Probing Reaction Mechanism of [1,5]‐Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier
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.