CONTROL OF DIASTEREOSELECTIVITY AND ENANTIOSELECTIVITY IN METAL-CATALYZED 1,3-DIPOLAR CYCLOADDITION REACTIONS OF NITRONES WITH ALKENES - EXPERIMENTAL AND THEORETICAL INVESTIGATIONS

The scopes and limitations of the catalytic effects of achiral and chi ral Mg(II) and Cu(II) complexes on the stereochemistry of the 1,3-dipo lar cycloaddition reaction of nitrones with alkenes have been investig ated. A remarkably high degree of endo-selectivity (endo/exo > 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 exo-isoxazolidi ne. By the reaction of an alkene bearing a chiral auxiliary, with diff erent nitrones and a catalytic amount of the Mg(II)-phenanthroline com plex, one of four possible diastereomers of the isoxazolidines is excl usively formed. The absolute stereochemistry of this product is also a ssigned by an X-ray crystallographic investigation. The presence of a catalytic amount of a chiral Mg(II)-bisoxazoline complex in the 1,3-di polar cycloaddition reaction leads to high endo-selectivity and occasi onally with an ee > 80%. The reaction mechanism of the Mg(II)-catalyze d reaction is discussed on the basis of the experimental results and s emiempirical 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 M g(II)-ligand-catalyzed reactions.