| Sumario: | Changing the counteranion along the series Br, BF, PF, SbF in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with lrH(PPh) to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to lr to give Ir with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir, not lr, because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. F,¹H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).
|
|---|