A mechanistic investigation of hydrodehalogenation using ESI-MS

3 Pages·2013·1.391 MB·3\3

by  Zohrab Ahmadi and J. Scott McIndoe

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A mechanistic investigation of hydrodehalogenation
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en using ESI-MS†
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22:38:15. nported Li RC4ei9tc,ee1itv1he4ids8:81C5htehmA.uCgoumstm2u0n1.3,2,013, Zohrab Ahmadi and J. Scott McIndoe*
2015 3.0 U Accepted22ndOctober2013
3/n
0o DOI:10.1039/c3cc46271d
d on 10/Attributi www.rsc.org/chemcomm
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oaon Therateofhydrodehalogenationofaryliodideswithapalladium alsosupportstheinvolvementofbetahydrideeliminationasa
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wm catalystinmethanolexhibitsastrongprimarykineticisotopeeffect keystepinthereaction.Ourinterestinthereactionstemmed
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er 2013. DCreative C afrommajboortrholCeDi3nOtDheanmdeCchHa3nOiDsm,s.uggestingthatdeprotonationplays fsarilodkmeynaroetuiaorcntoibownshereivnnatcitoohnnedtuhcocatptephdeyrid-nfrroemdeeehtShaoalnonogogelan,7sahatiinordanwwreeaascdtteihoceindm/eHdaejcotkor
ublished on 22 Octobe is licensed under a T(pfnbohayihrsyleZmldadardrisofmeikoudp,rim1elncahaia-cscnatehaaldmeotlyadgttelhiyencyeznitenheadlyotdedifxsocrtanoreaodn)nCsedds–hie-sXcaduolnoa(ubXgwpyelfa=Ninrnneaohtgqteliaaudonrlenie2dbnaoyeictnpf)tivracobooronoyldvmlsnui,hndcpagtaeslwni.todidixAnetihnsdgisawaetraiarevCsreaels–pycpaHrtdmooiondpbenisooctisihbnoeialnndde- csoigtsetortruhenstda-letbotiyrlnwuyciuttooiaoniuxcnuipctdseilhtielmnsee.gropoaOwrpntueeanisdtrroetrnnribaigcepynherpg,turotsobthfeiaynewcchtghethenaterikhtmqsoebuiimanercseaapoelltalfsaeatlpislyevkurtexeyicplcnshyeseduasa.ntirsHreiaozntylrenydtidsegroitossfhbdalyaemeilssaahepmvatillopneionoggieutneao,snfsubeuastusituiitttoohhminnees
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cc (Scheme1). charge-taggingandESI-MSisanincreasinglypopularapproach
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n ThesamemechanismalsowasreportedusingNi(0)/imidazolium toestablishingspeciationincatalyticmixtures.10
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Op chloride3andRu(II)4catalysts.Oxidationofthealcoholsolvent Examinationofthereactionofthecharge-taggedaryliodide
has been reported by Zhang5 in homocoupling as well as [Ph PCH C H I][PF ] (ArI) in methanol using Pd(PPh ) as the
3 2 6 4 6 34
hydrodehalogenationofarylhalides.Recently,anotherreport6 catalystandtBuOKasthebaseallowedustofollowtheabundance
onequivalentoxidationofsolventduringhydrodehalogenation ofstartingmaterial,product,byproduct(inthiscasethebiphenyl
productofhomocoupling)andallintermediatescontainingthe
charged tag that are present in reasonable abundance. The
reaction was repeated in CH OD and CD OD. The reaction
3 3
takes about 100 minutes to go to completion in CH OH at a
3
catalyst loading of 6 mol% under pseudo first-order kinetics
(k = 0.0292 min(cid:2)1).Inaddition tothe product (ArH), the homo-
coupling byproduct (ArAr) also forms in low yield (B2%). The
main palladium-containing species observed over the course of
the reaction are PdP (Ar)(I), PdP (Ar)(H), and PdP (Ar) . Fig. 1
2 2 2 2
showsintensityvs.timetracesforArI,ArH,ArAr,PdP (Ar)(I)and
2
PdP (Ar)(H)(thetwoPd-containingspecieshavebeenmultiplied
2
by100togetthemonthesamescaleastheotherspecies).Note
thatthebehaviourofthePdP (Ar) isdiscussedinESI;† thelow
2 2
Scheme1 Apossiblemechanismofhydrodehalogenationofarylhalides. yieldofthisbyproductmeanswehaveneglecteditascontributing
significantlytotheoverallprocess.
DepartmentofChemistry,UniversityofVictoria,P.O.Box3065,Victoria, Becausetwodifferentplausibleintermediatesareobserved,
BCV8W3V6,Canada.E-mail:[email protected];Fax:[phone];
the reactions that consume these compounds are both rela-
Tel:[phone]
tively slow, and both reactions are likely to contribute to the
†Electronic supplementary information (ESI) available: Experimental details,
numericalmodelling,additionalmassspectra.SeeDOI:10.1039/c3cc46271d overallrateofthereaction.Bothofthesespeciesappearinthe
11488 Chem.Commun., 2013, 49, 11488--11490 Thisjournalis c TheRoyalSocietyofChemistry2013
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5 2Un first-orderkinetics.Note:onlythefirst50minutesisshown,becauseafterthat
2013.0 catalystdecompositioncausessignificantdeviationfromfirst-orderbehaviour.
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ublished on 22 Octobe is licensed under a swqHratuaaotaesrwtlieoetfqavfoeutrairavp,PelplwdlyyePhass2elri(oamnAwnrit)c.l(haeHTer3ho:),rfeaetpihsanrtceeottneaidordumanycsteswldtioaaniwtasettehrtfeaoebppreetePhhardiratneePvdegi2o(eoAuinxffrrp).(iCeInFr)H,iimaga3l.Onle2dtnDhtc,arsoe.tjmheuemcpaarpsereaeasscttwittohhanees eteatthhiqvnoeaaudnrtti,lldiitwtbehhissreeiipsrualedmaiflnecootetrneePsecgrdmttPbhw2eeoea(dnAyitilrouayt)odt(reIniP)tdohid+esvePenlMrli(eogAefatltriOn.m)o(HdTIbi)hsttie2ionisrngvame?sad[saPT,ukdmhsmePoies2[p(PthAthpdioroaoPw)nn(s2Mos(ciAilseabrOn)irsl(eoiHMttalyhu)se]eot+OainordH+ainesb)Iep](cid:2)l+sroe.ofeHgitfoxitvoinhsaewatinnss--
Article. PThis articl ntuartnTioohnve,erwC-leHimw3OiotuiDnldgexespxtpeepericmitntevhnoeltvrweadtaesbto-peabliremtiecisnusaleatnirotlinyaliolnyrfuornremadffuaectctiivtveeed.eIwflihmthenie- Icfowmeprouunndthe(sceaetaFlyigti.cSr6eacint2ioEnSwI†i)thaonudtancoha[PrgdePd2(tAarg)((Mi.ee.OwHit)h]+P+hIIo(cid:2)r.
ss comparedtoCH OH.ThemonitoringofthereactioninCH OD MeOC6H4I rather than ArI), we see roughly equal amounts of
Acce revealeda1:1m3ixtureofArHandArDproducts(seeESI†).If3the [PdP2(Ar)(I)+K]+and[PdP2Ar]+.Thepotassiumionadductappears
Open wreeduwcotiuvledeelixmpiencatttihonisisreraecstpioonnstibolebefofratshteersltohwaenrtrhaeterienacCtHio3nODin, ibnetcearuacsteioonftbheetwpereensetnhceecoofmthpeleKxOantBduKb+asise,tydpeiscpailtlyeathweefaakctotnhea.tTthhee
[PdPAr]+ionisnotafragmentofthepotassiumadduct(seeESI†),
CD ODbutslowerthanthatinCH OH. 2
3 3
so represents an independent species, albeit present in very low
The fact that the rate was just as slow for CH OD as for
3
concentration. The modified mechanism appears in Scheme 2.
CD OD suggests instead that the key step was deprotonation.
3
If the deprotonation happens off the metal to make RO(cid:2), the Assuming this mechanism is operative, it suggests a couple of
amount of RO(cid:2) will be determined by the position of the avenues of further exploration. If dissociation of iodide is critical,
ROH +KOtBu2ROK +HOtBuequilibrium in solution, thiswill itsremovaloughttoacceleratethereactionandadditionofiodide
oughttoslowitdown.Bothoftheseexperimentswereconducted,by
adding AgNO at the start of the reaction to precipitate out any
3
iodide that is released, and by adding one equivalent to I(cid:2) to the
reaction.Thepredictedeffectsindeedplayedout(Fig.3).
The AgNO experiment was especially interesting because the
3
reactionwasinitiallyveryfastbutafterconversionofabout25%
rapidlyslowedtoacrawl.Wesuspectthisisduetoremovalofthe
catalystsomehow,perhapsacombinationofoxidationofPd(0)by
Ag(I), and co-precipitation of remaining Pd(0) with the Ag(0) so
formed. Thesolutiondiscolouredsubstantiallywhenthe catalyst
precursorwascombinedwithAgNO .Whateverisgoingontoslow
3
thereaction,theinitialburstofproductivitydoesnotcompensate
forthelaterslowdown,soAgNO isnotahelpfuladulterantinthe
3
long run. The slow reaction in the presence of I(cid:2) is predictable
given that we expect the iodide dissociation to be suppressed in
Fig.2 OverlayofproductbuildupinCH3OH(blue),CH3OD(red)andCD3OD(green).
Errorbarsaregeneratedfromduplicates(green)andtriplicates(redandblue). the presence of high concentrations of I(cid:2). Addition of halide has
Thisjournalis c TheRoyalSocietyofChemistry2013 Chem.Commun., 2013, 49, 11488--11490 11489
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notendeavourtomodelthehomocouplingpathwayexplicitly,since
itonlyinvolvedapproximately2%ofthefinalproduct.TheKIEfor
thedeprotonationstepneedstobe>7toaccountforthedramatic
change in reactivity, and the best fit is obtained if modelled at
k /k =16forthatparticulartransformation.Thisvalueisrelatively
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high, and may indicate that tunnelling is occurring or that our
estimatesoftherateconstantsofotherstepsarenotaccurate.Such
deviationsaretobeexpectedinsuchacomplexsystem;ourmain
e. hope is to generate a model that responds accurately to perturba-
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Octobder a amboodveelthsheonuolidseb;enolonwetehretlhesasn,tthheeiroatbhuerndinatnecremiendtihaetensutmhaetriacrael ourapproachtoavarietyofothercross-couplingreactions.
ublished on 22 e is licensed un oo(sbubb-besssleeitrrmavvneeintddiaa.,tliIwocnonhn)tichsihtsirsansiconuattgsstge:ue,irtsnttnhoseeveetdhPr-eadlidtPm2ttio(htAieenrm)gr(.OeuTalMahctteeei)ontnhiunemttneheroarmritcmeacdalolimnabstoeuedhmeaislveihsnoauoidrtt N21o(Aat.)eZMass.kSa.aVnnidcdiuP,.rGHe.efAleq.Guriresatsn,aJc.aeOndrsgS..CPh.eNmo.l,a1n9,7O8r,g4a3n,o1m6e1t9a–ll1ic6s2,02.001,20,
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11490 Chem.Commun., 2013, 49, 11488--11490 Thisjournalis c TheRoyalSocietyofChemistry2013