Direct, facile aldehyde and ketone α-selenenylation reactions promoted by L-prolinamide and pyrrolidine sulfonamide organocatalysts

A new catalytic method for direct α-selenenylation reactions of aldehydes and ketones has been developed. The results of exploratory studies have demonstrated that L-prolinamide is an effective catalyst for α-selenenylation reactions of aldehydes, whereas pyrrolidine trifluoromethanesulfonamide efficiently promotes reactions of ketones. Under optimized reaction conditions, using N-(phenylseleno)phthalimide as the selenenylation reagent in CH₂Cl₂ in the presence of L-prolinamide (2 mol %) or pyrrolidine trifluoromethanesulfonamide (10 mol %), a variety of aldehydes and ketones undergo this process to generate α-selenenylation products in high yields. Mechanistic insight into the L-proline and L-prolinamide catalyzed α-selenenylation reactions of aldehydes with N-(phenylseleno)phthalimide has come from theoretical studies employing ab initio methods and density functional theory. The results reveal that (1) the rate-limiting step of the process involves attack of the enamine intermediate at selenium in N-(phenylseleno)phthalimide and (2) the energy of the transition state for the reaction catalyzed by prolinamide is lower than that promoted by proline. This result is consistent with experimental observations. The role of hydrogen bond interactions in stabilizing the transition states for this process is also discussed.