Michael addition of chiral Fischer aminocarbene complexes to nitroolefins: study on the effect of the Michael acceptor structure on diastereoselectivity

The anions of the pentacarbonyl(chromium) trans-2,6-dimethylmorpholinyl(methyl)carbene and pentacarbonyl(chromium) trans-3,5-dimethylpiperidinyl(methyl)carbene complexes added to E- and Z-nitrostyrenes through a diastereoselective Michael-type reaction to give precursors of beta -aryl-gamma -butyric acid derivatives. The diastereoselectivity observed with the former carbene was dependent on the nature of the substituent present in the 4-position of nitrostyrenes and was higher when it was an electron-withdrawing group. The presence of 12-crown-4 ether in the reaction medium increased both reaction times and diastereoselectivity. Theoretical calculations were performed to rationalize the stereochemical outcomes of the reactions and to support the proposed transition state models.