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Mechanistic Details of the Fe + ‐Mediated CC and CH Bond Activations in Propane: A Theoretical Investigation
Author(s) -
Holthausen Max C.,
Koch Wolfram
Publication year - 1996
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19960790717
Subject(s) - chemistry , exothermic reaction , reductive elimination , computational chemistry , density functional theory , reaction mechanism , activation energy , endothermic process , elimination reaction , saddle point , medicinal chemistry , catalysis , organic chemistry , geometry , mathematics , adsorption
Quantum‐chemical calculations employing a density‐functional theory/ Hartree‐Fock hybrid method (B3LYP) have been used to explore the mechanistic details of the CC and CH bond‐activation processes in propane mediated by a bare Fe + ion. While the theoretically predicted results are in complete accord with all available experimental data, they give rise to a different mechanistic picture than envisaged previously. In contrast to earlier speculation, the activation barriers for the initial insertion steps of Fe + into a CH or CC bond are found to be significantly below the Fe + + C 3 H 8 channel. The rate‐determining steps for both, the CC and the CH bond activation branches of the [FeC 3 H 8 ] + potential‐energy surface rather occur late on the respective reaction coordinates and are connected with saddle points of concerted rearrangement processes. The CC bond activation, which leads to the exothermic reductive elimination of methane, occurs via the CC inserted species and not as a side channel originating from a CH inserted ion, as assumed hitherto. For the CH bond‐activation processes, which finally results in the exothermic expulsion of molecular hydrogen, two energetically similar reaction channels for an [1,2]‐elimination exist. The results clearly show, that an [1,3]‐H 2 ‐elimination mechanism cannot compete with the [1,2]‐elimination paths, in line with the experimental findings. Overall, a lower energy demand for the reductive elimination of methane compared to the loss of H 2 is obtained, straightforwardly explaining the preference of the former process observed experimentally.