MECHANISM OF OXYGEN ATOM TRANSFER FROM OXAZIRIDINE TO A LITHIUM ENOLATE - A THEORETICAL-STUDY

The asymmetric enolate oxidation protocol employing enantiopure N-sulfonyloxaziridines is highly successful in the synthesis of enantiomerically enriched alpha-hydroxy carbonyl compounds. Molecular orbital calculations at the HF/6-31 + G*//HF/4-31 + G level have been used on a model system to elucidate the structural and electronic features of the transition state for oxygen atom transfer. Oxidation of the lithium enolate of acetaldehyde proceeds by S(N)2 attack of the beta-carbon on the enolate along the O-N bond of the parent oxaziridine. In the transition state the lithium cation is coordinated to both the enolate and the oxaziridine oxygen atoms. Model studies suggest that the sulfonyl oxygen atom is also bound to the metal cation, influencing the stereoselectivity of the resulting alpha-hydroxy carbonyl compound.