).

Whenpropargylaminederivativesareused,thereactionleadstotheformationofsix-memberedcycliclactams(Scheme87).

9.6.VariousReactions

In2005,Oshima101reportedthesyn-hydrophosphinationofalkynesundercobaltcatalysis.Itshouldbenotedthatlanthanide-,102palladium-,ornickel-catalyzed103reactions

Cobalt-CatalyzedCross-CouplingReactions

Scheme86.Cobalt-CatalyzedIntramolecularReductive

Coupling/LactonizationofAcrylateswithPropargylAlcohols

Scheme87.Cobalt-CatalyzedIntramolecularReductiveCoupling/LactonizationofAcrylateswithPropargylAminesDerivatives

Table38.Cobalt-Catalyzedsyn-HydrophosphinationofAlkynes

EntryRR1Yield(%)

35/361pentMe8266/342Ph

Me7482/183o-anisylMe4180/204t-BuH81100/05PhH70/116

Et3Si

H

62

94/6

havealsobeendescribed.However,thestereoselectivityofthesereactionsishighlydependentonthenatureofthesubstrate.Onthecontrary,thecobalt-catalyzedreactionmainlyleadstothesyn-additionproduct35(Table38).Thisreactionisofparticularinterest,sincetheresultingalkenylphosphinecanbeusedforperformingWittigreactions(Scheme88).

Thesameyear,Oshima104reportedthepreparationof1,2-di-and1,1,2-trisilylethylenesviaacobalt-mediatedreactionofdibromomethylsilaneswithtrialkylsilylmethylmagnesiumreagents.Disilylethylenesareusuallysynthesizedbyhy-drosilylationofsilylacetylenes105ordisilylationofacety-lenes.106Contrarytothesereactions,thecobalt-mediatedprocedureishighlyregioselectiveandstereoselective(Scheme).

In2006,Cheng107reportedthecobalt-mediatedsynthesisofthioethersfromthiolsandarylhalides(Scheme90).Thiscouplingmethodisadvantageouscomparedtocopper-,palladium-,andnickel-mediatedreactions,108sincemild

ChemicalReviews,2010,Vol.110,No.31459

Scheme88.Cobalt-Catalyzedsyn-HydrophosphinationofAlkynes:ApplicationtotheWittigReaction

Scheme.Cobalt-CatalyzedSynthesisof1,2-Di-and1,1,2-Trisilylethylenes

Scheme90.SynthesisofThioethersviaCobalt-CatalyzedC-SCouplingReaction

reactionconditionsandlowcatalystloadingarenecessary.Moreover,theuseofsophisticatedphosphineligandisavoided.

Undertheseconditions,ethyl2-iodoacrylatecanalsobecoupledsuccessfully(Scheme91).

10.Conclusion

Sustainabledevelopmentpromptedorganicchemiststolookformoreecocompatibleandmoreeconomicaltransitionmetal-catalyzedprocedures.Thus,inthelasttenyears,agrowingnumberofiron-ormanganese-catalyzedreactions

1460ChemicalReviews,2010,Vol.110,No.3

Scheme91.Cobalt-CatalyzedCross-couplingbetweenThiophenoland2-IodoethylAcrylate

wereproposedtoreplacetheolderpalladiumandnickel-catalyzedcross-couplingprocedures.Ofcourse,intheframeworkofsustainabledevelopment,cobaltislessinter-estingthanironormanganese;however,itcomparesfavorablytonickelandpalladium.Thus,cobaltcanbeaninterestingalternativewhenironormanganesecannotbeused.Ontheotherhand,itshouldbeunderlinedthatseveralreactionsperformedundercobaltcatalysisarespecifictothismetal.Forinstance,iron-catalyzedcrosscouplingbetweenarylGrignardreagentsandfunctionalizedsecondaryalkylbromidesgenerallyfailedwhereasexcellentyieldsareobtainedundercobaltcatalysis.Thus,inthefuture,itshouldbepossibletodevelopmoredistinctreactionsusingcobalt-basedcatalyticsystems.

Inspiteofthenumerousinterestingreportsmentionedinthisreview,cobalt-catalyzedreactionsarestillintheirinfancy.Inthefuture,theyshouldbemoreextensivelydevelopedandcouldtakeasignificantplaceintherenewaloftransitionmetal-catalyzedreactions.

11.Acknowledgments

WethanktheCNRSforfinancialsupport.Ge´rardCahiezthankscurrentandformermembersofhislaboratoryfortheircontributiontothedevelopmentofcobalt-catalyzedcouplingreactions.Theirnamesappearinthelistofreferences.WealsothankJulienBuendiaandDr.OlivierGagerforproofreading.

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