Optically pure C-2-symmetric transition metal complexes. Steric consequences of flanking the cyclopentadienyl anion with a pair of bridged bicyclic terpene-derived hydrocarbon subunits

The method developed by Erker for the preparation of ''dibornacyclopentadiene'' 1 has been generalized to encompass two related cyclopentadienes flanked by bridged bicyclic subunits derived from (1R)-(+)-verbenone (4). Whereas lithium dimethylcuprate addition to 4 leads to 5a, reaction with diisobutylaluminum hydride and methylcopper affords the less substituted analog 5b. Both saturated ketones undergo the Shapiro reaction and subsequent in situ condensation with ethyl formate to deliver the dienols 7a,b, respectively. The dehydrative cyclization of these intemediates proved to be more difficult to accomplish than in the earlier work involving 1. Exposure of 7a to camphorsulfonic acid in benzene at 25 degrees C gives rise to 8 in 82% yield. These conditions proved much too harsh for 7b, which however undergoes ring closure to give 9 (68%) with iodine in ether. Following deprotonation, 8 can be silylated to give 11, but neither the lithium cyclopentadienide nor the silane can be coaxed into forming a variety of titanocene or zirconocene complexes. These reactants must consequently be more sterically crowded than 1, which does react with MCl(4) (Erker) and CpTiCl(3) (this work). Remarkably, hydrocarbon 9 completely resists deprotonation, a direct reflection of the extreme nonbonded congestion in the vicinity of its sp(3)-bound cyclopentadienyl proton.