cinnamonitriles_cinnamonitrile(3)

9.2.2.Ullmann-TypeCouplingandRelatedReactions

ItisnoteworthytomentionthattheUllmann-Goldbergcondensationofarylhalideswithphenolsandanilinesworkedefficientlyinthepresenceofcopperinwater.637Forexample,couplingof2-chlo-robenzoicacidwith4-chlorophenol(K2CO3/pyridine/copperpowder)gave2-(4-chlorophenoxy)carboxylicacid.638TheCuI-catalyzedtransformationof2-bro-mobenzoicacidintosalicylicacidhasalsobeenstudiedinaqueousmedia.639

Ullmann-TypeCoupling.Thehomocouplingofarylhalidetodiarylcompoundsisasyntheticallyusefulreactionandhaswideapplicationsinmateri-alsresearch.Suchcouplingshavebeenstudiedinaqueousconditions.In1970,arylsulfinicacidswerecoupledwithPd(II)inaqueoussolventstobiaryls(eq169).640However,thereactionrequirestheuseofa

stoichiometricamountofpalladium.Inthepresenceofhydrogengas,arylhalideshomocoupledtogivebiarylcompoundsinmoderateyields(30-50%)inanaqueous/organicmicroemulsion(eq170).641

In1999,VenkatramanandLireportecilecouplingofarylhalidesviaa

palladium-catalyzed

3136ChemicalReviews,2005,Vol.105,No.8reductivecouplingusingzincinairandaqueousacetoneatroomtemperaturebyusingPd/Casacatalyst(eq171).642Variousaryliodidesandaryl

bromidescoupledeffectivelyundertheseconditions.Subsequently,theyfoundthattheadditionofasuctantoracrownetherinwateraloneprovidedbetterisolatedyieldsoftheproduct.643Sassonandco-workersfurtherdevelopedthisreactionbyusingPEGasanadditiveandbyincreasingthereactiontemperature.Inthiscase,arylchloridealsoworkedeffectively.644Reductivehomocouplingofchloroben-zenestobiphenylsaffordshighyields(93-95%)inthepresenceofcatalyticPEG-400and0.4mol%ofarecyclable,heterogeneoustrimetalliccatalyst(4%Pd,1%Pt,and5%Bioncarbon).Thecompetingreductionprocessisminimized.645Theybelievethatdihydrogenisgeneratedinsitu.InadditiontoPd/C,Rh/Cisalsoeffectiveasthecatalyst.646Carbondioxidewasfoundtopromotethepalladium-cata-lyzedzinc-mediatedreductiveUllmanncouplingofarylhalides.Inthepresenceofcarbondioxide,Pd/C,andzinc,variousaromatichalidesincludinglessreactivearomaticchlorideswerecoupledtogivethecorrespondinghomocouplingproductsingoodyields.647

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