Carbon-Oxygen Bond Forming Mechanisms in Rhenium Oxo-Alkyl Complexes
2010 (English)In: Organometallics, ISSN 0276-7333, E-ISSN 1520-6041, Vol. 29, no 9, 2026-2033 p.Article in journal (Refereed) Published
Three C X bond formation mechanisms observed in the oxidation of (HBpz(3))ReO(R)(OTf) [HBpz(3) = hydrotris(1-pyrazolypborate; R = Me, Et, and iPr; OTf = OSO(2)CF(3)] by dimethyl sulfoxide (DMSO) were investigated using quantum mechanics (M06//B3LYP DFT) combined with solvation (using the PBF Poisson Boltzmann polarizable continuum solvent model). For R = Et we find the alkyl group is activated through alpha-hydrogen abstraction by external base OTf(-) with a free energy barrier of only 12.0 kcal/mol, leading to formation of acetaldehyde. Alternatively, ethyl migration across the M=O bond (leading to the formation of acetaldehyde and ethanol) poses a free energy barrier of 22.1 kcal/mol, and the previously proposed alpha-hydrogen transfer to oxo (a 2+2 forbidden reaction) poses a barrier of 44.9 kcal/mol. The rate-determining step to formation of the final product acetaldehyde is an oxygen atom transfer from DMSO to the ethylidene, with a free energy barrier of 15.3 kcal/mol. When R = iPr, the alkyl 1,2-migration pathway becomes the more favorable pathway (both kinetically and thermodynamically), with a free energy barrier (Delta G(double dagger) = 11.8 kcal/mol) lower than alpha-hydrogen abstraction by OTf(-) (Delta G(double dagger) = 13.5 kcal/mol). This suggests the feasibility of utilizing this type of migration to functionalize M-R to M-OR. We also considered the nucleophilic attack of water and ammonia on the Re-ethylidene alpha-carbon as a means of recovering two-electron-oxidized products from an alkane oxidation. Nucleophilic attack (with internal deprotonation of the nucleophile) is exothermic. However, the subsequent protonolysis of the Re alkyl bond (to liberate an alcohol or amine) poses a barrier of 37.0 or 42.4 kcal/mol, respectively. Where comparisons are possible, calculated free energies agree very well with experimental measurements.
Place, publisher, year, edition, pages
2010. Vol. 29, no 9, 2026-2033 p.
Alkane oxidation, Alkyl complexes, Alkyl groups, Bond formation, Carbon-oxygen bonds, Experimental measurements, Hydrogen abstraction, Hydrogen transfer, M-O bond, Migration pathway, Nucleophilic attack, Oxidized products, Oxygen atom transfer, Poisson-Boltzmann, Polarizable continuums, Protonolysis, Pyrazolyl, Quantum mechanics, Rate determining step
IdentifiersURN: urn:nbn:se:kth:diva-48871DOI: 10.1021/om900881xISI: 000277212300005OAI: oai:DiVA.org:kth-48871DiVA: diva2:488430
QC 201202072012-02-012011-11-232012-02-07Bibliographically approved