Nitrogen-containing xanthene-based chiral ligands: Synthesis, NMR and X-ray analyses, and catalytic applications of their palladium, silver and rhodium complexes
Résumé
We have synthesized and fully characterized three chiral ligands with a rigid xanthene backbone having phosphorus and/or nitrogen donor atoms, viz. the diimine 5 and the iminophosphines 15a and 15b. When the imine functions were derived from (1R)-camphor (compounds 5 and15a), their configuration was (E), whereas a dynamic mixture of (Z) and (E) isomers was observed in solution for the (1S)-fenchone derivative 15b. Attempts to coordinate palladium (II) with these ligands have led to untractable mixtures. By contrast, we isolated in good yields the well-defined silver (I) complexes 18–20, in which the ligands were bidentate, and the rhodium (III) complex 21 in which ligand 15a was tridentate. Complexes 18, 19 and 21 were characterized by X-ray crystallography. A fluxionality of the silver complexes owed to conformational (flip-flop) equilibrium was revealed by VT-NMR analysis. The rhodium complex was not fluxional. In the silver complexes, the camphimine was in the (E) configuration as in the free ligand, whereas it had isomerized to (Z) in the rhodium complex. Pd-, Ag- or Rh-catalyzed reactions implemented with our ligands have in general led to good conversions but modest enantioselectivities (0–36%).