Chemoselectivity in the Oxidation of Cycloalkenes with a Non-Heme Iron(IV)-Oxo-Chloride Complex: Epoxidation vs. Hydroxylation Selectivity

0507769 - ÚFCH JH 2020 RIV US eng J - Journal Article
Terencio, T. - Andris, E. - Gamba, I. - Srnec, Martin - Costas, M. - Roithová, J.
Chemoselectivity in the Oxidation of Cycloalkenes with a Non-Heme Iron(IV)-Oxo-Chloride Complex: Epoxidation vs. Hydroxylation Selectivity.
Journal of the American Society for Mass Spectrometry. Roč. 30, č. 10 (2019), s. 1923-1933. ISSN 1044-0305. E-ISSN 1879-1123
R&D Projects: GA ČR(CZ) GA18-13093S
EU Projects: European Commission(XE) 682275
Institutional support: RVO:61388955
Keywords : C–H activation * DFT calculations * Epoxidation * Gas-phase reactions * Iron complexes
OECD category: Physical chemistry
Impact factor: 3.255, year: 2019
Method of publishing: Limited access

We report and analyze chemoselectivity in the gas phase reactions of cycloalkenes (cyclohexene, cycloheptene, cis-cyclooctene, 1,4-cyclohexadiene) with a non-heme iron(IV)-oxo complex [(PyTACN)Fe(O)(Cl)]+, which models the active species in iron-dependent halogenases. Unlike in the halogenases, we did not observe any chlorination of the substrate. However, we observed two other reaction pathways: allylic hydrogen atom transfer (HAT) and alkene epoxidation. The HAT is clearly preferred in the case of 1,4-cyclohexadiene, both pathways have comparable reaction rates in reaction with cyclohexene, and epoxidation is strongly favored in reactions with cycloheptene and cis-cyclooctene. This preference for epoxidation differs from the reactivity of iron(IV)-oxo complexes in the condensed phase, where HAT usually prevails. To understand the observed selectivity, we analyze effects of the substrate, spin state, and solvation. Our DFT and CASPT2 calculations suggest that all the reactions occur on the quintet potential energy surface. The DFT-calculated energies of the transition states for the epoxidation and hydroxylation pathways explain the observed chemoselectivity. The SMD implicit solvation model predicts the relative increase of the epoxidation barriers with solvent polarity, which explains the clear preference of HAT in the condensed phase.
Permanent Link: http://hdl.handle.net/11104/0298739