A Chiral Bis(oxazoline) Phosphine with a Rigid Backbone: Ligand Synthesis and Characterization of Products Obtained in Highly Diastereoselective Reactions with a Rhodium(I) Complex and a Tetrahedral “RhRu₃” Cluster

A novel bis(oxazoline)−thienylphosphine ligand (3, abbreviated as (S,S)-BOTPHOS) has been synthesized in an enantiomerically pure S,S state and spectroscopically characterized. The reaction of 3 with Rh2(CO)4Cl2 in a dichloromethane solution affords the mononuclear Rh(Cl)2(CH2Cl)(κ3-(S,S)-BOTPHOS) octahedral complex (4), containing BOTPHOS coordinated to the Rh(III) center in a mer mode through the phosphorus atom and the two nitrogens of the oxazoline fragments. The coordination sphere of this complex displays an unusual rigidity, which is determined by intramolecular hydrogen bonding between the chlorides and protons of the organic ligands. Treatment of the CpRhRu3(H)2(CO)10 cluster with a slight excess of (S,S)-BOTPHOS results in the substitution of two carbonyls at a ruthenium atom and chelate coordination of the ligand through the phosphorus and a nitrogen atom of an oxazoline ring to give CpRhRu3(H)2(CO)8(κ2-(S,S)-BOTPHOS) (5). The bidentate coordination mode of BOTPHOS makes the phosphorus atom of this ligand chiral and simultaneously differentiates the ruthenium vertexes in the tetrahedral RhRu3 cluster. This differentiation generates an asymmetry in the cluster framework and introduces a fourth chiral center in the cluster molecule. A specific feature of this reaction consists of the quantitative diastereoselection induced by the chirality of the BOTPHOS oxazoline fragments. This induction produces only S configurations in both of the new chiral elements generated in the cluster molecule. Molecular structures of the complexes obtained have been established by X-ray crystallography and have been confirmed by the NMR spectroscopic study of these compounds in solution. CD spectroscopic measurements of 3−5 indicate that these compounds have been obtained in their enantiopure state.