Mechanisms of 1,4‐dipolar cycloaddition between propa‐1,2‐diene and imines catalyzed by PBu 3 : A DFT investigation

The mechanisms for the 1,4‐dipolar catalytic cycloaddition between ethyl 2‐methylbuta‐2,3‐dienoate and ( E )‐N‐ethylidene‐4‐methylbenzenesulfonamide were studied with M06‐2X‐D3/6‐311+G(d,p) level, and IEFPCM was applied to simulate the solvent effect of DCM. The computational results suggested that ethyl 2‐methylbuta‐2,3‐dienoate and tributylphosphine first had an interaction; then, an electrophilic reaction occurred, and one new C─C bond has been formed; lastly, there are two possible processes to achieve this reaction. The Gibbs free energy surfaces indicate that the favor path is that PBu 3 as the catalyst and oxidant can help to accelerate this reaction to yield the final product ethyl 6‐methyl‐1‐tosyl‐1,2,5,6‐tetrahydropyridine‐3‐carboxylate with lower energy barrier. Our results demonstrated that these reactions can take place at a certain condition, in good agreement with the experimental observation.