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.