Reaction mechanism of the reductive elimination in the catalytic carbonylation of methanol. A density functional study

Reductive elimination, the final step of the Monsanto and Cativa processes, has been studied using the density functional theory with the hybrid B3LYP exchange and correlation functional. To our knowledge, this is the first systematic computational study of the reductive elimination for which even the experimental studies are rare. We have studied different isomers of the anionic dicarbonyls [Rh(CO)2(COCH3)I3]− (1) and [Ir(CO)2(COCH3)I3]− (2). Several possible reaction routes for the elimination of CH3COI from 1 and 2 have been explored. In addition, different isomers of the neutral tricarbonyl [Ir(CO)3(COCH3)I2] (3) and possible reaction paths connected to 3 have been studied. Our results show mer,trans-1 to be the dominant intermediate in the rhodium system although its transformation to fac,cis-1 and the elimination from this seems to be the most likely reaction pathway. In the anionic iridium system, the dominating intermediate is proposed to be fac,cis-2. In the neutral iridium system, mer,cis-3 is proposed to be the dominant intermediate. While inspecting the iridium system as a whole, one could propose a transformation from anionic dicarbonyl to neutral tricarbonyl that would enhance the total rate of the reductive elimination. This observation is similar to that already verified in the 1,1-insertion in the Cativa process. In general, the geometrical arrangement of the different ligands has a large effect on the catalytic activity of the different possible intermediates of these processes.