Concerted Enantioselective [2+2] Cycloaddition Reaction of Imines Mediated by a Magnesium Catalyst

Enantioselective [2 + 2] cyclization between an imine and a carbon–carbon double bond is a versatile strategy to build chiral azetidines. However, α-branched allenoates have never been successfully applied in [2 + 2] cyclization reactions with imines, as they always undergo Kwon’s [4 + 2] annulation in previous catalytic methods. Herein, a simple in situ generated magnesium catalyst was employed to successfully achieve the enantioselective [2 + 2] cyclization reaction of DPP-imines and α-branched allenoates for the first time. Insightful experiments including KIE experiments, controlled experiments, Hammett plot analysis, and 31P NMR studies of initial intermediates indicate that the current [2 + 2] cyclization of imine most likely involves an asynchronous concerted transition state. Further mechanistic investigations by combining kinetic studies, ESI experiments, 31P NMR studies of coordination complexes, and controlled experiments on reaction rates under different catalyst loading amounts provided the coordination details for this [2 + 2] cyclization reaction between DPP-imines and α-branched allenoates. This new approach was applied to the synthesis of various chiral aza-heterocycles, including the enantioselective synthesis of the key intermediate of a lipid-lowering agent Ezetimibe