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.