Synthesis and Structure of C₂-Symmetric, Doubly Bridged Bis(indenyl)titanium and -zirconium Dichlorides

Abstract

The synthesis of the new C 2h -symmetric doubly bridged 1,2′-(ethane-1,2-diyl)-2,1′-(ethane-1,2-diyl)bis(indene) ligand 9 from 1,5-dimethyl-1,5-cyclooctadiene via a selenium-catalyzed oxidation reaction and a double Nazarov π-cyclization reaction is reported. Only one bridged C 2 -symmetric metallocene diastereomer was formed in the metalation of 9 because the geometry of the double bridge precluded formation of a meso isomer. The C 2 -symmetric [1,2′-(ethane-1,2-diyl)-2,1′-(ethane-1,2-diyl)bis(indenyl)]titanium and -zirconium dichlorides 10 and 11 were prepared in good (63 and 67%) yields, with no evidence of oligomeric metallocene products. The solid-state structure of 11 was determined by X-ray crystallography. The related bis(indene) 19, containing a bicyclo[3.3.1]nonane bridging moiety, was synthesized from Meerwein's diketone but could not be efficiently metalated.C 2 -symmetric ansa-metallocenes of group 4 transition metals are among the most prominent catalysts 2 for a number of organic transformations such as: epoxidation, 3 hydrogenation, 4 and carbomagnesation 5 of alkenes, reduction of imines 6 and carbonyls, 7 and diene cyclization. 8 Several synthetic groups have demonstrated that C 2 -symmetrical ansa-metallocenes with reduced flexibility and enhanced availability of the active metal center for the approaching monomer molecule are considered to be the best organometallic catalysts for the stereoregular polymerization of alkenes. 9 A synthetic challenge in this area has been to avoid the formation of undesired meso side products to give a selective synthesis of dl-ansa-metallocenes with well-defined geometries. Although the tethering of ligands at an unsymmetrical site (e.g. the 1-position of indenyl) has in some cases led selectively to C 2 -symmetrical ansa-metallocenes, these complexes are often conformationally too mobile to provide a well-defined geometry. 2,10 One successful approach for the selective formation of conformationally well-defined, C 2 -symmetric ansa-metallocenes has been the use of a biaryl bridge to link cyclopentadienyl or indenyl ligands at a symmetrical site (e.g. the 2-position of indenyl). 11 Another recent approach has been to use doubly bridged bis(cyclopentadienyl) ligands to give conformationally well-defined complexes. 12-14 Compounds 1-3 are recently reported doubly bridged "cyclopentadienylphane" type metallocene dihalides.In an effort to develop a facile, possibly general, and large-scale route to bis(ethanediyl)-bridged ansa-bis-(indenes), we have investigated the Nazarov cycliza-X Hollis, T. K.; Odenkirk, W.; Rheinhold, A. L.; Whelan, J.; Bosnich, B. Organometallics 1993, 12, 4391. (d) Huttenloch, M. E.; Diebold, J.; Rief, U.; Brintzinger, H. H.; Gilbert, A. M.; Katz, T. Prosenc, M.-H.; Rief, U.; Brintzinger, H. H.

Strategies for the Synthesis of Chiral Carbon‐Bridged Group IV ansa‐Metallocenes

“…The double-bridgedb is-indenyl metallocenes 75 and 76 which are structurally relatedt ot itanocene 31 (see Scheme 8) have been prepared as well (Scheme 15). [66] Here, the required ligand precursor 74 was not accessed throughadouble Skattebøl reaction, butt hrough aN azarov cyclization of 73,f ollowed by ar eductiona nd dehydration. [67] Interestingly,t he structurally more rigid methylene-bridged derivatives were not accessible.…”

Stereochemical Control by an Ester Group or Olefin Ligand in Platinum‐Catalyzed Carboalkoxylation of 6‐(1‐Alkoxyethoxy)‐ hex‐2‐ynoates

“…Solvent was then removed under reduced pressure and the residue was purified by silica gel column chromatography using 6% of ethyl acetate/hexane to provide 1h as a white solid; yield: 1.14 g (88%). 1 13…”

Enantioselective Hydrogenations with Chiral Titanocenes

“…This problem may also be resolved by incorporation of a double-bridge in the ligand, to form complexes such as 40-42 [47,48,49] or disilyl-bridged 43 titanocenes [50,51]. Because of the rigid structure of these complexes, only one titanocene enantiomer is obtained.…”