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Carbon−Oxygen Bond Forming Mechanisms in Rhenium Oxo-Alkyl Complexes

Cheng, Mu-Jeng and Nielsen, Robert J. and Ahlquist, Mårten and Goddard, William A., III (2010) Carbon−Oxygen Bond Forming Mechanisms in Rhenium Oxo-Alkyl Complexes. Organometallics, 29 (9). pp. 2026-2033. ISSN 0276-7333. https://resolver.caltech.edu/CaltechAUTHORS:20100524-150452201

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Abstract

Three C−X bond formation mechanisms observed in the oxidation of (HBpz_3)ReO(R)(OTf) [HBpz_3 = hydrotris(1-pyrazolyl)borate; R = Me, Et, and iPr; OTf = OSO_2CF_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 α-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 α-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 (ΔG^‡ = 11.8 kcal/mol) lower than α-hydrogen abstraction by OTf^− (ΔG^‡ = 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 α-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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/om900881xDOIArticle
http://pubs.acs.org/doi/abs/10.1021/om900881xPublisherArticle
ORCID:
AuthorORCID
Cheng, Mu-Jeng0000-0002-8121-0485
Nielsen, Robert J.0000-0002-7962-0186
Goddard, William A., III0000-0003-0097-5716
Additional Information:© 2010 American Chemical Society. Received October 9, 2009. Publication Date (Web): April 7, 2010. This work was supported partially by Chevron Energy Technology Company and by the Center for Catalytic Hydrocarbon Functionalization, an Energy Frontier Research Center (DOE DE-SC000-1298). Computer resources were funded from grants from ARO-DURIP and ONR-DURIP.
Funders:
Funding AgencyGrant Number
Chevron Energy Technology CompanyUNSPECIFIED
Department of Energy (DOE)DE-SC000-1298
Army Research Office (ARO)UNSPECIFIED
Office of Naval Research (ONR)UNSPECIFIED
Issue or Number:9
Record Number:CaltechAUTHORS:20100524-150452201
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20100524-150452201
Official Citation:Carbon−Oxygen Bond Forming Mechanisms in Rhenium Oxo-Alkyl Complexes Mu-Jeng Cheng, Robert J. Nielsen, Mrten Ahlquist, William A. Goddard III Organometallics 2010 29 (9), 2026-2033
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:18412
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:25 May 2010 15:06
Last Modified:03 Oct 2019 01:42

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