On the Role of Planar Chirality in Asymmetric Catalysis:  A Study toward Appropriate Ferrocene Ligands for Diethylzinc Additions

Employing a directed ortho-metalation route, chiral ferrocene-based hydroxyloxazolines have been synthesized, and their capability to serve as bidentate ligand precursors in the diethylzinc addition to benzaldehyde has been investigated. On the basis of the X-ray crystal structures of two ferrocenes, ( S, R p )- 6 and ( S, S p )- 10, it became possible to determine the general importance of the element of planar chirality for diastereomeric ferrocene complexes in this process. Comparison of the assumed transition states revealed a preferential catalyst conformation which led to the design of an adequate arene compound ( S )- 20 that displayed catalytic activity similar to that of ( S, R p )- 6 .