Formation of Fischer Carbene Complexes in Asymmetric Aldol-Type Condensation of an Isocyanide Component on Bis(oxazolinyl)phenylrhodium(III) Complexes with Aldehydes: Stereochemistry, Structural Characterization, and Mechanistic Studies

The reaction of (Phebox)RhCl 2 (H 2 O) complexes 3 (Phebox = 2,6-bis(oxazolinyl)phenyl) with methyl isocyanoacetate ( 1 ) and ( p -tolylsulfonyl)methyl isocyanide (TosMIC; 2 ) provided the corresponding isocyanide complexes (Phebox)RhCl 2 (L) ( 4, L = methyl isocyanoacetate; 5, L = TosMIC) in almost quantitative yields. These isocyanide complexes 4 and 5 readily reacted with aldehydes in the presence of t -BuOK to give the aldol adducts as diastereomeric mixtures of chiral Fischer carbene complexes 6 − 9 . NMR study and X-ray crystallography of these carbene complexes revealed that (1) the aldol reactions proceed with trans stereoselectivity with modest to good diastereofacial selectivities on the (Phebox)RhCl 2 fragments, (2) the carbene fragments act as basic σ-donor ligands, and (3) the hybridization of the carbene carbon atoms is sp 2 and the electron-deficient carbene carbon centers are π-conjugated with both neighboring nitrogen and oxygen atoms. We also succeeded in the isolation of both enantiomers of 2-oxazoline derivative 14 from the corresponding chiral carbene complexes 8 by treatment with AgBF 4 .