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A Rigid Cj-Bridged Ansa-Zirconocene-Derived

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A Rigid Cj-Bridged Ansa-Zirconocene-Derived A Rigid Cj-Bridged Ansa-Zirconocene-Derived Catalyst System Suited for Stereoselective Low Molecular Weight Polypropylene Formation Gerhard Erker*, Christian Psiorz, Roland Fröhlich Organisch-Chemisches Institut der Universität Münster, Corrensstraße 40, D-48149 Münster Dedicated to Prof. Dr. Dr. h.c. mult. Günther Wilke on the occasion o f his 70th birthday Z. Naturforsch. 50b, 469-475 (1995); received September 20, 1994 Homogeneous Ziegler Catalyst, Isotactic Polypropylene, Ansa-Metallocene, Fulvene 2,5-Hexanedione was converted into the bisfulvene 2, then treated with two molar equiva­ lents of methyllithium to yield the [4-cyclopentadienylidene-4,7,7-trimethyl-4,5,6,7-tetra- hydroindenyl]dilithio compound 4. Hydrolysis, followed by treatment with acetone/pyrroli­ dine, gave the corresponding fulvene system 5. Reaction of 5 with methyllithium followed by treatment with ZrCl4 furnished the ring-annulated Cr bridged ansa-metallocene 8, bearing a tert-butyl substituent at the Cp ring, as a 1:1 mixture of two diastereoisomers. Treatment of the fulvene 5 with LiAlH4 followed by ZrCl4 yielded the respective isopropyl-substituted ansa-metallocene diastereomers 9a and 9 b. Com plex 9 b was separated by fractional crys­ tallization and characterized by X-ray diffraction. Complexes 8 and 9 provide active homo­ geneous Ziegler-type catalyst systems upon activation with excess methylalumoxane produc­ ing low molecular weight isotactic polypropylene with high catalyst activities. Introduction Homogeneous group 4 bent metallocene methylalumoxane-derived Ziegler-type catalyst systems have become of enormous importance for the development of a-olefin polymerization [ 1], Most of the catalyst systems of practical impor­ tance are derived from ansa-metallocene precur­ I sors [2]. Developing ansa-metallocene derivatives and novel ansa-metallocene structural types has been very rewarding as this has turned out to be knowledge this is the smallest D -Z r -D angle in of prime importance for significantly changing and a bent metallocene so far [4]. Decreasing the controlling the catalyst properties. D -Z r -D angle results in a pronounced change We have recently developed a new ansa-metal­ of the electronic features of the bent metallocene locene system which is characterized by a very system and thus of the catalyst behavior [5], high backbone rigidity [3]. The Cp ring systems Homogeneous Ziegler-catalyst systems derived are connected by a one-carbon atom bridge which from 1 by MAO activation have rather specific at the same time is part of a Cp-annulated six- properties [3]: they are very active; they produce membered ring system. Binding a large zirconium rather low molecular weight polypropylene or pro- center into this specially fused bis-cyclopenta- pene oligomers at higher temperatures, respec­ dienyl system creates a very rigid ansa-metallo­ tively, and the polypropylene obtained is slightly cene system which is characterized by a very open syndiotactic (a ~ 0.30-0.35; chain end control bite of the bent metallocene wedge. [6]). Such catalyst features are potentially useful The ansa-zirconocene dichloride 1 exhibits a for practical applications. Among these is the pos­ D l- Z r - D 2 angle (D 1 and D2 denote the cen­ sible use in catalytic organic synthesis, i.e. catalytic troids of the Cp ring systems) of 116.2°. To our olefin coupling reactions to give new organic monomers. The zirconium complex 1 is chiral. However, the * Reprint requests to Prof. Dr. G. Erker. 1/MAO catalyst system does not produce polypro- 0932-0776/95/0300-0469 $06.00 © 1995 Verlag der Zeitschrift für Naturforschung. All rights reserved. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. 470 G. Erker et al. ■ A Rigid Cr Bridged Ansa-Zirconocene-Derived Catalyst System pylene by enantiomorphic site control. The lack of The dilithio compound 4 provides the basis for any chirality transfer from the bent metallocene a synthetic entry to derivatives bearing alkyl sub­ backbone to the final C-C-coupling product is a stituents at the Cp ring systems. It was hydrolyzed distinct disadvantage for a variety of potential to give a mixture of double bond shift isomers of applications of such a very active catalyst system. the neutral ligand system. This was directly sub­ Therefore, we have searched for ways to achieve jected to the fulvene synthesis (treatment with an effective influence of the bent metallocene’s acetone in methanol solution in the presence of stereochemical properties in the catalytic process. pyrrolidine) to yield a 1:1 mixture of two double As it turned out, attaching alkyl substituents at the bond shift isomers of the mono-fulvene system 5. Cp rings oriented toward the front side of the bent metallocene wedge leads to a drastic change in the stereochemical characteristics of the propene polymerization process. We here describe a typi­ cal example. 2CH ,Li Results and Discussion The synthesis of the parent compound 1 exhibit­ 1) h 2o h 3c ing the rigid annulated [4-(//5-cyclopentadienyl- 2 ~> h 3c c h 3 idene)-4,7,7-trimethyl-(?75-4,5,6,7-tetrahydro- 113 V-, 3 0 Li indenyl)]ligand system is outlined in Scheme 1. 1) LiAlH4 Reaction of the bis-fulvene 2 [7], readily available o (mixture of isomers) reflux h 3c 2) H20 0 Li by treatment of 2,5-hexanedione with cyclopenta- 3) 2 CHjLi diene in the presence of pyrrolidine [ 8 ], with two h 3c / CH , molar equivalents of methyllithium gives 4. In this H 3 Schem e 2. reaction sequence one equivalent of CH3Li acts as 7 a nucleophile, the other as a base. Treatment of 4 Reaction of the fulvene 5 with two molar equiv­ with ZrCl4 gives the ansa-zirconocene dichloride alents of methyllithium in ether proceeded with in low but reproducible yield (10-15% ). methylanion addition at the exocyclic fulvene sp2- carbon center [9] to give a tert-butyl group at the Cp ring system and deprotonation at the tetra- hydroindene moiety to yield the dilithiated ligand system 6. Subsequent reaction of this reagent with C H 3 C H 3L i ZrCl4 furnished the /m-butyl-substituted ansa- H,C metallocene complex as a mixture of two dia- stereoisomers 8 a and 8 b in a 1:1 ratio. Treatment of the fulvene 5 with LiAlH 4 in refluxing ether followed by hydrolysis gave the isopropyl-substituted ligand system. Subsequent CH 3Li lithiation with methyllithium and treatment with zirconium tetrachloride produced a 1:1 mixture of the isopropyl-substituted ansa-zirconocene dichlo­ H3C / CH 3 ride diastereoisomers 9 a and b. Crystallization ZrCI, gave one of these (9 b) isomerically pure. Accord­ H,C ing to an X-ray crystal structure analysis this dia­ stereomer is characterized by the stereochemical descriptor (4R*, p-R*, p -R*). In the crystal complex 9 b exhibits an ansa- Scheme 1. metallocene framework very similar to the one ob- G. Erker et al. • A Rigid C]-Bridged Ansa-Zirconocene-Derived Catalyst System 471 H,C. CH H?C ß H3 H,C H,C 6 [R = C(CH3)3] 8b 7 [R = CH(CH3)2] 9a 9b Schem e 3. Fig. 1. Two projections of complex 9b (4R *, p-R*. p-R*-diastereoisomer). served for the parent compound 1 (see Table I). planes is 72.3°, which is very similar to that in 1 The C(sp2)-C (sp 3)-C (sp 2) angle at the ligand- (71.4°). The electronic features of the ansa-zir- bridging carbon center in 9 b is 99.6(2)° (1: conocene complexes 9 b and 1 are probably closely 99.9(2)°). The angle between the cyclopentadienyl related as expressed by their almost identical D 1 -Z r -D 2 angles (9b: 116.6°; 1: 116.3°). In 9b the Cl 1-Z r-C 12 angle is 98.47(3)° [1: 98.3(1)°]. Table I. Selected bond lengths (A), angles (°), and tor­ sion angles (°) of complex 9 b. An inspection of the molecular geometry of complex 9 b (see Fig. 1) reveals that the newly in­ Z r-C l 2.599(2) C 7 -C 8 -C 9 123.1(2) Z r-C 2 2.530(2) C 8 -C 9 -C 4 122.0(2) troduced isopropyl substituent may very effec­ Zr-C3 2.421(2) C9-C4-C13 99.6(2) tively shield one quadrand (i.e. the “south-west” Z r-C 8 2.555(2) Cl 1 -Zr-C 12 98.47(3) Z r-C 9 2.444(2) D 1 -Z r -D 2 116.6 sector) at the front of the bent metallocene unit. Z r-C 13 2.434(2) Cp(A)-Cp(B) 72.3 There may be some additional shielding in the Z r-C 14 2.454(2) C 2 -C 1-C 8 -C 7 166.5(2) “north-east” sector envisaged potentially being Z r-C 15 2.561(2) C3-C9-C8-C7 -168.0(2) Zr-C 16 2.601(2) C 2 -C 3 -C 9 -C 4 161.9(2) caused by some unfavourable steric interaction be­ Z r-C 17 2.469(2) C 1-C 8 -C 9 -C 4 -162.4(2) tween the C ll methyl group and substituents of Zr-Cl 1 2.4459(9) C16-C17-C13-C4 -164.0(2) Zr-C12 2.4287(7) C 15-C 14-C 13-C 4 162.4(2) incoming prochiral a-olefin substrates.
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