Acyl Phosphonates: Good Hydrogen Bond Acceptors and Ester/Amide Equivalents in Asymmetric Organocatalysis

Abstract

This study demonstrates that unsaturated acyl phosphonates are excellent hydrogen-bond acceptors in enantioselective organocatalysis. By employing chiral thioureas or squaramides as catalysts, the acyl phosphonates are effectively coordinated and activated by hydrogen bonding, thereby providing successful relay of the chirality from the catalyst to the substrate. A variety of highly stereoselective conjugate additions to alpha,beta-unsaturated acyl phosphonates were performed, using different carbon-based nucleophiles such as oxazolones, indoles, and 1,3-dicarbonyl compounds. The reaction concept has been developed to be a double nucleophilic reaction, and it is shown that the acyl phosphonates serve as masked ester or amide equivalents, which upon quenching with the second nucleophile generate the parent structures in situ. Accordingly, formal C-C bond formation reactions of ester and amide substrates are achieved, affording a broad spectrum of optically active conjugate adducts in good yields and excellent enantioselectivities. Based on the experimental results, the mechanisms for the different reactions are discussed, including the approach of the oxazolones, indoles, and 1,3-dicarbonyl compounds to the acyl phosphonate coordinated to the catalyst and the role of the catalyst for the reaction course of the nucleophiles.