Lanthanides–pybox: An Excellent Combination for Highly Enantioselective Electrophilic α‐Amination of Acyclic β‐Keto Esters. Isolation of Ternary Pybox/Ln/β‐Keto Ester Complexes

High enantioselection is obtained in α‐amination of acyclic β‐keto esters in the presence of lanthanide triflates and pybox ligands. The Eu 3+ complexes proved more active than those with Yb 3+ and La 3+ and high dependence of ee on the type and steric bulk of the ester group was observed. Excellent results were achieved using a combination of europium triflate with ligand L4 (diphenyl‐pybox). Preparation of ternary complexes ( 8 ) [( L4 )(β‐keto ester enolate)Ln (OTf) 2 ] was accomplished. Complexes 8 and [( L4 )(β‐keto ester)Ln (OTf) 2 ] + , identified by ESI‐MS, are proposed as key intermediates of the α‐amination reaction.