Diastereoselective Synthesis of a Bis(tetrahydroindenyl)zirconocene Dichloride with “Ansa-Fused” Annulated Six-Membered Ring Systems:  A Specific Solution of the Synthetic rac-/meso-Group 4 Bent Metallocene Problem

• Published in: Organometallics 1997-06, Vol.16 (13), p.2900-2908
• Main Authors: Könemann, Martin, Erker, Gerhard, Fröhlich, Roland, Kotila, Sirpa
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om9700526

1-Phenyl-1,4-pentanedione (3) is selectively converted to the corresponding fulvene−ketone (4) by treatment with cyclopentadiene in the presence of pyrrolidine. Treatment with lithium dimethylcuprate results in selective CH3-addition at the fulvene carbon and subsequent ring-closure reaction to give 7, which in the presence of lithium diisopropylamide is rapidly dehydrated to yield the bicyclic fulvene 8. Its treatment with amalgamated calcium metal leads to reductive fulvene coupling and formation of the calcocene complex (10) of the substituted 7,7‘-bi(tetrahydroindenyl) ligand. Only the respective rac-isomer is obtained. Hydrolysis of 10 followed by double deprotonation with butyllithium yields the corresponding 7,7‘-bi(tetrahydroindenyl) dianion equivalent 16. In tetrahydrofuran, 16 is cleaved at the 7,7‘-linkage with formation of the “monomeric” disproportionation products 11-Li and 12-Li, and treatment of 16 with zirconium tetrachloride in THF leads back to the formation of the fulvene 8. Transmetalation to zirconium is, however, cleanly achieved when 16 is treated with donor-ligand-free ZrCl4 suspended in toluene. The corresponding diastereomerically pure rac-ansa-zirconocene dichloride 17 containing the ansa-bridge as part of the α-fused bi(tetrahydroindenyl) ligand system is obtained under these specific conditions. Complex 17 was characterized by X-ray diffraction, and the outcome of the preliminary propene polymerization experiments is reported.