Intermolecular [2 + 2] Cycloaddition/Isomerization of Allenyl Imides and Unactivated Imines for the Synthesis of 1‑Azadienes Catalyzed by a Ni(ClO₄)₂·6H₂O Lewis Acid

The intermolecular [2 + 2] cycloaddition/isomerization between allenyl imides and N-(2-methoxyphenyl) aldimine counterparts catalyzed by a Ni­(ClO4)2·6H2O Lewis acid at room temperature was discovered, providing a facile access to 1-azadiene derivatives with high atom economy. The incorporation of an 2-oxazolidinone group into allene amides resulted in unusual reactivity for the imine-metathesis and synthetic application to a chiral γ,δ-unsaturated β-ketoimide. A mechanistic experiment using density functional theory (DFT) computation in CH2Cl2 with the B3LYP functional rationalized the proposed catalytic pathway involving initial stepwise [2 + 2] cycloaddition to provide an azetidine species, two-time proton transfer to form a 2-azetine intermediate, and final conrotatory ring opening for trans-1-azadiene-based substances.