Synthesis and X-ray Crystal Structures of Bis(oxazolinyl)phenyl-Derived Chiral Palladium(II) and Platinum(II) and -(IV) Complexes and Their Use in the Catalytic Asymmetric Aldol-Type Condensation of Isocyanides and Aldehydes

Abstract

The transmetalation of (Phebox)SnMe3 (3; Phebox = 2,6-bis(oxazolinyl)phenyl) with PdCl2(PhCN)2 or K[PtCl3(C2H4)](H2O) gave (Phebox)MIICl complexes (4, M = Pd; 5, M = Pt) in modest to good yields. The (Phebox)PtIVCl3 complexes 8 were also synthesized by the reaction of 5 with CuCl2(H2O)2 as an oxidant. These chloride complexes, 4, 5, and 8, were in turn converted to the cationic complexes by treatment with the corresponding silver salts (AgX: X = BF4, OTf, OCOCF3). X-ray structure studies revealed that the octahedral structure of (Phebox)PtIVCl2 fragment G is isosteric with the (Phebox)RhIIICl2 fragment D, and the (Phebox)MII fragment H (M = Pd, Pt) is almost the same configuration with a square-planar structure. In these complexes, the cationic BF4 aqua complexes (9, M = PdII; 10, M = PtII; 11, M = PtIV) were found to act as chiral catalysts for the aldol-type condensation of isocyanides and aldehydes in the presence of i-Pr2NEt. Excellent trans diastereoselectivities and modest to good enantioselectivities were obtained in the reaction of tosylmethyl isocyanide (TosMIC, 1) by the use of [(i-Pr-Phebox)PtII(H2O)](BF4) (i-Pr-10).