Catalytic Enantioselective Addition of Cyclic β-Keto Esters with Activated Olefins and N-Boc Imines Using Chiral <i>C</i>₂-Symmetric Cationic Pd²⁺ N-Heterocyclic Carbene (NHC) Diaqua Complexes

The asymmetric addition of cyclic β-keto esters to activated olefins and N-Boc imines was realized by using chiral cationic C2-symmetric N-heterocyclic carbene (NHC) Pd2+ diaqua complexes 1a,b as the catalysts, producing the corresponding adducts in moderate to high yields (up to 95%) and with good to high enantioselectivities (up to 96% ee). Nevertheless, the most significant observation is that when the (R)-NHC Pd2+ diaqua complex was used in these reactions, different absolute configurations were observed in comparison to those obtained with catalysts obtained from (R)-phosphane ligands.以手性阳离子型C₂对称氮杂环卡宾（N-heterocyclic carbene, NHC）Pd²+二水合配合物1a、1b为催化剂，实现了环状β-酮酯与活化烯烃、N-叔丁氧羰基亚胺之间的不对称加成反应。所得目标加成产物收率可达中等至较高水平（最高达95%），对映选择性良好至优异（最高可达96%对映体过量值，enantiomeric excess, ee）。不过，本研究最显著的发现为：当使用(R)-NHC Pd²+二水合配合物进行该类反应时，所得产物的绝对构型与采用(R)-膦配体制备的催化剂所得到的产物构型存在明显差异。