NMR Investigation of Asymmetric Conjugate Additions Using Chiral 4-Phenyloxazolidinone as a Mechanistic Probe

The asymmetric conjugate addition of organocopper(I) reagents to α,β-unsaturated N-acyl-4-phenyl-2-oxazolidinones has been studied by and ¹³C-NMR spectroscopy using the Evans-type 4-phenyloxazolidinone 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 α- and β-protons of the metallo enolate 3 was measured as 150° which provided valuable information for examining the stereochemical effects of the β position on the chirality of the α center. Enolates 3 and 4 are reversibly temperature dependent. Enolate 3 is the major component at 253 K, while enolate 4 becomes the major component at 293 K. Therefore, temperatures lower than ~253 K are required for high stereoselectivity in the electrophilic bromination of the resulting enolate to build an α-chiral center. The free energy of activation (ΔG) and rate constant (k_c) of the equilibrium were measured as 15.0 kcal/mol and 1.4 × 10² s^-1, respectively.