The catalytic conjugate addition of nitroalkanes to a,b-unsaturated ketones is one of the most important processes in organic synthesis, [1, 2, 3] since the corresponding products are very useful intermediates for the synthesis of a variety of more elaborate structures, such as amino alkanes, amino carbonyls, lactones, and pyrrolidines. In the past decades, tremendous effort has been devoted to this area; however, to the best of our knowledge, most of these syntheses are promoted by organocatalysts [4] or phase-transfer catalysts, [5] although reactions catalyzed by chiral metal complexes, such as heterobimetallic chiral catalysts, [6a] lanthanum trisbinaphthoxide, [6b] and a salen-Al complex, [6c] have also been reported. Furthermore, catalysts in previous studies generally suffer from limitations in substrate scope that mean they are only effective for one particular type of enone and catalysts with high efficiency for both chalcone and "cinnamone" derivatives have rarely been reported. Therefore, finding a simple, efficient synthesis for both of these types of enone, with high enantioselectivity and broad substrate scope, is still challenging and interesting. Herein, we report that a Sc-A C H T U N G T R E N N U N G (OTf) 3 /N,N'-dioxide complex [7, 8] efficiently promotes the Michael addition of nitromethanes to enones, providing the desired adducts in excellent yields (up to 99 %) and excellent enantiomeric excesses (up to > 99 % for chalcone and up to 98 % for "cinnamone" derivatives).
The reaction between chalcone and nitromethane was selected as the model reaction for optimizing the conditions. A screening of different metals revealed that ScA C H T U N G T R E N N U N G (OTf) 3 was superior to any other at giving the product (Table , entry 5 vs. 1-4). Subsequently, the structure of the N,N'-dioxide ligand was studied (L1-L5). It was shown that ligands with bulky groups at the ortho position of the aniline functionality, such as isopropyl groups, provide good results (Table , entry 8 vs. 6 and 7). As for the chiral backbone, if an (S)ramipril-derived N,N'-dioxide was used instead of those derived from l-proline or (S)-pipecolic acid, the result was slightly improved (> 99 % ee; Table , entry 9).
To further improve the efficiency of the reaction, several other reaction conditions, such as additives and solvents, [a] L.