Bis(oxazoline) Ligands Containing Four and Five Spacer Atoms:  Palladium Complexes and Catalytic Behavior

Chiral bis(oxazoline) ligands with biphenyl ( A − E ) and diphenyl ether and dibenzofuran ( F and G, respectively) backbones were prepared, and their coordination chemistry to allyl palladium fragments was studied. The allylic complexes ( 1L and 2L, 1 for the allyl and 2 for the 1,3-diphenylallyl) were characterized, both in the solid state and in solution. X-ray crystal structures were determined for complexes 1C and 1F . The ligands coordinate to palladium in a N,N -chelating way, giving monometallic complexes, except ligand C, which acts as a bridge between two metallic fragments, by N,S -coordination to palladium. Ligand H, with four sp 3 carbon spacers, did not react with several allylic starting materials. Complexes 1L have been used as catalytic precursors in the model allylic alkylation reaction, producing up to 89% of enantiomeric excess. The catalytic behavior shown by these systems can be rationalized in terms of the coordination mode of the ligands toward the metal.