Chiral Transition Metal Lewis Acids Bearing Bis (oxazolinyl) phenyl (Phebox) as a Pincer Ligand: Synthesis and Application for the Asymmetric Reactions

摘要

Optically active bis(oxazolinyl) phenyl (Phebox) was designed as a nanionic N-C-N pincer ligand, and (POhebox )SnMe_3 was prepared as its stable precursor. The aqua complexes, (Phebox)Rh~(III)X_2(H_2O) [X = Cl, Br, F], [(Phebox)M~(II)(H_2O)] (BF_4) [M = Pd, Pt] and [(Phebox)Pt~(IV)(H_2O)] (BF_4), were syn thesized by the oxidative addition or transmetallation of Rh~I, Pd~(II) and Pt~(II) precursors with (Phebox)SnMe_3 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 Pd~(II) and Pt~(II) complexes are almost the same configuration with a square-planar structure. These aqua complexes acted as efficient catalysts for enantioselective allylation of aldehydes with allyltin reagents, hetero Diels-Alder reaction of Daniskefsky's diene with glyoxylates and the aldol-type condensation of tosylmethyl isocyanide with aldehydes. While (Phebox)Rh~(III) (SnMe_3)Cl complexes catalyzed the Michael addition of alpha-cyanopropionates to acrolein.