Novel Asymmetric Michael Addition of α-Cyanopropionates to Acrolein by the Use of a Bis(oxazolinyl)phenylstannane-Derived Rhodium(III) Complex as a Chiral Lewis Acid Catalyst

The rhodium complex prepared in situ by simply mixing [[RhCl(c-octene)2]2] and [(Phebox)SnMe3] (1) (Phebox = 2,6-bis(oxazolinyl)phenyl) was found to serve as an efficient catalyst for the asymmetric Michael addition of alpha-cyanopropionates (4) to acrolein under mild and neutral conditions. In the present catalytic system, both the temperature of catalyst preparation and the order of the addition of the substrates were very important for the catalytic efficiency and enantioselectivity. Detailed mechanistic studies of this catalytic system revealed that the [(Phebox)RhIII(SnMe3)Cl] complex (9), generated by oxidative addition of [[RhCl(c-octene)2]2] to 1, is an active catalyst and the turnover number (TON) of the present actual catalyst existing in a reaction mixture is greater than 10,000. The obtained (R) stereochemistry of the Michael adducts 5 can be explained by N-bonded enol intermediates C', which are formed by enolization of 4 bound to the Lewis acidic rhodium complex 9. We also found that the active catalyst 9 gradually decomposed in the presence of the remaining [[RhCl(c-octene)2]2] in the reaction mixture to form the catalytically nonactive [(Phebox)RhCl2] fragment A, whose structure was characterized by an X-ray crystallographic study after converting to the tBuNC complex 10.