Intramolecular [2 + 2] cycloaddition of bis(alkenylcyclopentadienyl)zirconium dihalides: a novel way of synthesizing ansa-metallocene complexes

Erker, Gerhard; Wilker, Stephen; Krueger, C.; Goddard, Richard

doi:10.1021/ja00053a057

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“…It seems that the 1-methylethenyl substituent was a good choice because it resulted in a very rapid photochemical ring-closure reaction. Under the applied reaction conditions, this substitution pattern achieved a very favorable photostationary equilibrium that was lying practically completely on the ansa-metallocene side, quite different from most early examples reported of this general reaction type at the bent metallocene nucleus (23,24). The interesting observation that the [2ϩ2]photocyclization is not adversely affected by the presence of even very bulky alkyl substituents at the Cp rings makes us hope for a broad application spectrum of this coupling method for ansa-metallocene formation and general carbon-carbon coupling at the stage of the intact early metal bent metallocene frameworks and other related sensitive organometallic systems.…”

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confidence: 80%

“…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 to result in a photostationary equilibrium mixture of the open and closed isomers of most investigated examples under practical conditions (23,24). We later showed that the [2-(1-methylethenyl)indenyl] 2 ZrCl 2 derivative (3) rapidly and completely underwent the intramolecular photolytic [2ϩ2]cycloaddition reaction when irradiated with Pyrex-filtered light to yield 4, from which an interesting homogeneous metallocene ZieglerNatta catalyst system was generated (25)(26)(27)(28).…”

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

Paradies

Kehr

Fröhlich

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

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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...

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

Paradies

Kehr

Fröhlich

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

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“…We had previously shown that the intramolecular [2+2] cycloaddition reaction of bis(alkenylcyclopentadienyl) ligands complexed to Group 4 metals constitutes a viable synthetic pathway to ansa metallocenes. [5] It had been demonstrated that this reaction could even be extended to the construction of ansa calcocenes and related systems (Scheme 1). [6,7] Since it was known that lithium cyclopentadienides undergo facile ligand disproportionation of the donor-ligand-stabilized monomer to give the corresponding anionic lithiocene structures (Scheme 2), [8,9] we decided to use this feature to extend the typical metallocene photocycloaddition reaction to the corresponding alkenyl-functionalized lithiocenes.…”

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Functional‐Group Chemistry of Organolithium Compounds: Photochemical [2+2] Cycloaddition of Alkenyl‐Substituted Lithium Cyclopentadienides

2006

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“…However, many of these systems displayed unfavorable photostationary equilibria. [5] We have now found that bis(2-alkenylindenyl)-ZrCl 2 complexes undergo a rapid photochemical olefin [2+2] cycloaddition to produce cyclobutylene-bridged ansametallocenes in which the annelated indenyl-arene rings are located at the front side of the bent metallocene wedge in a symmetrical fashion above and below the s-ligand system. A series of typical examples will be described herein.…”

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Formation of a Unique ansa‐Metallocene Framework by Intramolecular Photochemical [2+2] Cycloaddition of Bis(2‐alkenylindenyl)zirconium Complexes

et al. 2003

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Developing the chemistry of organic functional groups on bent Group 4 metallocene frameworks and related systems has been notoriously difficult due to the high sensitivity of such organometallic compounds toward the typical reaction conditions used.[1] Noteworthy examples include a Mannichtype carbon-carbon coupling reaction [2] and selective transformations of olefinic substituents attached to the metallocene cyclopentadienyl rings, such as metallacyclic olefin coupling [3] and olefin metathesis reactions.[4] Some time ago we had shown that several bis(alkenylcyclopentadienyl)-ZrCl 2 complexes undergo a photochemical [2+2] cycloaddition reaction to yield novel ansa-metallocenes. However, many of these systems displayed unfavorable photostationary equilibria.[5] We have now found that bis(2-alkenylindenyl)-ZrCl 2 complexes undergo a rapid photochemical olefin [2+2] cycloaddition to produce cyclobutylene-bridged ansametallocenes in which the annelated indenyl-arene rings are located at the front side of the bent metallocene wedge in a symmetrical fashion above and below the s-ligand system. A series of typical examples will be described herein.Two synthetic routes were employed to prepare the ligands.[6] Cross-coupling of 2-bromoindene (1) with the alkenyl Grignard reagents 2 a (R = Me) and 2 b (R = Ph) gave the corresponding 2-alkenylindenes 3 a,b in about 90 % yield each (Scheme 1). The cyclohexyl-substituted 2-alkenylindene was prepared by a complementary route starting from 2-indenyl magnesium bromide (4). Addition to cyclohexyl methyl ketone followed by acid-catalyzed dehydration provided both alkene isomers. Chromatographic separation furnished the required compound 3 c in 26 % yield. Deprotonation of 2-alkenylindenes 3 was achieved by treatment with n-butyl lithium (3 a,c) or LDA (3 b) in Et 2 O at 08C. Subsequent reaction of the resulting alkenylindenyl lithium reagents (5 a-c) in toluene with ZrCl 4 then yielded the bis(2-alkenylindenyl)-ZrCl 2 complexes 6 a (R = Me, 60 %), 6 b (R = Ph, 51 %), and 6 c (R = cyclohexyl, 27 %), respectively. Single crystals for X-ray structure analysis were obtained from the complexes 6 a (R = Me) and 6 c (R = cyclohexyl), in both cases from solutions in toluene/pentane at À20 8C. In the crystal 6 a features a "meso-like" rotational structure [7] in which the annelated phenylene rings of the two indenyl ligands are oriented toward the same side of the bent metallocene wedge (Figure 1, left). The CH 2 = C(CH 3 ) substituents at the indenyl 2-positions point to the other lateral sector. The CH 2 = C(CH 3 ) substituents are both oriented in the indenyl ligand plane (q(C9-C1,C10,C11) = À170.6(6)8) with the =CH 2 groups pointing toward the back side of the bent metallocene wedge.In contrast, the cyclohexyl-substituted complex 6 c attains an approximately C 2 -symmetric "rac-like" conformation [7,8] in the solid state, with the CH 2 = C(cyclohexyl) substituents oriented toward opposite lateral metallocene sectors (Figure 1, right). Again, the alkenyl groups are almost coplanar wit...

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Intramolecular [2 + 2] cycloaddition of bis(alkenylcyclopentadienyl)zirconium dihalides: a novel way of synthesizing ansa-metallocene complexes

Cited by 50 publications

References 1 publication

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

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

Functional‐Group Chemistry of Organolithium Compounds: Photochemical [2+2] Cycloaddition of Alkenyl‐Substituted Lithium Cyclopentadienides

Formation of a Unique ansa‐Metallocene Framework by Intramolecular Photochemical [2+2] Cycloaddition of Bis(2‐alkenylindenyl)zirconium Complexes

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