Torquoselectivity studies in the generation of azomethine ylides from substituted aziridines

Aziridines are useful precursors to the azomethine ylide family of 1,3-dipoles whose cycloaddition chemistry has been extensively exploited in the synthesis of heterocyclic targets. The torquoselectivity of aziridines that lack a plane of symmetry was investigated as an essential component of the calculation of the overall relative reaction rates and in prediction of the stereochemistry of the 2,3-trans compounds in 1,3-dipolar cycloaddition chemistry. It has been found at the MP2(Full)/6-311++G(d,p)//MP2(Full)/6-31+G(d) level that outward rotation is preferred for electronegative or anionic substituents while electropositive and cationic substituents favor inward rotation. After consideration of frontier molecular orbital theory, inductive, resonance, and electrostatic effects, an explanation of the preferred direction of rotation during ring cleavage that is based on substituent electron-withdrawing ability by means of a polar effect is presented.