NMR INVESTIGATION OF ASYMMETRIC CONJUGATE ADDITIONS USING CHIRAL 4-PHENYLOXAZOLIDINONE AS A MECHANISTIC PROBE

The asymmetric conjugate addition of organocopper(I) reagents to alpha ,beta-unsaturated N-acyl-4-phenyl-2-oxazolidinones has been studied by H-1- and C-13-NMR spectroscopy using the Evans-type 4-phenyloxazolidi none auxiliary as a mechanistic probe. Three chiral intermediates were observed directly. The two metal methyl groups in the olefin-copper(I ) complex 2, which is crucial for asymmetric induction, were assigned by both chemical shift and kinetic analysis. The dihedral angle of the alpha- and beta-protons of the metallo enolate 3 was measured as 150 degrees which provided valuable information for examining the stereoch emical effects of the beta position on the chirality of the alpha cent er. Enolates 3 and 4 are reversibly temperature dependent. Enolate 3 i s the major component at 253 K, while enolate 4 becomes the major comp onent at 293 K. Therefore, temperatures lower than similar to 253 K ar e required for high stereoselectivity in the electrophilic bromination of the resulting enolate to build an alpha-chiral center. The free en ergy of activation (Delta G double dagger) and rate constant (k(c)) of the equilibrium were measured as 15.0 kcal/mol and 1.4 x 10(2) s(-1), respectively.