Palladium Complexes with Chiral Diphospholane Ligands: Comparative Catalytic Properties and Analysis of (η3‐Allyl)palladium Species

The chiral diphospholane ligands Duphos (1) and Duxantphos (2), which can be differentiated by their bite angle, were applied to palladium‐catalysed asymmetric reactions, essentially allylic alkylations with symmetrically (3a−d) or unsymmetrically (5a,b) substituted allylic acetates as substrates. The most interesting results were found with the first series of substrates in which 2 was more reactive and/or selective than 1. The model complexes of the catalytic intermediates [Pd{(S,S)‐1}(η3‐cyclohexenyl)](BF4) (10) and exo‐[Pd{(R,R)‐2}(η3‐cyclohexenyl)](SbF6) (11) could be characterised by X‐ray crystallography. 1H and 31P NMR spectroscopic analyses of [Pd{(R,R)‐2}(η3‐diphenylallyl)](SbF6) (12) and [Pd{(R,R)‐2}(η3‐allyl)](SbF6) (13) revealed the presence of exchanging isomers in solution. Structural data provided by these techniques combined with the preferential rotation model led to a satisfactory interpretation of the catalytic allylic alkylation results. It appears that the bite angle plays a crucial role in the positioning of the proximal methyl groups, which determines the distribution of allyl isomeric intermediates and the balance between clockwise and anticlockwise rotations in the enantioselective step. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)