The oxidative addition and migratory 1,1-insertion in the Monsanto and Cativa processes. A density functional study of the catalytic carbonylation ofmethanol

Density functional theory with hybrid B3LYP exchange and correlation functi onal has been used to investigate the first two catalytic reactions, the ox idative addition and migratory I,l-insertion of the Monsanto and Cativa pro cesses. One of the main interests has been to study if the previously unide ntified trans forms of the active catalytic species [Rh(CO)(2)I-2](-) (1) o r [Ir(CO)(2)I-2](-) (2) have any significance in these processes. The oxida tive addition has been studied using both cis and ti ans forms of 1 and 2. We have also studied the oxidative addition of methyl iodide to [Rh(CO)(2)I -3](2-) (3). In addition, different isomers of dicarbonyls [CH3Rh(CO)(2)I-3 ](-) (4) and [CH3Ir(CO)(2)I-3](-) (5) and tricarbonyl [CH3Ir(CO)(3)I-2] (8) has been used in the l,l-insertion study to see if these could provide new , alternative reaction pathways. The calculated free energies of activation for the oxidative addition of methyl iodide to cis-l and cis-2 are 20.8 an d 16.9 kcal/mol, respectively. The corresponding free energy barriers for t rans-1 and trans-2 are 15.0 and 13.2 kcal/mol, respectively. The oxidative addition is the rate-determining step in the Monsanto process and the react ion with trans-1 is predicted to accelerate that step. The presence of 3 co uld enhance the addition even more; the free energy of activation is only 5 .6 kcal/mol. For the 1,1-insertions we have found similar activation energi es in fac,cis- and mer,tl ans-structures. In the rhodium system, the free e nergies of activation are in the order of 20 kcal/mol and in the iridium sy stem 30 kcal/mol. Interestingly, the insertions in,ner,cis-dicarbonyls have consider ably lower activation energies, half of those calculated for the insertions in mer,trans- and Sac,cis-structures. The iodide dissociation fr om fac,cis-5 could provide the path to mel,cis-5 and so significantly enhan ce the rate of the insertion in the iridium system. The rate of the inserti on should also increase if experimentally proposed tricarbonyl 8 is used in stead of 5. According to our calculations of different isomers of 8, this s eem to be true although the insertion barrier in mer,cis-5 is calculated to be even lower. Our results are consistent with the experiments and other c omputational results. These results show that the geometrical arrangement o f the ligands has a very large effect on the catalytic activity of the comp lexes and this suggests possible improvements to these industrially importa nt processes. (C) 2001 Elsevier Science B.V. All rights reserved.