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Control of Diastereo- and Enantioselectivity in Metal-Catalyzed 1,3-Dipolar Cycloaddition Reactions of Nitrones with Alkenes. Experimental and Theoretical Investigations
journal contribution
posted on 1996-01-12, 00:00 authored by Kurt V. Gothelf, Rita G. Hazell, Karl Anker JørgensenThe 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.