Use of bifunctional ureas to increase the rate of proline-catalyzed α-aminoxylations

The rate of the proline-catalyzed α-aminoxylation of aldehydes is significantly increased in the presence of a bifunctional urea. Structure-activity relationship data indicate that both an amine and a urea are crucial for rate enhancement. The evidence presented herein suggests that this rate enhancement originates from the hydrogen bonding interaction between the bifunctional urea and an oxazolidinone intermediate to increase the rate of enamine formation. Proline derivatives that are incapable of forming oxazolidinones exhibit no rate enhancement in the presence of the bifunctional urea. The rate enhancement is general for a variety of aldehydes, and the faster reactions do not reduce yields or selectivities.