Comparison of orbital symmetry allowed and forbidden ring opening of aziridines

The rates of reactions of N-aryl-cis-2,3-diphenyl aziridines (4a–d) with dimethyl acetylenedicarboxylate (DMAD) are dependent only on aziridine concentration suggesting slow conversion of aziridines to azomethine ylides which subsequently react in 1,3-dipolar cycloadditions to yield Δ3-pyrrolines. The Hammett reaction constant (−0.80) determinee for the N-substituents in the slow ring opening further suggests direct generation of azomethine ylides from these aziridines. The stereochemistry of the cycloaddition products derived from cis-1,2,3-triphenylaziridine with DMAD, maleic anhydride, dimethyl maleate, and dimethyl fumarate is consistent with the conrotatory generation of the ylides followed by their stereospecific cis-cis cycloaddition to the dipolarophiles.Similarly, cycloaddition of 7-aryl-6a,7a-dihydroacenaphtha-[1,2]-aziridines (5a–f) with DMAD yields bicyclic Δ3-pyrrolines and the rate is dependent only on aziridine concentration. This suggests the reaction proceeds via a slow disrotatory cleava...