Enantioselective Diene Cyclozirconation

Our objective is to use cyclozirconation to transform diene 40 into diol 44 in high enantiomeric purity. To this end, we have set out to computationally design a chiral bis Cp zirconium complex 41 such that one diastereomeric zirconacycle, 42 , would be substantially more stable than the other diastereomer, 43.

Two considerations guided our design. The first was that the complex must be usefully chiral. We predict computationally, for instance, that the Brintzinger complex 39 would show no thermodynamic selectivity in the cyclozirconation of 40 .

The second consideration was that the zirconocene must be quite open. The Brintzinger complex (39 ) participates only very slugglishly in intramolecular diene cyclozirconation. We thought that a good beginning place would be an ethano-bridged bis Cp such as 41 .

The question then was, how to make the ethano-bridged bis Cp usefully chiral? The key to this problem was to design substituents on the ethano bridge that would impart a sufficient chiral twist to the Cp rings. After some exploration, we have found that 1,2-bis-1-naphthyl substituents (e.g. 41 ) are predicted computationally to work well (calculated stability difference between the two trans diastereomers for R = H is 2.0 kcal/mol, for R = CH3 is 2.4 kcal/mol; in both series the cis-fused diastereomers are less stable than either of the trans diastereomers).

Douglass F. Taber . . . U. of Delaware

For an overview, see: computational catalyst design.

Home Page: Taber home page.

Questions and comments to: Douglass F. Taber