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)RhCl2(H2O) 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)RhCl2(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)RhCl2 fragments, (2) the carbene fragments act as basic σ-donor ligands, and (3) the hybridization of the carbene carbon atoms is sp2 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 AgBF4.