Michael Addition of Chiral Fischer Aminocarbene Complexes to Nitroolefins: Study on the Effect of the Michael Acceptor Structure on Diastereoselectivity

摘要

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 p-aryl-y-butyric acid derivatives. The diastereoselectivity observed with tpe 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 diastereoselec- tivity. Theoretical calculations were performed to rationalize the stereochemical outcomes of the reactions and to support the proposed transition state models.