Chiral transition metal Lewis acids bearing bis(oxazolinyl)phenyl (Phebox) as a pincer ligand: Synthesis and application for the asymmetric reactions

Optically active bis(oxazolinyl)phenyl (Phebox) was designed as an anionic N-C-N pincer ligand, and (Phebox)SnMe3 was prepared as its stable precursor. The aqua complexes, (Phebox)(RhX2)-X-III (H2O) [X = Cl, Br, F], [ (Phebox) M-II(H2O)] (BF4) [M = Pd, Pt] and [ (Phebox)Pt-IV (H2O)] (BF4), were synthesized by the oxidative addition or transmetallation of Rh-I, Pd-II and Pt-II precursors with (Phebox)SnMe3 and subsequent ligand exchange reaction. X-ray structure studies revealed that the octahedral structure of Rh-III complex is isosteric with the Pt-IV, and the P-II and Pt-II complexes are almost the same configuration with a square-planar structure. These aqua complexes acted as efficient catalysts for enantio selective allylation of aldehydes with allyltin reagents, hetero Diels-Alder reaction of Danishefsky's diene with glyoxylates and the aldol-type condensation of tosylmethyl isocyanide with aldehydes. While (Phebox)Rh-III(SnMe3)Cl complexes catalyzed the Michael addition of alpha-cyanopropionates to acrolein.