Ansa- polymerization catalysts derived SPECIAL FEATURE from [2؉2]cycloaddition reactions of bis(1-methylethenyl-cyclopentadienyl) systems

Jan Paradies, Gerald Kehr, Roland Fro¨ hlich, and Gerhard Erker†

Organisch-Chemisches Institut der Universita¨t Mu¨ nster, Corrensstrasse 40, 48149 Mu¨nster, Germany

Edited by Tobin J. Marks, Northwestern University, Evanston, IL, and approved June 22, 2006 (received for review April 11, 2006)

Bis(1-methylethenyl-cyclopentadienyl)zirconium dichloride (7a) was prepared by a fulvene route. Photolysis at 0°C with Pyrex- filtered UV light resulted in a rapid and complete intramolecular -2؉2]cycloaddition reaction to yield the corresponding cyclobuty] lene-bridged ansa- isomer (8a). This is one of the rare examples of an organic functional group chemistry that leads to carbon–carbon coupling at the framework of an intact sensitive group 4 bent metallocene complex. More sterically hin- dered open that bear bulky isopropyl or tert-butyl substituents at their Cp rings in addition to the active 1-methyl- ethenyl functional group undergo the photochemical ansa-metal- locene ring closure reaction equally facile. The metallocene systems used and obtained in this study have served as components for the generation of active metallocene propene CHEMISTRY polymerization catalysts.

photochemistry ͉ topochemical reaction

etallocene has become of great significance in Molefin polymerization, especially for polyethylene, ste- Scheme 1. reospecific polypropylene formation, and the production of some copolymers. The ansa-metallocenes of the group 4 metals and related systems play an essential role in this important and closed isomers of most investigated examples under practical development (1–3). Various methods have been devised and conditions (23, 24). We later showed that the [2-(1- applied to construct such bent metallocene frameworks that methylethenyl)indenyl]2ZrCl2 derivative (3) rapidly and com- feature a short carbon- or heteroatom-containing bridge be- pletely underwent the intramolecular photolytic [2ϩ2]cycload- tween their substituted cyclopentadienyl, indenyl, or fluorenyl dition reaction when irradiated with Pyrex-filtered light to yield . Because of the sensitive character of the organometallic 4, from which an interesting homogeneous metallocene Ziegler- group 4 metal complexes, construction and variation is Natta catalyst system was generated (25–28). This posed the usually carried out at the free ligand stage before the final question as to whether the outcome of the intramolecular ϩ transmetallation step to the transition metal in the practical [2 2]photocyclization reaction at the group 4 bent metallocenes preparative sequences. This is a serious synthetic limitation. It might significantly depend on the substituent at the alkenyl would be highly desirable to have an organic functional group functional group and that the use of, e.g., the 1-methylalkenyl chemistry developed for framework variation at the actual moiety might actually lead to a synthetically favorable situation. metallocene stage. Previously, some addition reactions to me- We have now prepared a small series of respective (1- tallocene frameworks had been reported, such as catalytic methylethenyl-Cp)-derived group 4 metallocenes and found that the systems tested rapidly and completely underwent the in- hydrogenation (4), hydrosilylation (5), hydroboration (6–9), or tramolecular [2ϩ2]cycloaddition to give their cyclobutylene- borylation (refs. 10, 11 and references therein, and 12). However, bridged ansa-zirconocene isomers, which were subsequently carbon–carbon coupling reactions at the intact group 4 bent used to generate active metallocene Ziegler-Natta catalysts metallocene frameworks were close to nonexistent before our (Scheme 1). work (13–15). Meanwhile, a few leading examples have emerged from the literature, using, e.g., olefin-metathesis (16–19) or even Results and Discussion a variant of the Mannich reaction (20–22) for carrying out The alkenyl-functionalized group 4 bent metallocenes (7) were carbon–carbon coupling reactions at the reactive group 4 bent synthesized by fulvene-derived routes (29–31). For the prepa- metallocene frameworks. Intramolecular photochemical [2ϩ2]cycloaddition reactions may become of a prime importance in this development. We had Conflict of interest statement: No conflicts declared. previously observed that bis(alkenyl-Cp)ZrCl2 complexes such This article is a PNAS direct submission. as, e.g., meso-1 underwent ansa-metallocene formation to yield Abbreviations: DSC, differential scanning calorimetry; MAO, methylalumoxane. ϩ meso-2 by intramolecular [2 2]cycloaddition when irradiated Data deposition: The crystallographic data have been deposited in the Cambridge Struc- with UV light. However, this specific reaction was not synthet- tural Database (CSD), Cambridge Crystallographic Data Centre, www.ccdc.cam.ac.uk͞cgi- ically useful for a clean ansa-metallocene catalyst development bin͞catreq.cgi (CSD reference nos. 615770–615772). because of its reversibility under the photochemical conditions †To whom correspondence should be addressed. E-mail: [email protected]. to result in a photostationary equilibrium mixture of the open © 2006 by The National Academy of Sciences of the USA

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0602627103 PNAS ͉ October 17, 2006 ͉ vol. 103 ͉ no. 42 ͉ 15333–15337 Downloaded by guest on September 29, 2021 Scheme 2.

ration of the ligands the 6,6-dimethylfulvene derivatives (5) were oriented cis to each other. The bridging C11–C14 bond length treated with lithium diisopropylamide to give the corresponding inside the four-membered ring is 1.543(4) Å [1.552(4) Å]. alkenyl-Cp lithium reagents (6), which were then reacted with The reaction of the 3-substituted lithium alkenyl cyclopenta- ZrCl4(THF)2 (32) to yield the (1-methylethenyl-Cp)2ZrCl2 com- dienide reagents (6b and 6c) with ZrCl4(THF)2 resulted in the plexes (7) (Scheme 2). formation of pairs of planarly chiral subunits and consequently to (Ϸ1:1) mixtures of the respective meso- and rac-7b͞7c ste- Photochemical ansa-Zirconocene Formation. Bis(1-methylethenyl- reoisomers. The pairs of isomers each show different spectra, but Cp)ZrCl2 (7a) behaves like a conformationally rapidly equili- their relative stereochemical assignment cannot directly be brating system of an averaged C2v-symmetry in solution (33) as derived from these because the averaged Cs- and C2-symmetric judged from its NMR spectra. The photolysis of 7a with Pyrex- species feature analogous NMR patterns. Fortunately, the dif- filtered light in toluene at 0°C led to a practically quantitative ferent solubilities of the rac-7c and meso-7c diastereoisomers in conversion to the [2ϩ2]cycloaddition product 8a within 2.5 h. n-heptane allowed a separation, and eventually complex meso-7c Because of the lower symmetry (C ), the product 8a now features could be characterized by x-ray diffraction (Fig. 3); with sup- s 1 1 porting H NMR information, this analysis allowed for tentative four separate C5H4 H NMR resonances (␦ 6.57, 6.35, 5.94, and structural assignments of the planarly chiral meso͞rac-7b͞7c 5.74) and a single CH3 signal at ␦ 1.11 (6H). The head-to-head cycloaddition has resulted in the formation of a bridging cy- diastereoisomers. 7c clobutylene ring system with each a pair of CH groups as well In the crystal, complex meso- attains a chiral conformation 3 with both tert-butyl groups pointing to one side but offset by as Cp ligands cis-1,2-attached to it. Consequently, the cyclobu- Ϸ1͞5 Cp rotation. The C(Me)ACH functional groups are tane OCH OCH O moiety exhibits different signals of meth- 2 2 2 likewise oriented at the opposite lateral sector of the bent ylene hydrogens oriented cis (␦ 2.16) and trans (␦ 1.70) to the Cp metallocene with the alkenyl planes oriented close to parallel to rings (Scheme 3). their adjacent Cp planes [dihedral angles: C7A-C6A-C1A-C2A, 7a 8a Both of the complexes and were characterized by x-ray Ϫ168.0(2)°; C7B-C6B-C3B-C2B, Ϫ3.4(4)°] [bond lengths: C6A- diffraction. The open metallocene 7a features a conformational C7A, 1.345(4) Å; C6B-C7B, 1.374(5) Å]. One of the CACH2 arrangement in the crystal where both the 1-methylethenyl groups (C6B-C7B) points to the front, and the other (C6A-C7A) substituents are C2-symmetrically oriented at the open front side points to the narrow back side of the bent metallocene wedge. of the bent metallocene wedge (for a comparison, see ref. 34). The rac͞meso-7b complex mixture (1:1) was photolyzed in The Zr-C(Cp) bond lengths are in a narrow range between toluene solution at room temperature. Within 1 h, a complete 2.476(2) Å and 2.566(3) Å [Cp(centroid)-Zr-Cp(centroid) angle, conversion to the respective cyclobutylene-bridged ansa- 129.8°; Cl1-Zr-Cl1*, 96.20(4)°]. The CAC double bond of the zirconocene isomers (8b) was effected. Principally, a mixture of alkenyl substituent is oriented in plane with its adjacent Cp ring [C4-C5A, 1.34(2) Å] (Fig. 1). The ansa-metallocene complex 8a is close to Cs-symmetric in the solid state (but not crystallographically). It features a pair of eclipsed Cp rings at zirconium [Cp(centroid)-Zr-Cp(centroid), 124.7͞125.5°; Cl1-Zr-Cl2, 97.59(5)͞100.80(5)°] that are linked by the newly formed cyclobutylene bridge. The bridge is located at the narrow back side of the bent metallocene wedge. Because the C2 bridge fits the geometry of the group 4 bent metallocene framework very well, the Zr-C(Cp) bond lengths are found in a rather narrow range between 2.484(5) Å (molecule A) [2.481(5) Å, molecule B] and 2.524(5) Å [2.518(6) Å]. The structure (see Fig. 2) confirms the formation of the cyclobutane ring in 8a by head-to-head [2ϩ2]cycloaddition. At the four-membered ring, the methyl groups are attached at adjacent carbon atoms and are

Scheme 3. Fig. 1. Molecular structure of complex 7a.

15334 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0602627103 Paradies et al. Downloaded by guest on September 29, 2021 SPECIAL FEATURE

Fig. 2. Molecular structure of the cyclobutylene-bridged ansa-zirconocene complex 8a (molecule B is shown).

three stereoisomers could be formed in this case, namely a rac

and two meso (cis-meso and trans-meso) isomers. However, we CHEMISTRY observed the formation of a 1:1 mixture of rac-8b with only one of the meso isomers. We have tentatively assigned this product the trans-meso-8b structure from 1H NMR NOE measurements. 1 It features a set of three H NMR C5H3 signals of the pair of Fig. 3. A view of the molecular structure of complex meso-7c. symmetry-equivalent 1,3-disubstituted Cp ligands [at ␦ 6.38, 6.00, and 5.56 (each m, each 2H)], two signals of the 13 1 cyclobutyleneOCH2-CH2O protons (␦ 2.15͞1.54), and one set ( C)], and their attached methyl groups are diastereotopic [ H of 1H NMR signals for the pair of symmetry-equivalent isopropyl NMR, ␦ 1.35, 1.22, 1.15, and 1.11 (each d, each 3H)]. substituents [␦ 1.28 (d, 6H), 1.11 (d, 6H, CH3), ␦ 3.40 (sept, 2H)], Photolytic conversion of the 1:1 rac͞meso-7c mixture to the as well as a singlet of the pair of CH3 groups at the cyclobutylene ansa-metallocene isomers rac͞meso-8c was complete after1hin bridge [␦ 0.98 (s, 6H)]. toluene at room temperature. Again, the formation of only one 1 213 The isomer rac-8b features a total of ten C5H3R R C NMR of the two possible meso-8c isomers was observed. Analogous to signals (␦ 146.4, 142.2, 141.3, 139.8, 122.0, 116.5, 109.6, 109.1, the isopropyl-substituted system 8b, this isomer was tentatively 105.4, and 104.9) because of its lower symmetry. Likewise, there assigned the trans-meso-8c structure according to its 1H NMR are a total of four cyclobutylene 13C NMR resonances [␦ NOE spectra (see Scheme 4). 1 49.8͞49.5 (quart), ␦ 30.8͞30.3 (CH2)] and the H NMR signals From the mixture the meso-8c isomer is easily recognized by 1 of a pair of attached methyl substituents [␦ 1.01͞1.00 (s, 3H NMR because of its Cs-symmetry [three H NMR C5H3 signals, each)]. The isopropyl substituents are spectroscopically differ- ␦ 6.52 (H-4), 6.10 (H-5), and 5.98 (H-2), each of 2H intensity; five 13 entiated because of their different position at the rigid rac-8b C NMR C5H3 resonances, ␦ 144.5 (C-3), 139.0 (C-1), 120.2 metallocene framework [␦ (CH): 3.41͞3.32 (1H); 29.1͞28.7 (C-4), 109.3 (C-2), and 108.5 (C-5)] and distinguished from the

Scheme 4.

Paradies et al. PNAS ͉ October 17, 2006 ͉ vol. 103 ͉ no. 42 ͉ 15335 Downloaded by guest on September 29, 2021 Table 1. Propene polymerization with the metallocene͞MAO catalyst systems Catalyst, † No. mg (mmol) Polypropylene, g Activity % mmmm Mn Mw PDI

7a 14 (0.037) 34.3 464 6 1,520 2,690 1.78 8a 22 (0.059) 36.6 309 2 3,490 7,990 2.29 7b 20 (0.044) 11.5 131 4 3,830 7,680 2.01 8b 24 (0.053) 66.8 630 24 1,910 3,580 1.87 7c 14 (0.028) — — — — — — 8c 14 (0.028) 2.9 52 42 680 1,070 1.58

In toluene solution, MAO activated, 20°C, 2 bar propene, 1-h reaction time. 7b, 8b, 7c, and 8c were employed as 1:1 meso/rac-mixtures. PDI, polydispersity index. †In g polypropylene͞mmol [Zr]⅐h⅐(bar propene).

1 C1-symmetric rac-8c isomer (six H NMR C5H3 signals, ␦ 6.53, us hope for a broad application spectrum of this coupling method 13 6.30, 6.15, 6.11, 5.87, and 5.80; ten C NMR C5H3 signals, ␦ for ansa-metallocene formation and general carbon–carbon 151.0, 142.3, 141.4, 138.7, 122.9, 114.2, 113.0, 109.9, 106.8, and coupling at the stage of the intact early metal bent metallocene 103.8). The rac-8c isomer features two 13C NMR signals of the frameworks and other related sensitive organometallic systems. cyclobutylene OCH2OCH2O unit (␦ 30.8 and 30.3; adjacent quaternary carbon signals at ␦ 50.0 and 49.3), whereas meso-8c Materials and Methods 13 exhibits only a single C NMR -CH2-CH2- resonance (at ␦ 30.2). Preparation of the (1-Methylethenyl-Cp)2ZrCl2 Complexes. For the Analogously, there are the signals of two different methyl groups preparation of the parent compound (7a), 6,6-dimethylfulvene in rac-8c (1H, ␦ 1.06 and 1.04; 13C, ␦ 26.4 and 25.1) and the (5a) was treated with lithium diisopropylamide to give 6a. The resonances of a pair of symmetry-equivalent CH3 substituents at alkenyl-Cp lithium reagent (6a, 10.5 g, 94.0 mmol) was reacted 1 the cyclobutylene four-membered carbocycle in meso-8c ( H, ␦ with ZrCl4(THF)2 (17.7 g, 47 mmol) in toluene (Ϫ78°C to room 1.02; 13C, ␦ 24.1). temperature) to yield 9.45 g (54%) of 7a after recrystallization 1 from dichloromethane. H NMR (500 MHz, 298 K, CD2Cl2): ␦ Propene Polymerization Reactions. Propene polymerization reac- 6.51, 6.32 (each m, each 4H, 2-H to 5-H), 5.38͞5.14 (4H, ACH2), 13 tions were carried out in toluene solution at 20°C. Catalyst 2.05 (dd, J ϭ 1.5 Hz͞0.9 Hz, 6H, CH3); C NMR (150.8 MHz, activation was achieved by treatment of the zirconocene dichlo- 298 K, CD2Cl2): ␦ 137.0 (C6), 131.4 (C1), 115.2͞114.7 (C2–5), ride complexes with a 500- to 1,000-fold excess of methylalu- 113.7 (C7), 21.5 (C8); mp 180°C [decomp 273°C, differential moxane (MAO). The catalysts derived from both the open scanning calorimetry (DSC)]; anal. calculated for C16H18Cl2Zr metallocene 7a as well as its ansa-metallocene isomer 8a were (372.4): C 51.60%, H 4.81%; found: C 51.14%, H 4.95%. X-ray quite active. Both gave atactic polypropylenes of a rather low crystal structure analysis (single crystals from a concentrated molecular weight (Table 1). The chiral metallocenes 7͞8 (b, c) solution in dichloromethane and storage at ϩ4°C): orthorhom- were used as 1:1 meso͞rac mixtures. The open metallocene bic, space group Fmm2 (no. 42), a ϭ 11.805(1), b ϭ 19.140(1), 3 Ϫ3 7b͞MAO system showed a moderate catalyst activity yielding an c ϭ 6.773(1) Å, V ϭ 1,530.3(3) Å , ␳calc ϭ 1.616 g⅐cm , ␮ ϭ 1.051 atactic polymer, whereas the catalyst derived from the 8b isomer mmϪ1, empirical absorption correction (0.710 Յ T Յ 0.979), Z ϭ was more active and gave a slightly isotactic polypropylene. The 4, ␭ ϭ 0.71073 Å, T ϭ 198 K, ␻ and ␸ scans, 1,818 reflections meso͞rac-7c͞MAO system was inactive under the applied reac- collected (Ϯh, Ϯk, Ϯl), [(sin␪)͞␭] ϭ 0.66 ÅϪ1, 887 independent tion conditions, probably because of steric hindrance. Consis- reflections (Rint ϭ 0.032) and 882 observed reflections [I Ն 2 tently, the isomeric meso͞rac-8c͞MAO catalyst was much less 2␴(I)], 60 refined parameters, R ϭ 0.020, wR ϭ 0.050, ACH2 active than the related less hindered 7a͞MAO and 7b͞MAO (C5A) and OCH3 (C5B) group refined independently with systems but produced a low-molecular-weight polypropylene of occupancy 0.5 for each; hydrogen atoms calculated and refined markedly higher isotacticity. It remains to be clarified which of as riding atoms. the respective substituted catalyst components, rac-ormeso-7͞8, The substituted derivatives were prepared analogously. In are predominantly responsible for the formation of the rather theses cases, Ϸ1:1 mixtures of meso͞rac-7b and meso͞rac-7c were narrow polydispersity polypropylenes. obtained. The reaction of 6b (10.3 g, 66.8 mmol) with ZrCl4(THF)2 (12.6 g, 33.4 mmol) in 150 ml of toluene and Concluding Remarks. Our study has shown that the intramolecular recrystallization from dichloromethane gave 11.5 g (76%) of photochemical [2ϩ2]cycloaddition reaction can in some cases meso͞rac-7b (1:1). Anal. calculated for C22H30Cl2Zr (456.6): C reliably be used for the preparation of ansa-metallocenes that 57.87%, H 6.62%; found: C 58.41%, H 6.94%; mp 107°C serve as transition-metal components to generate active olefin (decomp 205°C, DSC). 1H NMR (meso-7b, 600 MHz, 298 K, polymerization catalysts. This easily performed carbon–carbon d6-benzene): ␦ 6.37 (m, 2H, H-2), 5.95 (m, 2H, H-5), 5.69 (m, 2H, coupling reaction thus represents one of the few examples of a H-4), 5.17 (br d, J ϭ 2.2 Hz, 2H, H-7Z), 4.89 (br d, J ϭ 2.2 Hz, useful functional group chemistry at the backbone of a sensitive 2H, H-7E), 3.17 (sept, J ϭ 6.9 Hz, 2H, CHMe2), 1.86 (dd, J ϭ 1.6 early metal bent metallocene. It seems that the 1-methylethenyl Hz͞0.8 Hz, 6H, H-8), 1.17 (d, 6H) and 1.09 [d, J ϭ 6.9 Hz, 6H, 1 substituent was a good choice because it resulted in a very rapid CH(CH3)2]; H NMR (rac-7b): ␦ 6.50 (m, 2H, H-2), 5.98 (m, 2H, photochemical ring-closure reaction. Under the applied reaction H-5), 5.57 (m, 2H, H-4), 5.13 (dd, J ϭ 2.2 Hz͞1.5 Hz, 2H, H-7Z), conditions, this substitution pattern achieved a very favorable 4.86 (dd, J ϭ 2.2 Hz͞0.8 Hz, 2H, H-7E), 3.31 (sept, J ϭ 6.9 Hz, photostationary equilibrium that was lying practically completely 2H, CHMe2), 1.84 (dd, J ϭ 1.5 Hz͞0.8 Hz, 6H, H-8), 1.27 (d, J ϭ on the ansa-metallocene side, quite different from most early 6.9 Hz, 6H), and 1.10 [d, 6H, CH(CH3)2]. For additional data, see examples reported of this general reaction type at the bent Supporting Appendix, which is published as supporting informa- metallocene nucleus (23, 24). The interesting observation that tion on the PNAS web site. the [2ϩ2]photocyclization is not adversely affected by the pres- The 1:1 meso͞rac-7c complex mixture was prepared analo- ence of even very bulky alkyl substituents at the Cp rings makes gously from 500 mg (2.97 mmol) of 6c and 555 mg (1.48 mmol)

15336 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0602627103 Paradies et al. Downloaded by guest on September 29, 2021 of ZrCl4(THF)2 to yield 539 mg (75%) of product. Anal. four-membered rings, indicating some unsolved disorder that calculated for C H Cl Zr (484.7): C 59.48%, H 7.07%; found: was refined with geometrical constraints (SADI); two almost

24 34 2 SPECIAL FEATURE C 59.58%, H 7.00%; mp 183°C (decomp 294°C, DSC). X-ray identical independent molecules in the asymmetric unit; hydro- crystal structure analysis of meso-7c (single crystals from dichlo- gen atoms calculated and refined as riding atoms. For spectral romethane at ϩ4°C): monoclinic, space group P21͞n (no. 14), data, see the text and Supporting Appendix. a ϭ 11.746(1), b ϭ 14.878(1), c ϭ 14.452(1) Å, ␤ ϭ 110.35(1)°, Photolysis of a solution (2.00 g, 4.38 mmol) of meso͞rac-7b 3 Ϫ3 Ϫ1 V ϭ 2,368.0(3) Å , ␳calc ϭ 1.359 g⅐cm , ␮ ϭ 6.97 mm , (1:1) in 20 ml of toluene under similar conditions (1 h, room empirical absorption correction (0.873 Յ T Յ 0.979), Z ϭ 4, ␭ ϭ temperature) gave meso͞rac-8b (1.56 g, 78%) as a 1:1 mixture. ϭ ␻ ␸ 0.71073 Å, T 198 K, and scans, 25,904 reflections collected Anal. calculated for C22H30Cl2Zr (456.6): C 57.87%, H 6.62%; Ϫ1 (Ϯh, Ϯk, Ϯl), [(sin␪)͞␭] ϭ 0.66 Å , 5,624 independent (Rint ϭ found: C 56.51%, H 6.77%. Analogously, a 1:1 meso͞rac-8c 0.068) and 4,446 observed reflections [I Ն 2␴(I)], 252 refined mixture (1.85 g, 82% yield) was isolated from photolysis of 2.00 g parameters, R ϭ 0.036, wR2 ϭ 0.077. For spectroscopic data, see (34.13 mmol) of meso͞rac-7c (1:1) dissolved in 100 ml of toluene, Supporting Appendix. mp 143°C (decomp 214°C, DSC). Anal. calculated for C24H34Cl2Zr (482.1): C 59.48%, H 7.07%; found: C 59.40%, H Photolysis Reactions. The photolysis of a suspension of 7a (4.25 g, 7.04%. For spectral data, see Supporting Appendix. 11.4 mmol) in 30 ml of toluene at 0°C (Philips HPK 125-W, Pyrex Propene polymerization reactions were carried out in a ther- filter) was carried out for 2.5 h. Solvent was removed in vacuo, mostated Bu¨chi glass autoclave system, which was charged with and the product was washed with pentane to yield 4.12 g (97%) toluene (200 ml) and 20 ml of a 10.5 wt % MAO solution in of the ansa-metallocene product 8a, mp 207°C (DSC). Anal. toluene. At 20°C propene [2 bar (1 bar ϭ 100 kPa)] was added calculated for C16H18Cl2Zr (372.4): C 51.60%, H 4.87%; found: for 30 min with stirring and then a solution of 10 mg of the C 51.56%, H 5.00%. X-ray crystal structure analysis (single Zr-catalyst precursor in 20 ml of toluene. After1hofreaction crystals from a concentrated solution in toluene at room tem- time, the reaction was stopped by careful addition of a 1:1 perature): monoclinic, space group P21͞n (no. 14), a ϭ mixture of 2 M aqueous HCl and methanol. Excess monomer was 10.404(1), b ϭ 21.667(1), c ϭ 13.758(1) Å, ␤ ϭ 98.16(1)°, V ϭ vented, and the polymer was extracted with toluene, stripped, 3 ␳ ϭ ⅐ Ϫ3 ␮ ϭ Ϫ1 3,070.0(4) Å , calc 1.612 g cm , 1.048 mm , empirical and dried in vacuo. Characterization of the obtained polypro- absorption correction (0.744 Յ T Յ 0.818), Z ϭ 8, ␭ ϭ 0.71073 pylene samples was carried out by GPC and 13C NMR pentad Å, T ϭ 198 K, ␻ and ␸ scans, 27,782 reflections collected (Ϯh,

analysis (refs. 35–38; and ref. 39 and references therein); for CHEMISTRY Ϫ Ϯk, Ϯl), [(sin␪)͞␭] ϭ 0.66 Å 1, 7,333 independent reflections details, see Table 1 and Supporting Appendix. (Rint ϭ 0.055) and 5,603 observed reflections [I Յ2␴(I)], 347 refined parameters, R ϭ 0.054, wR2 ϭ 0.168, max. residual This work was supported by the Deutsche Forschungsgemeinschaft and electron density 1.60 (Ϫ1.07) e ÅϪ3 in the region of the the Fonds der Chemischen Industrie.

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