Origins of selectivities in proline-catalyzed alpha-aminoxylations.

The mechanisms, transition structures, regioselectivity, and stereoselectivity of proline-catalyzed alpha-aminoxylation reactions were studied with density functional theory (B3LYP/6-31G(d)). The most favorable transition structure involves the nucleophilic attack of the (E)-proline enamine to the oxygen of the nitrosobenzene with proton transfer from the carboxylic acid to the nitrogen. Enamine attack to the oxygen is favored over the attack on the nitrogen due to the greater basicity of the nitrogen. Previously proposed zwitterionic enaminium pathways are highly disfavored due to the energetic penalty associated with charge separations.