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
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-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
endo-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.