The mechanism of the iridium-catalyzed functionalization of a primary C-H bond at the gamma position of an alcohol5 is investigated by density functional theory (DFT) calculations. A new Ir-III-Ir-V mechanism is found to be more feasible than the previously reported Ir-I-Ir-III mechanism. 10 In the Ir-III-Ir-V mechanism, the reaction begins with the initial formation of (Me(4)phen) Ir-III(H)[Si(OR) Et-2](2) from the catalyst precursor, [Ir(cod) OMe](2) (cod = 1,5-cyclooctadiene). The catalytic cycle includes five steps: (1) the insertion of norbornene into the Ir-H bond to produce (Me(4)phen) IrIII(norbornyl)[Si(OR) Et-2](2) (R = -CH(C2H5) C3H7); (2) the Si-H oxidative addition of HSi(OR) Et-2 to form (Me(4)phen) IrVH(norbornyl)[Si(OR) Et-2](3); (3) the reductive elimination of norbornane to furnish (Me(4)phen) IrIII[Si(OR) Et-2](3); (4) the intramolecular C-H activation of the primary C-H bond at the gamma position; and (5) the Si-C reductive elimination to produce the final product and regenerate the catalyst. The highest barrier in the IrIII-IrV mechanism is 7.3 kcal/mol lower than that of the Ir-I-Ir-III mechanism. In addition, the regioselectivity of the C-H activation predicted by this new Ir-III-Ir-V mechanism is consistent with experimental observation.