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An Anion-Dependent Switch in Selectivity Results from a Change of C−H Activation Mechanism in the Reaction of an Imidazolium Salt with IrH 5 (PPh 3 ) 2

2005; American Chemical Society; Volume: 127; Issue: 46 Linguagem: Inglês

10.1021/ja055317j

1943-2984

Leah Appelhans, Daniele Zuccaccia, Anes Kovacevic, A. Chianese, John R. Miecznikowski, Alceo Macchioni, Eric Clot, Odile Eisenstein, Robert H. Crabtree,

Catalytic C–H Functionalization Methods

Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 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 Ir(III) to give Ir(V) 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(III), not Ir(V), 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. 19F,1H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).