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OFFICE OF NAVAL RESEARCH ELECTEi JUN 2 7,1995 F Cntract N J-1433 R&TCde Technical Reprt N. 25 Explring Factrs that Determine Cis/Trans Structure and Tacticity in Plymers Prepared by Ring
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OFFICE OF NAVAL RESEARCH ELECTEi JUN 2 7,1995 F Cntract N J-1433 R&TCde Technical Reprt N. 25 Explring Factrs that Determine Cis/Trans Structure and Tacticity in Plymers Prepared by Ring Opening Metathesis Plymerizatins Invlving Inititatrs f the Type Syn and Anti M(NAr)(CHCMe2Ph)(OR)2. Observatin f a Temperature Dependent Cis/Trans Rati by R. R. Schrck, J.-K. Lee,R. O'Dell, J. H. Oskam in press in Macrmlecules Department f Chemistry Massachusetts Institute f Technlgy Cambridge, Massachusetts June 14,1995 BTXO qitalot INSPECTED 8 Reprductin in whle r in part is permitted fr any purpse f the United States Gvernment. This dcument has been apprved fr public release and sale; its distributin is unlimited REPORT DOCUMENTATION PAGE frm fipprvea OMB N. 0/ ICOttKtO' lr«4t«a. «civtim iuga«tti * lf reducing Ihn turfa«n. fe ftmtwmt« ft«imai.35&fm*«emimi*n^^ '*»/.»ikm MC4. 4krtin0t V V»» ).»nd t tht (Xhct 0' M,«*jem«i«I 11. AGENCY USE ONLY (leave Wan«.REPORT DATE inif AND SUBTITLE Explring Factrs that Determine Cis/Trans Structure and Tacticity in Plymers Prepared by Ring Opening Metathesis Plymerizatins Invlving Inititatrs f the Type Syn and Anti M(NAr)(CHCMe2Ph)(OR)2. Observatin f a Temperature Dependent Cis/Trans Rati 6. AUIHÖRIJ) R. R. Schrck, J.-K. Lee,R. O'Dell, J. H. Oskam 11. Vtf PORT TrPE ANO DATES COVf R 0 Technical Reprt ' '. [S. FUNDING NUMBERS (Cntract NQ J-143: IR&T'Cb'de Kenneth. Wynne [ 7. PERFORMING ORGANIZATION NAME(S) ANO ADDRESS(ES) Massachusetts Institute f Technlgy Department f Chemistry, Massachusetts Avenue Cambridge, MA [ 9. SPONSORING/MONITORING AGENCY NAME(S) ANO ADDRESS(ES) Department f the Navy Office f Naval Research 800 Nrth Qulncy Street Arlingtn, VA PERFORMING ORGANIZATION REPORT NUMBER 110. SPONSORING/MONITORING AGENCY REPORT NUMBER 25 II. SUPPLEMENTARY NOTES 111 DISTRIBUTION 'AVAILABILITY STATEMENT Reprductin in whle r in part is permitted fr any purpse f the United States Gvernment. This dcument has been apprved fr public release and sale; its distributin is unlimited. I 1J. ABSTRACT (Vü/mum 200 wctf;} L I 12b. DISTRIBUTION CODE The factrs that determine the structure (cis/trans and tacticity) f plymers prepared by ring pening metathesis plymerizatin (ROMP) using well-defined initiatrs f the type M(NAryl)(CHCMe2Ph)(OR)2 (Aryl = a substituted aryl grup and OR = a variety f alkxides) have been explred. The main finding is a dependence f cis/trans structure n temperature; at lw temperatures (-35 C) all eis plymer is frmed and at high temperatures (65 C) up t 90% trans plymers are frmed. These results are cnsistent with frmatin f eis plymer frm syn alkylidene rtamers and trans plymer frm anti alkylidene rtamers in circumstances where syn and anti rtamers can intercnvert SUBJECT TERMS IIS. NUMBER OF PAGES 116. PRICE CODE 17. SECURITY CLASSIFICATION OF REPORT Unclassified KSN 7S10-OI-280-SS0O 18 SECURITV CLASSIFICATION OF THIS PAGE Unclassified 19. SECURITY CLASSIFICATION OF ABSTRACT Unclassified 120. LIMITATION OF ABSTRACT UL Standard Frm 298 (Rev 2-69) Pretcribcd by ANSI Std f jj.i Explring Factrs that Determine Cis/Trans Structure and Tacticity in Plymers Prepared by Ring Opening Metathesis Plymerizatins With Initiatrs f the Type Syn and Anti M(NAr)(CHCMe 2 Ph)(OR) 2. Observatin f a Temperature Dependent Cis/Trans Rati. by Richard R. Schrck,* Jin-Kyu Lee, Richard O'Dell, and Jhn H. Oskam Department f Chemistry Massachusetts Institute f Technlgy Cambridge, MA Accesin Fr NTIS CRA&I DTIC TAB Unannunced Justificatin By Distributin/ Dist H Availability Cdes Avail and /r Special Abstract The factrs that determine the structure (cis/trans and tacticity) f plymers prepared by ring pening metathesis plymerizatin (ROMP) frm 2,3-bis-(triflurmethyl)nrbrnadiene) (NBDF6) and several dicarbalkxynrbrnadienes using well-defined initiatrs f the type M(NAryl)(CHCMe2Ph)(OR)2 (Aryl = a substituted aryl grup and OR = a variety f alkxides) have been explred. The main finding is a dependence f cis/trans structure n temperature; at lw temperatures (-35 C) all eis plymer is frmed and at high temperatures (65 C) up t 90% trans plymers are frmed. These results are cnsistent with frmatin f eis plymer frm syn alkylidene rtamers and trans plymer frm anti alkylidene rtamers in circumstances where syn and anti rtamers can intercnvert. Several all eis plymers prepared frm achiral initiatrs that cntain varius flurinated alkxides were fund t be biased tward istacticity. In ne case, hwever, the plymer was biased tward synditacticity. INTRODUCTION Ring pening metathesis plymerizatin (ROMP) is a methd f making plymers frm a wide variety f cyclic lefins. 1 2 If the lefins are relative reactive cmpared t rdinary lefins (e.g., nrbrnenes and nrbrnadienes), and well-defined initiatrs are emplyed, then the plymerizatin can be living. 3,4 Well-defined initiatrs fr plymerizatins f this type that have been used ften are cmplexes f the type M(NAryl)(CHCMe2Ph)(OR)2 (Aryl = a substituted aryl grup such as 2,6-Me2C6H3). 5 A feature f such species, and perhaps classical metathesis catalysts als, 1 is the presence f syn and anti alkylidene rtamers that may, r may nt, intercnvert readily cmpared t the rate f plymerizatin (equatin 1). We recently prpsed 6 that syn and anti rtamers f cmplexes f this type play a majr rle in determining ROV Ar Ar I I N N II MOv TT V, MOv R' c^ ^s RO i ^ r RO I R ^a H anti K eq = k a/s /k s/a» ROV r (i) syn whether ROMP plymers prepared frm several nrbrnenes r nrbrnadienes 4 ' 7 ' 8 cntain eis r trans duble bnds. Syn and anti rtamers whse reactivities culd be evaluated (OR = OCMe(CF3)2) were fund t have vastly different reactivities and t intercnvert at rates that differed by up t six rders f magnitude, relatively rapidly (k s / a = 1 s' 1 at 25 ) when OR = O-t-Bu, but relatively slwly (k s / a = 10 5 s 1 at 25 ) when OR = OCMe(CF3)2- In mst cases that were examined K eq was fund t be 10 3. The rate f rtamer intercnversin was fund t be faster by apprximately three rders f magnitude in Atfe-mnsubstituted phenyl imid cmplexes such as M(N-2-t-Bu-C 6 H4)(CH-t-Bu)[OCMe(CF 3 )2]2 than in 2,6-disubstituted phenylimid cmplexes. 6 The single bulky substituent in the rth psitin was prpsed t prmte bending f the imid ligand and therefre a lwering f the barrier t rtamer intercnversin by making the rbital that lies in the N-M-C plane, the ne that is believed t stabilize an alkylidene in the prcess f rtating 180, mre readily accessible. The mnmer that has been emplyed ften in investigatins f this general type is 2,3- bis(triflurmethyl)nrbrnadiene (NBDF6). On the basis f detailed NMR studies 6 it was prpsed that syn-m(nar)(ch-t-bu)[ocme(cf 3 )2]2 (Ar = 2,6-i-Pr 2 C 6 H 3 ) reacts with NBDF6 by crdinating t the ex face f the unsubstituted duble bnd t generate a syn first insertin prduct having a eis duble bnd (syn+l c ; equatin 2), while the anti rtamer reacts (at least 100 times mre rapidly) t give a syn first insertin prduct having a trans duble bnd (syn+l t ; equatin 3). Trans-ply(NBDF6) prepared frm M(NAr)(CHCMe 2 Ph)(0-t-Bu) 2 OR, v ax v RO Ar N syn syn+l c M=C «V m (2) y J R0*y \CF \cr 3 CF 3 H ArN.^ ReqO^ Vl 1 ORax anti R0*y RO Ar N M=C «\CF 3 V CF 3 \ (3) H syn+l t was prpsed t arise frm reactins invlving nly (unbservable) anti rtamers, since the anti rtamer is kinetically accessible at rm temperature (k s / a «Is -1 ; equatin 1), and since studies invlving M(NAr)(CHCMe2Ph)[OCMe(CF3)2]2 suggest that the anti rtamer is much mre reactive than the syn rtamer. Since Ke q = 10 3 the anti rtamer actually must be at least 10 5 mre reactive than the syn rtamer in rder t generate 99% trans duble bnds. 6 On the ther hand, use f M(NAr)(CHCMe 2 Ph)[OCMe(CF 3 ) 2 ]2 as the initiatr gave cis-ply(nbdf6). 9 In this case syn-m(nar)(chcme2ph)[ocme(cf3)2]2, the predminant species present in slutin at 25, is reactive enugh t react with NBDF6. Since a syn insertin prduct that cntains a eis duble bnd is frmed at each step the plymer cntinues t grw frm syn rtamers t give all-cis ply(nbdf6). Anti rtamers f alkylidene intermediates, even thugh they are much mre reactive, are simply inaccessible n the time scale f the plymerizatin reactin in the OCMe(CF3)2 system. Tacticity is believed t be a separate issue, at least in systems that have been explred s far. The fur pssible regular primary structures f plynrbrnene (fr purpses f illustratin) are shwn in Scheme 1. It is believed that a mlybdacyclbutane cmplex is frmed when the mnmer appraches ne f the tw CNO faces f the pseud-tetrahedral catalyst (equatin 2 r 3). The tw CNO faces crrespnd t the tw sides f the M=C bnd. If the mnmer appraches the same CNO face in each step, an istactic plymer is frmed; if the mnmer appraches alternate CNO faces sequentially, a synditactic plymer is frmed. If a plymerizatin prcess yields all trans r all eis plymer, then it is pssible in sme cases t determine the degree f tacticity in a relatively simple manner by 13 C NMR. 1 Recently it has becme pssible t prepare highly tactic all eis plymers with mlybdenum initiatrs that cntain a racemic binaphthlate ligand 10 and t prve that related all eis plymers that cntain chiral grups in the ester f a dicarbalkxynrbrnadiene are istactic while all trans plymers that cntain chiral grups in the ester f a dicarbalkxynrbrnadiene are synditactic. 7 It als has been shwn that all eis plymers prepared frm several enantimerically pure 2,3-disubstituted nrbrnenes emplying mlybdenum binaphthlate initiatrs are istactic. 7 Mlybdenum alkylidene cmplexes that cntain phenxide ligands have been relatively little studied. Unlike cmplexes that cntain t-butxide ligands, bth syn and anti rtamers are ften bservable and they intercnvert with activatin energies between 15 and 18 kcal mj-l 11,12 Mlybdenum phenxide cmplexes that have been briefly investigated as ROMP initiatrs were fund t prduce plymers that cntain bth eis and trans duble bnds. On the ther hand, linked phenxide ligands such as binaphthlates and biphenlates ften give rise t highly eis and highly tactic ROMP plymers. 7,10 In rder t explre the rigin f eis and trans plymers we chse t examine phenxide initiatrs in this paper. In rder t explre hw the tacticity f sme all eis plymers varies with the mnmer, the imid ligand, and the alkxide ligand, we chse t examine plymers prepared with M(NAr)(CHCMe2Ph)(OR)2 initiatrs where OR = OCMe(CF 3 ) 2, OC(CF 3 ) 3, r OC(CF 3 ) 2 (CF 2 CF2CF3). RESULTS Mlybdenum 2-t-ButylphenyIimid Cmplexes We suspected that the rate f rtamer intercnversin in mlybdenum initiatrs that cntain the 2-t-butylphenylimid ligand wuld be in an intermediate range where changes in temperature might dramatically effect the utcme f the plymerizatin reactin. An initiatr that can be purified by recrystallizatin is mst desirable, s M(N-2-t-Bu-C6H 4 )(CH-t- Bu)(OAr) 2, 13 an il, is unsuitable. M(N-2-t-Bu-C6H 4 )(CHCMe 2 Ph)(OR) 2 cmpunds where R = 2-t-Bu-C6H 4 r 2,4-t-Bu 2 C6H 3 were als fund t be ils, 14 but M(N-2-t-Bu- C6H 4 )(CHCMe 2 Ph)(BiphenBu 4 ) (BiphenBu 4 = 2,2'-[4,4,,6,6'-t-Bu 4 ](C 6 H 2 ) 16 ) culd be prepared as a crystalline THF adduct, l(thf). THF culd be remved in vacu frm a sample f l(thf) disslved in a high biling, nn-crdinating slvent such as tluene, and THF-free 1 thereby btained. 1, l(thf), and several related new and knwn crystalline t-bu initiatrs used in this wrk are listed in Table 1. The l U NMR spectrum f 1 at 23 C shwed ne sharp alkylidene resnance at ppm, while the 13 C NMR spectrum shwed ne sharp alkylidene resnance at 279 ppm with a value fr l Jcu (120 Hz) characteristic f a syn rtamer. 5 ' 6 N significant change was bserved upn cling the sample t -65 C. Hwever, the prtn NMR spectrum f l(thf) in tluene-dg at 26 C shwed a brad alkylidene H a resnance at ppm (Figure 1). When the NMR sample f l(thf) was cled, tw new alkylidene resnances grew in at ppm and ppm (Figure 1) and sharpened at the expense f the ppm resnance upn further cling, ultimately t give a spectrum cntaining tw alkylidene resnances in a 1:3 rati. Additin f THF (-10 equiv) t a sample f 1 in tluene-ds at -65 C generated a sample having the same tw alkylidene resnances fund in the spectrum f l(thf) at lw temperature. Therefre we prpse that the H a resnances at and ppm are due t THF adducts f anti and syn rtamers, respectively, i.e., lanti(thf) and l syn (THF). (It is well-dcumented that H a resnances in alkylidene cmplexes in this family mve dwnfield upn adduct frmatin, and that anti H a resnances are fund dwnfield f syn H a resnances. 5 ) The tw unidentified tiny resnances at and ppm bserved near the resnances fr l ant i(thf) and l syn (THF) culd nt be identified. They culd pssibly be structural ismers in which THF crdinates t the metal in a psitin ther that what is believed t be the favred CNO face. 6 As the temperature f the sample cntaining l an ti(thf) and l syn (THF) is raised, THF appears t cme ff the syn rtamer mre readily than the anti rtamer, as judged by bradening f the ppm resnance first at -15 C (Figure 1) and lss f intensity as K eq shifts tward the THF-free cmplex. At higher temperatures all resnances calesce int a single brad alkylidene H a resnance between 11.0 and 11.1 ppm as the equilibrium shifts virtually entirely tward the basefree frm f 1 and THF exchange is rapid n the NMR time scale. Samples f 1 were phtlyzed at -75 C in tluene in an attempt t prduce an bservable amunt f land- (Phtlysis has been fund t cause intercnversin f syn and anti rtamers f M 6 and Re 17 alkylidene rtamers.) Hwever, n lanti accumulated after 18h f phtlysis. Either back reactin f l an ti t l syn (phtchemical r thermal) is t fast n the chemical time scale at -75 C, r phtlysis is nt an effective methd f effecting anti/syn intercnversin in phenxide cmplexes. The frmer seems mre likely t be the case, in part since bending f the mn-substituted phenylimid ligand is knwn t reduce the barrier t alkylidene rtatin significantly in OCMe(CF3)2 cmplexes. 6 Therefre we suspect that l syn and l an ti can equilibrate n the chemical time scale even at -75 C and that the equilibrium lies well tward the syn rtamer. It is interesting t nte that fr the analgus 2,6-diisprpylphenylimid cmplex (3(THF); Table 1) the antirsyn rati is 1:3 at rm temperature in tluene-ds- (On the basis f chemical shifts it is likely that the anti frm is the THF adduct and the syn frm the THF-free frm f 3.) Perhaps the additinal steric hindrance caused by the 2,6-disubstituted phenyl ring decreases the stability f the THF-free syn rtamer relative t the THF adduct f the anti rtamer in additin t significantly slwing the rate f intercnversin f anti and syn frms. Plymerizatins Emplying 1 as the Initiatr Plymerizatins f NBDF6 r DCMNBD (2,3-dicarbmethxynrbrnadiene) were carried ut with 1 as the initiatr in THF, tluene, r dichlrmethane (Table 2). In discussing these results we will assume that l an ti is in equilibrium with l syn n a time scale f the rder f the plymerizatin reactin, that trans duble bnds are frmed nly frm anti rtamers and eis duble bnds nly frm syn rtamers, and that nly pseud fur-crdinate THF-free cmplexes are reactive. 6 (We will later discuss whether these assumptins are valid.) The first six entries in Table 2 shw data fr samples f ply(nbdf6)i- Hmgeneus plymerizatins were bserved when THF was emplyed as a slvent, but in tluene anther phase frmed as plymerizatin prgressed. Yields were high at rm temperature r abve, but lw at lw temperatures, even with a plymerizatin time f 48 hr. At temperatures belw -10 we prpse that plymerizatin is slwed dwn t a significant degree simply by crdinatin f THF (cmpetitive inhibitin), mst strngly t the anti rtamer. (The anti rtamer is knwn t have a greater affinity fr a-dnr bases such as phsphines. 5 ) THF therefre selectively pisns the anti rtamer by cmpeting mre effectively with the mnmer fr the crdinatin site in the anti rtamer than in the syn rtamer. Cnsequently nly eis bnds are prduced via syn rtamers at -35 C since effectively nly the THF-free syn alkylidene is accessible. Hwever, the reactin is extremely slw since s little THF-free alkylidene is available. Cnversely, at 65 THF is dissciating readily t give bth anti and syn fur-crdinate alkylidene intermediates. Since syn/anti intercnversin is believed t be rapid, the majrity f the plymer arises via anti intermediates, and 76% trans plymer is frmed. Ply(NBDF6) prepared at 23 in THF has apprximately a 1:1 rati f trans and eis duble bnds. In tluene at 23 reactin f the mnmer with the anti frm is n lnger as inhibited by THF as it is at lw temperature and the % trans cntent therefre increases frm what it is in pure THF, frm 42% t 68% (Table 2). Plymerizatin f 2,3-dicarbmethxynrbrnadiene with 1 in tluene yielded analgus results (Table 2). Plymerizatin at -35 was carried ut in dichlrmethane in rder t enhance slubility f the plymer at that temperature. At -35 a 36% yield f 100% eis plymer was realized in 12 hr. At higher temperatures in tluene the % trans steadily increased t 81% at 80. A cntrl experiment ruled ut the pssibility that trans plymer results frm ismerizatin f eis duble bnds in the plymer (secndary metathesis). All-cis ply(dcmnbd) (100 mer) that had been prepared using 2(THF) as the initiatr 10 was added t the living alkylidene generated by adding 10 equivalents f DCMNBD t 1, and the mixture was heated fr 2h at 80 C. The trans cntent did nt increase t any significant degree, cnsistent with slw secndary metathesis, even at 80. In rder t cnfirm that syn and anti rtamer accessibility is the rigin f the dependence f plymer structure n temperature, it wuld be desirable t shw that the cis/trans structure f plymers prepared with OCMe(CF3)2 initiatrs, where a syn rtamer is knwn t generate syn+l c i s (equatin 2) while an anti rtamer generates syn+l tra ns (equatin 3) 6, is als temperature dependent. It is already knwn that bth syn and anti rtamers f OCMe(CF3)2 cmplexes are qualitatively much mre reactive in tluene than they are in THF and that the reactivity f an anti rtamer decreases mre significantly in THF than the reactivity f a syn rtamer in THF, relative t their reactivities in tluene. The best knwn OCMe(CF 3 ) 2 initiatr is M(NAr)(CHCMe 2 Ph)[OCMe(CF 3 ) 2 ]2 (7), which generates high eis ply(nbdf6) in tluene r THF at 25 C (Table 3). 9 At 65 the eis cntent drps t 88% fr ply(nbdf6) prepared in tluene and t 83% fr ply(nbdf6) prepared in THF (Table 3), cnsistent with greater participatin f the anti rtamer (t give trans plymer) in reactins at the higher temperatures. Plymerizatins emplying M(N-2-t- BuC6H 4 )(CHCMe 2 Ph)[OCMe(CF3)2]2 (4) must be carried ut at -35 C in rder t achieve a similarly high eis cntent because rtamer intercnversin in M(N-2-t-BuC6H4) cmplexes is abut three rders f magnitude faster than in NAr cmplexes. 6 When the temperature is raised t 65 the eis cntent drps t 29%. These results are cnsistent with the prpsal that a higher rate f rtamer intercnversin leads t higher trans plymer as a cnsequence f a greater accessibility f anti rtamers, and that THF reduces the reactivity f an anti rtamer mre strngly than a syn rtamer t prduce a plymer with a higher eis cntent cmpared t a plymer prepared in tluene. Predictably, plymer yields are lwer in THF r at lw temperatures as a cnsequence f slw and incmplete plymerizatin. Tw ther new 2,6-diisprpylphenylimid phenxide initiatrs (3 and 5) were used t make ply(nbdf6)i at varius temperatures and the results cmpared with thse btained emplying 6 as the initiatr (Table 3). The trend f the effect f temperature n cis/trans structure using all three initiatrs is tward higher trans cntent f the plymer at higher temperature and in tluene. The frmatin f predminantly eis plymer emplying 3 was expected since the analgus nepentylidene initiatr als yields virtually all eis plymer. 6 At 65 sme decrease in the eis cntent is bserved, analgus t the results emplying 7 (Table 3). The Tacticity f High Cis Plymers Prepared frm M(NAryl)(CHCMe2Ph)(OR)2 Initiatrs (OR = OCMe(CF 3 ) 2, OC(CF
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