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

The transmetalation of (Phebox)SnMe 3 ( 3; Phebox = 2,6-bis(oxazolinyl)phenyl) with PdCl 2 (PhCN) 2 or K[PtCl 3 (C 2 H 4 )](H 2 O) gave (Phebox)M II Cl complexes ( 4, M = Pd; 5, M = Pt) in modest to good yields. The (Phebox)Pt IV Cl 3 complexes 8 were also synthesized by the reaction of 5 with CuCl 2 (H 2 O) 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 = BF 4, OTf, OCOCF 3 ). X-ray structure studies revealed that the octahedral structure of (Phebox)Pt IV Cl 2 fragment G is isosteric with the (Phebox)Rh III Cl 2 fragment D, and the (Phebox)M II fragment H (M = Pd, Pt) is almost the same configuration with a square-planar structure. In these complexes, the cationic BF 4 aqua complexes ( 9, M = Pd II; 10, M = Pt II; 11, M = Pt IV ) were found to act as chiral catalysts for the aldol-type condensation of isocyanides and aldehydes in the presence of i -Pr 2 NEt. 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)Pt II (H 2 O)](BF 4 ) ( i -Pr- 10 ).