Palladium Complexes with Chiral Diphospholane Ligands: Comparative Catalytic Properties and Analysis of (η 3 ‐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 }(η 3 ‐cyclohexenyl)](BF 4 ) ( 10 ) and exo ‐[Pd{( R , R )‐ 2 }(η 3 ‐cyclohexenyl)](SbF 6 ) ( 11 ) could be characterised by X‐ray crystallography. 1 H and 31 P NMR spectroscopic analyses of [Pd{( R , R )‐ 2 }(η 3 ‐diphenylallyl)](SbF 6 ) ( 12 ) and [Pd{( R , R )‐ 2 }(η 3 ‐allyl)](SbF 6 ) ( 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)