Imidazole Side Chain Bridging Pentamethyl Cyclopentadienyl Ligand And Iv, Vice Family Transition Metallocene Complexes Of Synthetic Research

Synthetic pathways to the pentamethylcyclopentadiene ligands are described. The general synthesis starts from the reaction of the 2,3,4,5-tetramethylcyclopent-2-enone (4) with (1-methylimidazole-2-yl)methyllithium (5a) and 1-diethoxylimidazole-2-methyllithium (13). One intermediate named 1-(1-methylimidazol-2-yl)methyl-2,3,4,5-tetramethylcyclopent -2-enol (6) was isolated during the purification. Acid-catalyzed elimination of the hydroxyl group in (6) could only give the byproduct with an exo-cyclic C=C bond. The byproduct was characterized by ¹H-NMR, ¹³C-NMR, ROSEY and finally proved to be 1-methyl -2-{(E)-[(4RS,5RS)-2,3,4,5-tetramethylcyclopent-2-enylidene]methyl}-1H-imidazole (7). The preparation of potassium salt of l-memyl-2-{(E)-[(4RS,5RS)-2,3,4,5-tetramethylcyclopent -2-enylidene]methyl}-1H-imidazole (7) was thoroughly developed.During the process of the preparation of 1-(1-methylimidazol-2-yl)methy1-2,3,4,5 -tetramethylcyclopent-2-enol (6) and 1-(1-H-imidazol-2-yl)methyl-2,3,4,5-tetramethylcy-clopent-2-enol (14), two unusual kinds of products were isolated: 4-(1,2-dimethylimidazol -5-yl)-2,3,5,6-tetramethyltetrahydro-2H-pyran⁴-ol (9) (enantiomers 9a and 9b) and 4-(2-methylimidazol-5-yl)-2,3,5,6-tetramethyltetrahydro-2H-pyran-4-ol (16). Crystal and molecular structures of 9a and 16 were established by X-ray diffraction analysis.Synthetic approaches to lithium salt of the C2-bridged pentamethylcyclopentadiene ligand were also attempted. The direct reactions of 1,2,3,4-tetramethylfulvene (26) with (1-methyl-1H-imidazol-2-yl)methyllithium (5a) and [2-methyl-1H-imidazole-1,1'-diyl -κN¹,C^1']dilithium (30) were supposed to afford the corresponding {1-[2-(1-methyl -1H-imidazol-2-yl)ethyl]-2,3,4,5-tetramethylcyclopenta-2,4-dienyl}lithium (27) and {{1-[2-(1H-imidazole-2-yl)ethyl]-2,3,4,5-tetramethylcyclopenta-2,4-dienyl} -1,1 '-diy1-κN^1' ,C¹} dilithi urn (29).During the research work, complete syntheses of 1,2,3,4-tetramethylfulvene (26), pentamethylcyclopentadiene (22), lithium pentamethylcyclopentadienide (23), [η⁵-1,2,3,4,5-pentamethylcyclopentadienyl]triichlorozirconium(IV) (24), and 2,3,4,5-tetramethylcyclopent -2-enone (4) were performed. The isolation and purification procedure for 1,2,3,4- tetramethylfiilveve (26) was improved.