Mechanistic Studies and Model Compounds

The rate of methanol carbonylation shows zero-order dependence on the concentration of methanol, carbon monoxide, and acetic acid. As shown, it is first order with respect to the concentrations of rhodium and methyl iodide  

The rate-determining step in the catalytic cycle is the oxidative addition of CH3I to 4.1. Oxidative additions of alkyl halides are often known to follow Sa2 mechanisms. This appears to be also the case here. The net negative charge on 4.1 enhances its nucleophilicity and reactivity towards CH3I. 

In situ IR spectroscopy of the reaction mixture at a pressure and temperature close to the actual catalytic process shows the presence of only 4.1. As we will see, all the other complexes shown in the catalytic cycle (i.e., complexes 4.2-4.5) are also seen by spectroscopic methods under milder conditions. Under the operating conditions of catalysis they are not observed by IR spectroscopy, as their concentrations are much less compared to that of 4.1. 

Regardless of the initial source of rhodium, that is, whether it is added just as a halide or as a phosphine complex, under the reaction conditions 4.1 is formed. With phosphine complexes, the phosphine is converted to a phospho-nium (PRj or 11 PR, ) counter-cation. As chelating phosphines bind more strongly than the monodentate ones, the induction time for the formation of 4.1 with chelating phosphines is longer. 

At ambient temperatures and in neat CH3I as a solvent, IR bands and NMR signals for complexes 4.2-4.4 are seen. The stereochemistry of 4.2 as shown in the catalytic cycle is consistent with the spectroscopic data. The relative thermodynamic and kinetic stabilities of complexes 4.1-4.3 under these conditions have also been estimated. The data show that 4.2 is unstable with respect to conversion to both 4.3 and 4.1. In other words, 4.2 undergoes facile insertion and reductive elimination reactions. 

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