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Titanium( IV ) Tttetallocycles

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Titanium( IV ) Tttetallocycles 6529 Synthesisand Reactions of Bis(cyclopentadienyl)titanium( IV ) tttetallocycles JosephX. McDermott,r Michael E. Wilson,and GeorgeM. Whitesides* Contributionfromthe Deportment oJ'Chenti.strr', Massachusetts Institute oJ Techrutlogt', Cantbridge,Ma.ssachusett.s 02139. ReceiredJanuarr 16, 1976 Abstract: 1,4-Tctramethylenebis(cyclopentadienyl)titanium(lV)(l) was preparedbv rcactionbetween dichlorobis(cyclopen- tadicnyl)titanium(lV) and 1.4-dilithiobutane:it could be isolatedat low tcmperature.but its thermal lability was such that it could be characterizedonly through its reactions.Compound I and the analogoussix-membered metallocycle I .5-penta- methylcncbis(cvclopentadienyl)titanium(lV)(5) were much more stablethan an acyclictitanium alkyl. di-n-butylbis(cyclo- pentadienyl)titanium(lV)(3). Thermal decompositionsof 3 and 5 appearto take place by unexceptionalp-climination and reductionreactions involving titanium hydrides.These reactions yield, respectively'.u mixture of butenesand n-butaneand a mixture of pentenesand cyclopentane.By contrast,thermal decompositionof I (4 h,0'C) takesplace in part by a path that clcavesa carbon-carbon bond of the titanocyclopentanering and generatesethylene: I -butcnc is the secondmajor product of this decomposition.Carbonylation of I yieldscyclopcntanone; carbonylation of 3. 5. and dimethylbis(cyclopentadienyl)titanr- um(lV) takesplace less readily, if at all. Theseresults indicate that the high stabilit;-of the titanocyclopcntanering, presum- ably reflecting suppressionof the p-elimination of metal hydride, is accompaniedbl abnormally high reactivity in carbonyla- tion and in thermal decompositionby carbon-carbon fragmentation. Several functional equivalentsof titanocene (CpzTi- N:TiCp:, [CplTiH].-, [Cp:Ti]:. Cp:TiCl: reducedwith lithium naphthalenide)react directll'with olefinsand form titanocy- cloDentanerings. The best vields in thesereactions are obtained usine benzonorbornadiene.norbornene. and ethylene. A dominant decompositionreaction of transition metal SchemeI. hcparation and Reactionsof CprTi(CHr)o(l ) alkyls is the /3-eliminationof a metal hydride moiety.r Al- Li(cH2)1Li CO (l atm) though this reactionis fundamentalto much of transitionmetal ------------:- Cpr'l'iCl t'p Ti | o-( | catalysis.its very facility limits the importanceof other. po- Et20, -?8 "C \--./ -4O to 23 C \--/ tentially valuable,types of reactions,particularly those in- 2O7o volving carbon-carbon bond formation and cleavage.We have co previouslysuggested that one way of suppressingthe (l-hydride pentane Br, CFCITCFTCI -cc L eliminationreaction is to constrainthe M-C-C-H dihedral -40 0c TOVo angleto valuesfar from the optimal 0o. This suggestionwas 1('p-TiCl 1t'pTiI3r' o verilicd by preparingseveral platinum(ll) metalloc)'cles,and \_ establishingthat their thermal decompositionis dramatically 1- + /\ (-p- Ti ) Iessrapid than that of acyclicanalogues.r Here we describe llt\ B11 \-J --RrJI ') | dctailsof the preparation,characterization, and reactionsof "'n-.--J 2 bis(cyclopentadienyl)titanium(lV)metallocyclcs. and par- r t'r, ticularly of 1,4-tetramethylenebis(cyclopentadienyl)titani- *I um(lV). CpsTi(CH2)a (l) These studies reinforce the suggcstionthat the chemistryof metallocyclopentanesis sig- o<| \----l nificantly different from that of analogousacyclic organo- * Cp Ti{CO)- rnetalliccompounds, and that an important basisfor this dif- fercncc is suppressionof il-hydrideelimination.r yieldsarc basedon thc assumptionthat the reactionof I with bromineyields 1.4-dibromobutanequantitatively: viz., that Results this reactioncan be usedto assaysolutions for the quantity of Preparation and Characterizationof 1,4-Tetramethvlene- I thel' contain.The major part of the dcscriptivechemistry of bis(cyclopentadienyl)titanium(lV)(l ). Reactionof a suspension I was carried out by preparing I by reaction of 1,4-dili- ol'dichlorobis(cyclopentadienyl )titanium( I V ) (CplTiClz) with thiobutancwith CplTiCl:, isolatingI by low-temperature oC 1.4-dilithiobutanein diethylether at -78 yieldsa reaction chromatographyusing procedures outlined in the Experimental mixturecontaining I (SchemeI). Its isolationand purification Scction.and assayingan aliquotby brominationto determine can be accomplishedby chromatographyover alumina at -40 the quantitypresent. The conversionof Cp:TiCl: to a solution oC underargon using hexane as elucnt;crystallization may be containingI was normally ca.20o/". oC. cffcctedby'cooling thc eluentto -78 Compound I is a dark Two linesof cvidenceargue that I is a monomer containing orangccrvstalline solid. readily soluble in pentaneand ethereal a five-memberedtitanocyclopentane moiety, rather than a solvents.whrch decomposesthermally in CFCI:CClF1 solution dimer or higheroligmer having the sameempirical composi- oC. with a half-liieof ca.0.5 h at 0 tion. First.its thermalstability is much greaterthan that of Sincc the thermal instabilityoi I precludedtraditional an- acyclicanalogues. For cxample,CpzTi(CHtCH2CHzCH r): ali,tical proccdures.it was characterizedby its reactions. (3, videinfra)dccomposes rapidly at -55 oC, while t hasap- Trcatment of chronratographedsamples of I with HCI (gas) preciablestability at 0 oC. If I werea dimer or oligmercon- in hexancor CFICICFCl2 yieldedCp2TiClz and butanein taining a ring having more than five members,its thermal rnolarratio l:1.0:treatment with brominein thesame solvents sterbilitlwould be expectedto be determinedby the rate of yieldedCpsTiBrz and 1,4-dibromobutanein molar ratio l:1.2. /J-h-v-drideelimination. and to be approximatelythe sameas Thc lact that the ratio of titanium-coniainingproducts to that ol3.r The observationthat the stabilityof I is qualitatively productsderived from the 1,4-tetramethylenemoiety is ap- much grcatcr than that of 3 arguesstrongly that the 1,4-te- proximately1.0 in bothof theseexperiments indicates that the tramethylenemoiety of I is part of a five-memberedring. compoundhas stoichiometry [CpzTi(CH:)+]u. In what follows, Second.carbonylation of I (CO, I atm, warming from -40 IReprinted from the Journal of the American Chemical Societr'.98, 6529 (1976).] ('opvright 1976bv the American Chemical Societr.'andreprinted bv permissiono1'the c'opr.'right owner 6530 TableI. Productsot'Thcrmal Decomposition o1 Two aspectsof thcscdata deservecompari\()rt I rr'1.the \{etallocy clopcntane sa muchenhanced thermal stabilitl'of I relatir.'clt,3 r:.ruut)lll- 1-Butenc/ paniedby a changein the t) pesof hydrocarbonprtrllii!1. i.\r'()- A Compound Temp ('C) Ct l)\ lL'nL"' duccd on decomposition:thc carbon-carbonl'nrenrcri.rtlr)n (csHs),Ti(cHr)o(l) 20 9 that convcrtsI to cthylenehas no counterpartin tll! (ir'!,\nt- A .e.rct (') a positionof 3. Second.this increased stltbility pcnr)n: I t,' 150c 0.05 with carbonnronoxidc and gencratecvclopentan()ite [)rl,cn- (C,NIc.)rTi(Cll,)4 0 (4.s) zvlbis(c)'clopcntadienl'l )titanium( I V ) and ntcthr Ii.r.r .', . i',- l5 0.' (0.4) pentadicnyl)titanium(lV)chloride . in whrch,J-hidrrtle e :;''- (Cql\'Iea I:t) rTi(CHr)4 0 (6) 250c (0.6) inationis nrttpossible. ulso carbonvlatc:'r dintcthrIl.r. .'.gI1r- CprZr(ClI,)o It) 6 pcntadienl'l)titaniunt(lV)reacts onll slowlr \\rtlr,,,r'rr)!r 250. l rnonoxidcat roor.t'ttenrpcraturc. The observationtir,ri I r\ a Thcrmaldecompositions rvcrc carricd out in CIrClrccllr, solu- significantll nrore reactive tor.lardcarbon nton()\itit I . tion at conccntrationsoriginalll' ca. 0.02 1\lin rlrganometalliccor.n- Cp.TiN{e1suggests that thc fivc-mcmbercdmetallocrclt. t.iirr pound. b The absolutcyit:lds o1'oletins u'crc not mcasurcdin in- ol'I servesboth to suppress,J-h1'drideelimination and lri .,r,'- stanccsrndtcated bl'parcntlrescs; in othcr cases,thc irroductbalancc vidca purticul;rrlrrcuclivc grouping. \\,asgrcatcr than 80%. l,.l-tctramethy'lenc Thc rcmaining rnoieties The factorsinfluencing the partitioningof the 1.4-tctrrr- appcared asa mirture ot'ethancand butanc.Sincc irredotninantll' of I betweenethl"lenc and l-buteneon therntul the ratio of cthancto butane\\'as approxinratcly tltc sanrcas that ot' methl'leneunit cth-'-lcncto butcne,thcsc tbrmer matcrialsu'cre- probabll lormcd b1 decompositionwcre cxploredbriefly (Table I). Also for conr- redrrcti,rrrol lllf llttt'r.8.l)ec()ln11()sili()n\\'a\carri.'d rrut hr irrjce- parison,thc rclatcdnrctrllocvclcs 4 and 5 *'crc prcparcd.pu- tion dircctlf into a GLC inlct port at 250'C. riiicd by chrornatographr.antl itllow'edto decomposcthcr- nrallr.The thcrmalstabilitics ol' thcse substunccs arc intcr- mediatebetwccn those ol-l and 3. The productso1'deconrpo- to 25 "C) yields cyclopentanone in 80-90o/oyield. If I were a sitionvaried slightll u ith tcntperaturc.but alr',ay'sconsistcd dimcr or oligmer, the carbonylgroups would be expectedto predominantll'of pcntcncs.No rnorethan trace propylcne join diliercnt tetramethylenemoieties. The productionof cy- a of or cth)'lene\\'as observcd ()n dccomposition of 4. clopentanonein good yield is consistentwith the existenceof a titanocyclopentanemoiety in l. cyclo- The Thermal Decompositionand Carbonylation Reactions I 250 C 1-pentene + 2-pentene + pentane of I Produce Different Types of Products Than Do Analogous t'ofi 0.35 0.65 0.00 Reactionsof Di-n-butylbis(cyclopentadienyl)titanium(lV)(3). \--) (GL(l rntection) Reactionof n-butyllithiumand methyllithiumwith CplTiCll 4 at -78 oC is accompaniedby the samecolor changcs observed 250 C on reactionwith 1.4-dilithiobutane:CplTiMe2 is casilyisolated t'pr|-\ 0.30 (1.65 0.05 in 80o/nyield as a yellow-orangesolid which is moderatelystable \--l (GLC injection) at room temperature;r'6CpuTiBu:
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