Comparison of orbital symmetry allowed and forbidden ring opening of aziridines

Abstract

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 cleavage of the aziridine rings producing azomethine ylides which undergo subsequent addition to DMAD. The Hammett reaction constant for the ring opening step is +0.74 suggesting that the transition state for the ring cleavage is electron deficient in the aziridinyl ring but the cycloaddition reactions of 5a with isomeric dipolarophiles as well as several dienes and cycloheptatriene reveal only stereospecific 1,2-cycloadditions expected of an azomethine ylide intermediate.