The [1,5]-migration reaction has attracted considerable attention from experimentalists and theoreticians for decades. Although it has been extensively investigated in various systems, studies on pyrrolium derivatives are underdeveloped. Herein, a theoretical study on the reaction mechanism of [1,5]-migration in both pyrrolium and pyrrole derivatives is presented. The results reveal lower activation barriers in [1,5]-migration of electropositive groups (AuPMe3 and SnH3 ) in pyrrolium derivatives, although the bond dissociation energies of the Au-N bond (98.8 kcal mol-1 ) and Sn-N bond (81.7 kcal mol-1 ) are larger than that of the N-F bond (57.6 kcal mol-1 ). The unexpectedly lower activation barriers (4.5 and 4.9 kcal mol-1 for AuPMe3 and SnH3 , respectively) for [1,5]-migration of electropositive groups, in comparison with the [1,5]-fluorine shift, can be attributed to aromaticity stabilizing the transition states, as revealed by significantly negative nucleus-independent chemical shift (NICS) values. Further studies indicate that charge distribution and frontier molecular orbitals also play some roles in [1,5]-migration of pyrrolium derivatives.
Keywords: aromaticity; density functional theory calculations; heterocycles; migration reactions; substituent effects.
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.