Our starting point in this work was the bis-tetrahydroindenyl ligand of Brintzinger. Although the derived zirconocene complex 9 is often drawn as though the cyclohexane rings were flat, in fact they are half-chairs, so the flap of the cyclohexane ring proximal to the Zr can be either endo or exo. Thus, Brintzinger-derived complexes such as 9 are flexible, with three conformers, exo - exo (10), endo - endo (11), and endo - exo.
We had two objectives in designing a new, usefully chiral ligand for Zr. Given the efficacy of 9in many Zr- and Ti-mediated processes, we wanted to mimic the spatial arrangement of the ligand system around the metal as closely as possible. Especially, it seemed likely to be most useful to lock the ligand into the exo - exo conformation (10). We also wanted to make the ligand inherently chiral, so that it would not be necessary to resolve the derived complexes. These considerations led us to complex 12, in which the half-chair of the cyclohexane is locked exo - exo by bridging methylenes. The addition of gem-dimethyl substitution on the bridging methylene serves to direct metalation to the opposite face of the cyclopentadiene, assuring an inherently chiral ligand. This analysis was supported by computational studies - the ZINDO-minimized structure of 12 is predicted to be C2-symmetric, and exo-exo.
Activity of Catalyst 12: We have begau to explore the reactivity and selectivity, especially enantioselectivity, of 12. We started with catalytic hydrosilylation. Thus, reduction of 12 with two equivalents of n-BuLi, followed by the addition of polysilane and ketone 18, gave, after work-up, the secondary alcohol 19. This product alcohol was >97 : 3 one enantiomer (analysis by chiral europium shift reagent, 1H NMR). The reduction proceeded rapidly with 1 mol % of complex 12.
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