Dinuclear Allylpalladium Complexes of C2-Symmetric Pyrazolate-Bridged Bis(oxazoline) Ligands (pyrbox's): Structures, Dynamic Behavior, and Application in Asymmetric Allyllic Alkylation

A series of new chiral binucleating pyrazolate-based N-donor ligands (3a-d) with oxazoline side arms (coined pyrbox's) have been synthesized. Bimetallic methallylpalladium complexes (4a-d) of these ligands were obtained, and the solid-state structures of complexes 4a,c were characterized by X-ray diffraction. NMR spectroscopy revealed that in solution 4a-d exist as three different isomers that differ in the orientation of the two methallyl ligands. Exchange between the isomers (i.e., allyl rotation) was observed in two-dimensional NOESY NMR experiments as syn/syn and anti/anti interconversion of the allylic hydrogen atoms; kinetic parameters for the fluxional behavior have been determined. The catalytic activity of the complexes was tested in palladium-catalyzed allylic alkylation of rac-(E)-1,3-diphenylallyl acetate. By comparison of the set of complexes 4a-d that feature different ligand scaffolds, both steric and electronic factors were found to be important for enantiocontrol, and a model has been proposed for rationalizing the observed enantioselectivities.