Modular spiro bidentate nitrogen ligands - Synthesis, resolution and application in asymmetric catalysis

A family of new modular, chiral, bidentate, nitrogen-donor ligands containing the large spirobi(chroman) backbone 1 (SPAN) has been prepared. Enantiopure diamines containing the nitrogen donors at the 8,8′ positions of spirobi(chroman) 1 have been synthesized through the resolution of the 8,8′-diamino racemate 3. Accordingly, secondary and tertiary amines [(+)-6/(-)-6 and (+)-5/(-)-5, respectively] and imino derivatives [(+)-7/(-)-7] were obtained in both enantiomeric forms. A new SPAN derivative incorporating aldehyde groups at the 8,8′ positions of the spirobi(chroman) unit [(+)-9/(-)-9] has been synthesized with the aim to extend the ligand library by means of Schiff base chemistry. The derivatization of the isolated enantiomers of 9 obtained by diastereomer resolution gave derivatives containing the nitrogen atoms at the benzylic positions. The syntheses of ligands (+)-10/(-)-10, (+)-11 and (+)-12 illustrate the scope of this methodology. The coordination ability of ligand (-)-3 has been tested by the synthesis and characterization of complex [Pd(OAc)₂{(-)-3}] [(-)-13]. Some preliminary results related to the oxidative kinetic resolution of 1-phenylethanol with this Pd complex are reported.