Optically active C3-symmetric triarylphosphines in asymmetric allylations

A new class of optically active, monodentate, C3-symmetric ligands for asymmetric catalysis, 1–3, were prepared via routes involving asymmetric reduction of aryl ketones (). Single crystal X-ray structures of trans-PdCl2(phosphine)2 complexes of ligands 1a, 2a and 3a showed that the ligands do not adopt perfect C3-symmetric conformations in the solid state; in fact, all three have one aromatic ring lying ‘face-on’ to the metal. Circular dichroism studies of the same complexes provided some evidence for an aromatic ordering effect for complex 22, but none for 21 and 23. Palladium catalyzed allylation reactions using ligands 1–3 showed that encouraging enantioselectivities could be obtained, particularly for cyclic, allylic substrates.