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)SnMe3 (3; Phebox = 2,6-bis(oxazolinyl)phenyl) with PdCl2-(PhCN)(2) or K[PtCl3(C2H4)](H2O) gave (Phebox)(MCl)-Cl-II complexes (4, M = Pd; 5, M = Pt) in modest to good yields. The (Phebox)(PtCl3)-Cl-IV 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)(PtCl2)-Cl-IV fragment G is isosteric with the (Phebox)(RhCl2)-Cl-III 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 BF4 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-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)Pt-II(H2O)](BF4) (i-Pr-10).