Ansa-metallocene polymerization catalysts derived from [2+2]cycloaddition reactions of bis(1-methylethenyl-cyclopentadienyl)zirconium systems

Abstract

Bis(1-methylethenyl-cyclopentadienyl)zirconium dichloride (7a) was prepared by a fulvene route. Photolysis at 0°C with Pyrexfiltered UV light resulted in a rapid and complete intramolecular [2؉2]cycloaddition reaction to yield the corresponding cyclobutylene-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 hindered open metallocenes that bear bulky isopropyl or tert-butyl substituents at their Cp rings in addition to the active 1-methylethenyl functional group undergo the photochemical ansa-metallocene 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 polymerization catalysts.photochemistry ͉ topochemical reaction M etallocene catalysis has become of great significance in olefin polymerization, especially for polyethylene, stereospecific 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 development (1-3). Various methods have been devised and applied to construct such bent metallocene frameworks that feature a short carbon-or heteroatom-containing bridge between their substituted cyclopentadienyl, indenyl, or fluorenyl ligands. Because of the sensitive character of the organometallic group 4 metal complexes, ligand construction and variation is usually carried out at the free ligand stage before the final transmetallation step to the transition metal in the practical preparative sequences. This is a serious synthetic limitation. It would be highly desirable to have an organic functional group chemistry developed for framework variation at the actual metallocene stage. Previously, some addition reactions to metallocene frameworks had been reported, such as catalytic hydrogenation (4), hydrosilylation (5), hydroboration (6-9), or borylation (refs. 10, 11 and references therein, and 12). However, carbon-carbon coupling reactions at the intact group 4 bent metallocene frameworks were close to nonexistent before our work (13-15). Meanwhile, a few leading examples have emerged from the literature, using, e.g., olefin-metathesis (16-19) or even a variant of the Mannich reaction (20-22) for carrying out carbon-carbon coupling reactions at the reactive group 4 bent metallocene frameworks.Intramolecular photochemical [2ϩ2]cycloaddition reactions may become of a prime importance in this development. We had previously observed that bis(alkenyl-Cp)ZrCl 2 complexes such as, e.g., meso-1 underwent ansa-metallocene formation to yield meso-2 by intramolecular [2ϩ2]cycloaddition when irradiated with UV light. However, this specific reaction was not synthetically useful for a clean ansa-metallocene catalyst development because of its reversibility under the photochemical conditions t...

Functional Group Chemistry at the Group 4 Bent Metallocene Framework: [2+2] Cycloaddition of the Parent Bis(vinyl-Cp/vinyl-Ind)ZrCl2 Systems

“…This study has shown that the parent bis(vinyl-Cp)zirconium dichloride complex ( 6 ) readily undergoes a photochemically induced intramolecular [2+2] cycloaddition reaction under our typical reaction conditions. This observation follows the general trend for this reaction type, as we have seen recently , that the outcome of this organometallic photocycloaddition is strongly dependent on the size and steric bulk of the α-substituent at the alkenyl unit. It seems to be a rule that small substituents (H, CH 3 ) at this position lead to a photostationary equilibrium that lies completely on the side of the ansa -metallocene isomer, whereas large cycloalkyl or aryl substituents may shift this equilibrium substantially, so that in these cases we often find mixtures of the open and ansa -metallocene isomers.…”

“…cycloalkyl, aryl) resulted in photostationary equilibria that sometimes lay far on the side of the open starting materials (see Scheme ) . For us the introduction of the easily prepared Cp-C(CH 3 )CH 2 ligands marked a turning point: attachment of the small methyl substituent at the Cp-conjugated alkenyl moiety resulted in complete conversions to the respective ansa -zirconocene by intramolecular photochemical [2+2] cycloaddition reaction under simple and straightforward experimental conditions . This has made the [2+2] cycloaddition a suitable and reliable method for ansa -zirconocene synthesis …”

How diabetes and insulin therapy affects the lives of people with type 1 diabetes

“…Oft ist es wegen der Empfindlichkeit der metallhaltigen Verbindungen schwierig, an metallorganischen Gerüsten Umwandlungen typischer organischer funktioneller Gruppen durchzuführen. Dies gilt besonders für viele metallorganische Verbindungen “früher” Übergangsmetalle, bei denen erst in jüngster Zeit eine größere Zahl geeigneter Verfahren für solche Transformationen beschrieben wurde 61b–e. 83 Metallfreie katalytische Hydrierungen mit frustrierten Lewis‐Paaren gehören zu diesen hinreichend milden Methoden.…”

“…Das Ansa ‐ Metallocen 133 ist durch eine säurekatalysierte intramolekulare Mannich‐Reaktion leicht aus dem entsprechenden zweifach Enamino‐substituierten Zirconocen zugänglich 61b–e. Im analogen [3]Ferrocenophan 120 eröffnete die nachfolgende Hydrierung beider Doppelbindungen einen Zugang zu einer Reihe nützlicher Liganden für die asymmetrische Katalyse 62.…”

Zirconocene Functional Group Chemistry: Photochemical [4+4] Cycloaddition of Isoprenyl Sidechains to the Bent Metallocene Framework

“…This reaction type proved to be somewhat substituent-sensitive but use of a proper substitution pattern at the alkenyl groups allowed very efficient ansa-metallocene formation by the photochemical [2+2] cycloaddition route. [7][8][9][10][11][12] Photolysis of bis(butadienyl-Cp)ZrCl 2 systems takes a similar route. At ambient temperature this reaction rapidly and selectively leads to the formation of the isomeric cyclooctadienylene-bridged ansa-metallocenes by formal [4+4] cycloaddition.…”