Iodide and acetate promotion of carbonylation of methanol to acetic acid: Model and catalytic studies

Iodide and acetate salts increase the rate of reaction of Li[RhI₂(CO)₂] with MeI at 25° C in acetic acid solution, a model of the rate-determining step in catalytic methanol carbonylation. The effects of water, LiBF₄, and other additives are also reported. Iodide salts also promote catalytic methanol carbonylation at low water concentrations. In the case of LiI promoter, lithium acetate is produced in catalytic solutions via the reaction of LiI with methyl acetate. The promotional effects of iodide and acetate on both the model and catalytic systems are rationalized in terms of iodide or acetate coordination to [RhI₂(CO)₂]⁻, to yield five-coordinate Rh^I anions as reactive intermediates for rate-determining reactions with MeI.