Mechanism and electronic effects in nitrogen ylide-promoted asymmetric aziridination reaction

The mechanism and stereoselectivity of the aziridination reaction between guanidinium ylide and a series of para-substituted benzaldehydes have been studied by using density functional theory methods. The mechanistic details and analyses of the key elementary steps involved in (a) the addition of nitrogen ylide to benzaldehydes and (b) subsequent fragmentation of the resulting oxaspirocyclic intermediate are presented. The relative energies of important transition states and intermediates are found to be useful toward rationalizing reported diastereoselective product formation. The relative energies of the key transition states could be rationalized on the basis of the differences in steric, electrostatic, and other stabilizing weak interactions. The deformation analysis of the transition state geometries exhibited good correlation with the predicted activation barriers. The changes in cis/trans diastereoselectivity preferences upon changes in the electron donating/withdrawing abilities of the para substituents on benzaldehyde are identified as arising due to vital differences in the preferred pathways. The large value of reaction constant (ρ > 4.8) estimated from the slope of good linear Hammett plots indicated high sensitivity to the electronic nature of substituents on benzaldehyde. The formation of trans-aziridine in the case of strong electron donating groups and cis-aziridines with weakly electron donating/withdrawing group has been explained by the likely changes in the mechanistic course of the reaction. In general, the predicted trends are found to be in good agreement with the earlier experimental reports.