Ansa-metallocene polymerization catalysts derived SPECIAL FEATURE from [2؉2]cycloaddition reactions of bis(1-methylethenyl-cyclopentadienyl)zirconium 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-zirconocene dichloride 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 metallocenes 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 transition metal components for the generation of active metallocene propene CHEMISTRY polymerization catalysts. photochemistry ͉ topochemical reaction etallocene catalysis 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 ligands. Because of the sensitive character of the organometallic 4, from which an interesting homogeneous metallocene Ziegler- group 4 metal complexes, ligand 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.