DFT study of the ethylene hydroformylation catalytic cycle employing a HRh(PH3)(2)(CO) model catalyst

The potential energy hypersurface for ethylene hydroformylation catalyzed b y HRh(PH3)(2)(CO) was mapped out at the CCSD(T)//B3LYP and B3LYP//B3LYP lev els of theory using effective core potentials. Combining the results obtain ed for each elementary step there are a number of possible pathways for the hydroformylation catalytic cycle, originating from the trans (2a) and cis (2b) isomers of the active catalyst. At both levels of theory employed, a p reference was predicted for the pathways originating from the trans isomer of the active catalyst, 2a. The alternative pathways originating from the c is isomer, 2b, were discounted because of the large activation barriers pre dicted for the two migratory insertion reactions, arising from unfavorable interactions between the equatorial phosphine ligands and the migrating axi al ligand, Considering only those reaction paths originating from the trans isomer of the active catalyst, 2a, a strong preference was identified for the oxidative addition of Hz to the unsaturated Rh-acyl complex (6a) on the same side as the ethyl moiety of the acyl ligand, as opposed to the additi on on the opposite face (i.e,, on the same side as the acyl oxygen). In the final aldehyde reductive elimination step an energetic preference was pred icted for the migration of the hydride ligand trans to the CO ligand in the most stable Ha oxidative addition products; however, this migration would lead to tile generation of the cis catalyst instead of the trans catalyst. Therefore, either the less electronically favored hydride ligand trans to t he PH3 ligand migrates to the acyl carbon, thereby regenerating 2a, or ther e must be some interconversion between the pathways originating from 2a and 2b. This interconversion would most likely occur at either the eta (2)-ole fin adduct (3b)or the CO addition intermediate (5b), since previous researc h indicates that complexes of this type can undergo facile pseudorotation. For the energetically feasible catalytic cycle, the CO insertion step is pr edicted to be the rate-determining step with predicted activation barriers of 20.4 and 14.9 kcal/mol, at the CCSD(T)//B3LYP and B3LYP//B3LYP levels of theory, respectively. The experimental enthalpy of hydroformylation (-28 k cal/mol), corresponding to the energy difference between the end aldehyde p roduct and the constituent reactant species, C2H4, CO, and H-2, is overesti mated by about 7 kcal/mol (-34.7 kcal/mol) at the B3LYP//B3LYP level. Howev er, recomputing the energies of the species with the CCSD(T) methodology yi elds a value of -24.4 kcal/mol, which is more in accord with experiment.