Some of the key steps in the alkane carbonylation processes developed by Sakakura and Tanaka have been modeled by density functional theory. The catalytic carbonylation cycle involves photochemical activation of the precursor compound Rh(PR3)2Cl(CO) 1, resulting in the 14-electron species Rh(PR3)2Cl 2, which activates the C-H bond of hydrocarbons. The model precursor compound Rh(PH3)2Cl(CO) has a ground state structure with the phosphine ligands in a trans position, whereas 2 for R=H prefers a cis-arrangement of the phosphines (cis-2a) and has a closed shell singlet ground state. The model species 2 with R=H adds a C-H methane bond to produce Rh(PH3)2Cl(H)(CH3) 5, after the formation of the h2-methane complex Rh(PH3)2Cl(h2-CH4) 3. The trans conformation trans-2a of Rh(PH3)2Cl is more reactive towards the C-H methane bond than cis-2a and forms a stronger h2-methane complex. The activation product Rh(PH3)2Cl(H)(CH3) 5 reacts with another CO to form Rh(PH3)2Cl(H)(CH3)(CO) 6, which can either eliminate methane to form 1 or undergo further transformation to eventually form acetaldehyde and 1.

The elimination of methane is relatively facile with kinetic barriers of 72 kJ/mol (trans) and 57 kJ/mol (cis), respectively. In addition, the elimination reactions are exothermic by respectively 112 kJ/mol (trans) and 125 kJ/mol (cis). It is thus clear that alkane elimination seriously can impede the carbonylation cycle. The catalytic activity can also be reduced by dimerization of Rh(PH3)2Cl.

The productive part of the catalytic cycle involves migratory insertion of CO into the Rh-CH3 bond of Rh(PH3)2Cl(H)(CH3)(CO) 6, generating the rhodium acyl Rh(PH3)2Cl(H)(CH3CO) 11 and further, the addition of another CO molecule to 11 yielding Rh(PH3)2Cl(CO)(H)(CH3CO) 12. Finally, acetaldehyde is eliminated from 12 to regenerate 1. We find that the rate limiting step in the carbonylation of methane is the migratory insertion of coordinated CO into the Rh-CH3 bond, 6 Æ 11, with a barrier of 129 kJ/mol (trans) and 114 kJ/mol (cis), respectively, whereas the reductive elimination of methane from 6 has a lower calculated barrier of 72 kJ/mol (trans) and 57 kJ/mol (cis). Therefore, the carbonylation will be seriously retarded by the concurrent reductive alkane elimination. We find that the overall productivity of the methane carbonylation process is determined by the migratory insertion step.