Control of Diastereo- and Enantioselectivity in Metal-Catalyzed 1,3-Dipolar Cycloaddition Reactions of Nitrones with Alkenes. Experimental and Theoretical Investigations
1996-01-12T00:00:00Z (GMT) by
The scopes and limitations of the catalytic effects of achiral and chiral Mg(II) and Cu(II) complexes on the stereochemistry of the 1,3-dipolar cycloaddition reaction of nitrones with alkenes have been investigated. A remarkably high degree of <i>endo-</i>selectivity (<i>endo</i>/<i>exo</i> > 20) is induced in the 1,3-dipolar cycloaddition reaction by the presence of a catalytic amount of, especially, a Mg(II)−phenanthroline complex. The diastereochemical assignment of the product is confirmed by an X-ray crystallographic determination of the structure of the <i>exo</i>-isoxazolidine. By the reaction of an alkene bearing a chiral auxiliary, with different nitrones and a catalytic amount of the Mg(II)−phenanthroline complex, one of four possible diastereomers of the isoxazolidines is exclusively formed. The absolute stereochemistry of this product is also assigned by an X-ray crystallographic investigation. The presence of a catalytic amount of a chiral Mg(II)−bisoxazoline complex in the 1,3-dipolar cycloaddition reaction leads to high <i>endo-</i>selectivity and occasionally with an ee > 80%. The reaction mechanism of the Mg(II)-catalyzed reaction is discussed on the basis of the experimental results and semiempirical quantum chemical calculations. The calculations are used to account for the catalytic effect of the Mg(II)−ligand complexes and to determine transition state energies for both the uncatalyzed and Mg(II)−ligand-catalyzed reactions.