Promotion of Iridium-Catalyzed Methanol Carbonylation:  Mechanistic Studies of the Cativa Process

The iridium/iodide-catalyzed carbonylation of methanol to acetic acid is promoted by carbonyl complexes of W, Re, Ru, and Os and simple iodides of Zn, Cd, Hg, Ga, and In. Iodide salts (LiI and Bu₄NI) are catalyst poisons. In situ IR spectroscopy shows that the catalyst resting state (at H₂O levels ≥ 5% w/w) is fac,cis-[Ir(CO)₂I₃Me]^-, 2. The stoichiometric carbonylation of 2 into [Ir(CO)₂I₃(COMe)]^-, 6, is accelerated by substoichiometric amounts of neutral promoter species (e.g., [Ru(CO)₃I₂]₂, [Ru(CO)₂I₂]_n, InI₃, GaI₃, and ZnI₂). The rate increase is approximately proportional to promoter concentration for promoter:Ir ratios of 0−0.2. By contrast anionic Ru complexes (e.g., [Ru(CO)₃I₃]^-, [Ru(CO)₂I₄]^2-) do not promote carbonylation of 2 and Bu₄NI is an inhibitor. Mechanistic studies indicate that the promoters accelerate carbonylation of 2 by abstracting an iodide ligand from the Ir center, allowing coordination of CO to give [Ir(CO)₃I₂Me], 4, identified by high-pressure IR and NMR spectroscopy. Migratory CO insertion is ca. 700 times faster for 4 than for 2 (85 °C, PhCl), representing a lowering of ΔG^⧧ by 20 kJ mol^-1. Ab initio calculations support a more facile methyl migration in 4, the principal factor being decreased π-back-donation to the carbonyl ligands compared to 2. The fac,cis isomer of [Ir(CO)₂I₃(COMe)]^-, 6a (as its Ph₄As⁺ salt), was characterized by X-ray crystallography. A catalytic mechanism is proposed in which the promoter [M(CO)_mI_n] (M = Ru, In; m = 3, 0; n = 2, 3) binds I^- to form [M(CO)_mI_n+1]^-H₃O⁺ and catalyzes the reaction HI_(aq) + MeOAc → MeI + HOAc. This moderates the concentration of HI_(aq) and so facilitates catalytic turnover via neutral 4.